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Showing posts with label Antennas. Show all posts
Showing posts with label Antennas. Show all posts

2025-01-17

Five-band HF Linked Dipole [updated January 2025]

An improved build

This antenna was first described in the November-December 2023 Communicator. After a year of using it, I’ve developed some improvements. - Updated January 2025

I appreciate inexpensive but effective antenna design. Sure, you can spend hundreds of dollars to buy a multiband antenna for POTA or GOTA but they are so simple to build, why would you want to? You can enhance your skills and communication capabilities by constructing custom antennas.

One popular and versatile option is the multi-band HF dipole antenna often called a segmented or linked dipole. This antenna design allows for effective communication across multiple HF bands by connecting or disconnecting sections along its length making it an inexpensive asset for ham radio operators, particularly outdoors enthusiasts and those needing a light, packable wire antenna that can be erected almost anywhere, and with good results—better than an end-fed, because each segment of the antenna is pre-tuned during construction, and can be used safely even when no antenna tuner is available.

Ideally it is hung from a pole or tree branch at 20 feet (6m) as an Inverted V, with the center point high and the dipole legs spread at least 110-120 degrees. I have worked the globe with this set-up on sideband and 20 Watts.

 

Oly one side of the dipole is shown 

As before, each of the five sections is cut to the band so no tuner is required. It is a marvellous POTA antenna, far better than any vertical, Hamstick or commercial antenna that I have tried.

You can read/download the updated article at: https://bit.ly/LinkedDipole

~ John VE7TI



2023-12-31

The January - February 2024 SARC Communicator

A great way to start 2024!

The January - February 2024 Communicator, digital periodical of Surrey Amateur Radio Communications is now available for viewing or download.

Read in over 145 countries, we bring you 120 pages of Amateur Radio content from the Southwest corner of Canada and elsewhere. With less fluff and ads than other Amateur Radio publications, you will find Amateur Radio related articles, projects, profiles, news, tips and how-to's for all levels of the hobby.

You can view or download it as a .PDF file:  



Download the Communicator January - February 2024

Previous Communicator issues are at:

https://ve7sar.blogspot.com/search/label/The%20Communicator

and a full index is HERE.  

As always, thank you to our contributors, and your feedback is always welcome. 

The deadline for the next edition is February 15th.

If you have news or events from your club or photos, stories, projects or other items of interest from BC or elsewhere, please contact us at communicator@ve7sar.net

73,

John VE7TI
'The Communicator' Editor






2023-12-27

Restoring a rusty old tower (revisited)

Like being given a 'free' dog...

A BIG Project!

(from The Communicator - August 2013)

Three years ago I received a free tower and rotator. I’ll write about the rotator in another instalment but I’ve since discovered that some ‘free’ ham gear is like being given a free dog… you have to be aware of the hidden costs.

I had never had a tower but always wanted one. I’d been satisfied with my Carolina Windom, an off-centre fed dipole at 25 feet off the ground, that let me use most HF bands right up to 80m. It served me well, and still does but greater involvement in contesting and the desire to move up to a more directional antenna encouraged me to make the move.

Fortunately I have a wife who is also a ham, though not particularly active, but she knows the thrill I get when working a new country or breaking through a pile-up. She was supportive in my quest. Coincidentally I also had to replace the deck on our 30 year-old house. That provided another incentive to get the project started and find the right location. Our old laundry line was accessible from the deck at a corner of the house. The new deck, with glass panels around it would no longer allow this so the new tower would have to do double-duty as the anchor for the clothesline as well. I decided that it would be a light-duty tower as I didn’t have the space for a full-size tower, and I wanted to keep peace with my neighbours.

I spread the word among my fellow club members that I was looking. Pretty soon an offer came in of five ten-foot sections of light duty tower. I picked them up and it was immediately evident that some work would be needed before they would be safe and usable. Thick rust had eaten though much off the galvanized surface. Several of the cross-member welds had broken and one section was noticeably bent. I knew my wife wouldn’t be pleased if I attempted to place that within view.

Over the next several days I spent time washing down my new acquisition, which had evidently spent time behind someone’s shed, judging by the weeds, caked mud and dead critters in and outside the tubing. I also spent some Internet time researching whether I could revitalize this tower and how to go about it.



My daughter-in-law manages a paint store and was able to provide me with some technical advice on surface preparation and coatings. I knew I would have to paint the sections for my wife to accept them right outside her kitchen window. Colour would also be a factor.

I bought two brass wire wheels and some emery paper and set to work to clean off as much loose rust as I could with my power drill. It worked well and a day later I was done. Based on my Internet findings, and helpful reviews by previous users, I tried three products to tackle the remaining rust. From left to right on the adjoining photo they were Permatex brand ‘Rust Dissolving Gel’, ‘Evapo-Rust’ by Rust-stop Canada, and Rust Check brand ‘Rust Converter’. All were applied according to the provided directions and they performed their intended function. The gel, being thicker, clung to the parts better but was much slower and required a lot of re-coating to keep working. The other two products were thinner and more difficult to keep in place, but they produced faster results. If these were small pieces that could be submerged it would be no contest, but keeping to a short section at a time and using a paint brush to keep the area wet with solution clearly showed the Evapo-Rust product to be the most suitable, and the fastest. It also appears to be the most environmentally friendly of the three, though I wouldn't recommend doing this job on your lawn, as I started to do. Yes, the grass did eventually grow back.



The surface was now free of rust and, after another scrub, was ready for inspection. I looked closely at each crosstie and at every weld. Suspicious ones were marked. Several were obviously cracked or already split. With the assistance of Fred Orsetti VE7IO, the welds were repaired. I was ready for paint!


I would have used an oil based primer and top-coat but my expert advised me against it and she was correct. According to the product sheets for such coatings, it is not recommended that you use an oil based product on galvanized surfaces. The paint will release and peel off after a time—and I didn’t fancy the thought of doing this again in a couple of years. There are special coatings available in a spray can specifically for galvanized metal but they are quite expensive with small coverage, exacerbated by the necessity to get inside and out and into all the nooks and crannies around the welds. I decided a brush was the better applicator for that job.

We, (read-in wife-approval mandatory) decided the least noticeable colour on our wooded lot would be a camouflage green. As a result of my ‘colour-Googling’ I had actually suggested a multi-colour camo paint scheme but that was vetoed as being too ‘military looking’, and so the appropriate latex primer and top-coat were tinted. It took exactly one litre each of primer and top coat to paint the five sections twice, with extra coatings on the welds. I used an air sprayer on the legs for the final coat.


Next came even tougher work. I had to remove a section of my cement patio to make the appropriate foundation and dig a big hole. There were brackets available that could be surface mounted but I’m a ‘belt and suspenders’ kind of guy and I wanted this thing in a block of concrete. If guys are not used, the tower manufacturer recommends fastening a section to the house as high up as possible, in my case that was just near the top of the 2nd section. I visited my local scrap yard and purchased some heavy-duty angle aluminium by the pound. I cut pieces to make an equilateral triangle and bolted one to the top plate of the house, running two arms to adjacent tower legs where they were secured by U-bolts. It’s steady as a rock. I used stainless steel hardware for all the section to section connectors in case I ever want (or have to) take it apart. That time is approaching as I have completed a rebuild of a rotator and HF Yagi that will go up in the spring.

It has now been two years plus and the tower shows no signs of either rust or paint failure. It was a lot of effort but I’m pleased I did it. Even with my Carolina Windom centred at the top of the tower, much higher than before, I’m getting much more activity across all the bands.



The sections above the roof blend in nicely with the trees. The final touch was to place flower baskets on the rungs at each level. We now refer to it as the ‘Tower of Flower’ and surprise… the neighbours even say it looks good.

 ~ John VE7TI



2023-11-13

Restricted putting up an antenna?

 Hide it in your holiday lights!

For all their supposed benefits, homeowner’s associations (HOAs) have a reputation of quickly turning otherwise quaint neighborhoods into a sort of Stanford prison experiment, as those who get even the slightest amount of power often abuse it. Arbitrary rules and enforcement abound about house color, landscaping, parking, and if you’ve ever operated a radio, antennas. While the FCC (at least as far as the US is concerned) does say that HOAs aren’t permitted to restrict the use of antennas, if you don’t want to get on anyone’s bad side you’ll want to put up an antenna like this one which is disguised as a set of HOA-friendly holiday lights.

For this build, a long wire is hidden along with a strand of otherwise plain-looking lights. While this might seem straightforward at first, there are a few things that need to be changed on the lighting string in order to make both the antenna and the disguise work. First, the leads on each bulb were removed to to prevent any coupling from the antenna into the lighting string. Clipping the leads turns what is essentially a long wire that might resonate with the antenna’s frequency into many short sections of wire which won’t have this problem. This also solves the problem of accidentally illuminating any bulbs when transmitting, as the RF energy from the antenna could otherwise transfer into the lighting string and draw attention from the aforementioned HOA.

Tests of this antenna seemed to show surprising promise while it was on the ground, but when the string and antenna was attached to the roof fascia the performance dropped slightly, presumably because of either the metal drip edge or the gutters. Still, the antenna’s creator [Bob] aka [HOA Ham] had excellent success with this, making clear contacts with other ham radio operators hundreds of miles away. We’ve shared another of [Bob]’s HOA-friendly builds below as well which hides the HF antenna in the roof’s ridge vent, and if you’re looking for other interesting antenna builds take a look at this one which uses a unique transformer to get wide-band performance out of an otherwise short HF antenna.

https://www.youtube.com/watch?v=4uG_D0bDGuI

Antenna Hidden In Holiday Lights Skirts HOA Rules | Hackaday

~ Hackaday



2023-09-19

Working POTA: A beginner's guide and video

 

SARC in the park!

We had an interesting workshop on Saturday, September 16, 2023.  'SARC in the Park' was a presentation by Dmitry VA7DVO for our members interested in POTA activations. You will find more about getting started in POTA at their website, and in our free digital magazine 'The Communicator'.

With an easy to build and inexpensive segmented 5-band wire dipole antenna, we made several of the 17m SSB POTA contacts shown in this video, including Switzerland, Italy and with a mobile station in Northern England.

The antenna plan is at: https://bit.ly/SARC23Sep-Oct on page 45.



Dmitry VA7DVO presenting POTA at the OTC

The on-site demonstration took place at Serpentine Fen Wildlife Refuge

Alex VA7PVC, Dmitry VA7DVO , and Leandro VE7LSI at Serpentine Fen

Here is the presentation on video and a look at the activation

https://youtu.be/RTAKs40DHjQ


Do you want to know what else is happening at SARC?

All our events are now available through our 'live' calendar at: https://ve7sar.net/.

~



2021-04-01

Scientists Warn RF May Disappear Completely by 2040

  

A new study published in the science journal Standing Waves shows that RF signals are disappearing at an alarming rate. Some scientists are going so far as to say that if action is not taken immediately, the airwaves could be completely silent by 2040.

The study’s chair, Dr. Bunsen Honeydew said, “We looked at daily activity on the HF bands from 3.5 to 29 MHz over the last 11 years. For a while the bands were showing healthy growth with plenty of activity, but in just the last five years signals have become much weaker and some have even disappeared completely. Worse hit has been the 10 meter band where we haven’t observed a signal for over two years… the extent of the devastation is breathtaking.”

But what is causing it? Scientists have a few theories but the main culprit seems to be that there are simply too many antennas absorbing a limited supply of RF. As this simple formula shows, RF is depleted at a rate inversely proportional to the square of the distance between any two stations:

Scientists warn that, as cos (1/x) increases, we risk reaching “the point of no return” where RF levels will never recover.

But what does this mean to the average ham? The short answer is we must all help conserve RF. Where hams used to just have one radio, it is now common to own three or even four radios, each with an RF absorbing antenna.

Of course, some of the worst contributors to the crisis are the so-called “Big Gun” stations. These use aluminum farming techniques that have gotten way out of control… covering acres of land with multiple towers reaching up to 100 feet and scooping up every signal that goes by.

The International Amateur Radio Union (IARU) and member societies like ARRL are calling for urgent action and plan to table a number of propositions at the next WARC meeting in Geneva. Among them would be a limit on antenna farming, a program for offsetting RF absorption by deploying more transmitters around the globe, and requiring hams to turn off their receivers when not really listening.

Perilous times.

- Adrian VE7NZ reporting


Hello Adrian, thank you for this enlightening article and for drawing attention to this growing problem. I understand that this may lead to an RF preservation tax much like the carbon tax that is now in place. I for one will be installing reflectors on all my antennas, when they are not in active use, to bounce the RF back into the aether.

I will certainly include this timely article on page 13 of the next issue of The Communicator in the hope that it will spur others into action before its too late.

John VE7TI

Editor ‘The Communicator’
https://ve7sar.blogspot.ca 


2020-08-11

Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength


Back to Basics


This 'Back to Basics' may be of particular interests to you off-roaders...

The Canadian Basic Question Banks asks:

B-006-10-4  Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength vertical antenna for VHF or UHF mobile operations?

A. A 5/8-wavelength antenna has less corona loss
B. A 5/8-wavelength antenna has more gain
C. 5/8-wavelength antenna is easier to install on a car
D. A 5/8-wavelength antenna can handle more power

An ordinary 1/4 λ (wavelength) vertical is smaller and resonant without any loading coil or matching network. What's the advantage to a 5/8 wavelength vertical? Why 5/8 in particular, and not something longer or shorter?

Indeed, why? A 5/8λ isn't resonant where a 1/4λ or 1/2λ would be.

The reason is the radiation pattern. The pattern for a 1/4λ monopole is essentially a doughnut, a pretty good pattern especially for a VHF antenna used primarily for local work. Extending the antenna changes the current distribution. This flattens out the pattern, removing power from the useless (for VHF purposes) vertical dimension and giving more horizontal gain and at a lower angle. See the illustration lower left from the late L. Cebik. The 5/8λ antenna focuses energy somewhat better towards the horizon (lower radiation angle) than a regular quarter-wave antenna.


Depending on the source, they will quote anywhere from 1 dB to 3 dB gain over the 1/2λ design [3dB is a doubling!]. There has also been some discussion that in some areas (urban and mountainous terrain) the lower angle of radiation is a detriment and a standard 1/4λ or 1/2λ antenna is to be favoured. 

So, why 5/8λ? Why not long longer? After all more gain is better right? Well, inspecting the figure below, you will notice the appearance of high angle lobes. As you lengthen the antenna past 5/8λ these lobes become more pronounced and break up the pattern in undesirable ways. Making it shorter maintains a good pattern, but the gain is less. So, 5/8λ is about optimal for this style of antenna.


You may have noticed a pattern developing here. A quarter wave ground plane antenna has a radiation pattern that produces maximum gain at about 25 degrees and a half wave antenna drops that angle to 20 degrees, and the 5/8 wave antenna further drops that angle to 16 degrees. So why not just keep extending the antenna out to one full wave? Well it would be nice if it worked but unfortunately the wave pattern begins to create very high angles of radiation beyond 5/8λ. 

So we've reached the maximum gain at this point and extending the antenna any further just reduces the gain where we want it (low angles). Of course if you are interested in very short skip, extending the antenna will produce nice gains over a dipole. 
All antenna lengths depend on various factors. Some of these factors are: 

  • the height above ground;
  • the diameter of the wire;
  • nearby structures;
  • the effects of other antennas in the area; and
  • even the conductivity of the soil. 


If we calculate the length of a 5/8λ antenna for our SARC repeater (147.360 MHz) the formula is 178.308/147.36 which equals 1.21m (3.97 feet).

The answer to our question therefore is 2. A 5/8-wavelength antenna has more gain.

Our next Basic Course starts September 15th.

~ John VE7TI
   08-03




2020-05-24

Antenna Modelling... A Follow-up


VA7NF Helps Explain What is Going On Here… 

In the last post, I described my experience with modeling and construction of a quarter wave 80 m inverted L antenna.  I stated that, while the model reliably predicted the real-life behaviour of the antenna, there were also some puzzling results beyond my understanding with the use of various antenna analyzers,  so I asked Stan VA7NF to apply his technical expertise to interpretation.   Other readers’ comments are also invited. 

Part 1 – Measurements Made at the Antenna

Here is a summary of my questions and Stan’s explanations:
Because the feedpoint of the antenna is at ground level, I was able to make one set of measurements without the feedline present then a second set at the station location with the feedline present.  



An AIM 4170 analyzer was connected to the antenna at the feedpoint by a 1 m long RG8X jumper.  I first adjusted the series capacitor at the feedpoint to bring series reactance (Xs) to zero at 3750 KHz.  Initially, each scan and re-scan produced the SWR curves below, showing a large number of random spikes, with each re-scan producing another set of spikes at different frequencies.  The first scan [below] is in red; the re-scan is in orange.

Could the noisy behaviour be the result of strong signals being pickup up by the antenna and interfering with the reflected wave or electronics of the instrument?  To check for the presence of out-of-band signals, I used the AIM 4170 band sweep function from 0.500 MHz (the low end of the broadcast band) to 5 MHz (above the 80 m band) as shown in the graph on the next page. This illustrates two strong, off-scale signals in the AM band (CKST at 1040 and CFTE at 1410 KHz) plus another unidentified signal at 2.0 MHz.  

As I continued with my measurements, the odd behavior shown in the first graph abruptly ceased and I was unable to reproduce it again.  I believe that the improvement came about after all the connections were thoroughly tightened, but it was also coincidentally about the time the weather changed from a prolonged cold, dry spell to very wet.  From that time on, the curves looked “smoother”.




Stan’s comments: 
Your assumptions above are well founded.  Antenna analyzers will send short bursts of RF into the test device (antenna) while monitoring the strength of the return signal.  For short antennae this will normally only be the reflection of its signal; when longer (80M or 160M or our FD antenna 320m) there will be many other strong signals especially the broadcast stations as you noted.

This is where analyzers differ.  Newer ones have Rx filtering that follow the transmit frequency; older ones take a voltage reading of everything on the antenna.  That means the readings are significantly affected by “stray” signals, making them useless in strong fields anywhere in the spectrum.

Your comment about the “noise” changing appears to be directly related to tightening the connectors.  Those two AM stations will mix with themselves, other lesser strength signals, and noise presenting the peaks you saw.  By making a better connection the diode effect of the poor junction has been removed eliminating the mixing products you saw.  Aside from analyzers, any diode (rusty tower bolts, wire fence supports, etc) will mix these strong stations with noise that becomes several S unit Rx background noise.  Doing the math, the 4 large spikes (3620 -3760) appear to be the sum of (1040 + 1410) mixing with 4 other local broadcast stations; also a 3rd peak exists at 2450 being 1040 + 1410.
Incidentally, our 80M antenna at the OTC is producing such images and should also be investigated.

Once the noisy data quietened down, and again scanning with the AIM 4170, the graph [below] showed an SWR close to 1.0, reactance near 0 and R not far off 50 Ω at 3750 KHz.  It doesn’t ever get much better than this, but could I believe it?  I decided to check with another instrument.

Measurements were again made at the antenna, but now using the AA-600 analyzer – results were somewhat different.

As seen in the graphs below, the minimum SWR was now higher at 1.5 but also shifted lower in frequency from 3750 KHz.  Note that at 3750 KHz, the reactance was still showing zero but Rs was around 80 Ω, thereby accounting for the higher SWR.   These results were more in line with my expectations but why did two high-quality instruments show such different results?



Stan’s Comments:
A major part of your low SWR is by design; you have tuned out Xs by adjusting the series capacitor; at 3750 your Xs went from -ve to +ve (tuned resonance) and the Rx value at that frequency as reported by the AIM was 50 ohms.  The RigExpert agrees with AIM that Xs crosses over at 3750 but it says Rs is around 80 ohms and drops as the frequency drops.  The >50 ohm Rx and -ve Xs combine below 3750 for the lower SWR.  It is unknown why there a difference in Rs from 50 to 80 ohms.

Conclusion:  Because they develop different Rs values the SWR curves are different.  By observation there is significant noise still on the wire and that may be the basis of the differences.

Continuing at the antenna, measurements were now made with an MFJ 269 analyzer. 

At 3750 MHz regardless of frequency, the SWR was off-scale (∞) consistent with an unchanging Rs=0 and, in addition, Xs varying erratically, i.e. it was not possible to get a meaningful measurement (note it was confirmed that the analyzer returned appropriate readings when tested with a 50 ohm dummy load and at 20 m on the author’s beam, so the meter seemed to be “working”).   Was this possibly another strong-AM signal interference problem?

Stan’s Comments: 
Agree.  Too much broadband noise for this meter to function.  Not usable on a real antenna with a long length of copper.

Part 2—Measurements Made at Station

Next, a length of new LMR-400 coax plus a short jumper were attached to the feed point along with a second common mode choke close to the station end, but otherwise conditions were unchanged from earlier.  All the following measurements were made at the station end of the transmission line and jumper.


The AIM 4170 produced SWR, Rs and Xs curves across the band. [left]  Zero reactance occurred now at a higher frequency than previously, and instead of rising into positive (inductive reactance) territory at frequencies above 3750 MHz, Xs decreased, showing a net capacitive reactance. How can this be explained?

Stan’s Comments:
Coaxial cable when connected to a matching impedance (50 ohms) will not affect the apparent impedance regardless of length;  however:
1) When connected to a non-matching impedance there will be returning currents with a +/- Xs.
2) As the current flows back it will be a “normal” sine wave changing phase until at 1 wavelength it again matches the phase of the forward wave.
3) RF on a coax cable does not travel the same speed as RF in free space.  Specifications show this as a velocity factor (VF) which is .85 for LMR-400; this means the RF will cycle its phase over less length (1/VF wavelength).  As the test frequency changes this means that the length for a full sine wave will vary being longer at 3.5 MHz and shorter at 4.0 MHz.  Viewing it from a single point (like the feed point) the phase of the reflected signal will vary with frequency.

Putting this together, a reflected signal starting at the antenna connection will be out-of-phase with the forward signal (The +/- Xs as you measured at the antenna).  As that reflected signal moves down the cable its relationship to the forward signal will change, starting at (for example) a +ve phase that will shift to in-phase, then -ve phase, to in-phase, then back to the +ve.

Depending how long your cable is (in wavelengths times VF) the reflected signal will appear + or - phase (Xs) and at places exactly in phase with the signal going the other way.  This combination is what your analyzer will see.  In your image the transformer effect of the length of LMR-400 modifies what is seen.

Another effect of coax transformer effect will be, when the antenna has a high SWR, artificial dips in APPARENT SWR appear; these artificial SWR dips appear when the forward signal is cancelled out by the reverse signal with a resulting Xs=0.  This cancellation effect is frequency dependent so a long coax may show several artificial dips typically 5-10Mhz apart.

Accepted practice is to have feed lines that are ½ wavelength (after applying the VF) but only when attempting to measure at-antenna SWR.  They will NOT appear if the antenna is low SWR as there is no reflected signal to mix with the transmitted one.  This will not eliminate the artificial dips as the length will not be a ½ wavelength across all those frequencies.  This also explains why an antenna that will not tune, may tune if a short length of coax is added between  tuner/transmitter and the antenna.
Continuing with measurements at the station end of the transmission line using the RigExpert AA-600, both the SWR and Rs-Xs graphs now agreed fairly closely with the results for the AIM 4170. [below]



Next using the MFJ 269, the behaviour was once again erratic, with no minimum SWR discernable at any frequency and no meaningful SWR measurable.

I took one further step when I noted that with the AIM 4170 instrument, conditions at the antenna can be measured from the far end of the feedline, if its length, velocity factor and matched loss are known.   As per the Times Microwave specs for LMR-400 giving a velocity factor of 0.84 and matched loss of 0.1 dB/100 ft. (at 1 MHz),  the total length of the cable was known to be 112 ft.  Under these conditions, a “refer to antenna” set of curves measured from the transmitter end of the coax looked like the following.  It was reassuring to note that these curves were virtually identical to those obtained when the measurements were made right at the antenna. 



Stan’s Comments:
The AIM is calculating the coax length effect and applying it to the reflected signal.  Smart!

Conclusions:

On the assumption that strong signals in the AM band (not sufficiently filtered out by the instrument) were the cause of anomalous results, the MFJ 269 readings were rendered useless.

The AIM 4170 analyzer appeared to be influenced by the strong off-band signals, but considerably less so than the MFJ.  

Only the RigExpert AA-600 appeared to be immune to the strong AM signals, and give relatively consistent results at both locations of measurement.

When adjusting for resonance of the antenna/feedline combination, conditions at the station end of the feedline would appear to be the place that counts, as presence of the feedline will influence the resultant impedance seen by the transmitter.  

~ John Brodie VA7XB
  18-02






2020-05-21

Antenna Modelling… A Learning Experience




One of our Tech Topics 

My OCF dipole came down in the wind over a year ago, so I decided it was time to replace it with something else, at least for 80m.  I have lots of trees in the yard, but the high ones really don’t lend themselves to a horizontal wire because they are all clustered together.  I researched other options and decided I could probably make an 80 m inverted L work.   Since my trees are all over 30 m high with the lower branches stripped off, I figured they should accommodate 20 m or more wire in the air, especially since only part of the antenna is vertical.   The relative proportion between vertical and horizontal legs is apparently not too critical to its performance.  This antenna is actually a quarter-wavelength vertical therefore it requires counterpoise, or ground radials. 



While the weather was good over the summer I began the project by installing a 1½ ” ABS conduit below ground from the house to the location of the antenna feed point, so that the coax would not have to run over the surface of the ground.  Next challenge was to get attachment points up in the trees, a job which requires a tree climber – a big challenge as tree climbers are very casual about returning calls and showing up for the job.  Finally it all came together just as the weather started to turn cold in October.  I had him tie a pulley and rope as high as possible on two trees which are about 10 m apart.  In order that there be no obstruction to the future wire, he had to remove a few limbs but, what the heck, most of them were dead anyway.  The ropes and pulleys went up at about the 20 m level on both trees without a problem.  

I have always wanted to learn about antenna modeling and saw this as an opportunity to get my feet wet.  Several different modeling software packages are available, either free or at low cost, but I elected to use EZNEC demo v 6.0, as it is supposed to do almost everything that the more sophisticated products will do, it has good documentation and is free.  After reviewing the instructions provided with the package, I had no trouble working through the examples to get familiar with its features.  There are also several good tutorials available on You-Tube.   EZNEC does assume that you have a basic understanding of impedance, resistance and reactance which are expressed in the form of complex numbers (Z=R ±jX) as well as familiarity with x-y-z rectangular coordinate systems, but that’s about as complicated as it gets.   I was pleasantly surprised to find out how intuitive the software is to use. 

Taking my guidance from the ARRL Antenna Book, here is the basic configuration that I planned to model: the top portion of the antenna above ground is around ¼ λ long and the other ¼ λ is in the ground in the form of radials or counterpoise.  The bend to make the above-ground wire into an L can be done at any convenient place, but I intended to make most of the antenna vertical. This configuration under normal circumstances will provide a usable swr over most of the 80 m band, with the help of a tuner.  

However, due to the proximity of the wire to ground, its radiation resistance (R) is said to be lower than 50 Ω.  I wanted to aim for 50 Ω at resonance to match the feedline impedance.  If the antenna were to be made somewhat longer than ¼ λ, the R value can be increased to 50 Ω.  However, with the antenna now longer than ¼ λ it becomes inductively reactive.  So I would cancel out the inductive reactance by introducing an equivalent (of opposite sign) series capacitive reactance at the feedpoint – or, as expressed in mathematical form, the impedance would be Z = 50 ±j0 at  resonance.

My objective was to locate the minimum swr at 3.75 MHz, the centre of the 80 m band.  A critical parameter is ground loss but the demo version of EZNEC does not allow modeling of the counterpoise system.  Instead, the ground loss was estimated based on advice in one of the tutorials.  I chose a series load loss of 10 Ω to represent an acceptable but less-than-perfect ground radial system, since I hoped to get by with only 5 radials.  I was soon to find out that 5 isn’t a sufficient number.

After much EZNEC trial-and-error, here is the final antenna configuration:

The Far Field elevation and azimuth plots with the maximum gain 25 deg above the horizon, and the antenna is nearly omni-directional, not unexpected for an antenna which is basically a dipole turned on its side.   This antenna will not likely be effective at short range as there is little vertical component to the elevation pattern.

Wire #1 represents a 6’ length of copper pipe to which the counterpoise wires are attached at the bottom; wire #2 is a short horizontal connection from the top of the pipe to the bottom of the vertical wire with the “feedline box” located approximately midway between the two; wire # 3 is the main vertical radiator, and Wire #4 is the short horizontal leg.  The small rectangle shows the location of the coax feed point, and the small circle is ground. 

The total length of the above-ground portion (wires 1-4) is 22.3 m, which is longer than ¼ λ, as intended.  After compensating for the inductive reactance by insertion of a series XC  of -89 Ω in the model, the predicted SWR graph came out as shown below.  At the target frequency of 3750, R was 48 Ω and the reactance virtually zero.

I was also interested in the conditions at the feed point and capacitor.  Since I may want to run at full power, I specified the power as 1500 watts in the EZNEC Options menu.  The result shows that the series reactance-compensating capacitor should have a spacing on the plates that will withstand at least 540 v without arcing when operating within the band. To provide -89 Ω of XC, the capacitance would be about 480 pF based on the formula XC = 1/2Ï€fC.


And now for the construction

Three items were included in the feedline junction box: 
  • a common mode choke, 
  • a 30-1000 pF variable capacitor to introduce series capacitive reactance as described above, and 
  • a surge arrestor, all as shown in the photo.   


Five counterpoise wires, each 22.3 m long and buried 10 cm below the surface of the ground, were soldered to a 6 ft. ½ inch copper pipe driven into the ground and connected at the top to one side of the choke.   The ground wires ran from the base of the copper pipe all around the yard wherever I could fit them in, in all cases with bends to accommodate obstructions and the constraints of the property boundary. Instead of the store-bought choke I could have used a coax coil looped through ferrite toroids, which would have done the same thing.  I put insulation around the ground rod to protect children and animals from RF.


Since the feed point for this antenna is at ground level, it made it possible to conduct measurements right at the antenna without the presence of a feedline.  





Above, the scans  of SWR, R and X were made with the club’s RigExpert AA-600 analyzer. 

You will note that the minimum SWR is about 1.5 and it is also shifted slightly from 3750 kHz.  The higher SWR than predicted is because the impedance is not 50 Ω but 79 Ω – comprised almost entirely of R since the reactance was tuned out (observe the green reactance line crosses the 0 Ω axis at 3750 kHz).

Now this would actually be a very acceptable match across the entire band, but it does indicate that more counterpoise wires would be beneficial in order to match the model.  If we return to the model and simulate a poor ground by increasing the load R from 10 Ω to 30 Ω, the model confirms that the minimum SWR does in fact rise to 1.5.   So I plan to add more ground wires while I monitor the SWR and  impedance (both of which should come down) as the work progresses.  

I was gratified to find that changes made to the physical configuration were consistent with the model’s predictions.  This exercise has given me confidence in the modeling software and a better understanding of how the components of impedance inter-relate with the physical characteristics of the antenna. 

In the next installment I intend to outline a few things that did not make sense, and ask for advice in their interpretation, e.g. 
1) how 3 different analyzers gave me 3 distinctly different results and 
2) how introduction of the feedline also affected the results.



~ John VA7XB
  18/01





2020-05-10

Near Vertical Incident Skywave (NVIS) Antennas



HF emergency communications or otherwise, contact stations within the skip zone.

A previous post details Robert VA7FMR’s experiences with a dipole using Hamsticks and a dual bracket. Here is additional information on them and Near Incident Vertical Skywave (NVIS) antennas using Hamsticks. 

SEPAR and many like organizations may be called upon in an emergency to provide ancillary communications for emergency services or primary communications for those emergency service partners who do not have RF communications systems of their own, ESS, The Red Cross and Salvation Army, to name just a few. It is a given that the first stations will be on the higher bands above 50 MHz. Anyone with a Basic License may operate within that spectrum. This should provide reliable communications with modest antennas within the Lower Mainland—no, not just with a handheld and a rubber duckie. Even if many of the local repeaters fail, we should manage to set up a decent network within a few minutes to hours. But what if reliable communications are required for longer distances, for example  to Kamloops or to Seattle, or beyond? VHF and above may not be able to span that distance without gain antennas, placed high with at least 50 Watts of power. HF will be the ‘goto’ bands. We may not need a 60 foot or higher tower to communicate effectively either—this is where NVIS becomes an important emergency communications antenna, especially in the field.

If you recall the antenna theory that you should have picked up in your Basic course, you know that the orientation (horizontal, vertical or somewhere in between) of an antenna will affect its radiation pattern. Much like bouncing a ball off a wall. The greater the angle, the greater the distance the ball bounces away from you. Throw it straight at the wall and it should come pretty close to you on its way back. The same general idea applies to the NVIS antenna. If we cause the RF wave to travel nearly straight up or at a slight angle, it will reflect off the ionospheric layers and come back close to our point of origin. So, if we want to communicate on HF with stations within about 1,500 Kms, we use an antenna that radiates primarily straight up. A DXer on the other hand prefers to talk to stations far away, with a few hops, the farther the better, so DX antennas radiate at angles primarily horizontally to bounce and skip back off the ionosphere for the greatest distance. 



So, the NVIS antenna is one that provides the majority of its radiation at an extremely high angle. That is to say the major lobe is between 75 and 90 degrees to the earth's surface. This will provide excellent omni-directional communication out to a distance of up to about 1,500 Kms with no skip. The maximum frequencies involved will be as low as 1.8 MHz under very poor conditions to as high as 14 MHz under excellent conditions, with the most usable being between 3.5 MHz (80M) and 7.3 MHz (40M).



To summarize, NVIS works for frequencies lower than the vertical incident critical frequency—the highest frequency for which signals transmitted vertically are reflected back down by the ionosphere.  At or below the critical frequency the ionosphere will reflect an incident signal arriving from any angle, including straight up. Because the critical frequency is low, you must usually operate 40, 80 or 160 meters or possibly 30 meters to use NVIS propagation.

Under most conditions you can easily obtain coverage on one of these bands from zero to 350 miles or more with no skip zones. On 75 meters with 100 W and an antenna 15 feet high, contacts with stations over 1000 miles away with excellent signal reports are not uncommon. 

These are the characteristics we look for in an emergency-ready HF antenna for distances up to about 1,000 miles… No skip, easy set-up and take down and reasonably reliable communications.

When I first started looking at the NVIS antenna for "local", primarily emergency communications, the consensus seemed to be that it was a dipole-type antenna, near 1/8th wave at the operating frequency, above the ground. I purchased a set of HamSticks, mounted as a dipole, for this purpose as I was operating from a vacation area surrounded by high mountains.  NVIS antennas are commonly used by the military, as their needs fit these characteristics. There is an excellent, though technical article at https://region6armymars.org/downloads/NVIS-Antenna-Theory-and-Design.pdf

Every horizontal antenna has an NVIS component in its radiation. Similarly, every horizontal antenna has a component that is most useful for DX. Your decision then is to pick the configuration that either favours or optimizes the properties you want.  Reliable local communication on HF dictates NVIS. How then do we determine what NVIS antenna will best suit our needs?  Let’s examine the parameters that have a significant effect in antenna performance. This is information on how to make it work reasonably well, NOT a graduate degree treatise on the theory of NVIS.


Height above ground

The antenna height above ground seems to be the single most controversial subject in discussion of NVIS antennas. Some say anything below 1/4 wave works. Others say anything below 1/8th wave and yet others say ten to fifteen feet works very well. You will note that there is negligible difference in antenna gain between 1/8 wave and 1/4 wave height. There is however a significant difference in the logistics of placing an antenna at 70 some feet in the air versus 35 feet in the air.

Antenna guru L.B. Cebik (W4RNL), writing about NVIS antenna elevation, explains that the height, in the 1/8 to 1/4 wave length above ground, has very little difference in gain. In fact, if you roll in the next parameter, ground (detailed below), height can easily have much less effect than ground.

The Near Vertical Incident Skywave (NVIS) antenna is a half-wave dipole antenna, configured straight or as an inverted vee, mounted not over 1/8th wave above ground (at the highest operating frequency). While 1/8th wave works reasonably well, better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground. A second advantage of lowering the antenna to near 1/20th wavelength is a lowering of the background noise level. At a recent ARRL Section Emergency Test, communication on 75 Meters was started with a dipole at approximately 30 feet. They found communication with some of the other participants to be difficult. A second 1/2 wave dipole was built and mounted at 8 feet off of the ground. The background noise level went from S7 to S3 and communications with stations in the twenty-five and over mile range were greatly enhanced. Simply stated, you want as much of your signal going up as possible and ten to fifteen foot height has shown to function very well. It was also found that a network of stations, all using NVIS antennas experienced much stronger local signals.


Ground

Yet another consideration is the "quality" of the ground below your antenna. By this we mean the conductivity of the ground you are operating above. For any given height (1/4th wave length or less) poor conductivity will attenuate up to 3db more of your signal than high conductivity soil. A documented example is the ARES installation in Longmont, CO at the Emergency Operations Center. That antenna is mounted ten feet above a flat roof. The base for the roof is a grounded steel plate. This antenna consistently performs as well or better than any other in the state. The reason is simple; A full sized resonant dipole antenna mounted ten feet above an excellent ground.

A specific example of how well the Longmont EOC antenna works is one Sunday when testing the antenna, a local ham tried his Yaesu FT-817 running on the internal battery pack. As most know, that configuration produces 2.5 watts PEP maximum output. At that power level he received a signal report from NCS in Colorado Springs (90 miles South) of S9+10db, on 75M just before the net started.

Another example of how the conductivity affects your signals comes from Colorado where they regularly use NVIS antennas on 60M to communicate across the Continental Divide. Doing this on a twice weekly basis for several years now they have established a base-line for comparison. The week of 23 September 2004 they had a slow moving rain storm that put down more than one inch of rain, spread almost evenly over about 36 hours. For those that have thirty to fifty inches of rain per year, that would not be much. In Colorado that is one-fifteenth of their total annual precipitation. After the rain, under less than optimal band conditions, signals were UP 6 to 10db!

The chart by L.B. Cebik's (W4RNL) shows that any NVIS, above excellent ground, out performs an antenna above good ground at optimal height! Hmmm, does that imply that we have found the single most important parameter in NVIS?


Ground wire

Yet another approach is to run a "ground" wire at the surface where the antenna is mounted. A good discussion on this is found at an Australian site by Ralph Holland. He did some research on 160M and found that a ground wire at .02 to .06 wave lengths below the driven element produced the best gain. That translates to about 5 to 15 feet at 75M which would be consistent with the heights seen that have  produced the best NVIS performance. Others claim at least a 6db improvement with this same approach.

Experimenters  also notice an improvement if  you "water" the ground just prior to operation. Pour about one gallon of water on the ground around the ground rod or wire. If it seeps in very quickly, go get another gallon. This has made a noticeable improvement in both transmit and receive signals. 


Counterpoises

The high angle radiation of a dipole (or inverted vee) can be enhanced by adding a counterpoise wire below it, about 5% longer than the main radiating element, to act as a reflector. The optimum height for such a counterpoise is about .15 wavelengths below the main radiating element, but when the antenna is too low to allow for that, a counterpoise laid on the ground below the antenna is still effective.

A knife switch at the center point of the counterpoise can be used to effectively eliminate the counterpoise from the antenna system. This technique is useful for using a dipole for NVIS and longer distances, too. A counterpoise is installed at ground level, or as high as the switch can easily be reached, and a dipole is mounted .15 wavelengths above the counterpoise. When the switch is closed, the vertical gain will increase, and the noise levels will drop. When the switch is open, lower angle gain will increase, improving the antenna's performance for non-NVIS use.


Dual Ham-Stick

This is a portable antenna on a 5-foot mast that does well under ARES/RACES operating conditions. One person can put this up and have it operational in under five minutes! A side advantage of this antenna is its comparatively small size. It is only sixteen feet in length, which makes it much more reasonable for temporary installations.



‘HamStick’ antennas may be paired to make a very usable dipole antenna.
Mounting height will affect  the radiation pattern and therefore propagation.
Above, a typical HamStick and adjustable whip and the dual mount that makes it a dipole with directivity
. 

Inverted Vee

A dipole's close cousin, the inverted vee, is another good NVIS antenna which can be even simpler to support. An inverted vee will work almost as well as a dipole suspended from a slightly lower height than the apex of the inverted vee, so long as the apex angle is kept gentle—about 120 degrees or greater. An inverted vee is often easier to erect than a dipole, since it requires only one support above ground level, in the center.

This design has been successful for the author. It was developed by Dr. Jelinek and is in commercial use by the Armed Forces.


How do I select a frequency for NVIS operation?

The selection of a optimum frequency for NVIS operation depends upon many variables. Among the many variables are time of day, time of year, sunspot activity, type of antenna used, atmospheric noise, and atmospheric absorption. To select a frequency to try, one may use recent experience on the air, trial and error (with some sort of coordination scheme agreed upon in advance), propagation prediction software like VOACAP, near real-time propagation charts (available on the Internet) showing current critical frequency, or even just a good educated guess. Whatever the strategy used for frequency selection, it would probably be best to be prepared with some sort of "Plan B" involving communicating through alternate channels, or following some pre-arranged scheme for trying all available frequency choices in a scheduled pattern of some sort.

http://webclass.org/k5ijb/antennas/NVIS-low-antenna-regional-communications.pdf


A NVIS antenna on a wartime military vehicle.

Finally, this is also an antenna that should be in the ‘kit’ for Field Day or contests. We usually concentrate on working any and all stations however, skip actually works against us when it eliminates many potential contacts up to 1,000 miles or so. The ability to switch to an NVIS antenna will bring in those stations within the skip zone and enhance the score. This strategy has helped us place first in Canada in our 3A Field Day category for several years.

~17/10











CQ CQ CQ

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