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Tuesday, August 11, 2020

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




Wednesday, July 1, 2020

The July-August 2020 Communicator

Over 90 Pages Of Projects, News, Views and Reviews... 

Amateur Radio News from the South West corner of Canada and elsewhere. You will find Amateur Radio related articles, profiles, news, tips and how-to's. You can view or download it as a .PDF file from:  


http://bit.ly/SARC20JulAug






As always, thank you to our contributors, and your feedback is always welcome. The deadline for the next edition is August 21st.

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

Keep visiting our site for regular updates and news: https://ve7sar.blogspot.ca    

73,

John VE7TI
'The Communicator' Editor







Sunday, June 21, 2020

Surrey Celebrates Amateur Radio Week


Surrey City Council Recognizes
The Contribution Of Amateur Radio

Again this year Surrey City Council has proclaimed June 21 to 28th, the week leading up to Field Day, as 'Amateur Radio Week".


The Mayor and council have historically been strong supporters of our role in the emergency program and several VIPs from all levels of government and our served agencies  have visited our Field Day site in past years. Unfortunately, due to COVID-19, this year will be an exception but several members of Surrey Amateur Radio Communications (SARC) and the Surrey Emergency Program Amateur Radio (SEPAR) will be participating from individual home or field stations.

Since 1933, ham radio operators throughout North America and beyond have established temporary ham radio stations in public locations during an annual exercise called Field Day to showcase the science and skill of amateur radio. 

For more than 100 years, amateur radio — also called ham radio — has allowed people from all walks to experiment with electronics and communications techniques and provide a free public service to their communities during a disaster or emergency, all without needing a cellphone or the internet. Field Day demonstrates ham radio’s ability to work reliably under any conditions from almost any location and create an independent communications network. More than 35,000 people from thousands of locations participated last year in Field Day 2019 activities.

It’s easy for anyone to pick up a computer or smartphone, connect to the internet and communicate, with no knowledge of how the devices function or connect to each other, but if there’s an interruption of service or you’re out of range of a cell tower, you have no way to communicate. Ham radio functions completely independent of the internet or cellphone infrastructure, can send messages by voice, Morse code or digital mode, even email via radio. It can interface with tablets or smartphones, and can be set up almost anywhere in minutes. That’s the beauty of amateur radio during a communications outage.

The Surrey team placed first in their Canadian category for Field Day 2019 and have consistently placed in the top three stations in Canada, even achieving an all-time high score for using low power (less power than it takes to light a Christmas tree bulb) to contact other stations. In 2014 we even spoke to an astronaut aboard the International Space Station!

Field Day


In today’s electronic do-it-yourself environment, ham radio remains one of the best ways for people to learn about electronics, physics, meteorology and numerous other scientific disciplines. In addition, amateur radio is a huge asset to any community during disasters or emergencies if the standard communication infrastructure goes down. They also provide communications for relief agencies that do not have radio communications systems of their own and offer free communications services for non-profit events to practice these skills.

Surrey Fire Service Deputy Fire Chief Mark Griffioen, who coordinates emergency response for the City of Surrey adds: “Surrey Emergency Program Amateur Radio plays a supporting role in emergency communications during any major event for the city.  They train extensively and operate a communications room out of Fire Hall 1. During a major event, they provide enhanced life safety for the citizens of Surrey by providing a communication link with the Emergency Operations Centre.”

Stan Williams, President of Surrey Amateur Radio Communications, highlighted the value of Amateur Radio in its flexibility. “Amateur Radio is not centralized. There is no single point of failure; it does not rely on the Internet, a power utility or a communications company, cell towers or other infrastructure to provide service. It works when nothing else is available. We need nothing between us but air.” 

Anyone may become a licensed amateur radio operator. Worldwide there are more than 3 million licensed amateur radio licensees, as young as 9 and as old as 100. And with groups such as SEPAR and SARC, it’s easy for anybody to get involved right here in Surrey.  We offer regular amateur radio courses, classroom or on-line, see https://ve7sar.blogspot.com/2020/03/our-basic-amateur-radio-course.html 

To learn more about Amateur Radio in the Surrey Emergency program, watch the video at http://tinyurl.com/SeparsInfo.

For a look at our previous Field Day efforts visit our YouTube channel at https://tinyurl.com/SARC-Video




When All Else Fails, Ham Radio Works








Saturday, May 30, 2020

2020 IOTA Expeditioner of the Year Award



Award to SARC Member Mike Zavarukhin VE7ACN

The IOTA (Islands On The Air) Program is an exciting and innovative activity that has caught the interest of thousands of radio amateurs worldwide. Established in 1964, it promotes radio contacts with stations located on islands around the world to enrich the experience of all active on the amateur bands and, to do this, it draws on the widespread mystique surrounding islands. It is administered by Islands On The Air (IOTA) Ltd (called here IOTA Management) in partnership with the Radio Society of Great Britain (RSGB).


Mike (in the grey T-shirt) at SARC-SEPAR Field Day


Mike adding to our Field Day score

Our very own Mike Zavarukhin VE7ACN has been a very active participant in the program and he has received several awards for his participation over the past several years. This year he came in first so...

A big congratulations to SARC member Mike Zavarukhin VE7ACN/RW0CN on receiving the 2020 IOTA Expeditioner of the Year Award! The award was announced live on Ham Nation on May 27, 2020.






Read Mike's SARC profile in our newsletter 'The Communicator
and a description of his Russian station


~ SARC





Friday, May 29, 2020

Temporary Rule Waivers Announced for 2020 ARRL Field Day


Club scores will be permitted from distributed stations

With one month to go before 2020 ARRL Field Day, June 27 - 28, the ARRL Programs and Services Committee (PSC) has adopted two temporary rule waivers for the event:

1) For Field Day 2020 only, Class D stations may work all other Field Day stations, including other Class D stations, for points. 

Field Day rule 4.6 defines Class D stations as "Home stations," including stations operating from permanent or licensed station locations using commercial power. Class D stations ordinarily may only count contacts made with Class A, B, C, E, and F Field Day stations, but the temporary rule waiver for 2020 allows Class D stations to count contacts with other Class D stations for QSO credit.

2) In addition, for 2020 only, an aggregate club score will be published, which will be the sum of all individual entries indicating a specific club (similar to the aggregate score totals used in ARRL affiliated club competitions).

May/June 2020 Communicator
May/June 2020 Communicator
Ordinarily, club names are only published in the results for Class A and Class F entries, but the temporary rule waiver for 2020 allows participants from any Class to optionally include a single club name with their submitted results following Field Day.

For example, if Podunk Hollow Radio Club members Becky, W1BXY, and Hiram, W1AW, both participate in 2020 Field Day -- Hiram from his Class D home station, and Becky from her Class C mobile station -- both can include the radio club's name when reporting their individual results. The published results listing will include individual scores for Hiram and Becky, plus a combined score for all entries identified as Podunk Hollow Radio Club.

The temporary rule waivers were adopted by the PSC on May 27, 2020.

ARRL Field Day is one of the biggest events on the amateur radio calendar, with over 36,000 participants in 2019, including entries from 3,113 radio clubs and emergency operations centers. In most years, Field Day is also the largest annual demonstration of ham radio, because many radio clubs organize their participation in public places such as parks and schools.

Due to the COVID-19 pandemic, many radio clubs have made decisions to cancel their group participation in ARRL Field Day this year due to public health recommendations and/or requirements, or to significantly modify their participation for safe social distancing practices. The temporary rule waivers allow greater flexibility in recognizing the value of individual and club participation regardless of entry class.

ARRL is contacting logging program developers about the temporary rule waivers so developers can release updated versions of their software prior to Field Day weekend.

Participants are reminded that the preferred method of submitting entries after Field Day is via the web applet. The ARRL Field Day rules include instructions for submitting entries after the event. Entries must be submitted or postmarked by Tuesday, July 28, 2020.

The ARRL Field Day web page includes a series of articles with ideas and advice for adapting participation this year.

~ ARRL


Sunday, May 24, 2020

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






Thursday, May 21, 2020

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 = 1/2πfC.


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





CQ CQ CQ

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