Tech Topics: Review Of An SDR Dongle

The SDR Dongle 

SDR = software defined radio

Having already a conversation with VE7TI (John) about an older generation of SDR dongles I felt compelled to buy a new one, in 2018, a much smaller one, also from China. Most probably what I bought is a knockoff of a NooElec micro dongle. It was in sale at the time, for $7.87 CAD, shipping and taxes included, from aliexpress.com. It came with a remote control, an antenna and a CD with drivers. I discarded all those accessories, which are totally unusable if somebody wants to use the SDR dongle as a general receiver, and not as a DVB-T PC adapter, as intended.

I would like to start my review by underlining exactly that, the SDR dongle I am reviewing was not designed to be a general receiver, as I use it.

My first action was to install it on the computer, on a USB port, and to install drivers and software for it. I followed the instructions from www.rtl-sdr.com. It is tricky to have the drivers work in Windows 10, but if the instructions are followed exactly as in the given website, it works.

Some conclusions

• 
  The only software that completely works in Windows 10 is SDR sharp. It has various useful plugins, like a plugin for detecting the CTSS tones. Many plugins do not work with the last version of SDR sharp. It is free. A close competitor is HDSDR, which does not know how to decode stereo FM. All other programs I tried partially worked (they do not know all modulations types, have unclear settings, and so on).
  
  
• It has to be connected on the USB computer port with an extender, otherwise the electric noise generated by the computer makes it unusable, completely deaf for useful radio signals. I used my own accessories, in order to adapt the MCX antenna connector from the dongle to my antennas: 
  
  
• Caging the SDR dongle does not help much; if it is not case to case to the electric noise generator, but several centimeters apart, it is fine. I tried to cage it in metal and it did not make any difference in various test situations. I suspect it is already somehow shielded or partly shielded inside.
  
  
• In the commercial FM band it is a cheap stereo and more important, a RDS (radio data system) receiver. It knows how to display the name of the station, the songs that are played at that moment and whatever digital info the station sends in addition to the analog signal. The sensitivity in FM is way worse than 2 microvolts. Any dedicated commercial receiver-amplifier, including my roommate’s Yamaha 2 microV, every single FM radio in the apartment we have, including clock radios, and MP3 portables (the radio part) are more sensitive than the SDR dongle. I am using a proper horizontal dipole antenna on the balcony measuring 71 cm each leg, connected with coax cable to the SDR dongle, while all other 7 receivers have just a small piece of wire as antenna. I estimate the sensitivity in the 88 – 108 MHz band somewhere at 30 microvolts . It is expected the SDR dongle would be less sensitive in the FM band, due to the wide frequency bandwidth. I limited the bandwidth from 250 KHz to 180 KHz and there was a slight improvement.
  
  
  
• The sound in the FM band is not great. Even at 250 KHz, wide band FM (maximum in SDR sharp program), has audio quality that is just bearable. This is not exactly acceptable. I will not replace any of the radios with this SDR dongle, even though it displays data.
  
  
• The characteristics differ very much on the Rx bands and require adjustment at the RTL dongle settings. That means RF Gain; RTL AGC; Tuner AGC. 
  
  
• It is stable. I did not feel the need for a more stable oscillator. It did require adjustment in the software, 218 ppm as in the above picture for my dongle. This is considered a huge adjustment. I verified this with encapsulated quartz oscillators (32 MHz, 125 MHz, 150 MHz, the 28.197 CW beacon), and indeed it needs that huge adjustment.
  
  
• It seems it does not like the 50 MHz band, and the sensitivity is not great in this band. I confirmed the poor reports as everybody writing about this issue on the Internet experienced the same result, although I hear some local ham radios almost every evening. They never say their callsigns, so I just presume they are ham radios since they are in a ham band.
  
  
• On the 144 MHz band, with a good dipole, it receives everything the Kenwood 7950 and the Chinese walkie-talkie receives. It likes this band and it has a good sensitivity. All repeaters from Victoria, Port Angeles, Nanaimo, and Cowichan are 59. Probably the path is more important than the sensitivity in this case, too. I am at 103 meters above sea level. There are some images for strong local repeaters.
• It also likes the marine band, air traffic band and the weather band. They are all around 150 MHz and once the settings are done for one station, they can be kept for the weather, marine, 2 meter bands.
  
  
• The CB band and the beacon on 28.197 MHz (VE7MTY, Pitt Meadows, continuous, CW) are in a band where the RTL dongle is not so sensitive. The beacon (nearby me) booms in on my SONY ICF7600G portable radio, with its telescopic antenna. The SDR dongle with a CB whip on the balcony receives it almost OK, but only because I was hunting for the beacon and I knew where it was. The beacon’s signal barely produces a trace in the display spectrum, and I am nearby it (exactly 13.89 km).
  
  
• There are images everywhere. The FM band (88 – 108 MHz) can also be received on 30-50 MHz. The worse thing to do is to use an upconverter, as I saw so many do on the Internet, with a NE612 integrated circuit, and wide non-tuned input. I tried, and the images kill any useful signal. In the end I did 2 converters, in order to cover 3.5 MHz to 30 MHz, one for the lower part and one for the upper part. I used an NE612, attached to an amplifier with a BF998 in front. I have a tuning circuit just at the antenna, and a 2.2K resistor + coil in the output of the BF998’s drain. The source terminal is connected directly at the ground and the BF998 is supplied with 9 Volts (12 V is max in the datasheet, and it does burn after 12 V). The oscillator is an encapsulated 3.3 V powered oscillator, in a socket, to easily change it. The best the dongle worked for me is in the 150 MHz band, to stay away from the FM commercial band and to upconvert the shortwave into a sensitive band that the SDR dongle likes. I can adjust the signal from the oscillator to the value from the NE612 datasheet, but it actually does not make any difference, even if it is attached with 2 Volts (NE612 has a buffer in it before the mixer).

• The noise of the first element in the SDR dongle must be better than that in the  BFR91A. I tried a wide range untuned amplifier with one BFR91A, and it did not bring anything new, just noise. 
  
  
• The situation changed when I put a SAW 88 – 108 MHz (3 pin filter) in front of the BFR91A, and it helped.

• It does not run hot. Whatever other users noticed with old SDR dongles is no longer an issue with my 2018 SDR small dongle.

Final conclusions

• The SDR dongle is the cheapest 2 meter receiver a Ham can buy, and works as receiver on par with dedicated equipment, which is generally limited by line of sight, not by sensitivity. A beginner can listen to the weekly nets for some $8–11 CAD, shipping and taxes included.
  
  
• The SDR dongle is the cheapest FM commercial RDS receiver one can have, capable of displaying the digital data continuously and transmitted by almost all stations in Vancouver. 
  
  
• The SDR dongle was not meant as a general coverage receiver. It was designed as a DVB-T television European standard receiver, and it is probably better for that purpose.

A Postscript…

In the NI Multisim schematic you see a LED with a big resistor in series. It is not a mistake.

All LEDs I use are from China. The 3 mm ones I bought extremely cheap (I think they were 200 or 500 in the bag). The white ones are the most sensitive, and light at several microamps. I need to use resistors between 150 K (for BLUE) and 300 K (for WHITE) in series with the LEDs for 12 Volts power supply.

Now I understand what kind of LEDs they use in the portable lit antennas for walkie-talkies they sell for Baofeng. They light OK. Smaller resistances means burned LEDs. I tried the old values from various published schematics, and NO, they are not OK for the bags of LEDs I have.

~ Daniel VE7LCG

18/12

The November-December 2020 Communicator

 

Over 110 Pages Of Projects, News, Views and Reviews... 

Read in over 120 countries now, we bring you 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:  

https://bit.ly/SARC20NovDec

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

Not All Triplexers Are Equal: A Review of the Dunestar HF Triplexer

 

...Buyer Beware!

I had an opportunity to troubleshoot and repair SARC’s Dunestar triplexers. A couple of surface mount capacitors failed and had to be replaced. The triplexers are now in working order again. But...

These triplexers were very cheaply made. They contain only resonant circuits instead of filters. Better diplexers have low pass filters, band pass filters and high pass filters. So why are filters better than resonant circuit? Let’s see:

Here is a schematic diagram of our triplexer . There are 3 resonant circuits, one for 160, one for 80m and one for 40m. 

The frequency response curve of resonant circuits is narrow and may not cover a whole amateur radio band. In the case of our triplexer, the 80m bandpass is so narrow that does not cover both the CW and SSB portion of the bands. There is a jumper JP that adds more capacitance (C3) for the CW band by lowering the resonant frequency. Without the jumper the circuit is tuned to the SSB band. However, the jumper is located inside of the enclosure and it is very inconvenient to change.

Below are the 80m band graphs showing insertion loss (G) and VSWR of our Dunestar triplexer, with the jumper in the SSB position. As you can see it is useable only from 3.685 to 3.882 MHz if the practical VSWR limit is 2.  It would be slightly wider if the allowable SWR is 3, but it still covers only the centre of the band.

Triplexers which use filters (low, high, band pass) are much better. Below is a schematic diagram of a more sophisticated triplexer using filters rather than resonant circuits.

Let’s examine the characteristics of this 80 MHz bandpass filter. The graphs [above] show the scans taken the same way as for the Dunestar units. You can see that the band pass filter is flat and covers the entire 80m band (from 3.42 to 4.10 MHz) CW and SSB without need of a jumper. The low pass and high pass filters have similar characteristics; they are flat and cover the entire 160m and 40m bands.

So what have we learned here?

First, you get what you pay for.

Second, you have to be careful using this triplexer because outside the narrow pass bands you will run into high VSWR. It is worthwhile to check the specifications before you purchase any triplexer. 

As with any triplexer, you need to use extra band pass filters (one per band) because the isolation between the inputs is not sufficient (for example some RF power from the 80m transmitter could get into 40m receiver and damage the receiver’s front end).

~ Les Tocko VA7OM

18/12

Transceiver Foot Switches: A Better Solution

 

Enough of the light footswitch moving out of reach!

Foot switches were never a must-have Amateur Radio accessory… that is until I started contesting about 14 years ago. I used a desk mic and the built-in Push-To-Talk (PTT) switch on the mic base. It was fine for general chats. I switched to a headset sometime around 2000 and it did not have a built-in switch so I started examining alternatives.

My first trial was with a pushbutton hand switch.

It was useful but cumbersome and very unergonomic as I always had to have at least one hand on the button. Not a good choice for contesting, even with the paper logging I was using at the time.

Then I recalled my time in the  E-Comm 9-1-1 call centre. Radio Operators there use a foot switch exclusively, leaving both hands open for other tasks.  My first foot switch was a home-made affair. It worked just fine but did not have the right weight or ‘feel’ and moved around on the floor. I  then modified  a foot pedal from my woodworking tools by removing the AC socket and replacing it with a standard ¼-inch phone plug, the norm for PTT input. 

It was much better, had decent weight and a solid PTT contact as long as my foot hit the correct part of the pedal, something that doesn't always happen in the frenzy of a good contest pile-up or an attempt to get that rare DX.

It wasn’t until about 2008 that I noticed that the sustain pedal on my wife’s Roland piano used a ¼-inch phone plug as well. Although I don’t play myself, I found out that these are quite heavy and  was told that it did not normally move around.

I used that pedal for a while but, to avoid the inevitable: “Did you take my pedal again?” I decided to shop for my own. A trip to a couple of local musical instrument stores produced several good candidates. I tried some out… to questioning stares as I didn’t play a piano while doing so, but instead listened for a smooth and solid click and tossed it in the air a bit to judge the weight. I took one home for $25 with an assurance that I could return it if dissatisfied with the product. It turned out to be a Chinese-made item but it worked like a charm with all the right attributes, and it is still in use today.

As it turned out it also has a normal open (NO) and normally closed (NC) selector switch. Apparently this is because some pianos require that option. For Amateur Radio use the switch should be set to normally open (NO) to trigger the PTT when the pedal is depressed otherwise the radio would transmit constantly except when the switch is depressed.

Amazon has pedals starting around $20 and eBay has them starting at about $15. My recommendation is to visit your local music store and to try a few so you can determine if they tend to slide on the floor, if they have a nice solid click and if they are normally open.

~ John VE7TI

18/12

A Look At Modulation

A Back to Basics Column from November 2018

From the Canadian Basic Amateur Radio Question Bank

Back To Basics is a regular column in the SARC Communicator Newsletter, available at:  The Communicator Digital Edition: Amateur Radio Newsletter (ve7sar.blogspot.com)

It is a subject that is important because of the interference overmodulation can cause...

This month we’ll look at percentage of modulation and overmodulation. In all the exams I have administered, this topic is always covered. It’s important because it has the ability to cause significant issues on the air. The impact of this is highlighted by the fact that it is repeated a half-dozen times in the Canadian Basic Question Bank with slightly different wording, for example:.

B-001-019-004

The maximum percentage of modulation permitted in the use of radiotelephony by an amateur station is:

A. 100 percent

B. 50 percent

C. 75 percent

D. 90 percent

When you transmit a signal, you do so over what’s called a carrier frequency. This is a relatively constant oscillation, usually in the radio frequency band, that gets modulated (altered) by the signal. In terms of radio use, the modulation is generally (but not always) a waveform produced by the human voice, music or other audible means.

For example, either the amplitude or the frequency of the carrier gets modified (or “modulated”) by the signal, hence “AM” – (Amplitude Modulation) and “FM” – (Frequency Modulation).

When this modulation is so large that the carrier signal clips (distorts, in the case of AM) or the frequency changes to such a degree that it goes beyond the range that the receiver can pick it up or overlaps other carrier frequencies (in the case of FM), the signal is said to be overmodulated.

Likewise, if the signal is of such small amplitude or frequency variation that it cannot be picked up or adequately amplified by the receiver (because of background noise and/or the strength of the carrier frequency), it is said to be undermodulated.

Overmodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always considered a fault condition. In layman's terms, the signal is going "off the scale". Overmodulation results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal. This means that the envelope of the output waveform is distorted.

In the image, an amplitude modulated sine wave:

• At 0% unmodulated [top left], the sine envelope is not visible at all;
• Less than 100% modulation [top right] depth is normal AM use;
• At 100% modulation depth [bottom left], the sine envelope touch at y=0. Maximum modulation that can be retrieved with an envelope detector without distortion;
• At greater than 100% modulation depth [bottom right], "overmodulation" occurs and  the original sine wave can no longer be detected with an envelope detector.

Therefore, the answer to our sample question at the top of this article is A. 100 percent.

~ John VE7TI

18/11

Tech Topics: Filters, Diplexer and Triplexer Fundamentals

A Popular Communicator Article from 2018 

Several stimulating discussions around the Saturday morning club breakfast table have taken place recently in connection with our use of bandpass filters, diplexers and triplexers.  This article is designed to remove some of the mystery surrounding these devices, which we use both at the OTC and at Field Day.  Although the discussion relates to HF devices, the same general principles apply to VHF and UHF.

Introduction

As propagation conditions change throughout the day, week and year-to-year, HF stations need to have the flexibility to change to those bands which are open. Typically 20 m is open during the daytime hours with 160, 80 and 40 m opening up in the evening and nighttime hours.  In years when sunspot activity is greater, 15 and 10 m also open up during the day.  Currently we are near the sunspot low with the result that DX contacts are a challenge at any time of the day with only the low bands consistently productive for DX.

 

Ideally, a transceiver will utilize an independent antenna for each band on which it operates.  However this is not always possible, where space does not permit or when several transmitters are operating simultaneously (at Field Day, for example).  So we may deploy a multi-band antenna in conjunction with electronic devices that will allow more than one transmitter to use this single antenna, so long as each transmitter is operating on a different band.

SARC’s first exposure to these electronic devices was ca 2015 when we acquired a set of bandpass filters and triplexer for use with our 10-15-20 m TH7 beam antenna.  This was successful and allowed us to have the one antenna on a high tower serve multiple transmitters without significant mutual interference.   

Then a couple of years ago at Field Day, we began using an off-centre fed long wire for 40 and 80 m.  During the late evening hours these two bands were the only game in town, so the antenna was in demand by two stations simultaneously.  Again, a triplexer and bandpass filters allowed this to happen.  Alas, one of the devices failed at the critical time. 

In 2017, we acquired an identical set of the devices described above for use at the OTC, where we have a tri-band beam for 10-15-20 m plus an OCF dipole for 40 and 80 m.  Once again, the 160-80-40 triplexer failed during use.  

This could not continue as failures in these devices place expensive radios in danger of serious front-end damage (i.e. smoke) due to strong other-band signals not being adequately blocked.  It was time for serious reflection about our physical setup.  

A Review of Some Basics 

Inductors tend to pass lower frequencies and capacitors high frequencies.  In other words inductors have a low impedance to low frequencies and capacitors the reverse, the resultant reactance or impedance depending on the value of the inductance, capacitance and frequency.  

An inductor connected to a capacitor will have a unique frequency at which the pair resonates, called the resonant frequency.   At exact resonance, the inductive reactance equals the capacitive reactance expressed as XL = XC and the impedance will either be very low or very high depending on their parallel or series configuration.  The effect of resistance in any practical circuit does not change the resonant frequency but it does affect the sharpness (or Q) of the tuning.

In other words, an inductor in series with a capacitance has a low impedance at its resonant frequency, but the same pair connected in parallel exhibits a high impedance to the flow of current.  These properties are the basis of many types of radio circuits, used most notably for tuning purposes.  They can also be deployed in various combinations as RF filters and in power supply filters to change pulsating DC to “pure” DC.

A low pass filter will pass low frequencies and block high frequencies.  A high pass filter does the opposite.  Bandpass and bandstop filters allow a band of frequencies to pass or be blocked, respectively. The figures above show the generalized frequency response of the 4 basic filter types. 

Below are some simple examples of L-C circuits used in practice for the various kinds of filter devices.  The presence of R in the circuits represents loads but otherwise does not affect the general type of filter and can be ignored for the sake of this discussion.

Intuitively, it is not difficult to determine which type of filter it is by examination of the circuit, if you think of the way L and C respond to low and high frequencies, whether in isolation, in series or in parallel when presented with a range of different frequencies.  

More complicated circuits have been devised that improve the performance of these basic circuits and make them more useful.  A study of such devices will bring forth variations named for the engineers who studied their properties, such as Butterworth, Chebyshev, Cauer and Bessel.  More complicated circuits are not within the scope of this introductory article, but a comprehensive discussion can be found in any ARRL Handbook.

The complexity of a filter circuit is described in terms of its “order”, a measure of the number of L and C elements.  Here, for example, is a 4th order high pass filter:

Practical Devices

A diplexer allows two transmitters to feed one antenna or, conversely, two antennas to serve one transmitter (don’t confuse a diplexer with a duplexer, which is a different animal).  A diplexer simply consists of a low pass filter and a high pass filter operating in parallel, with the cutoff of each somewhere between the two operating frequencies.  With an HF unit used to separate 40 m (~7.0-7.3 MHz) from 80 m (~3.5-4.0 MHz), the cutoff frequency typically would be 5 MHz.   

A diplexer may be able to discriminate 80m from 40m signals by 20-40 dB.  While 20 dB represents a power suppression of the unwanted signal by a factor of 102  it is insufficient to protect the radio.  

That is why an HF diplexer is seldom used by itself.  A bandpass filter in series with the diplexer might suppress the unwanted frequency an additional 40-60 dB depending on its design.   So the diplexer and bandpass filter, operating together, would typically suppress the adjacent band signal by a total of 60-100 dB or a factor of 106-1010.  

If a triplexer rather than a diplexer, is desired to facilitate a third band, the problem becomes more complex.  The “middle” frequency would necessarily have to be a bandpass filter.

One problem is that the size of components for diplexers and triplexers for 160, 80 and 40m bands will be large.  This size factor and associated high cost generally make high power diplexers, triplexers and bandpass filters quite costly.  

Our Devices

Our Dunestar triplexers appear to be rather simple filter circuitry.  Why do these units fail repeatedly, even with the radios operating at 100 watts?  It can only be inadequate current or voltage ratings on the components or excessive SWR, or both.  This would suggest that the antennas connected to the triplexer should be close to resonant at the desired frequencies.  Operating at extreme ends of the band, especially under Field Day conditions when time does not always permit “tweaking” of their length, height or configuration may produce unacceptably high SWR.  

Here is the lesson we have learned: carefully research the characteristics of the diplexer or triplexer you are considering for purchase.  Not only are the band isolation and insertion loss important, but the need to have conservative voltage and current ratings on components is critical.  Then do not deploy these devices on antennas where a near resonant condition cannot be achieved.  

Conclusion

We will probably replace both our Dunestar 160-80-40 triplexers with more robust devices to ensure another failure does not happen.  Units available from VE6AM (www.va6am.com) and DX Engineering (dxengineering.com) and 4O3A (www.4o3a.com/products/high-power-filters/combiner/) are under consideration to meet this need. [In the end we went with VE6AM's product, which has given excellent service]

More good reading can also be found at: 

https://static.dxengineering.com/global/images/technicalarticles/lbs-pb-tp500_sn.pdf.

~ John VA7XB

18/09

Back to Basics: Transformers

The Communicator Revisited - October 2018

From the Canadian Basic Question Bank

Back To Basics is a regular column in The Communicator Newsletter. Past issues are available at The Communicator Digital Edition: Amateur Radio Newsletter (ve7sar.blogspot.com)

B-005-11-1 If no load is attached to the secondary winding of a transformer, what is current in the primary winding called?

A.    Magnetizing current

B.    Direct current

C.    Excitation current

D.    Stabilizing current

A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a varying electromotive force (emf) or "voltage" in a second coil. Power can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described this effect (See story Page 4). Transformers are used to increase or decrease the alternating voltages (AC) in electric power applications.

An ideal transformer is theoretical… lossless and perfectly coupled. There exists no lossless transformer though. Transformer energy losses are dominated by winding and core losses.  Magnetic permeability of the core results in the most loss, often felt as heat.

One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

A varying current in the transformer's primary winding creates a varying magnetic flux in the transformer core and a varying magnetic field impinging on the secondary winding. This varying magnetic field at the secondary winding induces a varying EMF or voltage in the secondary winding due to electromagnetic induction. The primary and secondary windings are wrapped around a core of high magnetic permeability so that all of the magnetic flux passes through both the primary and secondary windings. With an AC voltage source connected to the primary winding and load connected to the secondary winding, the transformer currents flow in the direction indicated in the diagram below.

According to Faraday's law, since the same magnetic flux passes through both the primary and secondary windings in an ideal transformer, a voltage is induced in each winding proportional to its number of windings. This is determined by the equation:

The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”. The transformer winding voltage ratio is thus shown to be directly proportional to the winding turns.

When connected to a source of AC power, current flows through the primary winding of a power transformer even when no loads are connected to the secondary winding. The primary winding remains an inductor and lets some AC current through despite its reactance. This minimal current is called "Magnetizing Current" Also known as the “Exciting Current”. This current establishes the magnetic field in the core and furnishes energy for the no-load power losses in the core. 

Therefore, the answer to our question is: 

A. Magnetizing Current.

~ 73, John VE7TI