Some more receive only transformer info

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Nulluser00
Posts: 9
Joined: Sat Jul 28, 2018 4:59 pm

Some more receive only transformer info

Post by Nulluser00 » Sat Jul 28, 2018 7:13 pm

I really enjoyed the series of posts by "glovisol" and after reading all the thread I was motivated to do some further research.

These two articles went into a bit more theory and offered some additional food for thought.

"Impedance Matching Transformers for Receiving Antennas at Medium and Lower Shortwave Frequencies" Bill Bowers, John Bryant, Nick Hall-Patch, VE7DXR Published on DXing.info on June 29, 2003, revised July 24, 2003

http://www.thomasn.sverige.net/Impedanc ... Bryant.pdf


This paper has data that suggest the best toroid ferrite transformer has the windings twisted together instead of separated windings on opposite sides of the toroid.


"Broadband Receiving Antenna Matching"
Mark Connelly, WA1ION – 15 July, 2003

http://www.qsl.net/wa1ion/bev/bb_antenna_matching.pdf


Mark Connelly has a paper that provides additional food for thought: "Another Look at Noise-Reducing Antenna Systems" Mark Connelly, WA1ION, - 06 JUL 1992"

http://www.radioscanner.ru/files/downlo ... nrants.pdf


The Mini-circuits transformer Mr. Connelly specifies have very small ferrite cores, extremely fine wires, and, are both very prone to causing IMD in the presence of strong MW signals and failing during lightning events. I experienced four 9:1 fail within six weeks. I suspect a Mini-Circuit 1:1 at the home end might be OK.

While Clifton Labs is gone, I did find a PDF of the article here:
https://groups.io/g/BITX20/attachment/2 ... ormers.pdf

FWIW the Crosscountry Wireless offers a SDR designed more for amateur radio operators then SWLs, it is interesting in the radio has a metal case and uses an insulated BNC connector with a 1:1 RF isolation transformer for the RF input port.


The BN-73-202 binocular ferrite core works extremely well for LF-MF-HF from 10KHz to at least 30MHz. As might be expected, different number of turns for the primary-secondary are optimal for different frequencies:

4 turns on the primary and secondary gives a 1:1 transformer that covers the ~1MHz to ~50MHz frequency range.
12 turns on the primary and 4 turns on the secondary gives a 9:1 transformer that covers the ~1MHz to 50MHz frequency range.


6 turns on the primary and secondary gives a 1:1 transformer that covers the ~100KHz to ~30MHz frequency range.
18 turns on the primary and 6 turns on the secondary gives a 9:1 transformer that covers the ~100KHz to ~30MHz frequency range.

6 turns on the primary and secondary gives a 1:1 transformer that covers the ~100KHz to ~30MHz frequency range.
18 turns on the primary and 6 turns on the secondary gives a 9:1 transformer that covers the ~100KHz to ~30MHz frequency range.


7 turns on the primary and secondary gives a 1:1 transformer that covers the ~100KHz to ~30MHz frequency range.
21 turns on the primary and 7 turns on the secondary gives a 9:1 transformer that covers the ~20KHz to ~20MHz frequency range.


8 turns on the primary and secondary gives a 1:1 transformer that covers the ~20KHz to ~30MHz frequency range.
24 turns on the primary and 8 turns on the secondary gives a 9:1 transformer that covers the ~10KHz to ~15MHz frequency range.


These values are based on a temporary 50 foot sloping antenna, the far end was elevated about 50 feet and it sloped down to the 8 foot tall structural member of the picnic shelter, at a company picnic and should be considered preliminary data subject to future verification.

I hope to refine the results when I get time to set up a test jig with a Signal Hound USB-SAB and the USB-TGA

At home I use a WinRadio 9:1 that is rated for operation from .1~30MHz, testing revealed it works fine down to at least 22KHz. The WinRadio uses a Ruthroff dual transformer design, which might account for the wide bandwidth. My primary antenna is a 40 foot vertical with 16 radials. I use a 500 foot run of low loss 1/2 hardline with a 6:6 turn 1:1 transformer at the house end. In my home setup the 6:6 works fine down to at least 22KHz. I can not explain the difference between the results at home and on the picnic. Again, I hope bench testing makes things a bit clearer.


Kits & Parts offers the BN73-202.
http://www.kitsandparts.com/
http://www.kitsandparts.com/toroids.php


For those who live in areas where the BN73-202 is unavailable, there might be another option. John Doty suggested using the binocular ferrite core from a 300:75 TV matching transformer. I haven't had a chance to more then check a couple of cores with 7:7 turns, and the bandwidth easily covered the 100KHz~15MHz frequency span.

I've had good success using the core from a TV matching transformer with 2:2 turns as a 1:1 VHF-lo to UHF isolation transformer to eliminate a ground loop problem I had with our dedicated police band scanner at work. I hope to refine the design with the planned test jig.

RF Common Mode Currents are a major source of interference. W1HIS has a masterful article dealing with this problem. I've found it is extremely useful to take every step possible to reduce the effects of my home grown EMI.

Martin Ehrenfried, G8JNJ, has an excellent page on active antennas and he has one idea I found to be extremely useful was the addition of chokes in the antenna coaxial feedline.
https://mediaprocessor.websimages.com/w ... 0balun.png
and
https://mediaprocessor.websimages.com/w ... tion-1.png

While mother nature sets the limit to the weakest signal we can receive, in most cases local EMI is significantly higher then the natural ambient RF Noise Floor. It is generally well worth the effort to lower both the EMI and the amount of EMI our antennas and feedlines pick up, and to keep our coaxial cable from carrying homegrown EMI to the antenna!

My primary interest is HF/SW "utility" reception, followed by ELF/VLF/LF, low power TIS [Traffic Information Stations] and NDB. I also have a dedicated RSP1a SDR for SID [Sudden Ionospheric Disturbance]. I monitor and use a laptop PC to log the signal level from the NAA, US Naval station in Cutler, Maine, that transmits on 24KHz.

One last thought. Many references state toroid transformers are inherently self shielding. While it is true they have vastly superior self shielding characteristics compared to axial inductors, they will pick up significant signals. I have a toroid I salvaged from a SMPS and soldered it directly to a chassis mount female BNC with the shortest practical leads. It makes a nice "EMI sniffer." The BN73-202 has no discernable EMI pickup.

In some situations it might be useful to install a 1:1 RF isolation transformer as close as practical to the SDR receiver.
While SDRs are wonderful, they require computers and computers are intense EMI sources.

I know this post has gotten a bit long winded, but my boss suggested I remind everyone that lowering your homegrown EMI footprint lower then the natural ambient RF noise level.

Noel Usher
Last edited by Nulluser00 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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glovisol
Posts: 658
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Some more receive only transformer info

Post by glovisol » Sun Jul 29, 2018 3:00 pm

The post by Nulluser00, who dug up very interesting technical information on antenna transformers, gives me the opportunity to further comment on the relationships among:
- low noise coupling between antenna and receiver,
- wide band coupling between antenna and receiver
- receiver's dynamic range and
- received noise minimisation

First of all we should not take the SDRplay receivers for granted. They truly represent, in my opinion, a major advance in this field of technology, which should drive us to see LF and HF reception under a new light. The articles cited in the post I am commenting were written at times when the ideas about this technology were just on the verge of dawning.
The RSP receiver provides the operator with a panoramic representation of a wide span of the frequency spectrum, while at the same time giving continuous indication of the receiver’s available dynamic range. Furthermore the RSP receiver affords a Hi Z (1000 Ω) balanced input: by properly exploiting all these features, it is possible to optimize low noise reception on the HF bands to a degree not previously possible with conventional equipment.
By looking at the PC display we can now immediately and effortlessly see and understand if a given change in our receiver's input system produced a S/N improvement or not. This is and was in the past hardly possible (not to say impossible?) with a narrowband conventional receiver. Keeping this in mind, we can now examine the proposed references.

"Impedance Matching Transformers for Receiving Antennas at Medium and Lower Shortwave Frequencies" Bill Bowers, John Bryant, Nick Hall-Patch, VE7DXR Published on DXing.info on June 29, 2003, revised July 24, 2003
This article is the definitive and most valuable tutorial on toroidal transformer engineering and construction. The emphasis is on insertion loss and on matching, but very briefly it takes antenna noise under consideration. At the time RSP receivers did not exist: if one does not take noise into consideration because his receiver does not easily provide this information, then he will just go for minimum insertion loss and best match. If I need a wideband transformer as interstage or output of a power RF amplifier, then even a 0.1 db of insertion loss is important, because power FETs and heat sinking are expensive and because the transformer must not blow up with dissipated power, but if I need a transformer to join antenna to receiver, when external noise may vary in a range of 30 dB, then I must go for lowest noise transferred & maximum dynamic range, hence the transformer must be optimised for these parameters only.

"Broadband Receiving Antenna Matching"
Mark Connelly, WA1ION – 15 July, 2003
We now come to the transformer bandwith issue: do we really need a 0.5 - 30 MHz bandwith between antenna and receiver? RSP receivers have wideband front ends, so for serious work ad hoc preselection should be used to preserve as large dynamic range as possible. If different passband filters are used for each band, then it is easy to switch different balun transformers (optimised for minimum noise transfer) along with the filters. We really should minimize PRI/SEC capacitance, not maximise bandwith. Under this light a 10 MHz bandwith is more than enough. It is the antenna that should be as wide as possible, not what comes behind it.
"Another Look at Noise-Reducing Antenna Systems" Mark Connelly, WA1ION, - 06 JUL 1992"
This article is representative of what could and should be done with a conventional receiver with umbalanced 50 Ohm input. But with this scheme the risk of the in between coax line becoming a noise antenna itself is ever present, so at best this and many more such schemes in the literature can work for one guy and make another unhappy. The comparatively inexpensive, but steel enclosed SDRplay unit, allows us to place the receiver and a preselector in any convenient place close to the antenna terminals, provided we use a buffered USB cable, and do away in one shot with all possible unwanted noise and interference sources, while we comfortably sit in the shack. Furthermore the balanced antenna input is a major factor in noise rejection right at the receiver's.


To conclude, I think it will take time for us all to fully understand the technical significance and the improvements brought about by the RSP technology: In fact I suspect that even the designers of the RSPplay class of receivers have yet to grasp all of its implications.

glovisol

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