Some more receive only transformer info

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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 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.


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