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Re: SDRuno S-Meter = S5 With No Antenna

Posted: Wed Aug 15, 2018 5:45 pm
by mark.
Thanks. My fault for not scrolling far enough. I didn't realize there were 2 charts. :oops:

Re: SDRuno S-Meter = S5 With No Antenna

Posted: Sun Sep 02, 2018 6:44 pm
by Murphyv310
I've been reading the thread with interest, to be fair I'm somewhat confused.
Over the years I've had many receivers through my hands and was a Radio & TV engineer to trade with all the relevant qualifications.
In a good comms receiver you would perhaps have two stages of RF amplification, frequency changer and two more IF stages followed by detector and audio amp. The IF amps and frequency changer would be controlled by the AGC line and in some sets the RF stages could also be AGC controlled or split so one or two have manual gain control. So when you increase the gain of the RF stages the noise floor increases it doesn't reduce as increased gain equals more noise. The noise floor in the receiver should never be so much as to bring in the AGC, it's poor design and unnecessary.
I would say the my new RSP1A is a very competent receiver but its inherent noise floor is excessive, weak signals are lost in the noise which is disappointing, a Racal RA17 has far superior low signal performance to the extent that the RSP often won't resolve a signal that the Racal takes in its stride.
My feeling is the RSP RF gain acts more like an attenuator than a true RF stage gain control.

Re: SDRuno S-Meter = S5 With No Antenna

Posted: Mon Sep 03, 2018 9:55 am
by glovisol
Hi Murphy,

As an old timer like yourself, in the fifties & sixties I used different receivers, amongst them the Racal RA17 and the Collins 75A4, at the time judged the best.

- With AM reception one kept AGC ON and audio level was adjusted with the volume control.
- With SSB reception one kept AGC OFF, audio level at a fairly high position and volume control was obtained by manually adjusting RF gain. With this procedure perceived noise was minimised, especially in the modulation throughs of the received signal, where received power, with low or no modulation, was at a minimum. This procedure minimized or avoided AGC amplification of internally generated or antenna noise.

With these receivers you only had one AGC circuit, controlling both IF and RF gain.

Comparing the two receivers, the Collins 75A4, a conventional double conversion superhet, is the less sensitive, but, with the formidable front end selectivity afforded by permeability tuning, it almost never overloads with strong signals. The Racal RA17, with its wideband front end and upconversion to a high IF, is more sensitive, but can often overload with strong signals, even far away from the receive frequency.

The RSP class of receivers, because of their structure, are impervious to front end overload and have two AGC circuits, one controlling the gain of the IF section and the other controlling the gain of the RF section: if the RF gain is low, then the self generated noise of the IF section (with the worse noise figure) will prevail. Increasing the gain of the RF section (better noise figure) its internal noise will prevail, minimising receiver's self generated noise. In any case present times antenna noise is at such a level as to render almost irrelevant the issue of internally generated noise. It depends on the operator's experience and skill to adjust the controls in such a way as to avoid antenna noise amplification, like I just found out to my surprise: ... f=5&t=3635

By using these procedures, with the RSP receivers you indeed easily copy signals at the same level as the antenna noise, a feat I was never able to accomplish with "conventional" receivers of time past.


Re: SDRuno S-Meter = S5 With No Antenna

Posted: Mon Sep 03, 2018 7:22 pm
by Murphyv310
Hi Glovisol.
Thanks for the reply and the tips.
I have given this a try and yes I do see an improvement.
My own set up from the aerials are: Modified Wellbrook ALA1530 loop, Long wire via balun and dedicated aerials for the frequencies I monitor (Band 1 Analogue TV, Low FM, Airband and 446PMR along with Dump 1090)
My main interest is Long, Medium and short wave DX where I use the loop and long wire.
At present I use a couple of Eddystone receivers, the RA17 (not mine but on loan) A CR100 and a Dynatron T99 tuner. My main interest is MW DX and the possibility of receiving the up and coming 405 line TV transmissions from the heritage TV chaps see here:
I did a bit of fiddling.... well adjustments and was pleased to say that I was able to reduce overall noise and resolve some weak signals on 20 meters, I still think the RA17 has the edge but perhaps after some more time with the RSP things will improve, I've only had it 3 weeks and have been busy too so its all down to the learning curve.