[Lowfer] RX improvements

[N1BUG]

Hi Clint,

As a veteran 160m DXer I realize de-resonating transmitting antennas 
which happen to be in the vicinity of receiving antennas is highly 
desirable. I must admit to incompetence, though. Over many years every 
attempt at de-resonating my 160m shunt fed tower has failed.

With 2200m I tried this a year ago. I didn't keep proper notes on 
everything I tried and results but my effort ended in making no 
significant improvement in the bottom line: S/N of 2200m signals. I seem 
to be getting very different results this time. Perhaps that "ground 
loop" problem I found with the Softrock Lite II had something to do with 
why I didn't get these results a year ago, or maybe something else I've 
changed since then is a factor.

This morning, on the advice of several who said the same thing you did, 
I reconnected the coax as normal to the transmitting antenna and 
disconnected the bottom of the resonating inductance from the impedance 
matching transformer. Since this shifted resonance much farther from 
2200m, there was a 10-11 dB drop in both noise floor and signals in the 
receiver. At first I was concerned, since the noise floor with antenna 
connected is now just 11 dB more than with a 50 ohm termination on the 
receiver. Usually for extreme weak signal work I like to see a bit more 
than that. However, I have been seeing S/N equal to yesterday with the 
other method or perhaps as much as 1 dB better. I have ordered a relay 
to switch this connection but will have to work on a control mechanism. 
I don't want to rely on RF sensing at the power level needed for one 
watt EIRP.

My receive spectrum during daytime now has fewer "RFI lines" than ever 
before. That will change with sunset as the big commercial signals HGA22 
and DCF39, along with strong east coast amateur signals come up and 
start making all sorts of 120 Hz spaced artifacts due to aggressive 
noise blanking I am forced to run. Without the blanker, very strong 120 
Hz impulse noise takes out all but the very strongest signals.

I appreciate all the input I have received on this. There is still much 
I don't know, but I am learning. I feel I have made real progress on 
receive performance here.

73,
Paul N1BUG


On 12/23/19 3:08 PM, Clint Turner wrote:
> Hi Paul,
> 
> I noticed this in my LowFer days a long time ago and have always had a 
> means of "de-resonating" the TX antenna during receive periods.  I have 
> long-used series variometers on my LF antennas (along with an 
> auto-transformer for TX matching) and I disconnect, with a relay (RF 
> sensing) the line between the series inductance and autotransformer - a 
> 100k resistor to ground remains as a static bleed:  I have a "bypass" 
> switch for those instances when I'm running SSB to prevent relay 
> chattering:  Being low impedance at this point, any old relay will do. 
> Since then, I have verified that this happens on 2200 and 630 meters as 
> well /(why wouldn't it?)/ - but a 630M resonant TX antenna does not seem 
> to have any obvious effect at 2200M RX and vice-versa.
> 
> Such a relay is innocuously depicted on this page: 
> https://ka7oei.blogspot.com/2017/12/now-qrv-on-630-and-2200-meters.html .
> 
> At my LF station, with a matched TX antenna (on the RX frequency) 
> connected, I notice I get what appears to be conducted noise from the AC 
> mains conducted onto it despite common-point grounding - and in my case, 
> this raises the receive noise floor by at least 6dB - but the desired 
> signals are "sucked out" by the TX antenna by several dB.  This sort of 
> effect is reminiscent of what anyone using a switched antenna "Doppler" 
> DF unit on VHF/UHF has seen: The switched antenna "space modulates" the 
> entire vicinity, affecting all signals on the same amateur band as 
> received on any radio nearby.  The effects of my rooftop DF array are 
> audible within about a 100' radius on any signal in the 2 meter band.
> 
> 73,
> 
> Clint
> 
> KA7OEI