[Lowfer] RX improvements

[Clint Turner]

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



On 12/22/2019 1:43 PM, N1BUG wrote:
> Hi LF, Lowfer,
>
> This winter it seems my TX is not breaking down every night so I 
> decided to work on my poor RX side. :-)
>
> The first thing I found is a ground loop (or lack of single point 
> ground) causing some RX noise problems. I fixed that.
>
> Then I discovered something which I do not yet understand. The TX 
> antenna is a vertical with top hat and a big coil/variometer at the 
> base. It is transformer coupled to the coax, with the coax side 
> floating (no connection to ground). The RX antenna is a short vertical 
> ("LNV") which is only about 15m from the TX antenna, so there is 
> strong coupling between TX antenna and RX antenna.
>
> If I disconnect coax at the TX antenna, I see less RFI on the RX and 
> S/N improves. Leaving the coax connected at the antenna but 
> disconnecting the end in the transmitter room does not get rid of the 
> RFI. Leaving the coax shield connected at the antenna but 
> disconnecting the coax center conductor also gets rid of the RFI. This 
> seems strange to me. I don't understand the mechanism by which the 
> coax is apparently injecting noise to the TX antenna.
>
> Anyway, I have less interference now. Not so many strong lines and 
> bright areas in the spectrum. It is difficult to evaluate real 
> improvement but I seemed to be hearing better last night. I was able 
> to decode N3FL on WSPR during the middle of the day which has not 
> happened in the past. I have to work on a relay system so I can RX and 
> TX without going to the antenna to disconnect that coax!
>
> Soon I will undertake to replace the RX crystal LO with a GPSDO 
> source. The TX already uses a GPSDO derived carrier oscillator.
>
> I think I should try a H field loop to see if that helps. Maybe some 
> of these local interference sources are stronger in the E field? The 
> e-Probe I built was completely useless in this environment...
>
> 73,
> Paul N1BUG FN55mf
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