[Lowfer] QRSS 60 tonight along with the boys on 137.776

[Clint Turner]

I had a similar problem several years with my LF-400B E-field whip 
which, in the past, had been fairly well-behaved in terms of BCB IMD, 
particularly when it was powered from a 18 volt supply.

What I finally found was that there was low-level re-radiation from the 
MedFER beacon transmitter/Antenna that is only about 10 feet from the 
antenna:  Several of the stronger local AM stations were getting into 
the powered-down final and mixing despite the fact that I have a fairly 
high-Q loading coil on the antenna.  Unfortunately, the only way to make 
them go away is to disconnect the antenna from the MedFER transmitter, 
but since this involves a trip to the roof, I just live with the problem 
for now - particularly when the (metal!) roof is covered with ice and 
snow:  In the summer I do less LF/VLF monitoring so it has long evaded 
my "to do" list.  It may be that you have low level re-radiation in a LF 
beacon (if you have one) and/or the front end of another receiver 
somewhere - maybe even an HF rig.

If I have a reason to open the MedFER beacon's case at some point (I've 
not done so for a decade...) I'll add a relay in series with the output 
of the MedFER beacon - the contact closed when it is powered up - to 
prevent this problem.  The other way to make this problem go away is to 
apply power to the beacon's output transistor without it being biased, 
but the design of my MedFER transmitter doesn't allow this:  On/Off 
keying for the QRSS mode is done by turning the MOSFET PA's voltage 
on/off while the gate drive remains - this producing a backwave that is 
only 20-25 dB down and even worse low-level mixing products!

73,

Clint
KA7OEI/CT

Douglas D. Williams wrote:
> Looking forward to monitoring.
>
> I've noticed that I seem to get a lot of spurs/images/mixing whenever I
> monitor the 137 kHz Part 5 band and there are several strong signals on.
>
> Any suggestions on how I can mitigate this?
>
> My receive antenna is a Clifton Labs active whip, which has good OIP2 and 3
> specs (OIP2 > +73 dBm and OIP3 +40 dBm).
>
> Not sure where the spurs are being generated. Receiver? Sound card?
>
> -Doug
>
>