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

[Ed Phillips]

Around here a lot of such spurs come from relay contacts in receive RF 
lines and sometimes working the relays a number of times helps.  Easy 
enough to try.  Unfortunately there are so many other sources of spurs 
that I can't do any useful lowfer listening.

Ed

W6IZJ

Clint Turner wrote:

> 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
>>
>>
>
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