Actually in some instance this mixing could be taking place outside of the
radio, such as a poor antenna connection, a corroded joint between a guy
wire and a tower leg or just about anywhere else where two metals join. The
signals are detected, mixed and the sum and or difference is re-radiated and
is being picked up by the Omni VI Plus. This is just the normal stuff that
us repeaters owners and operators deal with every day.
Since the LP filter ahead of the radio seemed to cure the ills, then likely
one of the switching diodes in the radio is the culprit. Perhaps a
tightening of the screws on the PCB mountings might correct the problem if
it is de-coupling caps floating up from ground.
73
Bob, K4TAX
----- Original Message -----
From: "Jack Mandelman" <modeler@bellsouth.net>
To: <tentec@contesting.com>
Sent: Wednesday, December 12, 2007 8:21 PM
Subject: Re: [TenTec] Omni VI+ receiver UHF intermod
> In the OmniVI only TX rf is routed through the LPFs on the LP filter
> board; receive does not see the benefit of these LPFs. However BPFs
> on the front end board are used for both TX and RX. But in the presence
> of a very strong UHF signal, could spurious mixing could occur in the
> switching diodes before any filtering? These diodes are used to steer
> rf from either the main or AUX antennas to the front end.
>
> Jack K1VT
>
> On Wed, 2007-12-12 at 18:26 -0500, Jack Mandelman wrote:
>> While tuning across 20m ssb with my recently acquired Omni VI plus I
>> noticed what first appeared to be S5-6 splatter from a nearby signal.
>> After closer inspection I realized that it was an FM modulated signal
>> centered at about 14.17980 MHz. I then switched the radio to FM and
>> was
>> able to make out certain softly spoken words, although the signal was
>> wideband FM while the Omni VI + is meant to receive narrow band FM.
>> The
>> FM signal appears to be from a local UHF TV station operating on about
>> 635MHz.
>>
>> To determine if the signal I was hearing was truly on the 20m band I
>> listened on both a TS-940S and a TenTec CorsairII. Neither the 940 or
>> C2 heard any such signal. Therefore I surmised I was hearing an
>> intermod product generated within the Omni VI +. I was able to
>> eliminate the intermod by inserting an old Johnson Viking LP filter in
>> front of the Omni VI+.
>>
>> Has anyone encountered or heard of a similar UHF intermod problem with
>> the Omni VI?
>
> I can sort of believe it happening. The LO is synthesized in that range
> and divided down to 5 MHz.
>
> I'd also check to see if the radio's internal band switched low pass
> filters weren't shorted out by faulty relays or diodes. There ought to
> be some low pass filter in the front end all the time.
>>
>> Jack K1VT
>> _______________________________________________
>
> 73, Jerry, K0CQ
>
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>
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