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Re: [TenTec] Omni VI+ receiver UHF intermod

To: <tentec@contesting.com>
Subject: Re: [TenTec] Omni VI+ receiver UHF intermod
From: "Jack Mandelman" <modeler@bellsouth.net>
Reply-to: Jack Mandelman <modeler@bellsouth.net>,Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Thu, 13 Dec 2007 11:51:10 -0500
List-post: <mailto:tentec@contesting.com>
The interference not being present on my TS-940S and CorsairII suggests 
that it is being created within the OmniVI.  I will check to see if the 
diodes are being properly switched and biased, and if everything else 
appears to be in order.  Have to open the radio anyway to install an 
INRAD filter in the next few days.

As an afterthought, my antenna is a CF Zepp that I use on all bands and 
fed with true (home made) open wire line.  The 600MHz TV signal creating 
the intermod is right off the end of my antenna.  On 600MHz there is 
very considerable gain off the ends, which may be resulting in a very 
strong interfering UHF signal.  One might expect that the 600MHz energy 
would not make it through my antenna tuner (an L-network in LP 
configuration).  However, at 600MHz parasitic lead inductance in the 
tuner may make it behave not at all as a LP filter.  Thus significant 
600MHz energy could be transferred to the Omni.

Thanks for all the feedback.

Jack K1VT

----- Original Message ----- 
From: "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
To: "Jack Mandelman" <modeler@bellsouth.net>; "Discussion of Ten-Tec 
Equipment" <tentec@contesting.com>
Sent: Thursday, December 13, 2007 10:59 AM
Subject: Re: [TenTec] Omni VI+ receiver UHF intermod

> 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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>> TenTec@contesting.com
>> http://lists.contesting.com/mailman/listinfo/tentec
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