Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[no6b]

At 2/23/2010 17:16, you wrote:
>Mark et al,
>Yes, this repeater is using the Motorola T1500 series bandpass cavities
>(two each for rx and tx). I've tried running rx and tx both duplex and
>seperate (borrowing a nearby antenna with permission). I can hear the
>interference underneath my signal when I'm about 2 miles away and
>monitoring my signal. When its strong enough, the PL encode of the
>repeater keeps it locked up until the modulation from the AM station
>overtakes the PL being looped (voice peak). Then the repeater drops
>since I have a tone panel in between and not continuous PL outbound.
>
>I have tried changing the receive frequency about 75Khz lower and the
>interference is not present (so a 4.925Mhz split), so that serves to
>prove to me that this indeed a mix.
>
>I can try adding an attenuator the next time I'm out at the site. The
>antenna is about 300 feet up and fed with 7/8" heliax, to a Polyphaser
>and then superflex to the duplexer. I've also tried without the Poly,
>but have the same result. I have some nice Mini-Circuit pads that should
>work in the receive side after the duplexer, but think the receiver is
>simply overloaded.

The cause of your interference problem is not RX "overload".  It is as 
others have suggested: a mix occurring somewhere in the near field of the 
antenna.  Pads may eventually mask the real source of the problem, once 
you've added enough to drop the signal below your RX's noise floor, but 
you'll end up with a deaf repeater.

How far away were the separate TX & RX antennas when you tried that?  I'd 
think if they were far enough apart that you would lose the mix.  OTOH if a 
tower joint is the source of the mix (likely since a lot of length is 
required to couple in the AM BC station), it might be all over the tower.

A similar problem was partially cured here by spraying some conductive 
paint into all the tower joints.  Each time it was done the interference 
would disappear for a few months, then return.

Bob NO6B

RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Gary Schafer]

Don't be too sure about that. Once the am station signal gets into the
receiver it can go anywhere and cause havoc. It could be getting into the IF
or the mixer once picked up by cables.

73
Gary  K4FMX

> -Original Message-
> From: Repeater-Builder@yahoogroups.com [mailto:Repeater-
> buil...@yahoogroups.com] On Behalf Of KT9AC
> Sent: Wednesday, February 24, 2010 4:42 PM
> To: Repeater-Builder@yahoogroups.com
> Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep
> UHF repeaters locked up?
> 
> If it was just audio then there would be no feedback of the PL/DPL
> tones, keeping the repeater locked up.
> 
> Good advice though.
> 
> Jeff DePolo wrote:
> >
> > >
> > > On 2/23/2010 3:11 PM, Jim WB5OXQ inb Waco, TX wrote:
> > > > Is it possible the AM signal is getting into an audio stage
> > > instead of the
> > > > receiver front end? I had that happen once.
> > > >
> > > Same here. All audio inteconnects are now tiny coax cables at
> > > that site
> > > now, installed with shield grounded at ONE end...
> > >
> > > Nate WY0X
> >
> > At AM broadcast sites or studios co-located with the transmitter,
> > hard-grounding the shield at one end and RF-coupling the shield at the
> > other
> > end to the equipment ground via caps (0.01 uF as a rule of thumb) is
> often
> > the most effective technique in many situations.
> >
> > --- Jeff WN3A
> >
> >
> 
> 
> 
> 
> 
> 
> Yahoo! Groups Links
> 
> 
> 

Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[KT9AC]

If it was just audio then there would be no feedback of the PL/DPL 
tones, keeping the repeater locked up.

Good advice though.

Jeff DePolo wrote:
>
> >
> > On 2/23/2010 3:11 PM, Jim WB5OXQ inb Waco, TX wrote:
> > > Is it possible the AM signal is getting into an audio stage
> > instead of the
> > > receiver front end? I had that happen once.
> > >
> > Same here. All audio inteconnects are now tiny coax cables at
> > that site
> > now, installed with shield grounded at ONE end...
> >
> > Nate WY0X
>
> At AM broadcast sites or studios co-located with the transmitter,
> hard-grounding the shield at one end and RF-coupling the shield at the 
> other
> end to the equipment ground via caps (0.01 uF as a rule of thumb) is often
> the most effective technique in many situations.
>
> --- Jeff WN3A
>
> 






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RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Jeff DePolo]

> 
> On 2/23/2010 3:11 PM, Jim WB5OXQ inb Waco, TX wrote:
> > Is it possible the AM signal is getting into an audio stage 
> instead of the
> > receiver front end?  I had that happen once.
> >
> Same here. All audio inteconnects are now tiny coax cables at 
> that site 
> now, installed with shield grounded at ONE end...
> 
> Nate WY0X

At AM broadcast sites or studios co-located with the transmitter,
hard-grounding the shield at one end and RF-coupling the shield at the other
end to the equipment ground via caps (0.01 uF as a rule of thumb) is often
the most effective technique in many situations.

--- Jeff WN3A

Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Nate Duehr]

On 2/23/2010 3:11 PM, Jim WB5OXQ inb Waco, TX wrote:
> Is it possible the AM signal is getting into an audio stage instead of the
> receiver front end?  I had that happen once.
>
Same here. All audio inteconnects are now tiny coax cables at that site 
now, installed with shield grounded at ONE end...

Nate WY0X






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RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[daniel haines]

How long is your feed line? Do you have any toriods on your feed line, where it 
comes in your shack?

 
> To: Repeater-Builder@yahoogroups.com
> From: kt...@ameritech.net
> Date: Tue, 23 Feb 2010 16:01:45 -0600
> Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF 
> repeaters locked up?
> 
> I was able to match up the audio coming through the repeater and the 
> local AM station. My latest theory is that their signal is so strong 
> that its blowing into the receiver's front end and multiplying/mixing 
> there (past the bandpass filters and all). They are heterodyne receivers 
> after all.
> 
> I'm considering an ICE broadcast high-pass filter that cuts off at 
> 1.8Mhz (model 402). I have an email into them to see how well it might 
> work at 448 Mhz.
> 
> Tony
> 
> tracomm wrote:
> >
> > Had a similiar situation at our site, a station on 106.7 MHz, music on 
> > hang time on many repeaters, intermod runs gave no clue to reason, did 
> > all the usual, grounding, filters no resolve.
> > Turned out to be the STL Marti system on 450.100 MHz, from an close 
> > studio site pointed right at our site, hitting our Rx multicoupler, 
> > mixing with our transmitters. Resolution was low passs & isolater on 
> > the STL system.
> > Make certain which station the broadcast audio is coming from and give 
> > the station engineer a friendly call, may reveal some info to help 
> > your issue.
> >
> > Chris
> > GMRS Inc.
> >
> >
> > --- In Repeater-Builder@ yahoogroups. com 
> > <mailto:Repeater-Builder%40yahoogroups.com>, Tony KT9AC  wrote:
> > Hi everyone,
> > A while ago I was troubleshooting a bad feedback or "growl" problem
> > that was impacting a UHF repeater, of which the short term workaround
> > was to not encode TX PL (PL or DPL would keep it locked until the 
> > signal dropped enough or timed out).
> >
> > In doing some more research, I found a 1250kHz AM station within a 
> > mile or two that changes pattern between day and night. The 
> > interference mentioned above would appear around drive times (like 
> > 5pm) so that had me chasing other sources. Still, it was puzzling that 
> > a 5Mhz signal could be causing the feedback (it didn't appear when 
> > doing normal receiver testing with a service monitor). The recent give 
> > away was that I could hear talking underneath my test signal (like a 
> > sports show).
> >
> > So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize 
> > that the 4th harmonic of a 5KW broadcast station isn't very powerful, 
> > but being in its nearfield might be enough to cause a mix with the UHF 
> > transmit output.
> >
> > Does this make sense? This phenomenon can be duplicated with both a 
> > 450 and 440 repeater system - both with standard 5Mhz offsets. I don't 
> > think any sort of filtering would work since the mix happens "in the 
> > air".
> > Only by having split PL's can the lockup be prevented, and equipment 
> > was both MSF5000 and Micor systems, through correctly tuned duplexers.
> > Thanks,
> > Tony
> >
> > 
> 
> 
> 
> 
> 
> 
> Yahoo! Groups Links
> 
> 
> 
  

Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[KT9AC]

Mark et al,
Yes, this repeater is using the Motorola T1500 series bandpass cavities 
(two each for rx and tx). I've tried running rx and tx both duplex and 
seperate (borrowing a nearby antenna with permission). I can hear the 
interference underneath my signal when I'm about 2 miles away and 
monitoring my signal. When its strong enough, the PL encode of the 
repeater keeps it locked up until the modulation from the AM station 
overtakes the PL being looped (voice peak). Then the repeater drops 
since I have a tone panel in between and not continuous PL outbound.

I have tried changing the receive frequency about 75Khz lower and the 
interference is not present (so a 4.925Mhz split), so that serves to 
prove to me that this indeed a mix.

I can try adding an attenuator the next time I'm out at the site. The 
antenna is about 300 feet up and fed with 7/8" heliax, to a Polyphaser 
and then superflex to the duplexer. I've also tried without the Poly, 
but have the same result. I have some nice Mini-Circuit pads that should 
work in the receive side after the duplexer, but think the receiver is 
simply overloaded.

Tony

Mark HARRISON wrote:
>
> Hi Tony,
>
> Are you using a duplexer on this repeater?
> A lot of cavity filters act as a short circuit to DC and low 
> frequencies, so additional filtering is unlikely to help.
> I can only think of one type of cavity that has a DC path between from 
> input to output (via an internal inductor) and not to ground. This 
> type could I guess pass low frequencies. It's simple to test - 
> disconnect the antenna and receiver leads and measure the DC 
> resistance with a meter between the centre and outer on the cavity 
> connectors. If it's 0 ohms then it's likely to be a very good high 
> pass filter for broadcast frequencies!
>
> Also, using a temporary attenuator you should be able to determine if 
> there is an intermod problem within the receiver, or parts of the 
> antenna filtering system on the receive side of a Duplexer. Inserting 
> an attenuator will reduce the interference (and desired signal) by the 
> same amount if everything on the receiver side of the attenuator is 
> functioning correctly. If instead the interference drops by 2-3 times 
> (in dB, and the desired signal drops only by the attenuator value) 
> then you've found your problem!
> Placing a power attenuator in the duplexed antenna line is more 
> complicated because you are attenuating both the Tx and Rx signals. 
> You would expect the interfering signal to drop by more than double 
> the attenuation value, and you can't really tell if the problem is in 
> the antenna, antenna feeder, or something external.
>
> 73,
> Mark VK3BYY
>
> -Original Message-
> From: Repeater-Builder@ yahoogroups. com 
> <mailto:Repeater-Builder%40yahoogroups.com> [mailto:Repeater-Builder@ 
> yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com>] On 
> Behalf Of KT9AC
> Sent: Wednesday, 24 February 2010 09:02 AM
> To: Repeater-Builder@ yahoogroups. com 
> <mailto:Repeater-Builder%40yahoogroups.com>
> Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM 
> keep UHF repeaters locked up?
>
> I was able to match up the audio coming through the repeater and the
> local AM station. My latest theory is that their signal is so strong
> that its blowing into the receiver's front end and multiplying/ mixing
> there (past the bandpass filters and all). They are heterodyne receivers
> after all.
>
> I'm considering an ICE broadcast high-pass filter that cuts off at
> 1.8Mhz (model 402). I have an email into them to see how well it might
> work at 448 Mhz.
>
> Tony
>
> 






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Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[KT9AC]

If you wouldn't mind...that would be interesting to see how it works.


DCFluX wrote:
>
> How about 1.25 MHz RF coming down the outer jacket of the UHF antenna
> and into the ground of the system? You have about 200 ft or so of
> coax? Try a mag mount antenna temporarily.
>
> Not really sure how you'd cure that though. Not sure if snap on RF
> beads would work on coax with a signal going in the center.
>
> 






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RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Mark HARRISON]

Hi Tony,

Are you using a duplexer on this repeater?
A lot of cavity filters act as a short circuit to DC and low frequencies, so 
additional filtering is unlikely to help.
I can only think of one type of cavity that has a DC path between from input to 
output (via an internal inductor) and not to ground.  This type could I guess 
pass low frequencies.  It's simple to test - disconnect the antenna and 
receiver leads and measure the DC resistance with a meter between the centre 
and outer on the cavity connectors.  If it's 0 ohms then it's likely to be a 
very good high pass filter for broadcast frequencies!

Also, using a temporary attenuator you should be able to determine if there is 
an intermod problem within the receiver, or parts of the antenna filtering 
system on the receive side of a Duplexer.  Inserting an attenuator will reduce 
the interference (and desired signal) by the same amount if everything on the 
receiver side of the attenuator is functioning correctly.  If instead the 
interference drops by 2-3 times (in dB, and the desired signal drops only by 
the attenuator value) then you've found your problem!
Placing a power attenuator in the duplexed antenna line is more complicated 
because you are attenuating both the Tx and Rx signals.  You would expect the 
interfering signal to drop by more than double the attenuation value, and you 
can't really tell if the problem is in the antenna, antenna feeder, or 
something external.

73,
Mark VK3BYY

-Original Message-
From: Repeater-Builder@yahoogroups.com 
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of KT9AC
Sent: Wednesday, 24 February 2010 09:02 AM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF 
repeaters locked up?

I was able to match up the audio coming through the repeater and the 
local AM station. My latest theory is that their signal is so strong 
that its blowing into the receiver's front end and multiplying/mixing 
there (past the bandpass filters and all). They are heterodyne receivers 
after all.

I'm considering an ICE broadcast high-pass filter that cuts off at 
1.8Mhz (model 402). I have an email into them to see how well it might 
work at 448 Mhz.

Tony

RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Mark HARRISON]

I would have thought good grounding practices on the feeder and equipment at 
the base of the tower would have pretty well bypassed any 1.25MHz stuff.

Ferrite 'beads' will reduce common mode pickup on coaxial cables without any 
effect at all on the signal inside the coax. 
Don't expect too much attenuation though - usually they are only good for 
8-15dB at UHF frequencies, and somewhat less at low frequencies (choose your 
ferrite material carefully!).

73,
Mark VK3BYY

-Original Message-
From: Repeater-Builder@yahoogroups.com 
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of DCFluX
Sent: Wednesday, 24 February 2010 09:35 AM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF 
repeaters locked up?

How about 1.25 MHz RF coming down the outer jacket of the UHF antenna
and into the ground of the system? You have about 200 ft or so of
coax?  Try a mag mount antenna temporarily.

Not really sure how you'd cure that though. Not sure if snap on RF
beads would work on coax with a signal going in the center.

Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[DCFluX]

How about 1.25 MHz RF coming down the outer jacket of the UHF antenna
and into the ground of the system? You have about 200 ft or so of
coax?  Try a mag mount antenna temporarily.

Not really sure how you'd cure that though. Not sure if snap on RF
beads would work on coax with a signal going in the center.

RE: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Jeff DePolo]

> I'm considering an ICE broadcast high-pass filter that cuts off at 
> 1.8Mhz (model 402). I have an email into them to see how well 
> it might 
> work at 448 Mhz.
> 
> Tony

Before you spend any "real money", you might just try a shorted quarterwave
stub.  If you want, I can make one up quick and see how much attenuation it
provides at 1.8 MHz.  I have to spend some quality time with the VNA tonight
anyway so it won't take but a second...

--- Jeff WN3A

Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Jim WB5OXQ inb Waco, TX]

Is it possible the AM signal is getting into an audio stage instead of the 
receiver front end?  I had that happen once.

- Original Message - 
From: "KT9AC" 
To: 
Sent: Tuesday, February 23, 2010 4:01 PM
Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF 
repeaters locked up?


>I was able to match up the audio coming through the repeater and the
> local AM station. My latest theory is that their signal is so strong
> that its blowing into the receiver's front end and multiplying/mixing
> there (past the bandpass filters and all). They are heterodyne receivers
> after all.
>
> I'm considering an ICE broadcast high-pass filter that cuts off at
> 1.8Mhz (model 402). I have an email into them to see how well it might
> work at 448 Mhz.
>
> Tony
>
> tracomm wrote:
>>
>> Had a similiar situation at our site, a station on 106.7 MHz, music on
>> hang time on many repeaters, intermod runs gave no clue to reason, did
>> all the usual, grounding, filters no resolve.
>> Turned out to be the STL Marti system on 450.100 MHz, from an close
>> studio site pointed right at our site, hitting our Rx multicoupler,
>> mixing with our transmitters. Resolution was low passs & isolater on
>> the STL system.
>> Make certain which station the broadcast audio is coming from and give
>> the station engineer a friendly call, may reveal some info to help
>> your issue.
>>
>> Chris
>> GMRS Inc.
>>
>>
>> --- In Repeater-Builder@ yahoogroups. com
>> <mailto:Repeater-Builder%40yahoogroups.com>, Tony KT9AC  
>> wrote:
>> Hi everyone,
>> A while ago I was troubleshooting a bad feedback or "growl" problem
>> that was impacting a UHF repeater, of which the short term workaround
>> was to not encode TX PL (PL or DPL would keep it locked until the
>> signal dropped enough or timed out).
>>
>> In doing some more research, I found a 1250kHz AM station within a
>> mile or two that changes pattern between day and night. The
>> interference mentioned above would appear around drive times (like
>> 5pm) so that had me chasing other sources. Still, it was puzzling that
>> a 5Mhz signal could be causing the feedback (it didn't appear when
>> doing normal receiver testing with a service monitor). The recent give
>> away was that I could hear talking underneath my test signal (like a
>> sports show).
>>
>> So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize
>> that the 4th harmonic of a 5KW broadcast station isn't very powerful,
>> but being in its nearfield might be enough to cause a mix with the UHF
>> transmit output.
>>
>> Does this make sense? This phenomenon can be duplicated with both a
>> 450 and 440 repeater system - both with standard 5Mhz offsets. I don't
>> think any sort of filtering would work since the mix happens "in the
>> air".
>> Only by having split PL's can the lockup be prevented, and equipment
>> was both MSF5000 and Micor systems, through correctly tuned duplexers.
>> Thanks,
>> Tony
>>
>>
>
>
> 
>
>
>
> Yahoo! Groups Links
>
>
>
>
> 








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Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[KT9AC]

I was able to match up the audio coming through the repeater and the 
local AM station. My latest theory is that their signal is so strong 
that its blowing into the receiver's front end and multiplying/mixing 
there (past the bandpass filters and all). They are heterodyne receivers 
after all.

I'm considering an ICE broadcast high-pass filter that cuts off at 
1.8Mhz (model 402). I have an email into them to see how well it might 
work at 448 Mhz.

Tony

tracomm wrote:
>
> Had a similiar situation at our site, a station on 106.7 MHz, music on 
> hang time on many repeaters, intermod runs gave no clue to reason, did 
> all the usual, grounding, filters no resolve.
> Turned out to be the STL Marti system on 450.100 MHz, from an close 
> studio site pointed right at our site, hitting our Rx multicoupler, 
> mixing with our transmitters. Resolution was low passs & isolater on 
> the STL system.
> Make certain which station the broadcast audio is coming from and give 
> the station engineer a friendly call, may reveal some info to help 
> your issue.
>
> Chris
> GMRS Inc.
>
>
> --- In Repeater-Builder@ yahoogroups. com 
> , Tony KT9AC  wrote:
> Hi everyone,
> A while ago I was troubleshooting a bad feedback or "growl" problem
> that was impacting a UHF repeater, of which the short term workaround
> was to not encode TX PL (PL or DPL would keep it locked until the 
> signal dropped enough or timed out).
>
> In doing some more research, I found a 1250kHz AM station within a 
> mile or two that changes pattern between day and night. The 
> interference mentioned above would appear around drive times (like 
> 5pm) so that had me chasing other sources. Still, it was puzzling that 
> a 5Mhz signal could be causing the feedback (it didn't appear when 
> doing normal receiver testing with a service monitor). The recent give 
> away was that I could hear talking underneath my test signal (like a 
> sports show).
>
> So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize 
> that the 4th harmonic of a 5KW broadcast station isn't very powerful, 
> but being in its nearfield might be enough to cause a mix with the UHF 
> transmit output.
>
> Does this make sense? This phenomenon can be duplicated with both a 
> 450 and 440 repeater system - both with standard 5Mhz offsets. I don't 
> think any sort of filtering would work since the mix happens "in the 
> air".
> Only by having split PL's can the lockup be prevented, and equipment 
> was both MSF5000 and Micor systems, through correctly tuned duplexers.
> Thanks,
> Tony
>
> 






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Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?

[Chuck Kelsey]

A lot of strange things can happen.

We had a situation where a school bus control station that was situated a 
couple blocks from an FM broadcast tower caused the control station to spur 
and get into our control station almost 20 miles away. When the offending 
control station was put on the bench (away from the FM broadcast station) it 
worked fine.

Have also had radio station Marti transmitters get into stuff as well. Was 
told by someone that Marti's were known to spur easily.

Chuck
WB2EDV



- Original Message - 
From: "tracomm" 
To: 
Sent: Monday, February 22, 2010 8:41 AM
Subject: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF 
repeaters locked up?


> Had a similiar situation at our site, a station on 106.7 MHz,  music on 
> hang time on many repeaters, intermod runs gave no clue to reason, did all 
> the usual, grounding, filters no resolve.
> Turned out to be the STL Marti system on 450.100 MHz, from an close studio 
> site pointed right at our site, hitting our Rx multicoupler, mixing with 
> our transmitters. Resolution was low passs & isolater on the STL system.
> Make certain which station the broadcast audio is coming from and give the 
> station engineer a friendly call, may reveal some info to help your issue.
>
> Chris
> GMRS Inc.
>
>
>
> --- In Repeater-Builder@yahoogroups.com, Tony KT9AC  wrote:
> Hi everyone,
> A while ago I was troubleshooting a bad feedback or "growl" problem
> that was impacting a UHF repeater, of which the short term workaround
> was to not encode TX PL (PL or DPL would keep it locked until the signal 
> dropped enough or timed out).
>
> In doing some more research, I found a 1250kHz AM station within a mile or 
> two that changes pattern between day and night. The interference mentioned 
> above would appear around drive times (like 5pm) so that had me chasing 
> other sources. Still, it was puzzling that a 5Mhz signal could be causing 
> the feedback (it didn't appear when doing normal receiver testing with a 
> service monitor). The recent give away was that I could hear talking 
> underneath my test signal (like a sports show).
>
> So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize that 
> the 4th harmonic of a 5KW broadcast station isn't very powerful, but being 
> in its nearfield might be enough to cause a mix with the UHF transmit 
> output.
>
> Does this make sense? This phenomenon can be duplicated with both a 450 
> and 440 repeater system - both with standard 5Mhz offsets. I don't think 
> any sort of filtering would work since the mix happens "in the air".
> Only by having split PL's can the lockup be prevented, and equipment was 
> both MSF5000 and Micor systems, through correctly tuned duplexers.
> Thanks,
> Tony
>