Batboard

Guys,

I was told today that I have one too many VHF radio's for the vehicle I have. I would think that 3 VHF radios, GM300; CDM1250; Spectra trunkmount, would be enough. For some reason or another the crossband repeat radio is passing the audio both ways but has a strong beating/pulsing signal to it, akin to the CHP Mobile Extenders but so bad it takes out whoevers talking on the local repeater.

GM300 is for APRS
CDM1250/GM300 combo is for crossband repeat for when I'm out of the vehicle, so that I can talk into the GM300 UHF to one of the local repeaters.
Spectra is for everything else.

One person said it was the APRS rig interfering, while someone else said it was due to having too many VHF radios in such a small space (Chrysler 2005 Town & Country). Tonight I was chatting with a few guys and they said you could build cavities out of old tin coffee cans, anyone have any ideas or suggestions on how to start such a project? The guys couldn't remember where they had read about it.

Any ideas or suggestions??

Regards,

-Frank C.

fchrist16 wrote:Guys,

I was told today that I have one too many VHF radio's for the vehicle I have. I would think that 3 VHF radios, GM300; CDM1250; Spectra trunkmount, would be enough. For some reason or another the crossband repeat radio is passing the audio both ways but has a strong beating/pulsing signal to it, akin to the CHP Mobile Extenders but so bad it takes out whoevers talking on the local repeater.

GM300 is for APRS
CDM1250/GM300 combo is for crossband repeat for when I'm out of the vehicle, so that I can talk into the GM300 UHF to one of the local repeaters.
Spectra is for everything else.

One person said it was the APRS rig interfering, while someone else said it was due to having too many VHF radios in such a small space (Chrysler 2005 Town & Country). Tonight I was chatting with a few guys and they said you could build cavities out of old tin coffee cans, anyone have any ideas or suggestions on how to start such a project? The guys couldn't remember where they had read about it.

Any ideas or suggestions??

Regards,

-Frank C.

Well one thing to bear in mind is that cavities are only good for a single frequency. If the radios never change channels,
then you might solve some of your interaction between the different radios. Cavities also need to be stable and as such,
I wouldn't jump to build you own coffee can cavities. The tuning changes with temperature and mechanical variations. If
you hung on going the cavity route, you would be better off to get decent cavities and do it right the first time.

Some cavities can be had by a visit to your local radio shops and looking through their pile of old odd stuff. Then again,
some radios shops will try to milk you for what ever they think they can get out of you. Shome looking on fleebay might
come up with results. You just need to plan well on what needs to be done.

Jim

A notch filter would work to keep out the APRS. Never seen it in a mobile application though.

Fowler

You haven't really provided enough detail for us to offer you suggestions for a solution. Some basic troubleshooting is called for.

First, you need to tell us under what conditions you experience the noise you describe, most importantly which radios are transmitting and on which other radio(s) recieve audio is the noise present.

Operating one radio at a time is a good next step. Maybe your x-band repeater is generating the noise itself, or the noise is coming from a non-RF source in the car (power supply noise, for example). If the noise is experienced when only the x-band repeater is on and operating, in which direction(s) is the noise present.

If the noise is only generated when some combination of radios is on, determine if it's when only one of the radios is transmitting, or when multiple radios are transmitting.

Look at how far your antennas are separated, and how much power each radio is transmitting.

RF cavities/filters are unlikely to solve your problem, as well as being not particularily suited to a mobile environment (multiple frequencies as others have said plus vibration issues).

It sounds like you may be dealing with desense, which in my limited experience is not going to be solved by adding filtering, if it's happening because your radios are in such close proximity.

Ok here is a full rundown, pictures to come when its light out again.

800Mhz & 900Mhz Maxtracs running for a specialized railroad program, RX ONLY
APRS GM300 running when ignition is on, Smartbeaconing enabled
6m Maratrac with CHP RX ONLY
110w VHF Trunkmount Spectra - primary radio for mobile
45w UHF Trunkmount Spectra - hardly used but always in RX Mode
CDM1250 VHF - crossband radio "A"
GM300 UHF - crossband radio "B"

For antennas they are all Larson Magmount 1/4 waves tuned to the radio it's connected too. The only exception is the VHF Spectra which has a 5/8 wave antenna. With the slight bit of roof real estate that I have, my 5/8 and lowband antennas are nearly right up on the front windshield. Across the frame between the drivers door and back doors are 3 antenna, 800/900Mhz and my scanner antenna. Along the rear roof-rack rail is my UHF and UHF crossband antenna. Along the rear of the van is my APRS and VHF crossband antennas.

Radios have nearly equal seperation. APRS & 800/900 are under the front passenger seat while the Maratrac, and two Spectra's are under the trunk bench seat. The crossband radios are set under and next to the drivers seat. The MW-520 is under the right side middle passenger seat.

As for the noise, it only happens when someone is keyed up on one local repeater, no other repater seems to do it. Doesn't matter what radios are on/off, it still happens.

Sorry for the longer posts.

Regards,

-Frank C.

fchrist16 wrote:Ok here is a full rundown, pictures to come when its light out again.

800Mhz & 900Mhz Maxtracs running for a specialized railroad program, RX ONLY
APRS GM300 running when ignition is on, Smartbeaconing enabled
6m Maratrac with CHP RX ONLY
110w VHF Trunkmount Spectra - primary radio for mobile
45w UHF Trunkmount Spectra - hardly used but always in RX Mode
CDM1250 VHF - crossband radio "A"
GM300 UHF - crossband radio "B"

For antennas they are all Larson Magmount 1/4 waves tuned to the radio it's connected too. The only exception is the VHF Spectra which has a 5/8 wave antenna. With the slight bit of roof real estate that I have, my 5/8 and lowband antennas are nearly right up on the front windshield. Across the frame between the drivers door and back doors are 3 antenna, 800/900Mhz and my scanner antenna. Along the rear roof-rack rail is my UHF and UHF crossband antenna. Along the rear of the van is my APRS and VHF crossband antennas.

Radios have nearly equal seperation. APRS & 800/900 are under the front passenger seat while the Maratrac, and two Spectra's are under the trunk bench seat. The crossband radios are set under and next to the drivers seat. The MW-520 is under the right side middle passenger seat.

As for the noise, it only happens when someone is keyed up on one local repeater, no other repater seems to do it. Doesn't matter what radios are on/off, it still happens.

Sorry for the longer posts.

Regards,

-Frank C.

A good start, but as AKARDAM has stated, the details make the difference. You mentioned APRS, well it would help if we
knew which frequency / band that GM300 was on. There are a number of different systems being used around the country.

Which "local repeater" frequency are you getting the "noise" on? What does this noise sound like? Is this by some chance
a problem with that repeater having a desense issue? If this is the case, no matter what you do, the "noise" won't go
away.

I would expect to have more problems caused by the VHF Spectra in transmit that most of the other radios you have. However,
you didn't say if this was a high power or mid powered transmitter.

Antenna separation can make a big difference in what issues you have between the different radios.

Jim

fchrist16 wrote:As for the noise, it only happens when someone is keyed up on one local repeater, no other repater seems to do it. Doesn't matter what radios are on/off, it still happens.

This still isn't entirely clear. What radio do you hear the noise on, in your x-band setup?

Is it present on the recieve audion from the "base" radio side (the VHF CDM)?

Is it present on the transmit audion from the "extender" radio side (the UHF GM300)?

Is it heard by other users on the repeater when you key up your UHF portable to talk to the "extender" radio?

Ok let me try this with a fresh mind...

The noise is only heard on the x-band radios, both the VHF and UHF. It doesn't matter who keys or when, the noise is there instantly on both radios. So it is present on the "base" and "extender" side each time the repeater 145.330Mhz - offset is keyed up. No one else can hear it, so it's local to my setup.

I don't think it's the repeater having a desense issue. The noise is a mix of squelch "white noise" doubling with someone and then pulsing. The APRS GM300 is set for 144.390Mhz.

Regards,

-Frank C.

Ok, now we're getting somewhere.

Disconnect the x-band connecting equipment and turn off all the other radios. Test the VHF radio by itself (and only radio on) as a regular radio subscriber to the VHF repeater. Test the UHF radio by itself (and the only radio on) on simplex with your UHF handheld.

If the noise is not present in either of these standalone tests, reconnect the x-band connecting equipment and turn on both x-band radios, but do not turn on any other radios.

You can also try placing the UHF radio on a dummy load instead of the antenna to which it is normally connected and see if the problem persists. If that eliminates the problem, put the UHF radio on a mag-mount antenna, with as much separation as possible from the VHF radio antenna. If that eliminates the problem, you have a separation/isolation issue. You can try lowering the power output on the UHF radio to its minimum.

Let us know the results of your tests.

I think it's narrowed down to the x-band stuff. One thing I forgot to mention was that when the x-band radios come onto the bench, which has 2 other 45w Spectra's and a GM300 VHF on 145.330 and a Maxtrac 800Mhz running, there is no interference and everything is fine.

I'll have to try the x-band stuff alone with no other radio's on and see if that does anything...

Results to follow.

Regards,

-Frank C.

fchrist16 wrote:I think it's narrowed down to the x-band stuff. One thing I forgot to mention was that when the x-band radios come onto the bench, which has 2 other 45w Spectra's and a GM300 VHF on 145.330 and a Maxtrac 800Mhz running, there is no interference and everything is fine.

I'll have to try the x-band stuff alone with no other radio's on and see if that does anything...

Results to follow.

Regards,

-Frank C.

Let me ask a question here on the cross band system. How was the RX audio coupled and level adjusted
into the next TX? My first guess without you saying anything is that your over driving the RX audio into the
cross band transmitter in both directions. The audio probably sounds sort of muffled as it probably is clipping
the pants off of the RX audio. In doing this, your just bringing up the background noise. You probably can
hear a person in the back seat whispering and have all sorts of road noise pickup from their mic.

You can try an experiment and have someone on one of the repeaters hold their mic close and say something
and then hold it about 2 feet away and see if the sound isn't much better.

My guess is that you didn't use a service monitor in setting up the cross band repeater system audio levels.
Unless you have some sort of interface board to adjust the audio levels and have built in timers to prevent
it from hanging up and tying up one or more repeaters, you might want to shut the cross band system off.

Jim

Jim,

I'm using the RA-2 Bi-Directional Rick from fleebay. There are audio pots on the RA-2 and I have them set for "middle ground", not too loud and not too soft. Sadly I'm not understanding your backseat/road noise statement. The way I understand it, with my GM300 UHF at minimum power, since I don't need it pumping out the full power, I wouldn't need a service monitor to adjust the audio levels with this controller.

This cable is one of those "everything is in it already" cables that has worked fine simplex to simplex, but the minute I throw it onto this repeater, everything falls apart on my end.

Regards,

-Frank C.

Well today it didn't exhibit the weird audio. Although the timing needs to be adjusted for drop outs of the audio so that it doesn't clip people off, thats another subject for another day though.

Regards,

-Frank C.

fchrist16 wrote:Jim,

I'm using the RA-2 Bi-Directional Rick from fleebay. There are audio pots on the RA-2 and I have them set for "middle ground", not too loud and not too soft. Sadly I'm not understanding your backseat/road noise statement. The way I understand it, with my GM300 UHF at minimum power, since I don't need it pumping out the full power, I wouldn't need a service monitor to adjust the audio levels with this controller.

This cable is one of those "everything is in it already" cables that has worked fine simplex to simplex, but the minute I throw it onto this repeater, everything falls apart on my end.

Regards,

-Frank C.

Let me try this again about the audio passage from the radio being used as the receiver to the radio being used as the
transmitter in one direction. You need to keep the audio going into the transmitter low enough to keep it from clipping
or limiting. If the audio is too high, the transmitter limiter will compress the audio and keep it from going outside the
TX band width of the channel it is sending on. When the audio is compressed by the limiter in the transmitter, it tends
to build up the background noise from the originating transmitter. This will bring up all the background noise and soft
spoken talk that the originating mic might pick up. If driving down the road, this tends to bring up all the background
rumbling of the vehicle. As such it is called high background noise.

The compressed audio has nothing to do with the transmitter power going to the antenna.

The simple way to adjust the "throughput" audio going from the receiver to the transmitter is to listen to an external
receiver tuned to your cross band transmitter output. Take a portable radio set to the cross band receiver input and
make a steady bellow sound like a long "AHHHHHHHHHHHH" or long drawn out "FIVE" to get a steady audio into your
system. Adjust the pot or variable resistor that controls the audio level to the cross band transmitter. You should
find a point or setting where the main level of the audio doesn't change amplitude. This is where you want to stop.

Without a service monitor, your relying on your cross band transmitter being adjusted correctly for no more than 4.8 KHz.
deviation. As with most ham radios and a number of older radios used on public safety frequencies, your at the mercy
of the last radio tech that adjusted the radio. With my many trips around the country visiting 911 dispatch centers, my
opinion of the average radios shop these places use is not very high. The technical expertise that I have seen used
on these radios is no way near what it use to be 10 years ago.

This is not rocket science on adjusting TX audio input, but do it wrong and is sounds like crap. Do it wrong on public safety
frequencies and your going to be outside the band width limit. This will cause interference to adjacent channel users and
could possibly bring a violation from the FCC down on who owns the license for the channels your playing with.

Jim

I'll do that and see what happens with the audio.

Thanks again Jim!

Regards,

-Frank C.