VHF mobile antenna question

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nhfirefighter

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I have an interesting problem I have never seen before and am curious if anyone might have some thoughts on what’s occurring. I have a xtl5000 VHF mid power radio with the latest and greatest firmware. It is used on conventional analog and P25 frequencies. I have not had it aligned yet - not sure if this is a contributing factor to my issue.

The problem at hand is it has very week receive on distant signals. Primarily analog signals but occasionally P25 as well. This can apply to simplex and repeatered transmissions. The radio is installed in a later model F150 with aluminum body if that matters. The radio replaces another XTL5000 I had unrelated issues with (01/90). The wiring - both electrical and antenna are the same but the receive performance is far diminished on this radio compared to the other. Closer (20 mile radius) transmissions receive fine.

Here is the really unusual part though. If I use a no frills quarter wave spike antenna it works great, even pretty good with receiving distant transmissions. As soon as I switch to a base load coil type antenna - 1/4 wave or 1/2 wave the reception ability tanks. The other XTL performed as expected with the same antennas. It would seem the coil antennas and even a 1/4 wave with a thicker rod such as a Laird ab150s lose the ability to receive well.

My question is has anyone seen this behavior before? If so what caused it? Would it be an alignment issue or a greater hardware issue with the radio? Thank you in advance for any ideas.
 

medic550

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topography? quarter wave i have found to work better in hills as the signal radiates upward. 1/2 wave seems to head straight out to the horizon.
 

Bill_G

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Without doing reproducible tests with a service monitor, we would just be speculating.
 
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nhfirefighter

nhfirefighter

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Yeah that’s what I figured. Considering the prices these radios go for versus the cost for shop time since I don’t have a monitor I was trying to rule things out first. Just strange how any antenna with a coil base seems to hurt reception on distant signals. I had never seen that occur before.
 

Bill_G

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It's possible the antennas work so well, they swamp the front end of the rcvr preventing you from hearing far stations. ie: desense. But, we'd have to prove that.
 

Astro Spectra

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Pop a DVM across the antenna inner and ground (no antenna connected) do you see voltage? Your basic quarter wave will be DC open, are the gain types a DC short?
 

Mars

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Probably not your issue, but I recalled this and wanted to share:

advanced-AGC.jpg
 

sarguy1941

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Check the antenna connector. I fought an issue on a UHF XTL for about 2 months. Customer would swap between his puck and a 1/4 wave and go back and forth on which worked well. At one point I hooked up my service monitor o the radio and all was well. I thnen took a NMO/N adapter and ran thru is mount thinking he had a bad mount. Wiggling cable around the base was fine. When i touched the cable at the radio it lost all signal. I went back to the my test cable and putting some wiggle with it loose resulted in same. Found the center pins on the Mini U in the radio appeared to be pushed out. My guess is maybe a bad connector at one point got shoved in the radio. I installed a spare radio and the old one is sitting on my shelf now.
 

jry

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wasn't there a VHF preamp option that was set in the flashcode ?

Just wondering if your new one did not have that enabled .

Think the HW is there regardless ...just a flash opt. IIRC
 
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nhfirefighter

nhfirefighter

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Haven’t tried a dvm yet but another drawer exact same config worked fine in the setup.

Confirmed advanced agc not on.

Confirmed preamp is on in codeplug.

Good idea on the mini u connector on the radio. Will check tonight hadn’t thought to look at that kept chasing my antenna coax and wiring in case there was a grounding issue.
 

Bill_G

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If someone inserted a real MOT mini-UHF connector with the hollow tip that you can either solder or crimp, and they did a poor job of it, the socket on the radio side can get spread so the smooth center pin connectors can never mate well afterwards. I've seen more than one Spectra, Maxtrac, CDM, etc ruined by a poorly constructed OEM connector on the cable. All it takes is once.
 

photo8

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I've got to side with @Bill_G on the connector as a possible fail point, but I wonder, too, if there's a problem with the antenna cable or the mount base. Is there a good ground return to the vehicle body from both the coax shield and the chassis mount outer threads? How does the radio perform if you use a temporary magnetic mount with 1/4 wave and base loaded antennas?
 
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nhfirefighter

nhfirefighter

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First and foremost thank you everyone for your thoughts and suggestions it has been very helpful in chasing this down. Unfortunately I am coming to the conclusion there is more than likely a problem with the receiver. It isn’t completely gone but it does suffer from week reception on distant signals. Transmit abilities have been fine even reaching distant repeaters I can’t seem to receive. Without a proper tuning I have changed the reference oscillator from every setting on the spectrum with marginal changes in results.

I can confirm it receives local transmissions excellently using 1/4 wave spike type antennas all the way up to coil based 5/8 antennas. I am confident in the antenna coax but to be safe I removed it and installed a brand new coax run with a factory installed mini uhf connector. Unfortunately it didn’t improve things. I actually already had two coax runs installed that worked flawlessly with a vhf and uhf XTL prior to this radio. This radio exhibits the poor rx symptoms on either coax run.

The only thing that changed when these problems started are the radio drawer and the remote control head. I upgraded from a W series to and O5 series. I can’t imagine the control head could cause a rx problem?
 
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nhfirefighter

nhfirefighter

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A follow up for anyone who finds this post in the future. After many hours chasing down many things and talking to others the problem has been located. Apparently this is a known issue with older M5/O5 control heads with the problem residing in the audio amp on the control head. It seems to only effect VHF band radios - XTL and APX. It generally manifests with distant or weak signals. Per Motorola to test for the problem they recommend the following -

1: Set the Radio 05 to open squelch with the volume turned off. Or set the mobile to receive a signal. Does not matter what band or frequency.

2: Take a portable vhf and set to receive a distant weather broadcast.

3: Now turn the volume on the mobile up slightly and the portable gets desensed. This also desenses the vhf mobile receiver through it's roof top antenna and thus the problem we are experiencing. (A service monitor straight to the APX receiver will of course mask the issue.)

4: Turn volume off and desense is gone. Audio amp issue?

Unfortunately I don’t know the date for the hardware change to correct this. My symptomatic unit is from 2011 and has the newer HLN1467/1468 hardware. As an aside I tried pulling audio off the J2 instead of the control head but unfortunately still experienced the same problem. I’m not aware of any way in the CPS to disable control head audio.

Credit to user “loband” at batlabs for some of these details.
 

RFI-EMI-GUY

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So is that OP Amp oscillating?


A follow up for anyone who finds this post in the future. After many hours chasing down many things and talking to others the problem has been located. Apparently this is a known issue with older M5/O5 control heads with the problem residing in the audio amp on the control head. It seems to only effect VHF band radios - XTL and APX. It generally manifests with distant or weak signals. Per Motorola to test for the problem they recommend the following -

1: Set the Radio 05 to open squelch with the volume turned off. Or set the mobile to receive a signal. Does not matter what band or frequency.

2: Take a portable vhf and set to receive a distant weather broadcast.

3: Now turn the volume on the mobile up slightly and the portable gets desensed. This also desenses the vhf mobile receiver through it's roof top antenna and thus the problem we are experiencing. (A service monitor straight to the APX receiver will of course mask the issue.)

4: Turn volume off and desense is gone. Audio amp issue?

Unfortunately I don’t know the date for the hardware change to correct this. My symptomatic unit is from 2011 and has the newer HLN1467/1468 hardware. As an aside I tried pulling audio off the J2 instead of the control head but unfortunately still experienced the same problem. I’m not aware of any way in the CPS to disable control head audio.

Credit to user “loband” at batlabs for some of these details.
 
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