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CB RF antenna damage

Josh380

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Mar 29, 2015
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278
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Orange County, CA
So.. I have an issue that needs a remedy.

My RCI 2970N2 (300w am, 150w SSB, 102" steel whip, LMR 200) keeps taking out my UHF antenna. The UHF mag mount (NMO mount) is a cheap Chinese made antenna mount, with a 1/4 wave UHF antenna

Initially I thought my CB was taking out the receive on my Motorola UHF radio because I had the Motorola powered up while transmitting on my CB, but a swap out of the mag mount proved the CB was taking out the actual UHF antenna (or burning up the center conductor of the coax of the UHF antenna), instead of the radio. The new mag mount suffered the same fate while transmitting on the CB with the UHF radio powered off.

So, I've decided to go with a permanent mount UHF (NMO). UHF antenna is mounted closer to the rear of the cab, and my 102" steel whip is mounted on the bed of the truck, at the rear of the bed, approximately 6' apart.

The coax is routed through the rear of the cab, in between the bed and cab. Both coax lines are routed through separate holes on opposing sides of the cab, but come close together near the radios (gotta keep coax clear of the drivers side).

I suspect the close proximity of the coax cables near the connection points is allowing some RF to get from my CB coax into the UHF coax.

Aside from keeping these two lines as seperate as possible, avoiding cheap Chinese junk, and keeping the UHF powered off while transmitting on the CB, what can I do to keep RF from my CB from damaging my UHF antenna/coax?
 

WB9YBM

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Niles, IL
I suspect the close proximity of the coax cables near the connection points is allowing some RF to get from my CB coax into the UHF coax.

Aside from keeping these two lines as seperate as possible, avoiding cheap Chinese junk, and keeping the UHF powered off while transmitting on the CB, what can I do to keep RF from my CB from damaging my UHF antenna/coax?
At first glance, I'd think either increasing antenna separation and/or throttling back on the power level...
 

mmckenna

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What indications are you seeing that it's the coaxial cable that is damaged? Can you see physical damage, or are you just seeing that the mount no longer works?

300 watts is a lot, and you do need sufficient separation, but I'd find it hard to believe 300 watts is physically damaging a separate run of coaxial cable.
 

Josh380

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Orange County, CA
What indications are you seeing that it's the coaxial cable that is damaged? Can you see physical damage, or are you just seeing that the mount no longer works?

300 watts is a lot, and you do need sufficient separation, but I'd find it hard to believe 300 watts is physically damaging a separate run of coaxial cable.
It's definitely the mount itself or the coax. The 1/4 wave antenna itself is fine. Haven't used a multimeter yet to see what is actually happening. I just know it's either the coax or mount or both.
 

BushDoctor

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Dec 19, 2002
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Strasburg, Va
In all my years of working on Motorola mobile units low band 39.50 100 watts RG8 I have never had that problem with any of my police departments who also ran 453.xxx or 460.xxx or 155.xxx 1/4 wave Mororola antennas with rg58c cable. At your higher power maybe that is the problem to close for that much power. Try 100 watts maybe over 100 is the straw that broke the camels back, I turn off my scanners in my car when i am using my 10 meters Motorola U71LHT on 29.60 or the repeater channels 29.62 29.64 29.66 29.68. I have a 2 meter Motorola Motrac in my car i never turn it off.
 

KevinC

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Somewhere other than home :(
Assuming you actually have 300w exiting the antenna, at that frequency and spacing you have about 7 dB of free pace path loss. That gives you around 50w at the other antenna, I doubt 50w could damage the coax or mount...but I could be wrong.
 

mmckenna

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It's definitely the mount itself or the coax. The 1/4 wave antenna itself is fine. Haven't used a multimeter yet to see what is actually happening. I just know it's either the coax or mount or both.
I'm a bit skeptical that the power output of the CB is damaging the coax or the mount. More than likely your coaxial cable is getting damaged where it enters the vehicle, it's corroded from being exposed to the outdoors, or the connector is faulty. I think some careful troubleshooting this weekend would be in order.

At 50MHz, Belden RG-58 is rated to 300 watts, more at lower frequencies. https://catalog.belden.com/techdata/EN/8262_techdata.pdf
 

jwt873

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Woodlands, MB
Many hams with towers, (including myself) run VHF/UHF antennas mounted not too far above an HF Yagi.

I have a dual band 144/440 antenna and the feed point is only about 3 feet above the feed point of my 3 element HF Yagi. I run up to 600 Watts on HF and have had no problems at all with the VHF/UHF system. It doesn't even bother the front end of my Kenwood TS-2000 that I use for receiving/transmitting VHF/UHF.

I know hams that run over 1000 Watts HF with similar setups. I've never heard of an antenna or coax being damaged due to simple proximity.
 

Josh380

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Orange County, CA
I had to buy a new multimeter..gonna do some checking today. It's just too coincidental that both times my receive and transmit on the UHF were working just fine for months with no issues, only to fail after transmitting on the CB.

I do know for certain that the Motorola has not been damaged. I know the front end of Motorolas are pretty tough. Not to mention I've tested the at fault antenna on a known working radio with the same issue..no RX or TX.
 

mmckenna

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I've tested the at fault antenna on a known working radio with the same issue..no RX or TX.
Yeah, so wait, was it the coax/mount that failed, or just the antenna?

What kind of antenna is it on the UHF side? Describe the mount in detail, photos help.
 

Josh380

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Mar 29, 2015
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Orange County, CA
Well, this is interesting. I have continuity between the center pole of the radio's rear coax connector and the ground (outer barrel). That shouldn't happen.
 

nd5y

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Dec 19, 2002
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Wichita Falls, TX
Well, this is interesting. I have continuity between the center pole of the radio's rear coax connector and the ground (outer barrel). That shouldn't happen.
Perfectly normal if there is an inductor from the trace to ground anywhere between the antenna connector and the first series capacitor.
 

mmckenna

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Well, this is interesting. I have continuity between the center pole of the radio's rear coax connector and the ground (outer barrel). That shouldn't happen.
Yeah, that's normal. You need to check the entire length of coax and the mount carefully. I'd also check the antenna coil.
 

RFI-EMI-GUY

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Only time I have seen RG58/u damaged is when I walked into a radio shop to test a 5 channel 800 MHz trunk system and found its combiner connected to the dummy load with 10 feet of it. Once we had all five 125 Watt channels going, the coax smoked pretty fast.
 

mrweather

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Dec 19, 2002
Messages
990
Yeah, that's normal. You need to check the entire length of coax and the mount carefully. I'd also check the antenna coil.
From the original post it appears the UHF antenna is a basic 1/4 wave whip. There shouldn't be any continuity between centre pin and barrel.

If OP is seeing that then somewhere along the way the centre conductor and braid became electrically connected. Could be a coax pinch or bad solder job on the PL259 connector.
 

Mike_G_D

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Well, this is interesting. I have continuity between the center pole of the radio's rear coax connector and the ground (outer barrel). That shouldn't happen.
As has been mentioned before - RF "ground" does not necessarily equal DC "ground"!

Inductor (coil) between center conductor and ground still looks like a short to DC but not to 27 MHz RF!

-Mike
 

Mike_G_D

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From the original post it appears the UHF antenna is a basic 1/4 wave whip. There shouldn't be any continuity between centre conductor and ground.
He mentioned at the radio not at the antenna or coax to the antenna, at least that's how I read it.

-Mike
 

mrweather

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Dec 19, 2002
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He mentioned at the radio not at the antenna or coax to the antenna, at least that's how I read it.

-Mike
Good point, I read it as coax cable end of things.

Is it normal for a radio antenna jack to show a DC short like that?
 
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