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GMRS / FRS - Discussions related to GMRS (General Mobile Radio Service) and FRS (Family Radio Service) communications

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  #1 (permalink)  
Old 03-31-2018, 6:38 PM
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Default NMO Antenna continuity / isolation problem?

Greetings all, complete newbie to all things radio, researched for a few weeks and settled on a GMRS system for my vehicle.

I'm installing a Midland MXT400 and paired it with the MXTA10 3db gain antenna. I've got my antenna cable ran to my NMO mount and have confirmed solid ground of the mount, as well as good circuit isolation between the NMO mount center pen and the threaded mount, as well as the vehicle. I've also checked at the radio end, the PL-259 has isolation between positive and ground, and has continuity with the ground and vehicle ground. So it appears that the cable and mount are fine.

So I went to measure SWR with a Kalibur SWR meter designed for a CB system. After adjusting the system to Low power, and adjusting for the needle to hit the Set at the end, I switched it to Ref and hit transmit and it immediately redlined. Now after that happened (and an immediate bad electrical smell from the SWR) I've done more research and I expect to be told that this SWR meter is no good for GMRS or for the wattage of this unit, or both. BUT...

Since it looked like I was getting 100% reflection, I thought I should double check for a short, and lo and behold, the whip on the antenna has continuity with the mount and ground. I re-checked my cable run and mount and it still seemed properly isolated so I tested the center pin on the antenna to its own inner threads, and there is continuity, a complete short. Same from the whip to the inner threads, and when screwed onto the mount, between the whip and vehicle ground. Shouldn't those be fully isolated from each other? It was hard taking the pictures while holding the leads, but I assure you the leads are not touching.

Any help is appreciated. I've unplugged everything in the meantime, as I'm not keen on starting a fire. :P
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Old 04-01-2018, 8:10 AM
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I see nothing unusual. Except that the SWR meter you used is for CB (27MHz). It will NOT read correctly on the antenna you have for GMRS.
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Old 04-01-2018, 8:47 AM
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Inside the black plastic part of the antenna are electrical components that will keep your antenna from showing an open when ohming out the center to shield. This is normal. With ant off, as long as you show no continuity with center to shield of cabling, you should be good.

Frank KK4YTM
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Old 04-01-2018, 9:44 AM
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One final check besides using a proper SWR meter would be check continuity between the center pin of the mount and the center pin of the coax connector at the radio.
prcguy
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Old 04-01-2018, 9:47 AM
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Well that's a relief, thanks for the fast replies.

Quote:
check continuity between the center pin of the mount and the center pin of the coax connector at the radio.
I guess this is the part that confuses me the most. Currently with no antenna installed, I have continuity with the center pin of the NMO mount and the center pin of the PL-259 radio-side, and both are isolated from ground. When the antenna is installed, the center pin of the PL-259 radio-side registers a closed circuit to ground, which I thought was a bad short that will ruin the radio and potentially be fire-inducing.

Last edited by nafai; 04-01-2018 at 10:12 AM..
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Old 04-01-2018, 9:58 AM
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A loaded antenna (base load) will normally show a short to ground when on the mount. A standard 1/4 wave whip will not. This is normal on most antenna's. I would find someone with the proper SWR meter to check. I find that 99% of the cut charts that come with a loaded/gain antenna get you pretty close to use efficiently. a 1/4 wave is the simplest.
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Old 04-01-2018, 12:40 PM
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Quote:
Originally Posted by nafai View Post
Well that's a relief, thanks for the fast replies.



I guess this is the part that confuses me the most. Currently with no antenna installed, I have continuity with the center pin of the NMO mount and the center pin of the PL-259 radio-side, and both are isolated from ground. When the antenna is installed, the center pin of the PL-259 radio-side registers a closed circuit to ground, which I thought was a bad short that will ruin the radio and potentially be fire-inducing.
Center pin to center pin should show continuity. Center to ground should show none. Ground (sleeve) to ground should show continuity. With the antenna in, it should show some between center and ground, that's how it works, and gets a ground plane. I've never had an NMO antenna that's properly installed, as yours is, "start a fire".
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Old 04-01-2018, 12:41 PM
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I would show you on one of mine, but I'm currently stripping my old truck and getting ready to install into my new one..
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Old 04-01-2018, 12:57 PM
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Got it, thanks for your patience with my lack of knowledge. Iíve ordered a 150w 120-500MHz SWR meter, should have it tomorrow and Iíll report back. Hopefully my error of using a CB SWR meter just led to some ignorant paranoia and Iíve got a solid install.
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Old 04-01-2018, 1:24 PM
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Let us know how it turns out for ya..

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Old 04-02-2018, 4:24 PM
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Ok here are my readings. First on Channel 1 @ 5w:



Next on Channel 22 @ 15w:



And lastly Channel 22 but @ full power 40w:



So if this meter is functioning properly, looks okay, yeah? Am I missing anything? If not I'm pretty stoked to have this SWR in a Jeep JK, all the owners I've talked to seem thrilled to get anything around a 2:1. Fender mount for the win.
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Old 04-02-2018, 4:28 PM
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Bolt her down and fire it up.. you're all set

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Old 04-02-2018, 4:45 PM
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Those look like really good readings for your antenna.
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Old 04-02-2018, 7:03 PM
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I've got a buddy that is about to do a GMRS install on a JK, which fender mount did you use? Thanks in advance!
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Old 04-02-2018, 10:23 PM
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Knoxradio, I went with this fender mount from Right Channel Radios. Of the ones I found that you could purchase separately without buying a full radio kit, it was the only one I found that was rounded (and shaped properly for the Jeep's hood). It also uses an existing fender bolt so it's no-drill.

Since that mount is made for CB, your friend will need to get a 3/8" compatible NMO mount. I snagged mine from the Antenna Farm.
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Old 04-03-2018, 6:28 AM
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Quote:
Originally Posted by nafai View Post
Knoxradio, I went with this fender mount from Right Channel Radios. Of the ones I found that you could purchase separately without buying a full radio kit, it was the only one I found that was rounded (and shaped properly for the Jeep's hood). It also uses an existing fender bolt so it's no-drill.

Since that mount is made for CB, your friend will need to get a 3/8" compatible NMO mount. I snagged mine from the Antenna Farm.
I have an XJ and my CB is mounted similarally, using an existing fender bolt, but went through the roof with a 1/4 wave NMO mount antenna for GMRS. Thanks for the info, that has probably saved us a lot of searching and trial and error.
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Old 04-03-2018, 10:01 AM
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Happy to help, Knoxradio! In my research, I came across this post, which pretty much sealed the deal on placement for me. It has a good explanation on the body challenges that JK owners face trying to get a ground plane on what is essentially a convertible with a fiberglass top. Here is where you should mount your CB and HAM antennas on your Jeep.
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Old 04-03-2018, 10:40 AM
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ground planes on jeeps?
thats why god made 1/2 wave antennas

Comet CSB-750
or
https://www.diamondantenna.net/sg7500nmo.html
or
http://www.mfjenterprises.com/Produc...ctid=MFJ-1422B
or
others

no ground plane required
great for Corvettes,boats and motorcycles to!
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Old 04-03-2018, 10:53 AM
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Quote:
Originally Posted by k9wkj View Post
ground planes on jeeps?
thats why god made 1/2 wave antennas
Those look pretty nice! I'd be worried about snapping a 41" center load where my JK roams though. And I'm in a mountainous and rocky area, so was trying to balance vertical vs horizontal radiation. Aren't those 5-6db gain more suited for open road / interstate travel and flat terrain?
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Old 04-03-2018, 11:26 AM
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Quote:
Originally Posted by k9wkj View Post
ground planes on jeeps?
thats why god made 1/2 wave antennas

Comet CSB-750
or
https://www.diamondantenna.net/sg7500nmo.html
or
http://www.mfjenterprises.com/Produc...ctid=MFJ-1422B
or
others

no ground plane required
great for Corvettes,boats and motorcycles to!
Anything longer than 24" (unless super bendable) wouldn't survive where I go. Thus why I only run a 1/4 wave.
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