Help needed with setting up my GMRS radio, please?

[WROZ317]

Hi all.  I have installed my GMRS radio in my 2015 Jeep Wrangler Sport a couple of months ago, but I have not gotten around to setting it up to be used yet.  I will need to have my radio working by July for the NJ Jeep Invasion, and I am not sure how to get everything working properly.  To help you understand my set up, this is what I currently have:

• Midland MXT275 MicroMobile Two-Way Radio.  It is attached to a Quadratec Quick-Disconnect Mount for a CB Radio (I believe it's the JK-CBMU), which I modified slightly for the GMRS radio.  The mic is hanging from the Quadratec CB Radio Mounting Bracket #CBM-75WX11.
• Larsen Low-Profile Unity Gain Antenna #LP450NMO on a Midland MXTA12 Magnetic Mount, which is positioned center on the crossbar, and clears my soft top.  I ran the cable of the magnetic base from the center of the crossbar to passenger side B-pillar, down the B-pillar, and under the molding for the front passenger door toward the file firewall on the passenger side, where it cross over  under the dash and back to the radio.  This is similar to how Jeep ran the antenna for the SiriusXM antenna, as well as the GPS antenna if equipped.  Since I no longer use SiriusXM, I remove the SiriusXM antenna at the same time I was running the antenna cable for the GMRS radio.

It is my understanding, similar to setting up a CB or other communications radio, that you need to tune or trim the antenna for proper operation and to avoid any damage to the radio.  So my question is, how does one tune or trim the Larsen Low-Profile Unity Gain Antenna?  I have searched for information when I have had spare time, but so far, no luck.  I also heard that I should not use an SWR meter for a CB radio, so if someone could recommend a SWR meter for a GMRS set up, I would greatly appreciate it.

[gortex2]

There is no tuning on the LP450NMO antenna. Slap it on and enjoy. I ran mine under the softop for a few years until i wanted a better antenna and went with a 1/4 wave on a mount outside.

 

[wayoverthere]

Not all antennas have provisions for tuning, and some ostensibly come pretuned. Mine have all been too close to perfect to mess with, including both the Midland and Laird 1/4 waves.

That said, it's still good to double check the swr as installed, to ensure there's no issues that may not be obvious...though uncommon, a coax feed that was crimped badly (shorting the shield and center conductor together) isn't unheard of.

On meters, I've had good results with surecom, both the sw-102 (better suited to mobiles) and the sw-33 (better suited to handhelds), though, like most things there are some quality control issues and therefore not everyone is a fan.

[WRQI583]

If ever you wanted to go with another antenna, possibly a little larger, such as a whip antenna, if there is a bit of distance between you and another radio or repeater such as in a rural setting, from experience, you will want to go with something like a MUF4505S GMRS antenna type of antenna. I worked for a medical transportation company many years ago and while most of the vehicles had these style antennas, a few had quarter wave antennas for UHF and the quarter wave antennas performed horribly. Receive was about 1/4 of the larger antennas. The transmit range wasn't as bad but it was diminished due to the antenna size. Our coverage area was covered by moderate size hills between 300-500 feet above sea level with flatter ground surrounding them.

I had a low profile antenna similar to what you are running and it worked well in more suburban or urban areas. When you get in the more rural areas, the larger antennas are better. If your coverage is going to be short or in the city, the low profiles work well.

[tweiss3]

That antenna requires a ground plane, in GMRS's case, you need atleast 6" or ground plane in all directions. You could place a 12" diameter metal disk between the magmount and puck antenna, as long as the center pin still makes contact. Your crossbar isn't going to be enough to provide an adequate and effective ground plane. 

I've experienced issues with middle of the roof install of a magmount and a ground plane dependent antenna. When I went to a no-ground plane antenna, it helped significantly. 

[WRQU306]

Cut a 6-10in circle out of your wifes favorite cookie pan for a ground plane. She may get mad so id recommend a Target giftcard.