ground plane metal matter? and coax suggestion.

[JCunningham17]

im running a Laird UTRA 4301S3NB for my GMRS 45w mobile. Im mounting the antenna on a Back Rack on my truck. Im going to get a round sheet of metal for the ground plane. can i use aluminum? 7" or 10"? and i might need to run 20' of coax. should i do LMR400 in my truck?

 

[SteveC0625]

im running a Laird UTRA 4301S3NB for my GMRS 45w mobile. Im mounting the antenna on a Back Rack on my truck. Im going to get a round sheet of metal for the ground plane. can i use aluminum? 7" or 10"? and i might need to run 20' of coax. should i do LMR400 in my truck?

Aluminum is fine.

Ground plane should be 1/4 wave which is about 6” for GMRS. so a 12” diameter disc is just about perfect. See if you can find an aluminum pizza pan. It’s got rounded edges so no sharp edge hazard.

Good quality RG58 is plenty adequate for mobile installs.

Be sure to protect the underside of the NMO mount against water intrusion.

 

[W0JOG]

Go to www.thewireman.com for your coax. Order RG6m/m. That stands for Marine/Mobile and Is a tougher outer shielding to withstand abraiding and abrasions in marine and mobile use. I've installed it in all my mobile instalations and never had a bit of problems with it in years and years. You can solder plugs on a right length piece of coax, can't you.

de W0JOG

 

[JCunningham17]

ok thanks. a 12" pizza pan it is. i tried looking up rg6m/m and didnt find it. I have a local radio shop ill check out. thanks

 

[bharvey2]

Go to www.thewireman.com for your coax. Order RG6m/m. That stands for Marine/Mobile and Is a tougher outer shielding to withstand abraiding and abrasions in marine and mobile use. I've installed it in all my mobile instalations and never had a bit of problems with it in years and years. You can solder plugs on a right length piece of coax, can't you.

de W0JOG

RG6 would be a 75 ohm coax. Maybe an RG8 ? (just guessing a typo was involved)

 

[SteveC0625]

And that vendor offers an RG-58 m/m.

 

[mmckenna]

You can purchase NMO mounts with longer lengths of cable. That'll be an easier solution.

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_266_1511&products_id=8842

As Steve said, you need to make sure the underside of the NMO mount is protected. The NMO mounts are designed to go through the roof of a vehicle where the coax entry point is protected inside the vehicle.
You can solve most of those issues with your installation by sliding a piece of 3/8" marine grade heat shrink tubing over the coaxial cable entry point. The marine grade heat shrink tubing has a hot melt adhesive on the inside. When you shrink the tubing, the adhesive flows and will seal everything up nicely.

 

[JCunningham17]

ok thanks. I was thinking Flexible Times Microwave LMR-400. Its going to be a run of 30ft. I cant seem to find a NMO mount that works with LMR-400/RG8. I bought a 1/8" thick 12" round piece of aluminum. I read that you should run a jumper of more flexible line to the radio because 400 is so stiff. not sure if that goes for the flexible LMR-400.

 

[bharvey2]

ok thanks. I was thinking Flexible Times Microwave LMR-400. Its going to be a run of 30ft. I cant seem to find a NMO mount that works with LMR-400/RG8. I bought a 1/8" thick 12" round piece of aluminum. I read that you should run a jumper of more flexible line to the radio because 400 is so stiff. not sure if that goes for the flexible LMR-400.

LMR400 is overkill for a mobile install. LMR240 is closer to the diameter of RG58. It's also available in a more flexible version usually referred to with a "flex" suffix.

 

[mmckenna]

Yeah, they don't make NMO mounts that accept LMR-400 directly. If you really wanted to use LMR-400, you can get NMO mounts that have a female N connector on the bottom and you just screw in your own coaxial cable.

But as was said by others, LMR-400 is way, way, way overkill for a mobile install, and you'll have a hard time routing it. The performance gains you'd see by using a larger cable like that would be minimal over that length, and you'd lose much of what you did gain in all the connectors you'd need. You'd be spending a lot of extra money and a lot of headaches trying to route that cable and end up with no benefit.

You can get NMO mounts with different cable types and different lengths. LMR-240, as suggested by bharvey is a really good solution. It's stiff on it's own, but not impossible to route, the LMR-240 flex stuff will have a tiny bit more loss, but much easier to run.

 

[JCunningham17]

ok thanks ill do lmr-240

 

[paulears]

Putting a ¼ wave on a small ground plane gives a match, but is probably going to give a pretty poor radiation pattern. I see some contradictions here - very low loss feeder on a pretty dreadful antenna. If you have that much of a cable run, is it not possible to put a better antenna somewhere else?

 

[cmdrwill]

LMR240 has a solid center conductor.
Make sure you get LMR240 Ultra Flex cable.

 

[JCunningham17]

i want to keep the truck under 7'. the 3.5" antenna puts me exactly at 7'. I have a Laird BB4505CR too that I will keep in the truck if I need it. Ill be talking on a repeater that can be accessed from a 4w Ht 40 miles away but is hilly. i was hoping the little antenna would work with my set up.

 

[JCunningham17]

I ended up doing LMR-240 and drilling the hole. 16" from the rear of the cab ahead of the spoiler was perfect. only problem is that the Laird UTRA 4301S3NB has a SWR of 2.0 and is not adjustable.

 

[mmckenna]

Make sure all your connections are good. Make sure you are well away from any conductive materials, like garage doors, lamp posts, thing that will reflect. Good place to test antennas is out in a field or large parking lot.
Make sure your doors are closed.

2.0:1 isn't really that bad, and for an antenna with that much claimed bandwidth, it may be expected.

If you really want to nail the SWR down low, do a simple 1/4 wave antenna. But trim it to length using your SWR meter. You should be able to get it right down near 1.

or, just go with a 1/4 wave:

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_206&products_id=767

or

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_206&products_id=9267

 

[JCunningham17]

o ya i got all that. thanks though. I have a BB4503 and a BB4505CR Too the BB4505CR will have to be trimmed a little right now its 1.2 on 462.5500 and 2.1 on 467.7250. the BB4503 is 1.5 on both. Im using a MFJ-874 meter in open field on top of a huge hill i can see for 30 miles in every direction 1675ft above sea level.

 

[bharvey2]

Looking at the specs on that antenna on Laird's website, thy only advertise it as less than 2.5 to 1 SWR so it appears to be meeting spec. You might look at a standard 1/4 wave antenna, like their QW450 will set you back about $10-20 and is only 6" tall. As a 1/4 wave, you'll probably end up with better SWR.

 

[merlin]

Flat metal ground plane will work as long as it is 1/4 wavelength from the antenna centerline or more.
20' run of coax, LMR 400 is overkill, decent RG 58 will do the job nicely.

 

[JCunningham17]

i used 20' LMR-240