UHF Antenna bracket mounted to plastic base....Will this work?

[SCPD]

I have a UHF motorola nmo style antenna base on a stainless steel L bracket. I have mounted the L bracket using three small screws to a plastic component on the vehicle. I then got a grounding strap from the automotive store and ran the strap from one screw to a solid ground on the vehicle. This doesn't seem to work very well... Has anyone had experience with such a setup and did it work or do I have to find a new way to mount it where it is not screwed to a plastic base???

Thanks!

 

[russellmaher]

Hi cmccluskey: I've read and re-read your post, but I'm not quite getting the message. Let's see if this is what you've got.

The nmo base is mounted on a steel L bracket and that's attached to something plastic on the vehicle.

My first question is....Why a plastic component? Isn't there any metal to attach it to?

Next....What purpose does the grounding strap serve? The strap just may act as an interuption to the circuit the antenna is supposed to provide. I don't know if I'm wording this correctly, but if you get what I'm trying to convey, maybe you can give us more details.

Russell

 

[mmckenna]

If everything is making good contact you may have a good electrical ground, but not a good RF ground. You can check continuity and it might be good, but the antenna really needs a suitable ground plane underneath it to work properly. A ground strap leading away from the "L" bracket isn't going to provide a proper ground unless it's a very short run to the ground bolt, like less than an inch or so.

What you need is a good 6 inch round ground plane underneath it to work correctly. You may need to relocate your mount where you can attach it to metal directly. Even then, it can be tricky.

"L" brackets on their own don't provide a ground plane. A wire from the bracket to the body ground will provide an electrical ground, but not a ground plane. To get the antenna to tune up correctly, you need the ground plane and the proper length antenna..

You could try running 6 inch long wires from the bracket screws off horizontally under the bracket in different directions and see if that helps.

Other than that, you should look at all your connections. Make sure the antenna base is making good contact with the center pin, too thick a rubber seal and it can keep the centers from meeting. Check your connector and the radio for opens or shorts. See if your antenna is close using the supplied cut chart that should have been included with the antenna (if you bought it new or it is a field tunable type)

Good luck.

 

[SCPD]

The nmo base is mounted on a steel L bracket and that's attached to something plastic on the vehicle. **CORRECT**

My first question is....Why a plastic component? Isn't there any metal to attach it to? **Trying to eliminate more antennas on hood. It is an SUV and there is limited space to mount something without drilling holes, which I don't want to do.**

Next....What purpose does the grounding strap serve? **HA HA HA HA HA, I thought it needed to be grounded i.e. mounted to metal, so I thought the strap may be a work around?***

The strap just may act as an interuption to the circuit the antenna is supposed to provide. I don't know if I'm wording this correctly, but if you get what I'm trying to convey, maybe you can give us more details.

**Thanks for helping! **

Russell[/QUOTE]

 

[SCPD]

If everything is making good contact you may have a good electrical ground, but not a good RF ground. You can check continuity and it might be good, but the antenna really needs a suitable ground plane underneath it to work properly. A ground strap leading away from the "L" bracket isn't going to provide a proper ground unless it's a very short run to the ground bolt, like less than an inch or so.

** SHUCKS! I was hoping!! **

"L" brackets on their own don't provide a ground plane.

** We regularly use L brackets as there is little other option besides drilling holes. They work fine on other things, but apparently not when mounted to plastic. **

Other than that, you should look at all your connections. Make sure the antenna base is making good contact with the center pin, too thick a rubber seal and it can keep the centers from meeting. Check your connector and the radio for opens or shorts. See if your antenna is close using the supplied cut chart that should have been included with the antenna (if you bought it new or it is a field tunable type)

** I will look at all of this, thank you for helping!! **

 

[russellmaher]

OK! Now I think we're almost on the same page.

I don't know how many antennas you have on the hood, but if you can find a few inches to put one in, my suggestion is an nmo trunk lip mount - that's what I'm using right now - my Spectra is trunk mounted on the Hyundai Sonata I currently drive.

Just as a side note....I work at Bradley Int'l. Airport and some of the airport operations and maintenance vehicles (mostly SUVs) run several antennas down the length of the hood using some L brackets and some use nmo trunk mounts. One guy drives a Ford pickup and he's got five antennas on the right and four on the left....so, can you beat that? I'm just sayin'!

Russell

 

[LtDoc]

The two 'halves' of an antenna ought to be close together. The further apart they are, whip and 'groundplane'/counterpoise/vehicle body(metal), the less 'reasonable' it will be to make work right. That connecting conductor between bracket and vehicle's metal becomes a part of the antenna system. Sometimes that isn't a biggy, but in other cases it can really cause problems.
Best solution is to just drill that hole. It may 'hurt' a little to start with (like pierced ears?) but really isn't unusual after that. (No, it doesn't lower values, and if it's done right there's no 'complications'.)
- 'Doc

 

[mmckenna]

I agree 100% with the above. Drill the hole. Much better results, easier to tune, etc. I've drilled many holes in my day, and I've used chassis punches on some to, and never had an issue with one leaking or failing. I have never had an issue selling any of my vehicles with antenna mounts on them. You can buy hole plugs from Larsen if you really want to remove them later.

I've also drilled leased vehicles for others, and they've never had an issue turning them in.

It is difficult the first time, but once you get it out of the way, you'll have an easy time doing the rest of them. Just get the proper hole saw that is specifically designed for NMO mounts. It's worth the money to get one of these over a standard hardware store hole saw as the NMO designed ones have a limiter that keeps you from going too far. Chassis punches are a good option if you can get to both sides, they make nice clean holes with no metal chips to deal with afterwards.