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-   -   Fundamental question about mounting the Antenna (http://www.risingsun4x4club.org/forum2/showthread.php?t=13231)

rhyary 06-04-2010 10:42 AM

Fundamental question about mounting the Antenna
Here is what make sense to me. Let me know what you think
I have the 4x4lab rear bumper with two swings.
The tire swing has the Hi-Lift mounting point and it is where my hi-lift is mounted.

The second swing, the Jerry can carrier, also has the hi-lift mounting bracket.

What I am thinking is fabricating a bracket that mounts to the hi-lift bracket. probably a C channel type that will go up to the roof level. On top I will have some sort of a T bracket with holes for antenna mounts.

This will give me a roof level mount that is sturdy for any antenna, and because this based will be at roof level, I can use 1/4 wave antenna, as many as I want I guess. It should be easily sturdy enough for a 5/8 wavelength of any wavelength.

Although it will be above or at the roof hight, it will not be above the truck roof. Does it matter?
Will it have less "ground plane" if it is not over the roof?

Can I make it from Aluminum?

Since the bumper is powder coated, can I used grounding strap instead of scraping the powder coat?

I looked for entry points for the coax and did not find anything that is easily used.

Although radical, I would like to drill new holes and use rubber grommet, instead of poking and disturbing existing grommets.

I am sure my issues have been discussed before, but what I want to do is fairly specific. I think.

Many thanks in advance


nakman 06-04-2010 11:43 AM

One place to run coax is in your rear fender behind the tail light- you've already got a grommet there for the sunroof drain that is ripe for the disturbing if you swing back that direction, and plenty of room for drilling if not. But that gets the wires out of harm's way as far as the hatch and tail gate operation is concerned..

Are you installing just a 2m radio?

rhyary 06-04-2010 12:18 PM

Yea I saw on both sides the rubber drainage hose. I suppose I can poke a hole in the grommet, like I did on the firewall grommets.

rhyary 06-04-2010 12:22 PM

Bracket location
2 Attachment(s)
Here is the location of the bracket

rhyary 06-04-2010 12:24 PM

Here is the bracket idea
1 Attachment(s)
In red is the bracket I am thinking about:

corsair23 06-04-2010 01:50 PM


You can get real fancy with fabricating the mount but I've had great success with my NMO latch mount...Thin coax goes right in the hatch and then down the interior rear panel to my rig mounted in the PS rear quarter panel area.

Here is a thread where different antenna mounts on 80's have been discussed:


Not trying to talk you out of your idea at all, just offering some alternatives.

As for your other questions...I'm a HAM novice so I'm sure the experts will chime in but no, your antenna doesn't have to be "above" the roof line. Dead center of the roof will give you the best ground plane but that is an impractical location for many so you go from there.

I don't see any reason you can't make your bracket out of aluminum and yes, I would suggest a grounding strap vs. scarping the powder coat. Maybe a grounding strap from the base of the antenna then down the bracket and to a good ground location on the frame.

As for an entry point for the coax. I use the gromments in the rear area that can be accessed under the rig and inside the rear quarter panels. As Tim mentioned, you already have drain tubes going out these grommets so you can usually squeeze wires, coax, etc. through the existing hole in the grommet. In my case, I have my CB coax, Hoppy harness wires, and a couple ground wires all going through my PS grommet.

rhyary 06-04-2010 02:55 PM

many thanks for your insights.
My radio should arrive to day from HRO but I am going to temp everything with a cheappo mag until I figure out what I want.

DaveInDenver 06-04-2010 03:00 PM

If you want more theory, be happy to type it up sometime. But as for your questions specifically...

The more radiating length that is above your roof line the better. The body of your truck is doing a couple of major things as far as antennas go and none of it is helped if the antenna whip is physically close to the sheet metal for relatively long lengths.

OTOH, I've mentioned this before and will repeat it, you have to come up with an antenna system that is workable and livable as much as anything because even an ideal antenna that is a pain won't be fun. Just understand that there are definite reasons that everyone says to drill a hole in the middle of the roof, that is absolutely the best location to install an antenna in all cases, but really very impractical for anything other than VHF and UHF unless you are really dedicated to mobile operating. So you do the best you can within the limits you have.

As far as using aluminum, that is fine. Make sure it's grounded just like any other conductive material since it's physically close to the antenna and will try to become part of the antenna and you don't want to be making too many stray caps and inductors.

Regarding grounding, use straps if there is any question. Unless the path is absolutely bomber it is likely that it will have some 'impedance'. Notice that I did not say 'resistance', because a fair to even good DC ground will probably not be a zero ohm impedance path, which is the combination of resistance and reactance (made up of capacitance and inductance). So a DC ohmmeter isn't necessarily going to tell you the whole story.

Other than the frame itself and large sheet metal pieces there are very few excellent RF conduction paths on your vehicle (read up about the 'skin effect' for EM fields), so ultimately what you have to do connect smaller things with braid to larger things and then make sure all the larger things are well connected. This is termed 'RF bonding' your vehicle. What you need to do is make sure a significant part has a good braid connection to its nearest large piece and then that larger piece has a couple of braids to the frame. Put a braid that bridges the swing arm to the bumper (the pivot might be OK for DC current but is terrible for AC), then a braid or two from the bumper to the frame. The only way to know that something is an excellent RF conductor is if it either has multiple continuous lengths or welds longer than say 6 or 12 inches, so if your bumper is painted and bolted it probably is a horrible RF conductor despite maybe being a great DC conductor.

wesintl 06-04-2010 03:05 PM

or there is the famous gamiviti mout


nakman 06-04-2010 03:11 PM


Originally Posted by wesintl (Post 151821)
or there is the famous gamiviti mout


which are back in stock, btw :D

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