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Old 04-30-2008, 04:30 PM
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Default Need antenna help

I bought a dual band antenna today and need an Elmer to help me with its placement. The first picture shows the locations circled in red. I would lean towards the more "inboard" position. The issue with this position is that the antenna could hit the hi-lift. I can mitigate this by ensuring that the rubber block on the hi-lift prevents this.

In general, I would prefer to have the antenna in more of an inboard position as I think it would make a minor ground plane difference. Comments?
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Old 04-30-2008, 04:46 PM
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Not an Elmer by any means but in my opinion the little bit you might lose by out boarding it would surely minor compared to the reduced clanking and damage potential of hitting the Hi-lift.
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Old 04-30-2008, 04:48 PM
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Sold! Thanks Kevin
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Old 04-30-2008, 05:51 PM
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Comet 1/2 wave on 2m right? Maybe a Diamond hard to tell by the pix. Either way no need to worry about a "Ground Plain", all you need is a return path for common mode current to keep the current off the coax shield. Looks like you've got that covered with your bumper design.

How thick is that mounting plate? looks like 1/4 inch. My concern is the plate might be to thick for a standard NMO mount. You might have to go with a thick NMO mount.

Last edited by Seldom Seen; 04-30-2008 at 06:16 PM.
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Old 04-30-2008, 08:33 PM
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Good observation Brian. Its a Comet and Its a PL mount, not an NMO. The hole for the NMO was going to be a PITA to drill. This stated, I think the plate is too thick for the mount design I have... I'll have to see if I can find a better (i.e., longer) design. The PL mount has about 5 threads, but I don't think the center conductor is appropriately engaged in the mount (I need to measure this). Thanks.
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Old 04-30-2008, 10:44 PM
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A quick search turned up NO hits for a thick SO239 mount. Are you set on using that antenna? Are you open to trading it for a NMO type? NMO's can be found for roofs up to 2" thick and I found several that mount in a 3/8" hole. Just search for "3/8 NMO". NMO to SO239 adapters are readily available if you are set on that antenna.
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Old 05-01-2008, 06:07 AM
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Brian, good observation. On Red_Chili we used a regular NMO mount and Bill used a piece of stock that wasn't all that thick as far as bumpers and sliders. I want to say no more than 3/16", but Bill would have to say for sure. It was right at the margin of working and the center pin of the antenna barely made it with a very thin o-ring. The standard NMO mount is made for sheet metal, you know, 16 gauge metal. But since Jeff used a PL mount, it sounds like it's not a problem here. But good thing to point out because Larsen and some others make thick material NMO mounts for situations like this, although it's not necessarily a stocked item at HRO or whatever. On my CB antenna I used a NMO mount and the base material is n aluminum that's very, very similar to, well, uh, you know, a city street sign. I'd say it's maybe 1/16" thick or so and that was about the limit to a standard NMO mount.

A couple of points about antennas.

An antenna will always try to find a return current path, it's up to you to help it find one easily. In the case of 1/2 wavelength antennas the designers will usually design the current path into the antenna itself to make them less dependent on you providing one during installation. But the antenna will still benefit from a ground plane if you provide one, primarily in radiating a more uniform pattern. But if the antenna is termed ground plane independent, then you don't need one, unlike a standard 1/4 wavelength antenna where you have to have one to work right. Sometimes manufacturers tell you this and sometimes not. The best advice I can offer here is to see what the Harley and Goldwing guys find work best on their bikes since those are gonna be the most ground plane challenged vehicle shy of your own two feet.

Second, with a metal whip you never want it to touch anything metal on the truck that might be at or relatively close to grounded. As annoying as the clanking would be on the Hi-Lift, I'd be worried that since it's bolted to the truck it is weakly grounded and that might eventually make your radio very unhappy.

Thirdly, an antenna radiates along it's whole length and anything conductive around it will act like a secondary antenna. This might help you if by chance the stars align (this is how a Yagi works, sympathetic elements focus the energy), but more likely it will just mess up your pattern. So you want to try and be away and/or above roll bars, sheet metal, spare tires and the like.

So with all that in mind, locating your antenna then becomes essentially impossible in the real world. What I usually tell people is to find a place that you can live with primarily, since if the antenna is annoying then ideal performance doesn't matter. Once you've identified a spot, then use the above rules of thumb to try and position it the best as possible. You notice that the roof is almost always the best spot performance-wise, but does have the downside of making your truck taller and finding tree branches becomes easy. And with a soft top, the roof in a way becomes as much of a challenge as any other spot to get a ground plane and solid mount. So everything is a compromise.
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Old 05-01-2008, 08:08 AM
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OK. I wish I would have posted this question before I drove 40 minutes to HRO to get the mildly surly service. anyway, some additional questions:

1: I asked about bonding and the guy said don't bother because I was probably grounded decently already. I am happy to do the bonding, but don't plan on putting an HF rig in this 40 any time soon. To bond, or not to bond; that is the question?

2: I am running my Kenwood TH-F6a and a Comet SBB7 dual band antenna. The goal was to put an antenna on the 40 that I could use today for the HT, and evenually with a V710A. when I posed this question at HRO the furst guy, Kent said that the performance of the HT with the SBB7 would be worse than with the rubber duck. The second guy said that I would have no decrease in performance, other than some dB loss on the conductor. The second answer seemed to make more sense to me. What do you think?

3: I had envisioned the antenna connection point on the tire rack above the spare. This would raise the antenna, get a lot further away from the hi-lift. I also wanted to put the antenna in the center of an 18" steel circle that would be used as a gound plane. Due to communications issues, the bumper was not built this way. Is the 18" GP overkill?

4: I can still modify what I have to accomodate most of the original design. This picture is below (Sorry I don't have a groovy Nathaniel CAD drawing).

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Old 05-01-2008, 08:45 AM
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Quote:
Originally Posted by Mendocino View Post
1: I asked about bonding and the guy said don't bother because I was probably grounded decently already. I am happy to do the bonding, but don't plan on putting an HF rig in this 40 any time soon. To bond, or not to bond; that is the question?
Yes, if you can get a piece of grounding strap, it would be best. This is true for any place where continuity might not be great, like across hinges, pivots, bolted connections, etc.
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2: I am running my Kenwood TH-F6a and a Comet SBB7 dual band antenna. The goal was to put an antenna on the 40 that I could use today for the HT, and evenually with a V710A. when I posed this question at HRO the furst guy, Kent said that the performance of the HT with the SBB7 would be worse than with the rubber duck. The second guy said that I would have no decrease in performance, other than some dB loss on the conductor. The second answer seemed to make more sense to me. What do you think?
The antenna has no idea what is feeding it, a 100W HF rig, 5W HT or 50W mobile. It does not care, RF energy is RF energy. The first guy is wrong. The only conceivable difference between an HT and regular radio with a SO239 jack is that you have to use a BNC- or SMA-to-SO239 adapter and that will have additional loss. But an external antenna's efficiency will far outweigh the little bit lost in the adapter.
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3: I had envisioned the antenna connection point on the tire rack above the spare. This would raise the antenna, get a lot further away from the hi-lift. I also wanted to put the antenna in the center of an 18" steel circle that would be used as a gound plane. Due to communications issues, the bumper was not built this way. Is the 18" GP overkill?

4: I can still modify what I have to accomodate most of the original design. This picture is below (Sorry I don't have a groovy Nathaniel CAD drawing).
That 18" steel circle will make a fine ground plane (for someone reading along, he picked that since it's 1/4 wavelength on 2m). Between that and the added height, that looks like a pretty decent mount. If it turns out to be visually unpleasing or just in the way, you could reduce the size of the circle as long as you keep the height. Height is the primary goal with VHF and UHF antennas since the SBB7 AFAIK doesn't absolutely need a ground plane to work.
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Old 05-01-2008, 09:08 AM
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Thanks Dave, much appreciated!!
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