Yet ANOTHER antenna/radio question...

[DaktariEd]

I have been searching past threads on antennas for 2m and dual 2m/70 cm radios...lots to learn! I will be getting a dual band radio when I pass my test...soon!

Although I will obviously be using this for on the trail communications, this will also be an emergency communication tool, so I want distance as well.

My concern comes back to the ground plane or counterpoise...I presume this refers to the same thing (?).

On my FJ40, the roof is fiberglass, so a roof mount is out. I thought about the bumper mount but there is no good ground plane.

It was suggested in another thread that I consider either a lid mount on the front hood, or a front fender mount.

The fender location would be better for a secure permanent mount. But what about the ground plane? Is that surface area sufficient to act as a good ground plane, or would the lid mount be better?

On the 40 the edge of the hood curves 90˚ with the seam pointing horizontal....you can see it below. Would a lid mount still work here?

What about engine noise? Is that going to be a problem?

Also...the Comet CSB 7700-7900 series of antennas (Comet) state, "They terminate to a PL-259 base and require a PO mount." But I can't seem to find what a PO mount is. Any ideas?

Thanks!

Ed

[Clay]

I've never had to do this, but here is what I understand about it. Center of roof is best not only for ground plane, but for direction as well. This is because nothing is blocking the antenna. If you mount the antenna to the hood or on the fender you are basically making the antenna directional, because the windshield will be blocking some of your wave. There are so many factors when it comes to antennas, I'm not sure that loosing some ground plane in favor of height wouldn't be a bad idea.

My suggestions would be mounting as high as possible, maybe a tire rack or on the brush guard on the front? Next I would suggest spending money on a good quality antenna and properly tuning it with an swr meter, then using quality coax as well. Other than that power is going to be your next step. More watts will get you further.

Hope this helps.

[DaveInDenver]

Yeah, I'm not sure where it came from, PO mount. But it's basically a wired bulkhead version of a SO-239 UHF connector. That is the female side of the PL-259. In other words, this means a PO/SO-239 antenna looks like the end of your coax and the antenna mount looks like the connector on your radio.

In a word, I would avoid it.

I don't know why manufacturers insist on using it. The connection is not inherently waterproof and pretty structurally weak. I would get the NMO version of the antenna. I have the Comet C-767 in an NMO mount. 'N'ew 'MO'torola was designed specifically to be an antenna mount, it's very weathertight and a much more secure connection. Not sure how to explain it better. I'll try to dig up some pictures of them, but NMO gives you a universal mount amongst all reputable antenna makers and it's designed to solve a problem and is very good at it.

Your other questions. A counterpoise does not specifically have to be the metal of your truck. It can be and that works quite well, but you don't have to technically have a metal bodied car to have a counterpoise. The transmitted current needs a return path to complete the circuit and as long as you provide one, you have made the radio happy. IOW, the amplifer excited some electrons and sent them up the antenna. These electrons bumped some electrons close to the whip (called the near field), which will bump some electrons a little farther out (called the far field) and so on. Those near field electrons, now finished doing their job want to come home and complete the circuit, so you need to give them a path to do so, which is called the counterpoise.

So a series of radiated wires, a sheet of metal or the body of your truck can all act like a counterpoise. It's just got to be a conductive (to electromagnetic energy) material that is insulated from the 'hot' side of the transmitter to the return side of the circuit (i.e. the shield side of the coax). The actual size of the counterpoise is somewhat tough to say. What works best will depend on the antenna, but I would recommend as a generalized wag that should work with most any antenna that the counterpoise should extend 1/4 wavelength from the whip, so a 2m counterpoise should be 0.5 meters around. So around 18" in all directions. See where this is going? You can use your roof if you put a 18" circle of copper under the mount and ground it to the same point as your radio and coax. This will acting swimmingly as a counterpoise.

BTW, not all antennas need an external counterpoise. But in general, unless the antenna is specifically designed not to need, you will have to have one. That's circular speak if I ever did type it.

Between the hood and fender, I would pick the fender only because it'll be easier to wire and tie down. No real advantage against ignition noise, other than the driver's fender is about the farthest metal point from the distributor and coil you'll get on a FJ40 without going to the back bumper or rigging something on the roof.

Post up questions, this is a good topic to hash out. There is probably a lot of misunderstanding about the ground plane and what it does. Gary from Ohio (I love that forum name, makes me think of ed fROMOHIO, the guitarist from Firehose and Whiskeytown) should be able to explain it better than me.

[pskhaat]

Ed, I did look at my 40. Your hood is still your best option for a counterpoise IMO.

1. Mount the antenna on a trunk-lip mount on the hood on the non-flanged lip all the way back. The rear 4"-most area will hold the lip mount perfectly.
2. Have a custom small bracket made that will bolt to the two through bolts on the hood latches on the hood (not on the body apron).

These will also allow you to avoid drilling into your beautiful 40. With the fact that I've seen your 40 with its rack completely consumed, I don't know if you'll have the luxury of a roof mount. Your antenna is absolutely key to performance, not so much the radio. I can talk near-line-of-sight on 5 watts between Prescott area and Mt. Lemmon, and I have a sub-par counterpoise. You don't get a whole lot better distance from a more powerful radio, but you certainly can with a properly installed antenna and feedline (coax).

[gary in ohio]

If you never take the top off then the top is still viable location. With a meta backing and some copper tape you can make a counterpoise.

It been a while since I have seen a FJ, but for jeeps I have seen mounts that bolt behind the tail lights then a meta mount goes up to the roof line. The antenna is then mounted to that. You sometimes see similar mounts when they put revoling lights on a jeep or other convertible.

[DaktariEd]

Thank you all for your thoughts. It helps a lot to hear different perspectives and get different "translations" of all this stuff...

OK....so here's where I'm at...

Scott, I like the idea of using the hood latch holes...a custom bracket. Great idea! But what about the ground plane? Does it matter that the antenna would be pretty far up front in relation to the hood itself?

I presume this is the latch you had in mind...see image below.

Here's another Comet antenna I found that is a dual-bander w/NMO mount. It also has a fold down hinge. That will be handy in brush and the garage....considering it's a 52" mast.

[DaveInDenver]

Scott, I like the idea of using the hood latch holes...a custom bracket. Great idea! But what about the ground plane? Does it matter that the antenna would be pretty far up front in relation to the hood itself?

I presume this is the latch you had in mind...see image below.

That's a fine place for it and there's plenty of hood and fender around that it will work acceptably. The only issue I can see with that spot is you'll have to be careful opening the hood and letting it rest on the front of the roof. If you use the prop rod, then it will work fine.

[pskhaat]

Yes, that's the latch to which I was trying to refer.

Yeah, I thought that too, Dave. I put my Diamond antenna up on my 40's hood all the way back against the windshield and it appears at first glance to not interfere. Of course, I have nothing on the roof my 40 :|. If you use the hood support (vs resting back against the windshield) the angle should be fine.

I defer to GiO and DiD as to any possible RF radiation problems in that area, but that location appears structurally sound and boltable.

[DaktariEd]

That's a fine place for it and there's plenty of hood and fender around that it will work acceptably. The only issue I can see with that spot is you'll have to be careful opening the hood and letting it rest on the front of the roof. If you use the prop rod, then it will work fine.

Yup...I'll have to keep an eye on that. It does have a fold-over hinge. That might be helpful in getting it out of the way.

Another thought is to have a small antenna in the mount when running trails with a group and distance is not that important. Then switching to a taller mast when out by myself and I want distance.

[DaveInDenver]

Yup...I'll have to keep an eye on that. It does have a fold-over hinge. That might be helpful in getting it out of the way.

Another thought is to have a small antenna in the mount when running trails with a group and distance is not that important. Then switching to a taller mast when out by myself and I want distance.

That all makes sense. I have two dual band antennas, the Comet C767 and the Larsen NMO270SH. The C767 is pretty similar to that SBB-7 that you are looking at. It's tall and has the fold-over hinge. The NMO2/70SH is a stubby little sucker with a spring, 19" overall with the spring. I use that on the trails and it works fine. I can tell it's limitations driving around on the commute, it's OK for local repeaters, but when you switch over to monitor the input side (to see if you can hear the original transmissions), it's basically deaf beyond about 15 or 20 miles.