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post #1 of (permalink) Old 05-05-2005, 03:33 PM Thread Starter
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No ground plane antenna needed for fiberglass tub?

Been doing some reading on ground planes for a cb antenna and realize my fiberglass tub probably has none. [img]/ubbthreads/images/graemlins/blush.gif[/img] Am I correct in thinking I need to purchase a "no ground plane" antenna as opposed to the usual antenna type?

The antenna itself would be mounted to the tire carrier/bumper at the rear of the Jeep.
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post #2 of (permalink) Old 05-05-2005, 03:47 PM
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Re: No ground plane antenna needed for fiberglass tub?

What I did on my fiberglass body was to run a heavy guage wire, I think 10 guage, from the one of the mounting bolts for the antenna mount and ran that to a good ground. It works fine.
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post #3 of (permalink) Old 05-05-2005, 04:02 PM
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Re: No ground plane antenna needed for fiberglass

'Ground Plane' is kinda misleading. Without going into a lot of details about propagation, antenna loading, VSWR, wavelength, impeadance, etc...
Imagine standing on a solid block and then jumping off. Block stays and you can jump pretty far. Now, put that block on rollers and try and jump off. You don't go as far. In simple terms, the groud plane sets up the reference for the wave propagation.

If your body is not grounded to the same point of the antenna base, then some of the energy is sort of 'absorbed' by the sheet metal. If you look at the propagation pattern on a vehicle, with the Antenna on the right rear corner, your stongest pattern is towards the left front. Put the antenna in the middle and it is more circular, but favors the front and rear. If you have a fiberglass body, then your frame acts as the ground reference or plane.

BTW...I have never heard of a "no ground plane" antenna. What you want is a fully loaded antenna...the taller, the better. Given the freq of CB's, about the longest you can get on there will be a 1/4 wavelegth.
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post #4 of (permalink) Old 05-05-2005, 04:45 PM Thread Starter
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Re: No ground plane antenna needed for fiberglass

Nice analogy DDawg! Given that the frame is steel, I should be good to go. One more question for anyone who cares to answer.....any harm in running the cb coax along the roll cage towards the front of the Jeep? I plan on placing the cb radio in an overhead configuration between the 2 middle cage stringers.
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post #5 of (permalink) Old 05-05-2005, 05:03 PM
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Re: No ground plane antenna needed for fiberglass

None at all...the coax is basically a 'pipe' for the RF. In fact, why not drill a small hole in the cage and run it inside. That way it is well protected.
On the passenger side, drill/cut a hole for a 1" bushing. Weld in the bushing and feed the coax through it. The reason for the bushing is to strengthen the hole....otherwise, the roll cage could bend at that point.

You will then have a hole up top for the coax to come out....along with power and ground wires.

The real advantage of this is that you don't end up with a bunch of tape or cable ties all over your cage.
post #6 of (permalink) Old 05-05-2005, 05:28 PM Thread Starter
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Re: No ground plane antenna needed for fiberglass

As they say in the Guinness commercials...."Brilliant!" [img]/ubbthreads/images/graemlins/laugh.gif[/img]
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post #7 of (permalink) Old 05-05-2005, 05:38 PM
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Re: No ground plane antenna needed for fiberglass

[ QUOTE ]
...That way it is well protected.
On the passenger side, drill/cut a hole for a 1" bushing. Weld in the bushing and feed the coax through it. The reason for the bushing is to strengthen the hole....otherwise, the roll cage could bend at that point.

You will then have a hole up top for the coax to come out....along with power and ground wires.

The real advantage of this is that you don't end up with a bunch of tape or cable ties all over your cage.

[/ QUOTE ]

Not a bad idea. How about going one better? If you use the roll cage as a conduit for the coax, then why not use a terminator for the cable at the output from the roll cage tubing?



This would let you use a small length to the antenna, and remove the antenna/cable if needed without a large piece of wire bundled/coiled/flopping around.
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post #8 of (permalink) Old 05-05-2005, 06:06 PM
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Re: No ground plane antenna needed for fiberglass

Outstanding idea Leve.

FYI....There are two types of bulkhead connectors, one that you solder on the coax, the othere that uses either a BNC or TNC connector. If you have the knowledge (not difficult) and the tools, I would go with BNC or TNC bulkheads...you can use the same tools to make the short cable...

And contrary to popular mis-belief, the coax length is not an issue....I've heard some people say that you can't shorten the coax because that will upset the VSWR.....WRONG. The coax has a fixed impeadance....regardless of the length...which is differnt from signal loss.

One last thing...it is actually real easy to fish the cable and other wires. Using an electricians snake (stiff wire used to pull wire through conduit), start at the top and push it down to your hole at the bottom. Attach 2-3 strings to the end and pull it back through. Use one string to pull the coax and power wires. Leave the remaing string(s) for future use.
post #9 of (permalink) Old 05-05-2005, 06:39 PM
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Re: No ground plane antenna needed for fiberglass

“Sherman set the way-back machine for 1975.”

“Where we goin’, Mr. Peabody?”

“To see C W McCall and hear him sing Convoy, fer sure, fer sure.”


<font color="green">Uh, Breaker One-Nine, this here's the Rubber Duck
You got a copy on me Pig-Pen? C'mon
</font>


Yep, about the time all this was goin’ on, they made no ground plane antennas for boats and such. Maybe you need one and maybe you don’t. Some people had good luck without them on ‘glass bodied cars, some could only talk if you were in front of him.

At the end of ’76, I think it was, maybe a year or two either way, 25 channels sets became obsolete and everybody and his brother got one of the marked down 25-channel sets for Christmas. There were experts all over the place and they all learned how to “pin your coax”.


You was doin’ pretty good there Dawg, even had the “V” in front of “SWR”. But;

[ QUOTE ]
And contrary to popular mis-belief, the coax length is not an issue....I've heard some people say that you can't shorten the coax because that will upset the VSWR.....WRONG. The coax has a fixed impeadance....regardless of the length...which is differnt from signal loss.

[/ QUOTE ]

Sorry, it’s true. You either tune the antenna to the coax length or trim the coax to tune to the antenna. Fixed impedance of the coax isn’t the problem.


I also would not drill a hole in the role cage. If it were square tube, channel or “I” it would be ok but with round it will weaken the tube.



<font color="green">Cause we gotta mighty convoy, rockin' through the night
Yeah we gotta mighty convoy, ain't she a beautiful sight?
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post #10 of (permalink) Old 05-05-2005, 06:56 PM
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Re: No ground plane antenna needed for fiberglass

[ QUOTE ]
...
Sorry, it’s true. You either tune the antenna to the coax length or trim the coax to tune to the antenna. Fixed impedance of the coax isn’t the problem....

[/ QUOTE ]

Uh... Taz, just one question. How do you trim (I'm assuming the term trim refers to cutting the cable and not electrically impedance matching) the coax to tune the antenna? Trimming the coax does no good... it's the same 50 ohm impedance at 3' or 50' until you reach the line loss at about 2 dB per 100 ft.

I'm waitin' to be edified!
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