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Topic: CB radio antenna tuning (Read 1117 times) previous topic - next topic

CB radio antenna tuning

Do any of you tune your own CB radio antenna? I am seeking information on how to get the CB Antenna SWR into the acceptable range on my 1996 U320.

The SWR on my Firestick antenna with a Cobra CB is about 10 (half way between 3 and infinity) which will cook the output transistors during transmission.

I have tested using another CB and/or a Larsen MNO27 antenna. The ground plane on my U320 is the aluminum tubing above the windshield. A one foot wire is used between the antenna and the frame. I have always mounted a CB antenna into the ground plane for good results. I can only get an SWR of 1.5 by setting the antenna on the roof with a temporary steel ground plane. 

Does any one have suggestions.
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: CB radio antenna tuning

Reply #1 –
From the Firestik website:

Tunable Antenna

All CB antennas are tunable in one way or another. Those antennas that have an easy, often mechanical, tuning device are referred to as a "tunable antenna". Accordingly, lots of people think the other antennas are not tunable and so they make no attempt to have the antenna tuned. This is a critical mistake.

Wire-wound antennas without mechanical tuners, that test electrically long, can be tuned by removing wire from the top. If SWR testing indicates the antenna is electrically short, the coils at the top of the antenna can be separated and spaced further apart or accessories that increase the physical length of the antennas (springs, quick disconnects) can be added.

Solid fiberglass antennas (straight or helical wire impregnated in fiberglass resins) that are electrically too long can have the cap removed and the top can be cut off with a hacksaw. If the SWR test on one of these antennas indicates that it is electrically short, the addition of a spring or quick disconnect (or both) is the only way to correct for the short condition.

Base loaded antennas with wire whips have a set screw (or two) just below the area that the whip is inserted into the loading coil. By loosening up the set screw(s) you are able to slide the whip up or down as required

Re: CB radio antenna tuning

Reply #2 –
Wyatt,
Don't think that the aluminum tubing is giving you enough ground plane.  In fact, aluminum is not a good metal for a ground plane.  Also, with the Firestick, are you using the proper "special" coax cable ("we call it a system because the antenna and coaxial cable are a matched set that may not be interchanged with other non-Firestik NGP antennas and coax assemblies")?  Take some time to read the following article which should offer you some insight:  http://www.cbradioschannel.com/tag/no-ground-plane/

The Larsen is a excellent antenna, but needs a good ground plane.

Just where is Jon when we need him?
1994 U225
build #4514

Re: CB radio antenna tuning

Reply #3 –
Quote
The SWR on my Firestick antenna with a Cobra CB is about 10 (half way between 3 and infinity) which will cook the output transistors during transmission.
If its a Firestick II, if you pull off that top cap (its red on ours with a white antenna) of the antenna there is a thumb screw in there that allows you to tune the antenna.  If its an original Firestick then you have to also remove the tip but either clip some of the wire off or spread out a couple of the coils depending on if you are shortening or lengthening the antenna.

Details are here: http://www.firestik.com/Tech_Docs/Setting_SWR.htm
2000 / 36' / U320 / WTFE
WildEBeest / "Striving to put right what once went wrong"

Re: CB radio antenna tuning

Reply #4 –
Thanks for these replies - I believe the problem is my motorhome windshield is two feet from the edge of the roof of my house which has a coated steel roof. DUH! I will check SWR on Friday because I will be driving Foretravel anyway.

The antenna/CB/coax was installed by Foretravel as far as I can determine. The antenna is a Firestick II but the rest of the printing cannot be read, so I do not know if it is a NGP antenna. Also, the coax is marked with RG58U with no other markings. The catalogue pictures I saw of Firestick NGP coax showed the same connector on both ends. The antenna end of the coax had a washer soldered to the center conductor and the shielding connected to a wire with a screw into the aluminum framing. I believe this is a ground plane antenna/coax.

There is a myth floating around that aluminum cannot be used as a ground plane. I contacted a local radio communications company everal years ago about this and they told me that a ground plane must conduct electricity and does not have to be magnetic. Good electrical conductivity must be maintained with the aluminum and that can be difficult.

I used the aluminum trim on my old motorhome very successfully for years as the ground plane. I maintained good conductivity by using dielectric silicon on the screw threads as well as urethane sealer on the outside. The Foretravel framing is larger then that trim and I believe any ground plane problems will be associated with maintaining electrical contact.

More to follow later.
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: CB radio antenna tuning

Reply #5 –
Quote from: P. Wyatt Sabourin –
so I do not know if it is a NGP antenna.
The NGP antenna has a yellow strip around it at the very bottom of the antenna.

 The coax at antenna end is not connected to anything.  You only connect the center connector, not the shielding.  They claim that the NGP antenna can only be used with their special NGP coax, and the their NPG coax can only be used with their NGP antenna.  From Firestik's web page: "The NGP coax assembly is black with identifying print in silver-grey. Assemblies made before February 2004 are printed with "Fire-Flex" RG-58A/U. Those made after February 2004 are printed with Firestik Super-Shield 50. The connector at the radio end has a soldered tip (not crimped). Also, at the radio end you will find the coax is covered with a piece of black shrink tubing that is about 18 inches (46cm) long. At the mount end of the assembly the center conductor of the coax has been preprocessed and ready to have the ring terminal installed by the user. The braided shield of the coax is not used at the mount end."
1994 U225
build #4514