CB SWR ..... Antenna Advice

[Simple Man]

Base mount on the bed passenger side rear window corner

I have my CB installed and hooked up. I was chasing the reason I could not get below 3 SWR. Tried a firestik and a short whip Tram...... Even took jumper cables and grounded the base.

I put my old school 8 or 10 foot whip on there and SWR is 1.5

I don't want a long whip sooo


My question is what antenna (if any) is shorter, but can mimik the tall whip?

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[Kevin_D]

Have you inspected your cable and connectors? A 1.5 SWR still isn’t very good.
While I work with high-powered transmission systems for a living, it’s been a while since I played with CB.
Make sure you’ve got a good ground to the body at the radio, and that the cab and box are bonded for a good, continuous ground.
I remember having to tune my 4’ fiberglass whip to get a good match.

Kevin

 

[prerunner1982]

It could be that the antenna is too close to the rollbar and is causing the signal to reflect back creating the elevated SWR, the longer antenna clears the roof by quite a bit so the signal that is reflected by the roll bar is minimal.
Also, shorter antennas have a steeper SWR curve, more like a V instead of a U. Sometimes it may be hard to find the dip in the curve. The long whip is more broad banded and has a very low SWR curve.

 

[sharpsicle]

Sounds like something else is going on. I have a similarly mounted Firestik and have a SWR maximum of 1.3, average 1.1 across all CB channels.

 

[hjdca]

Base mount on the bed passenger side rear window corner

I have my CB installed and hooked up. I was chasing the reason I could not get below 3 SWR. Tried a firestik and a short whip Tram...... Even took jumper cables and grounded the base.

I put my old school 8 or 10 foot whip on there and SWR is 1.5

I don't want a long whip sooo


My question is what antenna (if any) is shorter, but can mimik the tall whip?

I also had a problem with swr with the CB antenna in that location. The firestick, firefly, etc.. were too high swr. It turns out they are too short. I finally found one that works excellent - a Wilson 305-4FD Silver Load Series 4' CB Antenna Whip that I could tune to 1 swr.

Note: I am running both GMRS and CB now, and I like GMRS way better.

 

[Radio Guy]

The problem is often the lack of a good ground plane and that has nothing to do with grounding back to the battery but rather how much flat sheet metal is right under the antenna. Part of the formula is how much capacitance to ground the antenna whip has and substituting a longer one has more capacitance to ground and can sometimes allow a lower SWR to be achieved.

You can run a simple test to see if its the lack of ground plane by attaching a long piece of wire to the grounded side of the mount and stretch it out sideways like a ground radial. 9ft would be an ideal length but 4 to 6ft will usually answer the question. If the antenna tunes fine with this wire attached then your mounting location lacks a good enough ground plane. I have a mount on the top bed rail right below my rear window and its adequate for VHF and UHF antennas but a short CB antenna refuses to match there.

If your antenna mount is the type where you have a bare coax connection on the bottom using lugs instead of a coax connector you can cheat a little by adding a small value capacitor in the range of 25 to 50pf from the ground lug to the hot lug. A ceramic disc cap will work but I prefer a dipped silver mica and a voltage rating of 50 volts will be ok for a stock CB but if your running an amplifier up that to maybe 500 volts.

 

[Simple Man]

The problem is often the lack of a good ground plane and that has nothing to do with grounding back to the battery but rather how much flat sheet metal is right under the antenna. Part of the formula is how much capacitance to ground the antenna whip has and substituting a longer one has more capacitance to ground and can sometimes allow a lower SWR to be achieved.

You can run a simple test to see if its the lack of ground plane by attaching a long piece of wire to the grounded side of the mount and stretch it out sideways like a ground radial. 9ft would be an ideal length but 4 to 6ft will usually answer the question. If the antenna tunes fine with this wire attached then your mounting location lacks a good enough ground plane. I have a mount on the top bed rail right below my rear window and its adequate for VHF and UHF antennas but a short CB antenna refuses to match there.

If your antenna mount is the type where you have a bare coax connection on the bottom using lugs instead of a coax connector you can cheat a little by adding a small value capacitor in the range of 25 to 50pf from the ground lug to the hot lug. A ceramic disc cap will work but I prefer a dipped silver mica and a voltage rating of 50 volts will be ok for a stock CB but if your running an amplifier up that to maybe 500 volts.

Thanks for your help .... Will try that tomorrow.

 

[Simple Man]

I also had a problem with swr with the CB antenna in that location. The firestick, firefly, etc.. were too high swr. It turns out they are too short. I finally found one that works excellent - a Wilson 305-4FD Silver Load Series 4' CB Antenna Whip that I could tune to 1 swr.

Note: I am running both GMRS and CB now, and I like GMRS way better.

I might just get one of these and try it out

I am actually registered for GRMS use and will be installing a GRMS radio next.

Thanks,

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