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post #1 of 9 Old 07-02-2015, 07:13 PM Thread Starter
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CB electrical resistance

Hello everyone,
I am trying to tune my CB radio set up. SWR is dreadfully high - around 3:1, so I am trying to troubleshoot. I got the antenna base grounded to the body (checked). Then I measured the resistance between the antenna base and the active (upper) part of the antenna, thinking it should be completely isolated. To my surprise, the resistance was very much measurable - around 4.7 kOhm. I started digging through the connection points in my setup and found that resistance, when measured between the central pin and the outer casing of the antenna input of the CB radio itself, was that same 4.7 kOhm. Has anyone ever tried to measure that resistance value? Am I dealing with a faulty CB unit or is this a proper resistance reading?
Thank you!
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post #2 of 9 Old 07-07-2015, 09:34 AM
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I've never actually measured the resistance between hot and ground on an antenna/base/radio. I always just use a continuity tester.

Have you tested continuity/resistance of the ground and hot from one end of your coax to the other?

What coax, mount, stud, and antenna are you using?

Chase

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post #3 of 9 Old 07-07-2015, 03:30 PM Thread Starter
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The coax, mount, and the antenna are all connected properly - at least all the connections (including the ground) tested ok. The radio's antenna connection is the only one that I found strange.
The antenna is Firestik 2, 5 ft, the mount I do not recall... The coax is the recommended kind, forgot the name...
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post #4 of 9 Old 07-10-2015, 03:11 PM
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Post Antenna length

To adjust the SWR Standing Wave Ratio lengthen or shorten your antenna. If you spend a lot of time on a particular channel you can adjust it for your best reading. Otherwise try adjusting channel 20 for your lowest reading. It is not unheard of to have to cut a little off of an antenna to get it to adjust.

Some antenna's have a threaded portion on the top. Others you loosen a set screw and move the antenna up or down. If your near Southern California I have both SWR meters and an antenna analyzer and would be happy to help you get it adjusted in.

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post #5 of 9 Old 07-10-2015, 04:12 PM
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What are your SWR readings on channels 1 and 40? They are probably both over 3:1, but knowing which is higher will tell you which way to go.

I have always remembered it this way:

Channel 1 tells you which way your antenna needs to go. I.e., if your SWR on channel 1 is higher than on 40, the antenna needs to "go up," or, rather, be longer. If 1 is lower than 40, the antenna needs to "go down," or, rather, be shorter.

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post #6 of 9 Old 07-11-2015, 08:19 PM Thread Starter
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I removed the spring below the antenna to see the effect on SWR. It moved nicely from more than 3:1 to ~2.5:1. So I figured the antenna is too long. So I started to cut the wire windings on the very top, one by one and checking the reading after each (the screw was already removed). After disconnecting 3 or 4 coils SWR dropped to ~1.8:1, roughly the same on all channels. This is acceptable, right?
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post #7 of 9 Old 07-12-2015, 06:10 PM
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Arrow This is acceptable

Quote:
Originally Posted by GBel View Post
I removed the spring below the antenna to see the effect on SWR. It moved nicely from more than 3:1 to ~2.5:1. So I figured the antenna is too long. So I started to cut the wire windings on the very top, one by one and checking the reading after each (the screw was already removed). After disconnecting 3 or 4 coils SWR dropped to ~1.8:1, roughly the same on all channels. This is acceptable, right?
Your looking to get as close to 1:1 as possible. 1.8:1 is workable but you may try to get a tad lower too. I try to be 1.5:1 or less on the CB. But then I only us it 4bying. Most of the time I'm on the HAM radio.

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post #8 of 9 Old 07-13-2015, 02:24 PM
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Now that you're down to 1.8:1, I'd start taking it off 1/8" at a time until I got to 1.5:1 or slightly under.

An SWR of 1.8:1 isn't terrible and surely won't hurt your radio.

But at 1.8:1, you're losing some 8% of your power.

At 1.5:1, you're only losing 4%.

SWR Power Loss Table

Chase

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post #9 of 9 Old 07-13-2015, 07:19 PM Thread Starter
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I discovered something interesting. The biggest factor influencing SWR is not the grounding or the antenna height or even its placement. When I started repositioning the excess length coax cable (about two feet of it tucked under the seat), SWR started jumping wildly - all the way out to 3.5 (and from the lowest value of around 1.5) - depending on how I shape the cable. Which makes sense because what we're trying to do here is adjust the impedance of the system to minimize mismatch, and the shape of the cable has a major impact on it. Just something to keep in mind when doing the tuning...
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