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post #1 of (permalink) Old 02-27-2002, 12:23 PM
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CB question

I put a new CB antenna setup on the ZJ - going from a window-stick "looks like a cell phone antenna" to a real fiberglass Firestick II whip on a door jamb mount. I hooked it to an SWR meter, and am getting a reading of almost 3 with the adjustment knob all the way out. Screwing it back in only makes it worse. I'm running the recommended 18-ft. of cable along the inside of the Jeep from the front to the back, and the excess cable is VERY loosely coiled near the left rear quarter panel.

What can be done to improve the efficiency? The only other thing is that I had to make a new terminal at the antenna end of the cable and am not happy with the results - the terminal is a 259, but doesn't grip the wire tightly.

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post #2 of (permalink) Old 02-27-2002, 12:45 PM
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Re: CB question

I have a little advice and some questions for you to ponder that hopefully will help... I'm no radio guru so nothing I say here is sure to be 100% truth - anyways, why is 18ft the recommended length of cable? My understanding is that the approx CB wavelength is around 101 inches (8.42 ft) Going with that info, any multiple of 9 is good but not critical. I have a bone stock 12' manufactured cable from Radio Shack that is perfect length for my CJ7 with very little slack or tension. Try a store machine made wire.
Next, that wire is usually shielded pretty good with wire mesh, this material will often have (maybe need?) an electrical isolation or maybe a ground through the end connectors. I've found that this is difficult to get with any home crimped or basically non-machined fashioned wire end. Also, I believe that I was advised to ground the antenna connector or base ?? and when I did it improved my rx and tx along with removing a lot of noise I was getting. Just some ideas, open for comments.

Sully 84 CJ7 258/HEI/Weber

post #3 of (permalink) Old 02-27-2002, 12:56 PM
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Re: CB question

Thanks for the input. I don't know where they got 18ft, but that's what's on the antenna packaging. I had to drill a hole in the sheetmetal for the antenna mount, and would imaging that's as good a ground as any.

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post #4 of (permalink) Old 02-27-2002, 01:18 PM
 
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Re: CB question

If you want as close to a 1:1 SWR you have to tune the antenna. Length produces inductance, proximity to sheet metal produces capacitance. You need to find out whether your antenna is too short or too long. THe length of the feed cable has no effect on the actual SWR, just your perception of it, as you may find the cable length positions your radio at a node or a peak in the waves. Here's how to start. The high channels are higher in frequency, and consequently shorter in wavelength. The low channels are lower in frequency and longer in wavelenght. Antenna work best when they are some multiple fo the wavelenght. 1/4 wave whip is the simplest and that is what you are building. CB is 11 meter, roughly 33 feet or 1/4 wave is 8.25 feet. Roughly. As it's a multiple of the wavelenght, high frequencies use short antennas, and low frequencies use long antenna, generally. Test the SWR at both upper and lower extremes and compare the SWR. If the antenna is too short, the SWR will be lower at higher frequencies. Test the theory by either adding length to the antenna with an extension (the fiberglas whips have a wire down the middle) such as a spring base, or you can disconnect the connection at the base of hte antenna and add a coil of wire to the centre terminal. Use heavy copper wire wound around a large diameter approx toilet paper roll size. Wind a dozen widely spaced turns around a core like a piece of plastic pipe. connect one end to the base of the antenna centre, and connect an alligator clip to the centre of the coax cable on a short lead. Tune the antenna by positioning the clip closer or further from the end connected to the antenna. When you add more turns to the coil, it's like having a longer antenna, more inductance. You can either fabricate a longer antenna, or build a base coil having the same no. of turns and spacing as your tuning coil. You should be able to get it down to close to 1:1 by this method and greatly improve the efficiency of your antenna. You may also try positioning the antenna elsewhere. It needs a ground plane and works best on the centre of a metal roof to radiate equally in all directions, but will work in other locations but the proximity of the ground plane will affect the transmission pattern. You might want to try other locations from where you have it now.

First point though is to make sure you can proper connections on all points. Perhaps as other poster pointed out, a quality factory crimped cable might help. As I said, length plays not role other than fooling you into thinking it is better because the length of the cable happens to produce a position in a node of waves cancelling each other out. Also, excessively long cables produce losses in signal that both reduce output and diminish the return of the standing waves (so you are once again fooled into thinking the swr is lower). SWR has to do with reflected power back from the antenna. A poor antenna will radiate less and reflect back down the cable more signal. High swr means the antenna is working poorly and can even damage your equipment (not likely on CB;'s having low wattage).

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
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post #5 of (permalink) Old 02-27-2002, 01:23 PM
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Re: CB question

About the grounding of the antenna... did it come with one of those lock washers with the teeth and a bunch of washers? Like I mentioned earlier, when I first installed mine I disregarded all that hardware and just screwed the base nut to the antenna and plugged in the wire - that's when I had all the noise. I think one of the Radio Shack guys told me that the washers help isolate the mesh shielding and the washer with teeth helps ground the antenna (both would help aleviate noise). Other than that and the wire quality, TiminMb pretty much covered exactly what the guy at Radio shack tried to teach me :-)

Good Luck
Sully

post #6 of (permalink) Old 02-27-2002, 01:49 PM
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Re: CB question

The length of the cable DEFINITELY has an affect on SWR (or more correctly VSWR or voltage standing wave ratio, commonly referred to as viswar). It is a measure of the amount of signal being reflected back to the source in transmit mode. A direct short at exactly 1/4 wave length of cable away will be seen as an open at the transmitter and vice-versa. Back in the '70s a lot of CBs (old 23 channel) didn't have the antenna trim and you adjusted the VSWR by trimming the cable length. Try a web search for VSWR to see what all this means.

You need to fix the connection at the antenna before going any further. Then if you run out of adjustment in one direction on the trim, then go the other. Even if it goes higher at first, as you keep turning, it should go back down.



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post #7 of (permalink) Old 02-27-2002, 03:35 PM
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Re: CB question

If you're talking about the adjustment on the antenna, it's just a brass screw in the tip. If you unscrew it any further, it just comes out completely, and there's no wire attached to it. The antenna is already on a spring at the base.

The reason I had to cut the factory crimped terminal off is to run the wire through a small hole leading out of the vehicle to the antenna. Regardless of it's effect on the problem (if any), I need a better style terminal (this one is a solderless screw on).

'96 ZJ with stuff
'82 CJ-7 with more stuff
post #8 of (permalink) Old 02-27-2002, 03:44 PM
 
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Re: CB question

this may contain info you already know, BUT firestik.com has a page of ant trouble shooting ideas. talks about testing with a VOM and stuff like that.

it taught me ALOT, and whenever I encounter High SWR, I refer back to that page.

I have a recurring HIGH SWR problem also on a base unit, Ive eliminated everything I tihk except for lack of suffeceitn ground plane.



OzarkJeep

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post #9 of (permalink) Old 02-27-2002, 03:47 PM
 
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Re: CB question

Try removing the base spring to see if it improves the SWR. If so, the antenna may be too long and require trimming. For test purposes you could also install something in place of the test spring that is shorter or longer, such as a piece of threaded rod with a nut to save unnecessarily trimming the antenna when it was actually too short.

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
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post #10 of (permalink) Old 02-27-2002, 04:11 PM
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Re: CB question

<blockquote><font size=1>In reply to:</font><hr>

If you're talking about the adjustment on the antenna, it's just a brass screw in the tip. If you unscrew it any further, it just comes out completely, and there's no wire attached to it. The antenna is already on a spring at the base.

<hr></blockquote>

No, I understood your reference to an "adjustment knob" as being on the radio.



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