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Old 01-31-2016, 06:16 PM
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Why i get high swr only on extremes?

Installed a firefly antenna for a midland cb. I get 1.4 - 1.5 swr in channels 3 to 37 but 1-2 and 38-40 i get 2 or even 2.5. Any ideas? Bad grounding? Lack of patience tuning the firefly? Cheap swr meter? Thanks!

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Old 01-31-2016, 06:41 PM   #2
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That's ok, don't worry about it since it's clear your antenna is tuned since its lowest SWR is in the middle of the CB band. Some antennas just have a narrower bandwidth than others which means the SWR gets worse faster as you get away from the mid-point of the band than happens on an antenna with a wider bandwidth.

I make it a point to insure the antennas I tune have the lowest possible SWR on channel 20 and don't worry about or even check what it is on 1 or 40. If it's at a good low SWR on 20, that's as good as it's gonna get on channels 1 and 40.

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Old 01-31-2016, 06:43 PM
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Quote:
Originally Posted by Jerry Bransford View Post
That's ok, don't worry about it since it's clear your antenna is tuned since its lowest SWR is in the middle of the CB band. Some antennas just have a narrower bandwidth than others which means the SWR gets worse faster as you get away from the mid-point of the band. I make it a point to insure the antennas I tune have the lowest possible SWR on channel 20 and don't worry about or even check what it is on 1 or 40. If it's at a good low SWR on 20, that's as good as it's gonna get on 1 or 40.
thanks!
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Old 02-01-2016, 07:00 PM   #4
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What Jerry said is true, of course. Your antenna will operate best at one specific frequency, and it gradually performs worse and worse as you move further away from that "sweet spot" frequency in either direction. If you were to draw a plot of your SWR, it would be kinda U-shaped... and for best overall performance on all channels, you want the lowest point of the U to be right in the middle of your frequency range (ie--channel 20). Like this:


(NOTE: different antenna types have a narrower or wider U-shape than others.)

This is why most people suggest you test your SWR only at channels 1 and 40. Why? Because you want the SWR to be equal on these two channels--when it is, channel 20 will be at the "sweet spot" and thus as low as possible... no matter how narrow or wide the u-shape of your specific antenna.
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Old 02-01-2016, 08:39 PM   #5
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I think testing the SWR at one point, ch 20, is easier than testing it at 2 points, 1 & 40. Both methods end up with the same result, one takes an extra step.
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Old 02-01-2016, 09:55 PM   #6
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In general, I would agree with you, Jerry. The only reason I suggest testing at both ends of the spectrum is because it can be tricky to see subtle tuning changes at channel 20 whereas it is very easy to see tuning changes at channels 1 & 40 from even small antenna adjustments.

But, yeah.
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Old 02-02-2016, 09:38 PM   #7
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I have always check 1and 40 first, reason being that it tells you whether your antenna is long or short depending on which channel is higher.
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Old 02-03-2016, 03:11 AM   #8
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I also agree I check 1 and 40 then anything else if I feel like it.
I would rather check than have cut too short...You cant add length to metal or fiberglass without replacing it somehow.
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Old 02-03-2016, 05:55 AM   #9
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Yeats ago I moved my rig from a full size blazer to a Chevy celebrity and couldn't tune the antenna, showed that it was short. The antenna was a new Wilson 5000 come to find out that the celebrity did not provide enough ground plane and had to order a special whip that was significantly longer. Had I only tuned on Chanel 20 I wouldn't have known it was too short to start with and wasted a lot of time and a good whip that I sold to a buddy when he messed his up.

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