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102" whip and swr

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#1
I am setting up a Cobra 75wx in my pick up to use during hunting season, and need some help. My antenna setup is as follows. I have a 102" ss whip mounted on a Workman single groove mirror mount with a SO-239 stud that I actually screwed to the inside edge of my truck bed about 4 inches from the front of the bed on the left side rail. I connected my CB and antenna with an 18' Astatic Mini 8 cable coax cable. I used an Astatic PDC1 to check swr and decided to add a Workman KA52 barrel spring.

Here's where I need some help. I added the spring because my Ch1 swr was higher than my Ch40 swr and the spring brought my Ch40 swr down below 1.5, but my Ch1 swr only moved down below 2. Should I try and get it down lower than that on Ch1 or should I just leave it alone and be happy with what I have?
I can add a picture if needed.
 
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#2
The spring is effectively adding length (among other changes) to the antenna, so it is fairly easy to re-trim the antenna to a lower SWR. Whether that's worth your bother is something else again. With a plain stainless whip, tuning it would mean chewing off little bits from the tip and most of us would be reluctant to do that.

Another option might be to use a 3' or 4' helical stick (like a Firestick) which is actually a longer wire, helically wound on a fiberglass rod so it is not as long. These are often underappreciated but helicals tend to work damn well without having the worries about hitting incoming geese out of season.(G)
 

NC1

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#3
Trimming the antenna down will make channel 1 worse.

Since installing the spring lowered the frequency, you may want to look for a connector to go between the threads on the antenna and into the top of the spring. That should give you a little more length and lower the SWR somewhat more.

I used to have one of those connectors around here for just that purpose but cannot find it to post a picture.
A search through a CB retail site or eBay should turn up something.
 
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#5
My antenna stud came with a barrel that I took off at first when I added the spring, but then I put it back on to get my Ch1 swr down(which it did some). But like I asked is it even worth fooling with? Should I just be happy or keep working?
 
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#6
The spring is effectively adding length (among other changes) to the antenna, so it is fairly easy to re-trim the antenna to a lower SWR. Whether that's worth your bother is something else again. With a plain stainless whip, tuning it would mean chewing off little bits from the tip and most of us would be reluctant to do that.

Another option might be to use a 3' or 4' helical stick (like a Firestick) which is actually a longer wire, helically wound on a fiberglass rod so it is not as long. These are often underappreciated but helicals tend to work damn well without having the worries about hitting incoming geese out of season.(G)
Haha, I hear you on the geese, but several members of my hunting club have tried the firesticks with not so great results. They have all ended up going back to a 102" whip.
 
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#7
Mounting an antenna too close behind the cab will cause issues. Since 2 or 3 feet of the antenna is in close proximity to the sheet metal of the cab, you are probably seeing some issues from that.
Moving the antenna a bit further back would probably improve things a bit.
Making the antenna a bit longer, as others have suggested, is a good idea. You should be able to go to a hardware store and get a 3/8" x 24 threaded rod coupler and a short length of threaded rod (you'll likely need to trim down a longer piece with a hack saw) to lengthen it a bit.
 
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Beamsville, Onatrio
#8
Mounting an antenna too close behind the cab will cause issues. Since 2 or 3 feet of the antenna is in close proximity to the sheet metal of the cab, you are probably seeing some issues from that.
Moving the antenna a bit further back would probably improve things a bit.
Making the antenna a bit longer, as others have suggested, is a good idea. You should be able to go to a hardware store and get a 3/8" x 24 threaded rod coupler and a short length of threaded rod (you'll likely need to trim down a longer piece with a hack saw) to lengthen it a bit.
I have to agree with mmckenna. If you check out my post "CB antenna question" you'll see a picture of my home made mount that sounds kind of like what you described just yours is a little lower. This did not work for me, caused me very high swr.

I used a stake pocket mount with my 4.5 ft whip I got off a freightliner and it instantly got my swr down to 1.3 on CH 1 and 1.1 on CH 40.

Just my 5 cents since pennies are obsolete in Canada.
 
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Hobart, LA
#9
This picture is a little dark, but the barrel on top of the spring is 1 inch tall. There is a piece of all thread thats its screwed onto and if i unscrew it all the way its about an inch long. With it like that the Ch1 swr is around 1.7 and the ch40 swr is around .8, so i'm thinking if i can make a barrel at least 2 jnches long i should be good.

Sorry i cant seem to turn the picture the right way.
 

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Sep 11, 2016
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Hobart, LA
#10
I have to agree with mmckenna. If you check out my post "CB antenna question" you'll see a picture of my home made mount that sounds kind of like what you described just yours is a little lower. This did not work for me, caused me very high swr.

I used a stake pocket mount with my 4.5 ft whip I got off a freightliner and it instantly got my swr down to 1.3 on CH 1 and 1.1 on CH 40.

Just my 5 cents since pennies are obsolete in Canada.
I had already read your post before i posted, and yes my setup is very similar to yours. If i may make a suggestion to you though, you can run your cable up through the bed and have it come out under the plastic rail cover. That will keep the cable from chafing where it comes over the bed, and reduces the possibility of it being pinched in between the bed and the cab. Nice fab work on the mount btw, I have used the single groove mirror mounts several times in the past to mount antennas because they are cheap and very sturdy.
 
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1158 W. Valley Circle, Ash Fork, AZ 86320-482
#12
But like I asked is it even worth fooling with? Should I just be happy or keep working?
Trying to bring the SWR down, any more than it is, is probably not worth the effort. Transmitters usually don't start folding back the power until the SWR is over 2:1.

All of your transmitted power, minus the loss in the cable and loss due to SWR, will be radiated by the antenna. With only 18' of coax, your loss is going to be less than 0.30 db. If you then add in the loss due to SWR, it's only going to add less than 0.10 db. So, overall, you will lose less than 1/2 watt, between the radio and the antenna.

Just leave it the way it is.

Martin - K7MEM
 

wa1nic

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Westfield, Ma
#15
The numbers hint that it wasnt to be longer but if you have never cut it that is not likely needed.

The proximity to the cab might be causing your problem, but to be honest, typically anything under 2 is "good enough".

If you takes some insulated thing (broom handle maybe) and move the antenna closer to and further from the cab, does the SWR change?
 
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