cb radio, antenna, tuning, swr help...

i have a cobra 29 ltd classic, with a firestik original so its the "trim to tune".
i was just adjusting the swr on my radio and tuning the antenna with it.

it says to place the switch on the "calibrate" position and hold the mic down and calibrate, then while still holding down the mic place to the "swr" position and you will be able to see if the swr is too high or not.

1st attemt without doing anything to the antenna: swr was barely too high
2nd attempt and trimmed about 1/8" off: swr slightly lower but ch 1 was still a bit high
3rd attempt and about another 1/8" off: swr went way up and now all the channels have a really high swr.

whats going on here? did i trim the antenna too much? i only trimmed about 1/4" off

thanks
-Shomsky

in the JP mag for this month, it has a really good article by Randy (the electrical guy) about tuning CB antennas

Shomsky462 said:

i have a cobra 29 ltd classic, with a firestik original so its the "trim to tune".
i was just adjusting the swr on my radio and tuning the antenna with it.

it says to place the switch on the "calibrate" position and hold the mic down and calibrate, then while still holding down the mic place to the "swr" position and you will be able to see if the swr is too high or not.

1st attemt without doing anything to the antenna: swr was barely too high
2nd attempt and trimmed about 1/8" off: swr slightly lower but ch 1 was still a bit high
3rd attempt and about another 1/8" off: swr went way up and now all the channels have a really high swr.

whats going on here? did i trim the antenna too much? i only trimmed about 1/4" off

thanks
-Shomsky

Click to expand...

I usually use a 3 channels to get an idea what's going on 40,18,1
Channel 40 is the highest frequency, 18 is sort of in the middle, and 1 is the lowest. Since the CB band covers about 1 mhz, there is almost no way of getting a flat response all across the band, so I tune for the middle and it slopes up on either side.

Like I said, Channel 1 is the lowest frequency, so that equates to a longer antenna. Channel 40 is the highest frequency and so that equates to a slightly shorter antenna.

How does that translate? If the SWR is higher on 40 than it is on 1, then your antenna is too long. Conversely, if the SWR is lower on 40 than on channel 1, your antenna is too short. Trim or add accordingly (it's just wire!).

Now, IIRC, fudgesticks use a fixed lenght (no trimming it as it is part of the matching network) of 52 ohm coax to do their magic right. You also need to make sure that you have a good ground at the antenna side, AND that the antenna is away from the body. If you don't start with that, it probably won't match no matter what you do.

More help here >> http://www.firestik.com/Tech_Docs.htm

ok so i probably cut the antenna too short then cause there is definitely a higher swr on 1 then 40. and i dont know exactly what cable i got. just one from a truck stop and didnt really know there were differences.

Zuki-Ron said:

I usually use a 3 channels to get an idea what's going on 40,18,1
Channel 40 is the highest frequency, 18 is sort of in the middle, and 1 is the lowest. Since the CB band covers about 1 mhz, there is almost no way of getting a flat response all across the band, so I tune for the middle and it slopes up on either side.

Like I said, Channel 1 is the lowest frequency, so that equates to a longer antenna. Channel 40 is the highest frequency and so that equates to a slightly shorter antenna.

How does that translate? If the SWR is higher on 40 than it is on 1, then your antenna is too long. Conversely, if the SWR is lower on 40 than on channel 1, your antenna is too short. Trim or add accordingly (it's just wire!).

Now, IIRC, fudgesticks use a fixed lenght (no trimming it as it is part of the matching network) of 52 ohm coax to do their magic right. You also need to make sure that you have a good ground at the antenna side, AND that the antenna is away from the body. If you don't start with that, it probably won't match no matter what you do.
More help here >> http://www.firestik.com/Tech_Docs.htm

Click to expand...

im pretty sure i just have a crappy coax or a bad mount.
what would be a good coax, cable combination or kit? its going to be mounted on the bed of my comanche at the back.

thanks for the help
-Shomsky

They sell the coax in a 18ft length. They didn't just pull that number out of their arse, 18ft is about a 1/2 wave feedline. The coax should say RG-58, or RG-8 on it.

Just for grinns, get out your soldering iron and add more wire to your antenna and see if you can get it to come around. Chances are though, if you are using an odd-lenght feedline, you may not be able to get it to go below 2:1.

Recheck your ground. The coax should make a ground connection at the antenna end, and the center should make a nice connection at the whip.

Edit: I just saw your second post.

Firestick sells an install kit. Depending on what your are going to use the CB for (Trail as opposed to DD), you may want to opt for a baseload whip like a Larsen or a K-40.

The Antenna should extend beyond the bed, so mounting it on the bed rail would be an option, but I would not mount it IN the bed itself. Best option really would be the fender cowl (under hood lip mount which mounts to the inner fender and the antenna appears to sit on the fender) or on tip of the cab. I use a Larsen baseload NMO mount which I have mounted in the place of the AM/FM antenna. It works nicely for what I use it for and puts up with getting banged around on the trail nicely.

Trimming your antenna? I've never heard of anyone trimming their antenna and I'm even a HAM operator.

:anon:nevermind

bosco01xj said:

Trimming your antenna? I've never heard of anyone trimming their antenna and I'm even a HAM operator.

Click to expand...

bosco01xj said:

:anon:nevermind

Click to expand...

:roll::roll::roll::roll::roll::roll::roll:

bosco01xj said:

:anon:nevermind

Click to expand...

we're all human.:doh: