K-40 Center Loaded Antenna SWR Problem

Well first of all, you got a great buy if it’s the k40 antenna I’m thinking of. You have to remember that the same antenna can have a different swr reading on a different vehicle. As far as trimming the antenna will fix the swr problem?, I can’t answer that without more info. You say you checked the swr with the antennas whip almost completely down. What happens when you lower it completely down? Here’s how I’ve always adjusted my antennas, some may not agree with it. I always lower the steel whip completely down and then check my swr on channel 20 (center of the band). I take a reading and make note of it. Then I raise the whip up about 1/4th of an inch, and note the second reading. If the 2nd swr reading is higher then the first, I know the whip is to long. Then I remove 1/4th of an inch from the steel whip, and lower it completely down. Then take reading again and it’s usually lower then the very first reading. I keep repeating this procedure until I get the swr where It should be. Now when you first start this procedure and there’s no change on your swr readings, don’t start cutting the steel whip until you resolve the problem with the antenna system first. Hope this makes sense.

 

Wonderful, glad to help. Now you can enjoy your new antenna. For me this has always been the easiest way to figure out if the antenna is too long or too short.

 

It is very difficult to get a true ground plane with a mirror mount antenna, even though most trucks use them! Because of that, any new or different truck, or general positioning will require re-tuning. Load coils always cut effective transmission power, even though they can cure SWR issues. Using a full 1/4 wave (at center frequency) antenna without a load coil is always best but not usually practical. But hey, as long as your under 13-6, why not?

Now I'm a licensed HAM, highest license allowed and can transmit to satellite stations. We all are allowed to design and create our own antenna systems, and enjoy it when we have the time.

You might get yourself a HAM radio HF band antenna tuner for your CB system. You can then adjust and optimize for best SWR and transmission power on any channel while you actually use the radio. Most are automatic now days.

 

Yes, a whip is without a load coil. You can do the math on the length. It's 1/4 the wavelength of the desired frequency. While I was once a CB nut back in the mid seventies, and once installed commercial mobile communications systems, it's been a while, so I don't remember the exact lengths. The type of COAX you use also makes a big difference. Make sure you have the 50 ohm because there is a lot of 75 ohm used in video, and that will create havok with SWR. There are also some specialty COAX impedances as well for special systems.

Also, the shield of the COAX can be a big problem in any radio system. Today most COAX uses an aluminum wire and or foil shield. This stuff is TRASH! A big corner cut to avoid the cost of copper. The aluminum guarantees connection problems WILL occur sooner or later. Usually sooner. We used to always use copper jacketed cable and soldered all shield connections, which does take some skill. If you look hard enough, you can still find the good stuff. Road salt will trash today's COAX connections very easily. Good job security for all of the system installers though.

 

If you go 102 whip, be sure to use the big fat spring too, it needs to be 108" to work right.