Help tuning CB Antenna
JK Jedi Master
Joined: Mar 2007
Posts: 14,800
Likes: 374
From: Midwest City, OK
I'm not certain about this. A ground plane needs to be below the base of the antenna. With his two-foot antenna mounted on the third break light, the mass of the softtop frame (you can ignore the fabric--it's virtually invisible to RF) is mostly to one side of the antenna. Therefore, it's going to work more like the forward elements on a directional antenna array. More power will be thrown forward (which may be desirable). However, some will be reflected because the frame is not "tuned" for the specific frequencies involved. IOW: It's going to take an antenna engineer (and he's going to want to run tests) to determine the affect that the softtop is having on the antenna. The best advice in this situation: OP has what he measured. If he's unhappy with that, the best thing for him is a longer antenna that puts more of it above the roofline.
JK Jedi
Joined: Aug 2007
Posts: 5,813
Likes: 0
From: Kenai Peninsula, Alaska
I'm not certain about this. A ground plane needs to be below the base of the antenna. With his two-foot antenna mounted on the third break light, the mass of the softtop frame (you can ignore the fabric--it's virtually invisible to RF) is mostly to one side of the antenna. Therefore, it's going to work more like the forward elements on a directional antenna array. More power will be thrown forward (which may be desirable). However, some will be reflected because the frame is not "tuned" for the specific frequencies involved. IOW: It's going to take an antenna engineer (and he's going to want to run tests) to determine the affect that the softtop is having on the antenna. The best advice in this situation: OP has what he measured. If he's unhappy with that, the best thing for him is a longer antenna that puts more of it above the roofline.
If the OP was to get a tuneable antenna, and if he still gets the same results, there is a good chance though that he could tune the antenna for a "happy medium" swr (under 2) with the top up as well as down.
Or......he could at least take readings and make adjustments to the antenna for best swr with the top up and down, and make position marks on the antenna. Then it would only be a matter of loosening the set screw and move the antenna to the mark when he puts the top down.
JK Freak
Joined: Mar 2009
Posts: 699
Likes: 0
From: Omnipresent
I'm not certain about this. A ground plane needs to be below the base of the antenna. With his two-foot antenna mounted on the third break light, the mass of the softtop frame (you can ignore the fabric--it's virtually invisible to RF) is mostly to one side of the antenna. Therefore, it's going to work more like the forward elements on a directional antenna array. More power will be thrown forward (which may be desirable). However, some will be reflected because the frame is not "tuned" for the specific frequencies involved. IOW: It's going to take an antenna engineer (and he's going to want to run tests) to determine the affect that the softtop is having on the antenna. The best advice in this situation: OP has what he measured. If he's unhappy with that, the best thing for him is a longer antenna that puts more of it above the roofline.
Thread Starter
JK Freak
Joined: Aug 2009
Posts: 500
Likes: 0
From: North Hills, CA
FYI, even with the 2' antenna I have almost a foot extending past the roof line. I have the antenna way up on the third brake light. My only concern is the drastic difference between top up and down puts me about 3 swr and I know that is bad for my radio. I will have to try a new antenna latter.
