No folding of the braid needed here! All you need are some chokes, coax, and a bit of wire.
I saw this idea mentioned in several antenna books and thought I'd give it a try. Basically all I did was cut a quarter wave wire (234 / F Mhz) * 12 = inches -- and attached it to the center conductor of 50 ohm coax. The shield was left free.
Down the coax, I just choked it off a quarter-wave away down with some snap-on Radio Shack ferrites instead of hassling with folding back the braid etc. I didn't really have to take any velocity factor into account, since we're dealing with common-mode current that flows on the outer skin-depth of the braid - I just choked it at the point calculated for a quarter wave. So now I've got the two quarter-wave elements to make the dipole.
I used a variety of chokes, and the MFJ antenna analyzer agreed that this was a good thing. I'm not good enough at antenna modeling to try this in EZnec, although I think the choke is a win-win, as it serves to form one of the dipole elements, and helps to decouple the rest of the feedline from the antenna, preserving the pattern and perhaps cutting down on any inbound noise ingress on the shield.
I tried all of these chokes from Radio Shack and they had a pretty good effect when doubled up. They were all the snap-ons and fit around different sizes of cable.:
#273-105
#273-067
#273-069
The 067 and 069 series was new to me and very welcome as I also use these to choke smaller power leads and audio cables - even sometimes purposely using a quarter-wave choke on those for handhelds with rubber ducks. I guess in tough cases, you might want to use more than two like I did. In some cases like the 273-105, it still seemed to work even if it was a bit loose on the cable. I just tie-wrapped a little stopper to keep it from sliding.
I have only really built VHF versions, and my analyzer stops at 170mhz, so I'm not sure if these chokes are effective at anything higher, especially UHF. Has anyone tested these up there?
So far so good. It is quite a bit cleaner to just have the whole antenna in a vertical plane, unlike my others that had a feedline coming off perpendicularly.
I only concentrated on one band, but now that I think about it, I wonder if a fan-dipole might work if I placed other chokes at different 1/4 wave points along it.... then again, I might balk at putting $50 worth of chokes on it.
I saw this idea mentioned in several antenna books and thought I'd give it a try. Basically all I did was cut a quarter wave wire (234 / F Mhz) * 12 = inches -- and attached it to the center conductor of 50 ohm coax. The shield was left free.
Down the coax, I just choked it off a quarter-wave away down with some snap-on Radio Shack ferrites instead of hassling with folding back the braid etc. I didn't really have to take any velocity factor into account, since we're dealing with common-mode current that flows on the outer skin-depth of the braid - I just choked it at the point calculated for a quarter wave. So now I've got the two quarter-wave elements to make the dipole.
I used a variety of chokes, and the MFJ antenna analyzer agreed that this was a good thing. I'm not good enough at antenna modeling to try this in EZnec, although I think the choke is a win-win, as it serves to form one of the dipole elements, and helps to decouple the rest of the feedline from the antenna, preserving the pattern and perhaps cutting down on any inbound noise ingress on the shield.
I tried all of these chokes from Radio Shack and they had a pretty good effect when doubled up. They were all the snap-ons and fit around different sizes of cable.:
#273-105
#273-067
#273-069
The 067 and 069 series was new to me and very welcome as I also use these to choke smaller power leads and audio cables - even sometimes purposely using a quarter-wave choke on those for handhelds with rubber ducks. I guess in tough cases, you might want to use more than two like I did. In some cases like the 273-105, it still seemed to work even if it was a bit loose on the cable. I just tie-wrapped a little stopper to keep it from sliding.
I have only really built VHF versions, and my analyzer stops at 170mhz, so I'm not sure if these chokes are effective at anything higher, especially UHF. Has anyone tested these up there?
So far so good. It is quite a bit cleaner to just have the whole antenna in a vertical plane, unlike my others that had a feedline coming off perpendicularly.
I only concentrated on one band, but now that I think about it, I wonder if a fan-dipole might work if I placed other chokes at different 1/4 wave points along it.... then again, I might balk at putting $50 worth of chokes on it.
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