Coaxial dipole revisited with only Radio Shack chokes

[hertzian]

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.

[hertzian]

Here's a thought for owners of the so-called "Sputnik" RS antenna #20-176

How about removing the radials, and attaching 2-4 rf chokes 20 inches down from where the radials were mounted?

Mine is broken as it has come totally apart. May have to pick another one up and see. Could be very handy for indoor use without those radials..

[hertzian]

Best DX to date on homebrew choked dipole: Helicopters 50 miles away under visual flight rules. Yeah, it's working!

(Actually I'm running a 3/4 wave version mounted at the ceiling at 8 feet hanging down with high elevation angles. Happy!)

[N_Jay]

Did you do a test both ways?

[hertzian]

Testing was interesting, and I was encouraged that it followed the same overall setup as the horizontally-fed dipole(s).

The regular half-wave dipole works fine, although for my targets, I've got a lot of buildings in the way and I'm also down in a valley. So while I could hear them, they were just a bit weaker. I've GOT to get a receiver with a real S-meter. I'd love to live out in the clear or have a normal half-wave up on top of the house instead of the 3/4 low in the attic.

From the MFJ analyzer, I immediately got about 2:1 to 3:1 across the 118-136 band. This is without any trimming of the element or sliding the chokes around. Close enough for me for rx-only work - at least with my short runs of cable at VHF.

For the 3/4 wave element version, I used 75-ohm coax and got around a 1.5 - 2.5:1 swr across the band. (1/4 wave choked on coax) Again, close enough. Remove the chokes, and there's nothing but reactance. I'm using a 50 ohm device to 75 ohm cable, so reading will be off somewhat.

This made me laugh - when constructing another one, I had the receiver on, and only had the coax laying on the floor without the whip. Everything is nice and quiet. When I put the chokes on at the 1/4 wave point, the receiver came alive. Not great, but at this stage, I've essentially got a 1/4 wave attached to the shield due to skin-effect. Not recommended for permanent install, but maybe something for out in the field in a total emergency where no wire is around and all you can do is try and choke it.

I'd really love to model this in EZnec, but I haven't mastered transmission lines, or the choke as loads. So for now, I'm going by ear and what the MFJ tells me.

[hertzian]

What if I used the same common-mode technique for the main vertical element instead of wire? Any magic? Yes and no.

Instead of seeking magic, I just wanted to use a larger element than #18 speaker wire, but most importantly take advantage of the distributed capacitance of the coax to make it less sensitive to its surroundings indoors.

This time, instead of wire, I cut up a bnc jumper for the top element and then when done just barrel the two elements of coax together. (this was done to cut down on waste in case it didn't work - it does, so I could have used a continuous length without the barrel to make up the whole antenna.)

At the base of the jumper, I cut a 1 inch gap in ONLY the braid and trimmed it away making sure that there are no braid frog-hairs still crossing the gap.

I then cut the coax taking velocity factor into account (multiplied the calculated quarter wave by 0.66 for my solid-dielectric coax) and shorted the ends together as measured from the where the shield got cut at the top of the 1-inch gap. Great - that takes care of the inside of the transmission line - but because I'm using outer braid skin depth as the antenna, I had to make up for the shortening by attaching a small pigtail of wire to make it simulate just a plain wire (234 / f Mhz) * 12 = inches.

You'll see this technique used for so-called "double bazookas" on the HF bands where the bandwidth has increased. But I don't really care about bandwidth and want to utilize distributed capacitance.

The other element is the usual 2 - 4 radio shack chokes snapped around the coax a quarter wave down from the bottom cut of the braid at the 1-inch gap feedpoint described above.

Well, what do you know - the bandwidth has increased (usually an indicator of inefficiency, so I'm skeptical at this point). Yawn - I was covered pretty well already without this. I suspect that just the mere change from #18 wire to a larger outer diameter of coax is also helping contribute to the broadening.

But the goal of reducing the interaction with surrounding elements has improved! I have a very very weak ATIS station that I could knock out just by getting my hand near the former antenna with bare wire. Now, the station is just as weak, but I can place my hand anywhere on the coax and not make it drop out. Seems to be working!

The other good point is that this version seems to have the same vertical lobes just like my bare-wire whip version. I don't like surprises either good or bad.

This is a very long way to go just to reverse-feed a piece of coax to make it emulate a bare wire! (This is exactly how my "shielded coax loops" for HF work. The main advantage is the distributed capacitance for balance, not any sort of shielding, and that's what got me thinking about the vertical.....)

Where this thing is hanging is pretty free from obstacles, so I'm going to revert back to the wire. Performance is similar.

What it did do is give me an option for window mounting which happens to be an aluminum frame. This way I can try and reduce the coupling to the frame. I might even get brave enough to try the half-square broadside array made out mostly coax like this. Maybe next month!

[hertzian]

By total accident, the Icom R20 handheld helped me tune the chokes !!

It is one thing to see a change on the antenna analyzer when the chokes are attached at the 1/4 wave length down the coax. But what about something in the real world, that I can see?

To make a long story short, the RS #273-105 snap-ons definitely work at 127 mhz!

Initially I planned to use the R20's s-meter to do some adjustments on my standard weak test-signal - an atis station that I can make go away by merely looking at the antenna.

Using the S-meter didn't pan out, so I just put the antenna back on the Pro-136 desktop. Then I placed the whip back on the R20 (which is 23" long - a nice quarter-wave for aircraft) and put only one choke back on the coax. When I accidentally swung the whip near the base of the antenna - it made the atis signal totally drop out. Take the choke off, and I can NOT detune the bottom element and the atis signal returns weakly.

Ok, looks like this resonant whip of the R20 was detuning what was a 1/4 wave wire in the making on the coax antenna. Here it comes:

With only one choke installed, I could detune the coax antenna by holding the whip parallel about 3 inches away from the bottom element.

With 2 chokes, detuning took place about 9 inches away.

With 3 chokes, detuned at about 14 inches away.

With 4 chokes, detuning was at about 18 inches away!!

Then I ran out of the 273-105 chokes.

Unbelievable - something is working! It didn't improve my weak atis signal that much really, but I'm glad nobody saw me sitting on the floor waving that whip around like a crazed orchestra conductor.

I also tested some of my other chokes, like 4 to 6 #273-069's, and they didn't perform nearly as well. So for me, I'm going to stick with the 273-105's. I believe they might be "type 43" ferrite material, so I may go looking for cheaper alternatives.

Nice to see the choking action actually working by accident. Next best thing to lobe-spray.