20M "end fed" dipol...
 
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20M "end fed" dipole from RG58


Jed Baer
(@kd0ymg)
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Joined: 5 months ago
Posts: 27
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I have a 1st attempt at building the 20M dipole from coax, described by A5DX - Phil Salas, in the May 2022 QST. This the antenna I posted about, a while ago. Not 100% finished, as the next step is to find a spot to hang it up and check the SWR. Once that's done, I'll do a little more sealing up at the joint between the coax and the wire, as recommended by Phil.

I decided to put the choke on a platform, since I wanted options for using a line to hang it up, and also take strain off the BNC connector. I might look at some strain relief for the coax as well.

I used a Vishay 561R series NPO class 1 ceramic, instead of the mica cap in the original plan. I think the ESR will be acceptably low. If I can get my hands on an ESR meter, I'll measure it.

So far, the one thing I'd do differently is leave some of the center insulator hanging out the end where the long wire attaches, just because it'll be a little more neat. I don't know whether it matters for the performance of the antenna, but I guess I'll find that out, if this one is a dud.

If anyone has a nice backyard spot where I can get this up in the air, I'd appreciate it. e-mail is my call at arrl.net

20m ef cf 1

 

 


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Jed Baer
(@kd0ymg)
Eminent Member Trial
Joined: 5 months ago
Posts: 27
Topic starter  

Figured I'd do an update. I don't have the low SWR that Phil - A5DW showed in the QST article. I found a low dip in the SWR above the 20M band, and was able to move the dip down to 14.225 by soldering more wire to the end, but the reactance is inductive, so I don't see how I can both move the dip down by making the wire longer, and lower the SWR, since a longer wire adds inductance.

My RF Elmer, Bill - N0CU, has suggested try to tune the antenna (X=0) to resonance. Adding another foot of wire is no big deal, but I'm dubious. Maybe I'm misunderstanding the Smith chart. The printed charts I have indicate the top half is inductive.

a5dx 20m efcf 2
a5dx 20m efcf 3

Added: Curiosity got the better of me. Initially, I had thought I wouldn't worry about the VF difference between the coax and the wire, especially since Phil - A5DX posted that nice SWR plot. But I just looked up the VF for the JSC 3110 RG58 coax that HRO sells, and it's 66%, which is quite a difference from the assumed 95% for plain braided wire. Thus, it would seem that what I've built is an overly long off-center fed dipole. That doesn't fully explain things, but it implies that the coax shield, past the choke, should be 11'5.5" long, rather than 16'6". I'll likely test that out this Saturday.

This post was modified 3 days ago 2 times by Jed Baer

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