In the May 2022 issue of QST, there's an article on a portable 20M "end-fed center-fed" antenna, where the author talks about it as if it's a dipole. For anyone who doesn't get QST, it's a free download: End-fed Center-fed 20-Meter Portable Antenna.
From the article, basic construction:
To me, that looks more like a Zepp, i.e. a wire radiator with a transmission line impedance transformer. But with the 1/4 wave wire, it ain't gonna work like a Zepp, and I can only assume that choke also does something with phase, such that the feed impedance is close to 50 ohms.
This is the 2nd time I've heard an end-fed antenna referred to as a dipole. Dave Casler also uses that wording, when talking about a typical end-fed halfwave, because the shield on the coax feed makes the other leg of the dipole (or, if used, a counterpoise, but then maybe you have a tri-pole). I think this usage is incorrect, but then I'm not James Maxwell. 🙂 Perhaps, at some fundamental level of physics, all antennas can be considered as dipoles, simply because there has to be a return path to the transmitter, even if it's just earth ground. But it seems to me to be non-helpful to use that terminology that way, because I think most hams, if they don't simply ignore it, will just get confused, rather than try to think about the physics of antennas. I admit that I'm uncertain about whether our traditional use of the term "dipole" has more to do with physical construction or fundamental RF theory. Maybe it's a little of both. But I'm uncomfortable with the fuzzing of terminology that forms a useful framework for practical discussions among hams, especially newcomers to the hobby. (How's that for a curmudgeonly approach? 🤪 )
So, what do y'all think?
I figured I'd post a follow-up, since this is crazy stuff.
This antenna is, in fact, a dipole. Whenever I have RF questions, I call on Bill - N0CU, who really knows this stuff. After a few e-mails, he pointed me to this article about how coaxial cable works. I knew about skin effect, but not how it worked in cable shields at RF. Wow.
So, the short version is that at the end of the 1/4-wave coax segment, the shield is cut, and the interior current crosses over to the exterior of the shield, thus turning the shield into the other leg of the dipole. Technically, this is common-mode current, and the choke keeps it from coming back into the radio, making it "end" similarly to how the wire of dipole isn't connected to anything at the end. Sure, there'll be some common-mode current coming back, because of course the choke isn't perfect.
I had no idea there are separate, different, currents flowing on the inside and outside of a coax shield.
And thus, this antenna is physically end-fed, and electrically center-fed!
Anyone interested in building one? Group buy?
- RG58 is 20 cents / foot at HRO
- 82pF 1kV mica cap, $4.45/ea with break to $3.586/ea at 10 pieces at Digikey
- T157-2 toroid, $4.20/ea at Amidon
