An End-Fed Dipole?

Dipole antenna
Voltage and current distribution in a dipole antenna. From AA5TB.com

I’ve seen some comments on social media and elsewhere commenting on how the label “end-fed half wave dipole” is an oxymoron, and if an antenna is end-fed, it can’t be a dipole, and if it’s a dipole, it can’t be end fed. I’ve thought about this, and while I disagree with this assessment, I understand it. A typical dipole has two radiating elements, and if it’s end-fed, there is obviously only one radiating element. Let’s leave aside the fact that an end-fed antenna still needs a counterpoise to work effectively.

However, I think the name dipole refers to the electrical, rather than physical characteristics of the antenna. If you recall your license manual, the dipole antenna has equal voltages of opposite polarity at either end – a positive and negative pole, if you will. Thus, the name dipole refers to the two electrical poles, rather than the two pieces wire. In fact, the reason that most dipoles are center fed is because that’s where the lowest impedance occurs. But there’s no reason it has to be fed at that point, if you also add a matching network of some type. Off-center fed dipoles are a good case in point. As you move from the center towards the end, the impedance increases. Carry this mental exercise out to the end, and you can imagine the feedpoint being at the end of the wire, and the impedance being very high. At least that’s my thinking — I’m still a little tentative in my thinking.

Anyway, I’m still pretty new to ham radio and electronics in general, so I decided to ask the expert: ARRL’s antenna doctor himself, Joel Hallas, W1ZR. Here’s what he had to say:

Scott,

I think an end fed half-wave antenna can be called a dipole, perhaps an extreme off-center-fed dipole, although calling it an end-fed half wave might be more descriptive.

Keep in mind that a single end-fed half-wave doesn’t really quite work. There needs to be something else for the other side of the source to hook to. If it isn’t provided, the antenna system will provide something, usually common-mode current on the transmission line. It is usually best to provide some kind of choke on the transmission at least a few feet from the feed. Then it really is clear that it is an OCF antenna.

GL & 73, Joel

Joel R. Hallas, W1ZR

So I guess we can all just agree to say “end-fed half wave antenna” and leave it at that. In the end, the important thing is that you get on the air and make some contacts, right? If it works for you, who cares what it’s called.

If you’re interested, check out AA5TB’s excellent write up on end-fed antennas.

73,

Scott WZØW

One thought on “An End-Fed Dipole?

  1. I believe you’re right in stating that the term DiPole is brought over from an electrical standpoint. Electronics is very specific and to the point when it comes to defining anything electrical. It has to be.

    Some great examples of common misinterpreted antennas, are dipoles on handy talkies, or our ever popular BuddiPole that many of us like to use. The buddipole is a great example because you actually have to lay out the counterpoise. The length of the counterpoise is determined by the frequency range you plan to transmit on. I usually lay my counterpoise at 1 quarter the wave length. or approximately 20 feet of counterpoise for 20 meters.

    Which brings me to the next part of my comment. A wave length is determined by the frequency in use. So if and antenna is labeled as being a halfwave antenna. Its proximate length would be half the wave length of the frequency. So if you’re transmitting on 14.070 (also referred to as 20 meters). The length of a half wave dipole antenna would be approximately 10 meters.

    Here is a Youtube video that explains how to determine a wave length of a frequency. Once you determine that you can use it to build your antenna design.

    https://youtu.be/iPU7itDdpEs

    enjoy.

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