What happens if you move the feed point in a dipole?

Foundations of Amateur Radio

One of the recurring topics in on-air discussion is that of antennas and if we were to graph the topics of conversations, antennas would be the clear winner in any line-up. As a beginning Amateur this phenomenon bamboozled me for a very long time.

Why are these people talking about antennas all the time and what's there to know that you can't say in 30 seconds?

From the mouths of babes...

I've mentioned in the past that Amateur Radio is to a very large degree magic. Another way of expressing that is to say that there is an Art to being an Amateur and antennas play a big part.

A friend of mine loaned me his antenna kit called a Buddipole. It's a portable set-up that is akin to Meccano or Lego in that you can build up an antenna from parts and make a large range of antennas from the same basic parts, two coils, a feed point, a balun, two telescopic whips and some extension pieces.

For me this particular antenna has been temperamental and I couldn't get my head around how to make it work. This all changed last weekend when I had a spare 15 minutes, literally, 15 minutes when I went into the shed to have another look. This was spurred on by a note that I'd read that pointed out that the Buddipole is asymmetric, that is, both legs and coils are not the same.

This important tid-bit of information made things click in my mind and all of a sudden I realised that I didn't need to make both sides the same length, or adjust both sides in the same way.

Until that moment I'd always thought of the Buddipole as a dipole on a stand and expected like any traditional dipole it would have both legs at the same length.

What if you could move the feed point along the length of your dipole, what would happen? What if you kept the overall length the same, but by making one end longer and the other end shorter, you in effect were moving the feed point along your dipole?

Wonderful things start to happen, that's what.

What I'm saying is that you don't have to make a dipole have equal length legs and that sometimes this is desirable.

Previously I've mentioned that the height of a dipole, the wire thickness, the ends, the angle and so on all affect the feed point impedance. Turns out, that where you place the feed point also affects this.

If you recall basic antenna theory, you might recall that the middle of a dipole is the lowest impedance and that the end of a dipole is the highest impedance.

Each of these values are on a continuum, that is, they vary as you change things. That means that between the two extremes of impedance there are other in-between values. If you have a balun, you can use this to get a great match for your antenna by tweaking these values.

Another example of this continuum is a loop antenna. If you make it twice as high as wide, the feed point impedance is 50 Ohm, but if you use the same loop and squash it flat, the impedance is 300 Ohm. Varying the shape changes the impedance.

In essence this means that there is an infinite number of antennas that can be made just as a dipole and another infinite number of antennas that can be made as a loop.

So, just two antenna types alone already gives you a lifetime supply of options and that's ignoring the height, soil or wire.

Now you understand why antennas are tricky and why we talk about them so much.

It also explains why the Internet is full of different explanations on antennas, since they are all based around the local conditions under which the author is describing their adventure.

Next time you hear an Amateur going on about their antenna, perhaps there's something to take away.

I know I won't be anywhere as impatient listening to others talking about their contraptions.

Final thought. A vertical is a dipole too. The radials are one half, the vertical the other. You can change the length of either, or both, but you can also feed the antenna in a different location.

I'm Onno VK6FLAB