Foundations of Amateur Radio

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

Posted: 22 Oct 2016 10:00 AM PDT


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
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3