The biggest category subject in Ham Radio publications and on-air discussions is Antennas. Here is a very interesting excerpt on Antenna impedance from Onno, VK6FLAB’s podcast called Foundations of Amateur Radio, October 23, 2016, episode.
Foundations of Amateur Radio by Onno VK6FLAB
One of the recurring topics in the 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. 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. 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? 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. 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. 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: You can change the length of either or both, but you can also feed the antenna in a different location.—I’m Onno VK6FLAB
The full episode is here: http://podcasts.itmaze.com.au/foundations/
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