Friday, 23 October 2015

Brain Bender

All week I've been trying to work out what this is supposed to do all week. It came from the base of the my vertical HF Antenna. It's some kind of inductor that has one end connected to the shield and the other end connected to the actual antenna. The centre of the coax is connected to the same point on the antenna through the middle of the inductor. To see me dis-assemble the antenna or if you have any suggestions keep reading past the break.

First thing to do was to get the big metal stick down. The thunder had just rolled away so I figured it would be safe enough.
No proper tools here. No point going to the shop just to get a tek bit. An 8mm socket works fine.
I finally got organised with a table out the back so I actually have space to work on a project like this.
Not usually a good start to dis-assembling something when I need to use pliers on knurled parts.
Turns out only one screw was holding the base of the pole onto the bracket. With that removed and a little twisting it all came off. So far so good.
Hmm. This one not so good. This bolt was a little bit burred and there was no way I was going to hold the the bolt still. So I tried to drill it out. Unfortunately the only drill bit I could find was a little too small for the bolt and only mangled the head of it. For my next trick I drilled out the nut instead. While not the conventional method as you can see it did work.
And this is where I started to get confused. But this is a trapped antenna so maybe there is a good reason for it.
But this is where I completely lost the plot. This inductor will be acting a bit like a transformer but I don't know if it would be making a significant difference. Being an inductor it's reactance(?) goes up with frequency. That means that possibly at the frequencies we are looking at it will look like an open circuit. But then maybe it could appear as a reactance which would produce some kind of voltage divider action.

Most of that's probably wrong. It's time to stop guessing and find out. Over the weekend I'll probably run some simulations or something and see how it would behave under various situations and I'll see if I can make up some useful graphs.

If you have any idea why my antenna has a coil in it's base it'd be great to hear from you.


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