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Sunday, 19 August 2012

Top Band, what happens there then?

Well,

Following some inspiration from my ever patient chum Vince, G0ORC, we built a "sloper" at his QTH for top band. The basic design was inspired from this site here:

http://www.mds975.co.uk/Content/amateur_radio_antennas_02.html

This is the site of Mike, M0MTJ. He has done some fine work on sloper antennas. I sent Mike some email to ask about the inductance being used in his Top Band antenna and he pointed me here:

http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html

Now this is an amazing site including the java based calculators. So assuming that my "sloper" is half a loaded dipole I have done some calculations (this is after the first experiment at the QTH of G0ORC).... and I have come up with this design based on the space available in my back garden.


So, the inductor was wound on a piece of drain pipe which has an outside diameter of 63mm. Using the extremely handy Ring Core Calculator:

http://www.dl5swb.de/html/mini_ring_core_calculator.htm

And using the part for "Air Cores" I calculated that using my drain pipe I would need about 30 turns to create a 42uH inductor. So using some hard drawn copper wire I had here I made the coil.

I got hold of some "flex weave" antenna wire and cut an appropriate length. The coil looks like this in reality:


And this is the ground arrangement; I've driven two earth spikes into the ground which I have soldered an old piece of pipe between. To this I have soldered the outer of the coax feeder and also the end of a wire which I have trailed round the garden at ground level:


The antenna looks like this, although its rather hard to see - its the wire on the left hand side of the picture:


You can see above that the pole is looking rather bent, the co-linear that's on the top could come down as I don't use that for anything now, so I really should try and put up a thicker pole with some less bendy joints! It will do for now I'm sure.

But much to my total disbelief, when I stuck the antenna analyser on the end of this new sloping antenna; this is what I saw:


So it seems by some complete fluke I have managed to create a top band antenna. If it actually hears any signals will be a different matter, but only time will tell.

I also took the beam down again this weekend and have added a plastic owl to the top; his name is Oliver... I'm hoping his presence will reduce the amount of bird droppings on my back sitting area! I also hope it won't deter birds from the garden completely. You can see him on the top of the pole here:



We shall have to see.

Fun though, egh?

1 comment:

  1. G'day
    Gus here wonder how wide the bandwidth of your 160M?
    my Ground here is sandy soil resistance may 120 OHM hi hi
    please let me know Thanks

    ReplyDelete