Homebrew Antenna construction discussion...

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Homebrew Antenna construction discussion...

Post  G7VGG on Mon May 04, 2009 8:02 pm

If you have links or other information about constructing homebrew Amateur Radio antennas, or perhaps on new experimental Antenna designs then please feel free to post them here.
73
Alf (G7VGG)ormation o

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The G7VGG "FAT JIM" antenna design for 6m and 4m...

Post  G7VGG on Thu May 07, 2009 12:45 pm

The need for a simple, high gain, vertical omnidirectional antenna to cover both 6m and 4m on a single length of coax led me to design an antenna I called the "Fat Jim".
I published the original design for the first time on the 70mhz.org (four metre band forum) on the 19/3/2009, as a yet to be proved design concept:



Experiments on my first prototype, made from 300 Ribbon cable and enamel coated 16 SWG wire, immedietly showed that the shorting stubs at the top and bottom of my original design were causing severe de-tuning problems and so I revised the design by ommitting them and subsequently built two more prototypes using the revised design.
After two weeks of expeiments I was confident enough in their performance to claim that the Fat Jim design actually works.
However, I still have'nt been able to obtain an antenna analyser and so it has been impossible to know where the prototype antennas where actually resonant. Therefore the tuning process of the Ribbon cable and wire protypes was almost completely hit and miss and also very labour intensive.
For this reason I decided that the only way to make the Fat Jim easier and quicker to tune was to construct one from Copper tube, elbows and T-Peices:



As you know, Copper tube is'nt as flexible as Aluminium tube, so there is a risk that the tube can get permanantely bent by strong winds.
Whilst 15mm Copper tube is more than strong enough to resist wind bending with self supporting 2m Slim Jims, where the overall length is only 1.5m, its not really strong enough to resist wind bending with self supporting 6m Slims Jims where the overall length is around 4.5m. So if the Copper tube Fat Jim antenna is to remain self supporting outdoors without the risk of getting permanently bent by the wind, I suggest positioning the 4m side at right angles to the 6m Slim Jim as this will greatly increase the strength of the whole antenna. The main problem with this is the added complexity of the feed bracket, but with a little ingenuity I'm certain this should present little difficulty to the keen homebrewer.
Unfortunately, I cant afford the tubing to build such a prototype right now and as an antenna analyser is still a must to tune it in properly it may be some time before I can carry out further experiements.
However, I welcome any offers to build a Fat Jim as long as the builder reports back on the results.
73
Alf.

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