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Author Topic: Restoring an old antenna  (Read 1522 times)

Posts: 133

« on: August 10, 2008, 05:22:38 AM »

When cleaning my garage, I found a 2m Isopole antenna which was made about 20 years ago. A light application of's Aluminum Brite removed most of the corrosion, but I remember this antenna came with a conical radial which was made of sheet aluminum, and I sold the cone for scrap because it got all smashed up.

This antenna looks like it can work again, but I'd need radials for it. The stick is adjustable from 29" to 49", and the cone as I remember it was about 18" long. I can easily machine an aluminum collar to hold any number of rod radials at the base of the antenna, but I wonder if there's an equation I can use to figure out how long to made the replacement radials. Unlike the old cone, they will have to be perpendicular. The cone was roughly half as long as the antenna if the stick was adjusted for mid-length, but I also have a 10' Diamond vertical for 2m whose perpendicular radials are only 21" long and made of stainless. I have both stainless and aluminum rods, if one is better than the other. Should I just start out making the Isopole radials half the length of the stick and then go from there?

Posts: 2086

« Reply #1 on: August 12, 2008, 08:36:43 AM »

I had one many years ago. It was however a dual bander with two cones. It does seem to me that the 2 meter cone was about 18/19 inches long.

73 de Lindy

Posts: 17476

« Reply #2 on: August 14, 2008, 11:03:14 AM »

Actually there are two cones at various locations down
the mast from the feedpoint.  Each cone is 1/4 wave long,
though the cone shape means the exact length isn't too
critical.  I'd guess that 20" of aluminum would include
space to clamp it to the mast plus the flared out portion.

Basically the antenna is 5/8 wave above the feedpoint and
5/8 wave from the feedpoint down to the bottom of the
upper cone (or the ends of the radials if they are horizontal.)
The second cone/radials is placed 1/4 wave below the upper
one - this is not part of the radiator but serves to
decouple the RF currents on the mast.

So the actual antenna is the top whip (about 48") plus
the mast down to the top cone/radials (about 29") plus
the cone/radial length (about 19").  Sloping radials
(at least for the top cone) will increase the gain
slightly as they will add to the vertically polarized
radiation, which horizontal radials won't.  The lower
cone/radials provide a fairly effective RF choke for
the mast, helping to avoid unwanted radiation that can
mess up the pattern.

A quick set of radials might be two wires folded in
half with the folds secured to the mast with a hose
clamp.  At least that should be adequate to confirm that
it is working.

Commercial ground plane antennas that used two sets of
radials typically staggered them around the mast and
used different numbers of radials in each group to avoid
close coupling between the radials.  I've seen this
done with (I think) 4 sloping radials at the feedpoint
and 5 horizontal radials a quarter wave down the mast
for decoupling.
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