Re: [IRCA] Antenna Project

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

I've had some time to work on the copper tube flag antenna.  The current
configuration is a three foot square loop made of 1" and 3/4" copper tubing.
The vertical uprights are split in the middle and a length of 3/4"
fiberglass rod is inserted to make a roughly 3" gap.  Right now I have two
9:1 transformers feeding to two SO-239 jacks, one from each side.  These are
symmetrical.

Here's a picture of the antenna:
http://www.am-dx.com/Graphics/miniflag.jpg

And, one of the two identical feeds.  Sorry it's a bit fuzzy:
http://www.am-dx.com/Graphics/flagfeed.jpg

I spent a fair bit of time with various resistors, but could only come up
with about a 20db null.  It is a nice cardioid, though.  I also tried
hooking the two outputs to a phasor.  That really didn't work at all, for
some reason.

The best results were obtained by connecting the output to one side, and the
other side to an old home-made antenna tuner.  This provided a good null on
several locals.  Here are the results:

Station    Null    180 degrees from null

WPRO-630   S-7        S-9 +32db

WSKO-790   S-8        S-9 +38db

WHJJ-920   S-8 1/2    S-9 +30db

WRNI-1290  S-7        S-9 +28db

What I did was to get the best null on the station, record the reading, and
then simply turn the antenna 180 degrees without changing anything else.
That gives a pretty good measure of the front to back ratio of the antenna.

Gain is very good.  I am using a DX Engineering preamp, but it probably
wouldn't be needed for most use.  The receiver is a Drake R8B.

Next I will make a specific tuner with a variable pot in series with a coil
and variable capacitor.  This will give the desired complex termination.  I
suspect this needs both a resistive and reactive component to work properly.

Overall, I am very pleased so far.  It's been worth the effort.  Full
article to follow soon.

Craig Healy
Providence, RI


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