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Author Topic: DDRR antenna for 2 meter/70cm  (Read 2925 times)
K0PRP
Member

Posts: 25




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« on: January 03, 2006, 06:26:43 PM »

Does anyone have any experience with the small loop type antenna identified as a DDRR?

I have a height issue on a mobile installation. I've found this antenna but can only find two references to two meter. One is for a G-Whiz built by N3GUC (http://www.zelie.com/~geno/ddrr.htm) and an article in the Antennex site (http://guests.antennex.com/rooms/preview/2mddrr.htm).

Using Google, all I find are links to these two or references to use on 40 meters.

Does anyone know of other links or have any experience using the antenna?

Thanks

Paul P
K0PRP
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K5LXP
Member

Posts: 4536


WWW

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« Reply #1 on: January 04, 2006, 06:29:01 AM »

This might be what you're looking for:

<http://www.untenna.com/>

Mark K5LXP
Albuquerque, NM
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WB6BYU
Member

Posts: 13573




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« Reply #2 on: January 06, 2006, 01:13:23 PM »

I've built my own, and think I have one of the "Untenna"
models out in the barn somewhere (waiting for the proverbial
"round tuit" to fix it up and test it out.)

There are other commercial antennas that are straight
rather than bent into a circle, but otherwise work on the
same principle - these are sometimes called "shark fins".

In general they can be tuned over a wide frequency range
via some sort of variable capacitor on the end of the
radiator, but have a comparatively narrow bandwidth without
retuning.  Because of the high Q, they will have a low
radiation resistance, making them poorly suited for use
as a mag mount (which is the variety of "UnTenna" that I
have.)

If you want to build your own, I'd ignore the common
designs using a closed bottom loop and mount it on a
sheet of brass or printed circuit board instead.  From
this, the radiator can go up a couple inches (the higher
it is, the more efficient, since the vertical part does
most of the radiating) and the remainder running
parallel to the base plate.  The exact shape doesn't
matter - if you are using wire it can be run straight
or bent into various shapes.  Even a disk with a
circumference of 1/2 wavelength will work.  The end of
the radiator is a high voltage point, so some sort of
tuning disk capacitor is better than one with a solid
dielectric.

If you REALLY want a low height antenna, you can build
various types of annular slot antennas that are actually
flush with the roof, but that takes a bit more metal
working than I care to try on my car.


Will this really help?  I know one ham who had only 2"
of clearance between his car and the garage door.  He
ended up using a 1/4 wave whip that was flexible enough
to bend over that close to the base.  It survived several
years of driving in and out, and outperformed any
alternative that he had tried.

Good luck!
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