Ed,
This looks like a helical vertical. Given the spiral pitch it is resonant
around 5 MHz. If so, the base impedance is close to 12.5 ohms. Usually one
would place a 1:4 unun at the base or some matching network to transform 12.5
ohms to 50 ohms. However, another way to feed such a thing is to parallel four
50-ohm coaxial cables to create a 12.5 ohm t-line. Could this be the purpose
of the four coaxial cables? There may be an antenna tuner at the shack.
Without a tuner the useful bandwidth of such an antenna is about 100 kHz.
Dave Cuthbert
Linear Technology, Colorado Design Center
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From: [email protected] [mailto:[email protected]] On Behalf Of Price, Ed
Sent: Tuesday, April 10, 2007 7:08 AM
To: [email protected]
Subject: Unknown Antenna
Today's pop quiz is on antennas!
Specifically, does anyone know anything about the odd antenna? Here's a link
to a picture:
http://members.cox.net/edprice/Antenna.jpg
<http://console.mxlogic.com/redir/?2C-M
ONtASyUepvdw0Rvl8v6DmG7Y3sZ1qcHvwx2vMxL
aN4TvHTKCMMOUUCrKrGoxYi_GSbHkDo57OFeDNc
b__w09J5dxxwSyyesjhhKyMrhvdxe1m5j43h08gDY8qrDUvf0srhdECQn1NEVhjpd79LUUo>
Here's all I can tell you:
1. It's a vertical monopole, about 25 feet high and about 10" diameter.
Construction appears to be mostly epoxy fiberglass.
2. Yes, there is some kind of spiral (maybe coax) running part-way down the
outside from the top. I doubt that this is a circularly polarized beam,
pointed straight up, so the spiral must be some kind of broad-banding effort.
3. The antenna is sitting on a concrete base,
4. There are four heavy coax cables, and probably another control or power
cable, coming out of the ground and heading into the base of the antenna.
5. Those three toroidal humps near the base; mechanical re-inforcement or
loading coil covers or?
6. That wire near the concrete base is not a counterpoise, it's a roll of
concertina razor wire.
7. It seems like a lot of trouble for a receive-only antenna. The coax size
makes me think there might be several transmitters connected to the antenna.
8. The site had about a half-dozen of these antennas, with no apparent
spatial
relationship.
9. The diagonal yellow line is a guy rope for something unrelated.
10. And I can't tell you where it is or who pays the bills or how I got in
or
out. Sorry about the picture quality, but what can you expect from a nose
camera?
So, any guesses about frequency range and what this might be doing?
Ed Price
<mailto:[email protected]> [email protected]
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (FAX)
Military & Avionics EMC Is Our Specialty
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