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Author Topic: "shower head" propagation???  (Read 1048 times)
KB0TXC
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Posts: 79




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« on: August 27, 2013, 04:28:56 AM »

Hi again,

I have a friend who served in the US Army in the eighties and nineties in Germany. He was a staff Sgt, in charge of comms gear for his unit. When he retired, he showed me an antenna that was being tested for the armed forces (it had not been officially deployed), that was basically a spool of two wires that was unwound to the correct length for the frequency being used, in a dipole configurartion. Nothing unusual. However, it was meant to be laid on the ground! I made a statement that the radiation angle would be almost straight up, and he said "yeah, its called "shower-head" mode..."

Does this sound real or was I being BSed?

Best,

Joe KB0TXC
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KI5FJ
Member

Posts: 40




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« Reply #1 on: August 27, 2013, 04:44:26 AM »

Joe, Many years ago I read the military manual for a on-the-dirt antenna. This version was available for the USCG.
If I can remember correctly the wires formed a H. The design is used to limit the signal strength at distances not near the antenna.
NVIS propagation. 73 Joe O NNNN
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WX7G
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Posts: 6079




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« Reply #2 on: August 27, 2013, 09:46:12 AM »

A friend of mine has the on-ground Army antenna. He reports that it doesn't work very well on 75 meters.
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WB6BYU
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Posts: 13288




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« Reply #3 on: August 27, 2013, 11:49:10 AM »

The Army and other groups used such antennas long before the term NVIS came into
prominence.  High angle radiation as obtained by laying an antenna on the ground (or
mounting it at a low height) has a number of advantages for military applications:

1) more difficult for the enemy to DF signals

2) easier to set up and less noticeable under hostile conditions

3) less ground wave propagation that may cause interference at short distances
compared to higher antennas

The downside is lower efficiency, but if you have enough power available and
choose the proper operating frequency it can work fairly well.  Over some poor
soils such as sand the effective ground may be well below the surface, and such
antennas will work like a higher dipole.

The resonant frequency depends a lot on the earth conditions, so most antennas
are used with a built-in antenna tuner anyway.

So, yes, it can be done, especially on bands below the Critical Frequency (typically
160m to 40m, depending on current conditions) for relatively short range paths.
But mounting the antenna higher in the air nearly always improves signals.


And it isn't just the US that uses such antennas:  here is a Russian version, called
a "spreading antenna", used to restore communications following damage to an
antenna structure or during repairs, etc.:
http://www.antentop.org/008/files/sas008.pdf
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KB4QAA
Member

Posts: 2382




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« Reply #4 on: August 27, 2013, 12:46:53 PM »

Shower Head Antenna theory is all wet!

In all seriously, laying it on the ground will probably work, but the earth losses are maximized.  Raise it 10-20 feet and it's an ordinary NVIS.
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