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Author Topic: NVIS antenna for 160m  (Read 1640 times)
SV1KGA
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Posts: 11




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« on: November 12, 2002, 08:41:14 AM »

I am coming back to ask for your thought on a NVIS 160m antenna. Is a shortened vertical base fed with the coil on the top and without capacity hat sufficient for NVIS?
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AC5E
Member

Posts: 3585




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« Reply #1 on: November 12, 2002, 08:55:40 AM »

Regrettably, a short vertical is an inefficent antenna on 160, albeit one with a low angle of radiation. It works OK for ground wave, but it's much better suited for DX than local work.

If you can manage it, a dipole at about 3 meters with a full length reflector wire at or just under the ground will give you the high angle of radiation you need for an NVIS antenna.

73  Pete Allen  AC5E
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K8KAS
Member

Posts: 570




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« Reply #2 on: November 12, 2002, 10:01:06 AM »

Verticals on 160m, short or long result in a lower angle of radiation and are usually 10 to 15 db down within 400km on top band. A dipole 30 meters(1/4wl) or lower is a good NVIS antenna on top band. I have great local signals with a dipole 20 meters high at night at distances of 75 to 150km. The short vertical you describe would require a very good ground system and a good loading inductor and base matching circuit if you expect any peformance at all out of it at all, long or short distances. The local big guns usually have full wave loops less than 1/8Wl high, these are great NVIS local 160 meter band antennas.
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WB6BYU
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Posts: 13567




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« Reply #3 on: November 12, 2002, 12:51:17 PM »

NVIS generally requires a horizontally-polarized
antenna.  The vertical will not work well in this
application.  However, if you add a horizontal wire
as a "top load" and the wire is close to a quarter
wavelength, you may get a reasonable amount of high-
angle radiation.

If you are short of space, I would recommend the
following:  Put up as long of a wire dipole antenna
as possible.  (Hopefully this would be 10 to 20m long.)
In the center, add a large enough inductance to make
the antenna resonant on 160m.  (A dip meter is handy
for checking this.)  Then wind a coupling link of
several turns of insulated wire around the loading
coil and connect that to your coax.  Adjust the loading
coil for the desired resonant frequency, and add or
remove turns from the coupling coil to get a low SWR
on the coax.

Using a loading coil in the center of a dipole is not
as efficient as putting the load closer to the ends,
but it avoids the problem of needing separate coils in
each side, and it makes it much easier to get a low
SWR on the antenna.
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