In a message dated 6/7/2002 5:28:53 PM Pacific Daylight Time, n3dok@pgh.net
writes:
> Subj:Re: [Antennaware] Re: NVIS -- modeling, mininec vs nec-2 disparity
To: <antennaware@contesting.com>
> Date:6/7/2002 5:28:53 PM Pacific Daylight Time
> From:<A HREF="mailto:n3dok@pgh.net";>n3dok@pgh.net</A>
> To:<A HREF="mailto:i4jmy@iol.it";>i4jmy@iol.it</A>, <A
> HREF="mailto:antennaware@contesting.com";>antennaware@contesting.com</A>, <A
> HREF="mailto:n4zr@contesting.com";>n4zr@contesting.com</A>
> Sent from the Internet
>
> Here is the information about the NVIS Antenna.
> http://www.vcars.org/CARL/NVIS.html Also this link is excellent:
> http://www.tactical-link.com/
>
Bob N3DOK>
> Why use an NVIS antenna?
> To work the Skip Zone {out to about 1000 miles}.
> Areas behind obstructions and in dense foliage.
> To hear the near in stations just beyond ground wave range.
> Great for Field Day and contesting as a "Gap Filler" antenna.
> Easy antenna for HF mountain topping and camping trips to get RF out of
> deep
> canyons.
> Limitations
> Must work frequencies below the Maximum Usable Frequency (MUF).
> This is not a DX antenna.
> Needs to use an antenna tuner for good match.
> Power Limited to about 200 Watts.
> Beverage Mode Losses limit achievable gain.
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>
Bob: Very good summary. Why the benifits of NVIS antennas have evaded so
many I don't know. I really like the horizontal quad loop as a NVIS antenna
as the Rr is about 3-4 times higher than a low dipole, it's quieter and a
more balanced pattern than a low dipole. I plan to run some tests when I get
moved to SD where I will have 3 sets of 2 horizontal quad loops cut for
160,80/75&40m over regular ground and then over quad loops on the ground of
varying length to give good grouind reflections and compare them. I have
room for this. The ground or loops act as a reflector and gives 4-6 dB gain
straight up. It reflects off the ionosphere and comes back to earth not
touching any ground on the way to the receiving antenna at first contact.
The RF gets there with the least loss and no skip zones. It does great
during the day also. At about .1-.15 WL the normal single loop Rr is
lowered to around 50-75 ohms. It's a great antenna.
I have a way using a 80/75m quad loop on 160m to get a 50 ohm load at the
end of open wire line of a certain length with the same pattern (1/2 wave on
80/75m-1/4 wave on 160m). I will write this up for one of the mags soon.
There is a simple way also to get a very low SWR over the whole160m band (or
use a 40m quad loop over the whole 80/75m band). I used the 80/75m loop also
on 160m 20' high in the late 40's to 1956 in SD, in 1998-99 in SD and the 40m
quad loop on 80-75m 20' high in Seattle from 1956 to the present. I call it
my "Slam Dumk Antnena." One of my best reports has been been "every time you
come on with that damn Slam Dunk Antanna, you knock the papers off my table."
I have some radials under it also here. It's a great Net Control Antenna
also.
If you operate a 2F, 3F, 4F etc and higher, the pattern is "End Fire"
with progressively more gain all the way around on each band. CQ about a
year ago had a write up of this antenna cut for 160m mounted on 50' poles fed
with open wire line and a tuner is needed to match the Z at the end of the
feedline. For ease of matching with a tuner at its end, I'd suggest an open
wire feedline 136' long. An 80m loop is resonant with a low Z on all bands
above although the Rr does rise.
It's a great Field Day Antenna. A suggestion is to use the poles on
Fooball Fields. Use a kite with 2 strings to drop a string over the poles
and use that to pull a rope over them to support the ends of the quad loop.
It can be a square or triangle but make it horizontal. 71' on a square side
for 80-6m or 140' on a square side for 160-6m. Vary the height from 25-100'.
I'd also like to try the "K7GCO 2X Magnum Model." It will be cut 284' on
a square side (full wave on 900 KHz). On 160m it will have a couple dB gain
end fire in a clover leaf pattern in free space and the additional and normal
ground reflection gain on 160m. Mount this at the top of the poles. This
should be a great 160m antenna to cover the US. Have this mounted on one end
of the Football Field and the normal 160m quad loop 141' on a square side on
the other end of the footbal field for dual coverage.
The 2X model will have even more progressive gain on each higher
band--all the way around. A better antenna just doesn't exist.
Another wrinkle is to have an all band vertical like the GAP mounted in
the middle of the loops--for complete coverage. I've done this. There is a
way to feed them together in phase or 90 degrees out for the Turnstile
affect. I'll build this in SD again and report on it also. I was hoping to
be there for field day this year.
A "Semi NVIS Antenna" is a 2 element quad or Raibeam at 30-40' high for
20-10m. The vertical pattern is about 50 degrees to the half power points
and it fills in the higher angle Reflection Factors as a result. It gives
optimum coverage 1000-3000 miles at a low cost of $$$ and headaches. The
next step up for performance is a 5-8 element beam 75-100' high at many times
the cost. k7gco
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