[TowerTalk] NVIS

[Tim Duffy]

I have lots of experience with NVIS antennas and will share my observations.


I am involved with Western Pennsylvania traffic nets on the high end of 80
meters (75 meters) every morning. I have an NVIS half wave coax fed dipole
that is resonant at 3.988 MHz (1.1 to 1 VSWR). There are three 3/8 wave
radials below the antenna element on the ground. The dipole antenna is flat
(not inverted Vee) at 20 feet above ground. 

I have worked very hard to get my RX noise level to less than S zero.

I hear very well with this set up - even stations running less than 5 watts.
When we do state wide drills - I hear very good out to 650 miles. This NVIS
design works for me.

You can find me at 3.983 MHz at 9 AM Monday through Saturday and 3.9905 on
Sunday.

73
Tim K3LR



-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
Wilson Lamb
Sent: Wednesday, June 10, 2020 7:08 PM
To: undefined
Subject: [TowerTalk] NVIS

I'm claiming no expertise, but taking the nice figure with colored lines I
make the following observations:

7' is BAD, for everything.
28' wins the vertical race, out to 60 deg.
28, 42, and 56 are equal (minor rounding) from 60 to 50 deg.
Below 50deg, higher is always better, for the heights discussed.
Between 90 and 60 deg, 14, 28, and 42 are within a 3dB range.
At 30 deg, 28, 42, and 56 are within a 3dB range.
At 30 deg, 14' is down 6dB
The flattening pattern of the 56' antenna is down only 6dB from 28', at
vertical.
Since you aren't talking to yourself, I hope, things are better for the 56'
antenna as you go off vertical.

NOW, I have no idea of the height of the "layer" that may be in use for NVIS
and, therefor, the distance to stations enjoying signals radiated by
reflections from 90 to 60 deg from vertical.
Nor do I have any idea of the reflectivity of said layer and, therefor, how
many Watts would be needed to make it usable.

Conclusions:
Unless you REALLY NEED NVIS, you want the highest antenna you can get, up to
56' in this set.
Even if you really need NVIS, 400W on the 56' antenna is as good as 100W on
the best one.

This confirms the remark I saw "somewhere" that one gives up a lot of
everything else to get NVIS performance.
These data also show why a really low antenna performs remarkably well when
condx are good.
Tune across a band most anytime and look at signal strengths.  Then see how
many would have been unreadable if two S units weaker!

OK, tell me where I slipped up.  My two 80m antennae are at 55 and 65 feet.
I sure hope I don't have to lower them!
WL
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