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[TowerTalk] How accurate is EZNEC?

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Subject: [TowerTalk] How accurate is EZNEC?
From: (David Jordan)
Date: Thu, 06 Sep 2001 07:34:45 -0400

Built an interesting Labor Day special antenna...

Sort of an W8JK....

Two 140ft dipoles mounted 20ft above the ground and spaced 70ft apart.  I feed
them in phase with HB open wire line.  The antenna and feed line is made from 10
guage solid wire.  I used 5.5inch spacing for the feedline (I had a box of new
old stock Johnson ceramic insulators that were begging to be put into use).

At the center of the feedline (35ft) I have a 20ft drop of the same open wire
line that terminates into a HB ultimate transmatch.  I matched the feedling
using my MFJ259B  at the input to the matcher (saw 52ohms with 0 xl and SWR
1:1at 7.220.  The SWR at 7.130 and 7.310 is just under 2:1. So, it's a
set-it-forget-it antenna for my SSB phone operations.

I have an 80m dipole at 55ft that is also fed with open wire and I have been
using it on 40m for the past two years. It has been a super antenna. Reports
from 80m dipole on 40m have always been very good.  Always better than my
full-wave loop in the city.

I read that this low to earth array would be better for short skip on 40m which
is what I was looking for since I work a lot of mobiles and nets on 40m. It's
called an NVIS (Near Vertical Incidense Skywave) antenna.

The NVIS array seems to work even better than I could have imagined.  Typically,
the difference between the two systems for close-in stations (within 400miles)
is remarkable. The phased dipoles are about 5 -10bd better than the 80m dipole.
However, at 900miles out the dipole is "S"9 +10 and I can hardly be heard on the
NVIS (in the noise).  I've never seen such dramatic differences in low band HF
antennas but it sure is fun to be a witness of very high angle radiation versus
lower radiation patterns.

I wonder if EZNEC would have predicted these results?  I don't have any computer
modeling software so I have to wing it and believe me after spending 20hrs
hauling all that 10 guage wire around my property in the country I was pretty
anxious about the results, HAHA.


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