Topband: 160 Antenna's Modeled

Tom Rauch
Thu, 19 Apr 2001 21:29:10 -0400

        I'm sending this again since for some reason Majordomo did not
copy me via E-mail.  It appeared on the archive page OK but here's
another copy in case the first one did not make it via E-mail.

                                                 73,  Bill  W4ZV

> I've been busy lately modeling a variety of 160 meter antenna's on
> EZNEC 3.0. For ground type I used real minimec and ground description
> (conductivity) medium, frequency 1850 kc. From what I understand real
> minimec approximates 32 1/4 wave radials. If you antenna modeling
> professionals see any discrepancies in my data, I'm still learning how
> to use the program.

Mininec does not, to my knowledge, assume 32 1/4 wl radials. A 
mininec ground assumes zero earth connection loss, but does 
include some induced losses near the antenna.  

One of the most misused bits of information in Eznec is TO angle. 
Too many people waste time paying attention to TO angle. 

It is the absolute level at a given angle that is important, not the TO 

Also, Eznec calculates FS at a very large distance and considers 
the earth "flat", not rounded. That's why you see no FS along the 
earth, even though we all know (or should know) that verticals have 
a very useful groundwave signal for quite a distance on 1.8 MHz.

The problem with the programs we typically use, and with results 
we report, is it is almost impossible to verify performance. It's a 
constant argument because it is almost always emotional data, 
how we "feel" about our systems compared to other unknown 
systems. is tough to sort out ten dB that way, let alone 5 dB.

As for actual wave angle, it is a lot lower than we often assume. I 
have had various dipoles at heights up to 310 feet, and at best they 
equal a 200 foot tower with 100 200 foot long radials. More typically 
the vertical wins. The break-even distance between a low dipole 
and my omni-vertical is about 200 miles at night time.

A 318 foot tall vertical ties the 200 foot vertical at about 1000 miles 
or more at night.

My conclusion is the wave angle is almost always a lot lower than 
we think even at modest distances, and the FS of a vertical at low 
angles is a lot better than Eznec predicts. 

The worse single investment I ever made was because I thought 
the 25-35 degree radiation peak of a dipole 5/8 wl high would be 
better than the low angle of a vertical.

As for radials, most actual measurements show a small elevated 
system is about 5 dB down from a nearly-perfect system of 60 1/4 
wl radials. Your results may vary.

73, Tom W8JI 

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