[Antennas] ground mounted vertical radials question

Sat Nov 11 21:17:32 EST 2006

Dave...
You have received a number of responses to your question...and this has been 
discussed at length on the reflector before... but the topic does bear 
discussion again on occasion. You will receive lots of shoot from the hip 
solutions to radials for vertical installs, but the plain fact is... there 
is nothing better than experiment to determine exactly what you need. When 
radials are elevated, the situation is far different from radials laying on 
the ground/ buried under the ground. Since you have selected to use the 
latter, my comments apply only to that situation.
There are a number of factors that are in involved to determine the number 
of radials you should consider... the biggest are ground conductivity and 
frequency. If you know the ground conductivity, there are modeling programs 
that will help you determine the right number. If you don't, there is a 
very simple experimental way to figure it out. You can think of the radials 
as the other half of the vertical antenna (this isn't exactly correct, but 
useful for this example)... you can connect a meter to the antenna (Autek, 
MFJ, etc) to measure impedance. Add radials and plot the impedance as you 
go for the LOWEST frequency of intended operation. When you get to the 
point of adding more radials and do not see a change in impedance, you know 
that adding radials will not change the antenna (electrically) any further. 
This is the point at which adding more radials will only impact radiation 
angle (a whole new topic).
The other variable to consider here is radial length. Curiously, the 
definitive work for radials was done back almost 75 years ago by George 
Brown in his investigation of broadcast installations. An excellent summary 
of his work...and others was written in July 2000 QST by Rudy Severens. 
Brown's experimentation shows that the current density in radials is a 
function of length and number together. His experimentation (and the eqns) 
show that a higher current density will occur further from the base of the 
vertical with more radials than less radials. In simple terms, it means the 
more radials the better... and if you have a choice... make them at least 
quarter-wavelength. More radials is more important than longer radials over 
one quarter wavelength.
Summary: 1) people that recite radial numbers without knowing your antenna 
design parameters (eg ground conductivity) are full of B.S., 2) radials 
should be 0.25 wavelength or longer, 3) more radials are better, and 4) use 
experimentation to determine exactly the number you need as outlined above.
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
Email: bill at wjschmidt.com
WebPage: www.wjschmidt.com
"If you drink... don't drive. Don't even putt" - Dean Martin.
----- Original Message ----- 
From: "David Ashworth" <fathom at dslextreme.com>
To: <antennas at mailman.qth.net>
Sent: Friday, November 10, 2006 11:37 PM
Subject: [Antennas] ground mounted vertical radials question
> Hello to all. Will be installing a Hustler 5BTV vertical on the ground, 
> up
> in the country where there is no problem with people tripping over 
> radials.
> The radials will NOT be buried. They will be placed upon the ground. 
> Been
> reading my reference material on radials, but would like to hear from
> operators that have been there/done that. The lowest band will be 80 
> meters
> and the highest 10 meters. Hustler recommends at least two radials per
> band. Then, been reading W2FMI's book about short verticals. Think he 
> was
> happy with about 40 radials. Am I on the right track, the more radials, 
> the
> better, up to a certain point? The soil there is very dry unless it has
> been raining. As soon as the sun comes out, there goes the moisture 
> content
> of the soil. So, would it be better to spread, say 40 radials at 1/8
> wavelength, or maybe 20 at 1/4 wavelength? Would you spread some for just
> 80, or do a combination of the different bands? If this was salt water, 
> bet
> you could throw a coat hanger in there, but it is not. Any good ideas? I
> am leaning towards less radials and longer radials, but experience is the
> best teacher. Thank you for your comments/recommendations, Dave, NC6P.
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