[Antennas] ground mounted vertical radials question

[Dr. William J. Schmidt, II]

This is a repost of my original note.  It is valuable to understand the 
underlying physics and do some experimentation...

----- Original Message ----- 
From: "Dr. William J. Schmidt, II" <bill at wjschmidt.com>
To: "David Ashworth" <fathom at dslextreme.com>; <antennas at mailman.qth.net>
Sent: Saturday, November 11, 2006 8:17 PM
Subject: Re: [Antennas] ground mounted vertical radials question


> 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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>