Topband: Adding radials, a question

[Richard Detweiler]

Hi Dave,

While I am no expert on the issue of radials,  There are many good articals, 
  I've found some of the best about vertical theory in dealing with the 
Butternutt Verticals.

http://www.bencher.com/pdf_download.html  ( at the bottom of this page is a 
very good section on Vertical theory under Tech Notes )

As I understand it for ground mounted verticals, as you put in more radials, 
the 'Q' gets higher.   Especially for for low bands (160 & 80) Thus as the 
radials increase, the radiation resistance increases ( the good kind ) and 
and the ground loss or ground resistance decreases ( the bad kind ).  Excuse 
my teminoligy here.  So the total resistance approaches the more perfect 35 
ohm vertical impedence.  Obviously, it can never be fully acheived but the 
point is this,  when the Q stops rising appreciably,  you've probably put in 
enough radials for the amount of wire you have in the air.

Of important note here is that the length of the radial isn't really 
important as the ground detunes them,  but the need to be somewhere about as 
long as the height fo the vertical.

The way I measued was to see how wide the 2:1 SWR frequency band width using 
259 analyzer.  The more narrow the 2:1 width, the better the radiation 
resistance will be. Then use a small amount of power,  20 to 50 watts to 
double check.  The 259 is very suseptable to AM radio stations at those 
frequencies even if they are not near by.

In my case the best answer was 40 radial going out 30 feet 9 degrees apart.

All of this changes fairly significantly when the vertical is raised above 
ground.  So keep that in mind.

Best 73's
Rich
K5SF







>From: W5UN <w5un at wt.net>
>To: topband at contesting.com
>Subject: Topband: Adding radials, a question
>Date: Thu, 02 Sep 2004 00:14:36 +0000
>
>I have a question for any of you who have experience with adding radials
>and making measurements of the changing impedance on a 1/4 wl ground plane.
>
>I have a 1/4 wl 80 meter GP made from #12 AWG copper wire, with a 3/16 wl
>radius ground screen beneath it on the ground surface. The vertical is
>suspended from a Dacron line strung between two towers approximately 250
>feet apart. As it is, the impedance of the GP measures 43+j0 ohms at 3.525
>MHz. I am thinking of converting this antenna to a "T" vertical on 160
>meters. I know that the ground plane needs some improvement for this, so I
>cut some radials to 1/4 wl on 1.8 MHz, and began to add them in groups of
>four. While doing this, I thought I might as well see how the additional
>radials were affecting impedance of the 3.5 MHz GP. To my great surprise,
>the impedance went from 43+j0 ohms with only the ground screen, to 48+j0
>with the ground screen plus four radials.  Then jumped to 51+j0 with eight
>of these new radials plus original ground screen. The resonant frequency
>did not change, nor did I expect it to.
>
>This seems to fly in the face of theory, which says that the antenna
>impedance should approach 36 ohms, or so, as losses are reduced to near
>zero. Can someone explain what is going on here?
>
>FYI, the ground screen consists of a grid of #14 AWG house wiring, on four
>foot centers, bonded at each crossing intersection.
>
>
>
>73, Dave - W5UN						w5un at wt.net
>
>homepage: http://web.wt.net/~w5un    Grid: EM23mg
>
>RR 10, Box 27
>Mount Pleasant, TX 75455
>
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