Towertalk
[Top] [All Lists]

Re: [TowerTalk] TowerTalk Digest, Vol 168, Issue 64 raised radials

To: towertalk@contesting.com
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 168, Issue 64 raised radials
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 25 Dec 2016 13:37:13 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Just to clarify -- the primary reason for adding more radials is to reduce ground loss. N6LF has shown that 1) the more equally the current divides between radials, the lower the total loss will be; 2) a greater number of radials tends to improve that division; 3) making elevated radials slightly shorter than resonant tends to improve that division; and 4) as the number of radials increases, the current divides between them, but because power is I squared R, the total power lost in the radial system drops in proportion to the number of radials used. Rudy also showed that we don't want radials longer than a quarter wave but shorter than a half wave, because that range of lengths will produce a current maximum on the radial at some distance from the feedpoint that is actually greater than the current at the base.

An antenna like this can be seen as a simple series circuit, where the radiation resistance, Rr, is in series with the wire resistance, Rw, and the loss coupled from the earth, Rg. Rr is determined by the electrical height of the antenna, and I squared Rr is the radiated power, while I squared (Rw + Rg) is the loss. Rr is much lower than 50 ohms, so a lossy ground (radial) system will look like a great match, while increasing the number of radials will increase the SWR.

I like the suggestion, made by others, to tune the length of radials by measuring pairs running in opposite directions (and, per N6LF, tuning them a bit high in frequency). Some modeling I did years ago in NEC showed that radials buried or laying on the ground typically have VF in the range of 0.7 - 0.75, depending on soil, but that VF rises quickly as radials are raised, so that by the time you're 3-4 ft off the ground VF is getting pretty close to 1.

73, Jim K9YC

On Sun,12/25/2016 11:00 AM, Marv Shelton wrote:
Elevated radials.
I know all the science is good (I am an engineer after all) but what I found 
easiest to do when I constructed my 160m inverted-L was to attach a 500' spool 
of wire to the base and reel out the wire along previously installed insulated 
posts, until my analyzer indicated resonance at the desired frequency.  I then 
cut off the wire from the spool and terminated it. I plotted the swr curve to 
verify things.
I repeated the process for as many elevated radials as I had room for. The 
result of adding radials was to widen the useable bandwidth of the antenna.

I think this 'practical' method is advantageous because it instantly accounts 
for all the difficult to determine factors and variable that effect a 
calculated system design (ground resistance/loss, height of radials above 
ground, twists and bends in radial runs, type of wire, inductive affects of 
nearby objects, etc)
Rather than trying to measure, estimate or calculate things, I just start 
running thru the woods with my spool of wire. Much easier and faster, 
especially in the middle of the winter contest season!   smile

Marv - wa2bfw


Sent from my iPad
And YES it's cool!
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>