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

[Jim Brown]

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!
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