[UK-CONTEST] 'Remote' radials...
David, G3YYD
g3yyd at btinternet.com
Tue Jun 16 09:54:07 PDT 2009
Rather than think of radials it is better to think of screening the very
lossy earth from the RF current generated by the vertical. The highest
loss is where the maximum current is. In the case of quarter wave or
shorter vertical this is at the base of the antenna. The further out you
move from the base of the antenna the RF current will drop. From this
bit of knowledge the most important place to have your wire for the
ground screen is near the base of the vertical to keep the highest
current from entering the lossy soil. As the ground screen moves further
out from the base the "gain" from using more wire drops off rapidly.
The magic figure to remember is 2.6 metres at 160m. If you use this
spacing for the distance between the radial ends you are maximising the
return for the amount of wire you are putting down. Using radials that
are only a sixteenth of a wavelength long e.g. 10m at 160m will loose
less than 2dB compared to using a half wave radials spaced as suggested.
Increasing the total wire length by 4 times (doubling radial length)
gains another 1dB, i.e lot of work/cost for little gain. The magic
figure changes as the band goes up so 80m would be 1.3m between the
ends, 40m 0.65m, etc. This magic number is independent of radial length
as it is about how much of the RF field will go through the ground
screen into the lossy soil and at 2.6m at 160m the additional loss
compared to a solid copper disk of the same radius is less than 0.5dB.
My suggestion for a temporary ground screen is to use chicken wire
(galvanised steel netting) and just roll it out and peg it down with
some bricks/pegs. At the end of the contest roll it up and store it for
the next one. The loss in the zinc coating is very small as each strand
has a very low current due to the number of strands in parallel. Do not
leave it down as the zinc disappears very quickly and then the steel
soon rusts out.
Have fun
David G3YYD
More information about the UK-Contest
mailing list