G'day,
<snipo>
| Here's an interesting optimization question..
|
| Say you have a maximum height limit (call it 33 feet) and you want to
put up
| a vertical antenna for 40m. You have limited space for a ground radial
| array (think suburban back yard, not 40 acres of tidal plains in a salt
| marsh).
|
| You could either put up a standard 1/4 wave monopole with ground wires,
etc.
| The ground radial array is going to be non-optimal, and moderately
lossy.
| Maybe 10-20 ohms loss? Rr would be around 30-35 ohms (or pretty close, I
| would assume, depending on the diameter), so the system would be 60-70%
| efficient.
| Any comments, thoughts, practical observations, etc.
<snipo>
I use a 30 foot vertical with the base 3 feet off the ground on 30M. This
is too long on 30M so the vertical is brought onto resonance with a pair
of shortened radials. The radials are gull wing style, now the standard
for DXpeditions using vert on the beach. The radials slope up from the
base of the vert at 45 degrees to a height of 6 feet then horizontal.
This removes them from the influence of the potentially lossy ground and
you can also walk under them. Useful for spanning a narrow lot without
getting in the way of human traffic. There are very good arguments for
not making any radials a quarter wave, better balance of currents.
The long vert and the short radials results in a better match to 50 ohm
cable 1.1 in my case. The feeder is decoupled by a coax choke wound
on 4.5 inch sewer pipe.
I also use this occasionally on 40M by simply inserting a small loading
coil in series with the common point of the radials. The match isn't so
good because the vertical is short on 40M but it never the less works OK,
the auto ATU in the K2 deals with it.
The EZNEC predictions fell out spot on.
The Force-12 web site has quite a bit to say on this subject, also ON4UN
"Low Band DXing and Les Moxon "HF Antennas For All Locations. Cebik also
has quite a bit on his web site about shortened verticals with end loading
similar to the Force-12 Sigma series.
My vert cost me about $100, x6 for the Force-12 40M offering.
On 30M I regularly work JA short and long path, one direction out to sea,
the other up the hill behind me so it isn't just because I'm close to the
sea.
Regards,
Mike VP8NO
Stanley.
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