My experience is with a 50ft tower with F12 C3 beam on top. The yagi has
elements isolated from the boom and probably gives less capacity hat top
loading than most other hf yagis .
I put a 20ft alumin tube topmast above the yagi and from this ran a more or
less horizontal laing wire, length around 80ft as I recall.
Started with a fan of radial wires of varying length but only a few over
100ft.
Soil clay and broken rock.
Tried delta and various gamma matches with no success until I made a sort of
low dipole set of radials and ran these out along boundary fences. Grid
dipped to 1830.
Using this found I could get quite a variety of gammas to load (SWR at feed
point approx 1:1) without too much difficulty.
So my advice before working on gamma systems or whatever is to get the best
ground pland/radial system you can manage installed before playing with
matching system.
If ground system is not very good would suggest st least one approx quarter
wave radial so that there is something in the ground system that is likely
to resonate somewhere near the target frequency.
While playing with matching systems I searched literature on gamma matches.
Wide variety of fixes, seems almost anything can be made to work after a
fashion.
There may be a gamma modelling program out there somewhere, think the
antenna programs popular with amateurs could do the job, too. But unless one
was an experienced antenna modeller would treat the results with caution.
There are many variables in a gamma match.
The limiting case is where the gamma attaches to the top of the tower which
makes a folded monopole.ie half a folded dipole.
Folded dipole rules should apply.
In the case of the tower of 70 ft or so with substantial top loading from
yagis my suggestion would be to get the ground plane system as good as you
can get, then run a wire up the tower and attach as high as you can get
below the lower yagi. Space the wire say 3ft from the tower.
At bottom of wire make a little link of hookup wire between the new gamma
wire and the feedpoint connection to the groundplane.
Sweep with a gdo.
The lowest frequency is the one of most interest, and there should be other
resonances more or less harmonically spaced at higher frequencies.
These should give an indication of the natural resonance of your system.
If not too far from 1.8 mHz all should be well.
If significantly higher might be worth trying a loading wire off the tower
below the yagis.
Once you have a resonance somewhere near 160 you are in a position to start
playing with various gamma configurations plus series C.
As mentioned before the number of variables is great.
Before trying SWR bridges and antenna analysers at the feedpoint would
suggest checking with good old GDO to get an idea of where you are.
Also remember that hf broadcast stations can induce significant currents in
your new 160m vertical even from many miles away. This can mess up sensitive
instruments, another reason for checking with GDO or power oscillator before
using more sophisticated gear.
Think you will find the effort worth it.
Most people who try 160 and play around to get the best out of whatever they
can manage for an antenna are pleasantly surprised with what they can
achieve on the band.
73
Barry ZL1DD
PS Verticals are not the best rx antennas for the lower bands, but you could
well be impressed if you hook a good quality AM broadcast rx on to your 160
m vertical one evening!
--
Barry Kirkwood PhD ZL1DD
barry.kirkwood@gmail.com
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