Hi Guys,
Following the the thread on shunt feeding I'll offer some food for
thought. The approach is somewhat empirical but works.
I have top loaded towers in the height range of 50-100ft.
The largest 100ft with 4el 20/5el 15/4 el 10m at the top. This tower had
guys insulated at the tower with "Johnny Balls" and broken every 25ft to
ground level. (guys every 24ft) There were no dipoles or wire antennas
suspended of this structure. All feed lines and cables were brought down
to ground level, bypassed or grounded at base, on the opposite side of
the tower from the feed point.
My approach to matching is to add 2 inch aluminum gamma rod extension
at about 25-30 ft level. With three foot spacing from the tower I drop a
8-10 gauge copper wire to ground level and secured with insulator to a
stake or ground rod. I then either grid dip the tower for 1/4 wave
resonance or look at feed point with my original model MFJ antenna SWR
analyzer. I'll note again I'm looking for electrical quarter wave
resonance.(this tower by itself was 1/4 wave resonant at target range)
My experience tells me the antenna will work well if quarter wave
resonance is at or near 1 Mhz. (Yes 1 Mhz) With this approach the 100ft
top-loaded tower will match quite easily to 50 Ohm feed line with a
series capacitor.(200-300 pf) A omega match capacitor could be used from
feed point to ground or gamma matching section could be moved in/out as
necessary to achieve a 50 ohm match.
I have done the same thing to a 55ft crankup tower with VHF vert and
Modest tribander at 65 ft level. In this case the the 1/4 wave resonance
was achieved by adding 2 approximately 90 ft wires to the top hat by
attaching to standoff arm at 55ft level and folding within the property
lines with insulated ends forming a triangle insulated from ground at
about 5 ft level. The feed point tap again was about 27ft with 3 ft
spacing. This tower with minimal ground screen was matched only by
series cap and by slightly juggling the triangle loading wires total
length. Again I specifically was looking for the whole mess to resonate
at about 1 Mhz. No other wires or dipoles were supported by this tower.
Each of the towers had a minimal lossy ground system equivalent to
ground screen with thirty foot radius from base of tower. Each provided
100 Khz 2:1 SWR bandwidth. Each worked their fair share of DX and
Contest QSO's at near legal limit power 1.4 KW. Each were on city lots
of half acre or less. Getting out was never the problem. I was however
noise limited by urban environment.
With this approach I have been able to get a tower working quite well on
top-band in about 30 minutes.
I have not had success in folded loaded monoploes without top-loading.
Also, I have not had success using gamma or omega match to short towers
with gamma match running to the top of the tower. :) ????
Regarding Tree's query re: large antenna in middle of tower. The effect
should be negligible. Example: K7CA uses shunt fed 2M EME array pointed
at the Sky for 160!
73
Bob KG7D
--
GIF89aI!
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