TopBand: Shunt fed tower with wide band width

[Earl W Cunningham]

Lee, KT4ZX  writes:

"I decided to take down my inv-L that was hanging beside my 72 ft Trylon
tower and try to gamma feed the tower.  (The L also interfered with a
sidemounted antenna!).  To keep the L out of the path of the sidemounted
antenna, I tried moving the L closer to the tower (it was about 5 ft
away).  But as I got closer the real part of the impedance went upwards
of 100 ohms (according to my MFJ-259B).

"I got the gamma feed working last night, but I'm surprised at the
bandwidth.  Its around 260 kHz!!!  I was expecting something much
sharper.

----- DETAILS -----
Gamma wire:  #14 stranded copper.  4ft spacing.  48ft tall.
Feed:        235pf series cap (2 470's in series)
Tower:       72 ft self supporting   -  face width:  4ft at base - 18" 
at top
Antennas:    5el 10m beam at 50ft. 24 ft boom.  grounded elements.
	     F-12 20/40m beam at 73 ft.  30 ft boom.  insulated elements.
	     F-12 15m beam at 84 ft.  24 ft boom.  insulated elements.
Ground:      45 radials ranging from 20 to 130 ft.
Feedlines:   Attach to tower at 30ft level, then slopes off to house 30
ft away.

"I added a horizontal wire (50 ft long over to a tree in the yard) to the
top of the tower (70 ft level) so that I would have some high angle
radiation like the L.

"SWR dips at about 1.1:1 at 1900.  2:1 at 2000 & 1740.

"Without the horizontal wire, the SWR still dipped at about 1900, but
with a 1.4:1 match.

"I was able to work HI8 last night with 100 watts and a few calls.  I
heard a few US stations working into Europe, but I couldn't hear the EU
stations.

"So what do you think???  Is the antenna really that broad, or do I have
a lot of loss in the ground system (I'm planning on adding more radials
anyway)."
========
Comment:  Maybe your structure is close to 1/4-wave resonant.  That's
where you get maximum bandwidth.  But the 235 pF to match it makes it
sound likes it's not 1/4-wave resonant.  If it was, the capacity requires
would be about 600 pF.  Ground losses would also make it appear to be
broad-banded.
========

"Are conditions on 160 much worse than last year???  Last year I was able
to work into EU fairly easily with the inv-L and 4 elevated radials."
==========
Comment:  Here on the west coast, condx are definitely not as good as
they were at this time last year.
==========

"Any other ideas for improving the system????
===========
Comment:  Yes

1) Run your feedlines and rotor cable (now leaving the tower at 30' up)
all the way down to the ground before they leave the vicinity of the
tower.  There's no telling what effect they're having.

2) After doing 1) above, retune the gamma match.  To get the 1:1 SWR
point lower in frequency, either increase the spacing of the gamma wire
or tap it higher onto the tower (or both).  To raise the frequency, do
the vice-versa.

3) Add more 130' radials.

A variable capacitor is a big help in determining where the frequency of
the 1:1 SWR point is.  A small BC variable with your MFJ analyzer will
suffice.  Tune the gamma capacitor on various frequencies, trying to get
a 1:1 match at 50-ohms.  Once found, then you know which way to adjust
the gamma wire to bring the 1:1 frequency up or down.  After the final
adjustment, you can replace the small variable capacitor with a higher
voltage fixed cap of the same value as the variable when tuned.

73, GL, de Earl, K6SE


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