[TowerTalk] Top loading HF-6V for 160

[Terry Conboy]

At 15, AM 2007-10-19, Rick K6VVA wrote:
>My understanding is that use of the 160m loading coil for the HF-6V limits
>the power to 500w on 160m.
>
>Q: What if the 160m coil is left off, and 4 sloping top hat wires were
>attached to the top of the vertical?  Any idea what the length(s) would have
>to be, or would this even work?  Is it true that top loading narrows the
>bandwidth?
I haven't seen any more follow-up regarding optimization of this 
configuration, but I'll throw in the results of my modeling in EZNEC 
(NEC-2 engine).  I used the parameters discussed, and varied the 
length of the four sloping (at -30 degrees from horizontal) top 
loading wires from 10 to 65 feet long in 5 foot steps, plus just 
under 66 feet where the system is resonant without a loading coil 
(and the ends almost touch the ground).  I ran with a top load at 90% 
and a base load for ground system losses of approximately 2 and 10 ohms.

I'll send a copy of the Excel spreadsheet (which includes charts of 
gain, bandwidth, loading inductance, and feed R) to Rick and the gang 
who posted comments, but to summarize, the best gain (-1.17 dBi) is 
for a top load with wires 30-35 feet long for 2 ohm ground.  For 10 
ohm ground and top load, 15 foot wires give best gain (-4.42 dBi), 
although the loading inductor may be impractical.  For base loading, 
the gain is -1.44 dBi with 35-40 foot wires and 2 ohm ground.  For 
base loading and 10 ohm ground, peak gain is -5.49 dBi with 25 foot wires.

The self resonant (no loading inductor) configuration has poor gain, 
but lots of bandwidth, but a more realistic configuration with the 
ends of the top load wires somewhat higher than the 1 foot (which 
results from a 30 degree slope) might do OK.

I also looked at the effect of varying the number of top hat wires 
from 2 to 8.  Using 3 or 4 wires definitely pays off (at least 1 dB 
better than 2 sloping wires), but going from 4 to 8 wires makes only 
a small difference in gain (0.3 to 0.5 dB) and a negligible increase 
in bandwidth.

For comparison, I ran numbers with 25 foot non-sloping top hat wires 
(good for about 1 dB) and a full size 1/4 wl vertical (about 2 dB 
better than the best sloping top hat version over 2 ohm ground and 
about 4.5 dB better over 10 ohm ground).

To state the obvious, an EXCELLENT ground system with lots of copper 
on the ground is a must with these short antennas.  It is also 
necessary to make sure the loading coil doesn't arc or melt at the 
1500 watt power level.

If anyone else wants the spreadsheet or modeling details, reply off list.

73, Terry N6RY

PS - I have a 26 foot version of this antenna, and I think I'll 
shorten my 50 foot top hat wires a bit and increase my loading coil 
to pick up a little gain.  Where they fit, a few full-size 1/4 wl 
radials will be added as well.