On Wed,2/4/2015 10:52 AM, Paul Christensen wrote:
On 160m, a simple L network network at
the base will get us 50+j0.
For about 7 years, I've tuned my Tee vertical well below the band (by
making the top loading wires longer), raising the drive point impedance
to 50 + jX Ohms. The antenna looks inductive, so it's simple to add
series capacitance to tune out jX, and you end up with a nice match.
As noted, this moves the peak of the current up a bit. BUT -- don't
forget that it will also change the current distribution along the
length of the radials, which CAN increase loss in the radials.
N6BV (retired ARRL Antenna Book editor) has NEC4, and modeled this for
me with the radials. He said that from a radiation point of view, there
was no benefit to moving the current up the tower, but it didn't hurt
either, so the tuning method doesn't hurt antenna efficiency.
Until about a month ago, the vertical section was 86 ft, with about 130
ft horizontal (Tee). My tree climber, who is also an arborist, told me
that the big Madrone tree that held up one end of the Tee was dead and
in danger of falling on my water tanks, and that we should take it down.
He did, and found a young Douglas Fir, about 120 ft, to move the antenna
to. He did, and I ended up with 100 ft vertical. I now have about 82 ft
horizontal, and it tunes with about 900 pF.
I modeled the antenna in EZNEC, and dimensions came out within a foot or
so from what I measured. SWR Bandwidth of an antenna is increased by
making the conductors larger, and for quite a while, I've used two
parallel lengths of #10 THHN spaced about 10 inches. Both versions (old
and new) are giving me better than 1.8:1 up to about 1910 kHz.
Prior to lengthening the top section, I measured feedpoint Z of 33.8
ohms at resonance, about 1710 kHz. I've got about 60 radials laying on
the ground, varying in length between about 67 ft and about 130 ft, and
the soil here is quite poor -- very rocky, mountainous. Looking at a
graph in the ARRL Antenna Book for radiation resistance vs vertical
height, I'd guess that I have about 10 ohms of resistance in the radial
system.
73, Jim K9YC
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