Here's another approach. I put up an Inverted-L for 160m and measured
the complex impedance at the feedpoint. I loaded those values into TLW,
the free transmission line program that comes with the ARRL Antenna Book
and looked for the position on the coax feedline where the real part of
the transformed impedance was 50 ohms and the reactive part was
negative. I then wound a coil of the appropriate dimensions, mounted it
in a new 1 gallon paint can with SO-239 barrels on each end, and
inserted it into the feedline at that point.
The additional loss due to the SWR for what turned out for me to be a
relatively short length of coax between the antenna and the coil was
negligible (according to TLW), and a coil is a lot cheaper than a
capacitor. And in my case at least, breaking the feedline at the proper
point to put the coil was a lot easier than raising and lowering the
antenna to tweak the length since the horizontal top portion of my
Inverted-L is strung across a large ravine.
73,
Dave AB7E
On 1/7/2020 12:31 PM, Jim Brown wrote:
This is exactly what I've done since moving here in 2006. In NEC, I
tweaked the length of the horizontal top of the Tee to achieve that 50
+jX value at the center of my operating range, computed the series C
value to tune out that jX, then added parallel caps with low values of
series R to hit that value. The same technique would be used for an L.
73, Jim K9YC
On 1/7/2020 11:06 AM, Paul Christensen wrote:
My 160m T is 100 ft high with an 80 ft. flat top. Next season, the
flat-top will increase to 120 ft. where RF current magnitude is
uniform across the entire radiator length with a slight current peak
half-way up.
I initially selected an 80 ft top because it made matching super
easy. The base Z is 50+j200 and only requires a series capacitor to
achieve a 1:1 SWR. The series cap will be replaced by a low-pass L
network as the R value is expected to rise up to about 65 ohms.
Resonance presently occurs at 1500 kHz but will shift down to 1400
kHz when the flat-top length increases. Easy to match with a
low-pass L network.
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