I've not run quantitative tests of the performance, but subjectively it
appears the same as with the ladder line. I didn't change any of the
switching or matching transformers from those supplied by DXE. The F/B
ratio
varies with time of day and the distance of the received station, but is
in
excess of 20dB on 1.8MHz for JA stations in the morning. The antenna is
very
quiet, and I can easily copy stations I can't begin to hear on my
transmitting vertical or my 80m inverted vee. I feel quite good about the
mechanical benefits of using the field telephone wire instead of ladder
line, and am confident it will wear much better. I've had a few strong
winds
(40mph) in the past week and it barely waves in the wind.
Jim and all,
Impedance of the feedline as a transmission line primarily affects
termination impedance in the null direction when the null direction is at
the far (remote) end of the antenna, away from the feedline end.
Line impedance has no effect on F/B ratio or pattern with the antenna aimed
in the direction of the feedline end. A mismatched transmission line
antenna mode simply results in a small additional loss of both signal level
and noise at equal rates, with no change in pattern.
As with any transmission line, the effect of that mismatch is periodic with
frequency as it relates to electrical length. The effect is absolutely no
different than connecting any terminating resistance to the far end of the
antenna through a very long transmission line.
When the antenna far end is "matched" to the termination resistance and
transmission line impedance, it operates like a system with low SWR. The
termination (at the far end) is uniform in value regardless of frequency.
This is the ideal design case.
When the antenna wire's transmission line mode impedance mismatches the
termination system impedance, the termination now appears though a very long
mismatched transmission line. This results in a cycle of impedances that
will cross the optimum termination impedance when the antenna length happens
to be at "lucky multiples" of length.
It is really not much different than feeding a dummy load through a very
long transmission line and watching SWR vary with frequency.
What this all means is with some antenna electrical lengths and line
impedances in differential (transmission line mode), the system behaves
almost like a perfectly designed system. You will not notice the mismatched
transmission line mode impedances. At other frequencies, generally peaking
at or around odd-quarter waves of electrical length in transmission line
mode behavior of the antenna line, the mistermination will be at a division
or multiple of SWR on the line. If the line transmission mode impedance is
100 ohms (and we consider it lossless for this simple discussion) the
termination in a 500 ohm designed system will vary from 20 ohms to 500 ohms
with frequency.
The same thing happens, of course, when the transmission line mode is
inserted on the receiver side of the antenna system, but we generally have
so much signal and noise level we never notice the degradation caused by
mismatch.
A snapshot at one frequency in one direction doesn't mean anything for some
other frequency and/or direction. It hurts the far end termination most
when the electrical transmission line length is around an odd quarter wave,
and doesn't hurt at all when the line is around 1/2 wave electrical. It
hurts the far end termination pattern, but not the near end termination
pattern.
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|