Lee,
Your modeling of the 75-meter reversible wire beam looks to be very sound.
The loss relative to a flattop version is real, since the elements are
always below the peak height. The TO angle is correct, allowing for
variations due to terrain and ground clutter.
There is a program in the HAMCALC collection, available from VE3ERP, that
calculates for any given feedpoint impedance and feedline Zo (and velocity
factor) the voltage, current (and their phases), the R and the jX along a
transmission line. You can select a precise length to check or ask the
program to make a table at every 5 degrees of electrical length. The
program assumes lossless line, but for a short line such as the one you
propose, the connectors will introduce more error than the absence of the
loss factor (meaning negligible error in both cases).
You actually have two choices that you may wish to explore before freezing
the design. One is using an open capacitive stub to shorten one element
to be a director. The other is to use a shorted stub to lengthen one
element to be a reflector. You may wish to check both to get the best
combination of gain, F-B, and feed Z for your situation.
What you will get out of the final design is most relevantly compared to a
single element of the same type. That generally reflects the building
limitations of a given site. In your case, it appears that you are
limited to Vees, so the relevant comparator is the single element Vee.
However, you may also wish to consider end supports for the wires that
elevate the ends as high as practical for your installation. The
difference between 8' up and 25' up at the ends can make a difference that
you might judge significant relative to the work of placing the supports.
There is no magic in any particular angle for a Vee, just a continuum
between the flattop and the most extreme angle. The flatter the top, the
higher the feed Z for any given spacing.
If the native feed Z reaches 40 ohms or better, you may not need a
matching network at the switching position. Otherwise, a simple L-network
is likely to be the most efficient (or least lossy at 99% efficiency or
better). Whether you make it with fixed or variable components will
depend on the operating bandwidth you need. There is an L-circuit
component to the antenna matching network program by ZL1LE, also included
in the HAMCALC collection.
Incidentally, the current version of HAMCALC is 30, released just a few
weeks ago. Callbook address for VE3ERP is good. He requests a 5$
donation to cover the cost of disks and Canadian air mail postage and
contributes anything over his cost to the Canadian National Institute for
the Blind amateur radio program. However, distribution of the program
collection is open and free, in case a local friend has a copy.
To put the antenna in place, you will likely want to use the independent
resonant frequencies of the elements as your reference. That allows you
to compensate for the effects of surrounding objects that cannot appear in
the model.
I hope these notes are useful. Good luck with the project. I am putting
these notes on the list in case they might be useful to someone else as
well.
-73-
LB, W4RNL
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