Hi Ford,
> Tom Rauch questioned the formula for incorporating "tilt" into the
> equation. No attempt was made to measure or compensate in any way for
> tilt.
The tilt is caused by the fact the wave itself is not lightspeed near
earth. Maybe I'm missing something, but I don't see how the
optimum length can be selected without considering the change in
wave velocity near the antenna.
> 20 feet doesn't seem to matter much. Second, the Vp of your line is damn
> near impossible to measure in nature--how to you get channel A and channel
> B of a scope at both ends of the beverage at the same time! Third, it is
Vp is easy to measure. All you need is a tape measure, and a
typical antenna analyzer. You slightly misterminate one end of the
antenna and measure the repeating of impedances at the other end.
As frequency is swept, the impedance will go through gyrations
that repeat periodically. The frequency difference between two
repeating points (say the Zo maximum where X crosses zero) will
tell you the electrical 1/2 wl frequency of the antenna, and include
the Vp.
Say that spread is 1.1 MHz. You know the antenna is 492/1.1 =
447 feet long electrically at whatever frequency range you swept.
Take the physical length over that calculated electrical length and
you have Vp. Assume the antenna is 398 feet long. 398/447= .89
Vp for that antenna.
My Beverages are 1 wl long electrically on 1.8 MHz when they are
about 500 feet physically. Vp is about .9
> difficult to model the effective charge contribution per unit length the
> wavefront in air contributes to the wavefront in the wire. The
> spreadsheet looks only at phase relationships along the length of line for
> signals arriving in line with the antenna.
Enzec and other NEC based programs take all of this into account.
The NEC based programs seem to be off a bit on Vp and tend to
underestimate loss in conductors near earth, but Beverages are
very non-critical anyway.
To terminate the antenna, do the same sweep test and adjust the
termination for minimum SWR change with frequency. You will
never get the reactance out (except at periodic frequencies)
because lossy transmission lines (which is what the Beverage is)
have "internal" reactance, but adjusting for minimal SWR change
as frequency is swept will be darned close to optimum termination.
If you can measure the input impedance, the surge (self)
impedance of the antenna is the geometric mean of the highest
and lowest impedance in the same test that determines Vp.
Multiply the impedance maximum and impedance minimum, and
find the square root of that number. Say the highest impedance is
700 ohms and lowest is 400 ohms. The antenna Zo is 529 ohms.
73, Tom W8JI
w8ji@contesting.com
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