At 11:33 AM 2008-09-01, VE6WZ_Steve wrote:
>I am planning to add a second EU Beverage wire parallel and staggered
>about 25 m behind my existing EU wire and use the phase box in the shack
>for phasing the two together.
>
>Since the phasing provides the F/B pattern, is it necessary to have the
>Bev. wires terminated?? In other words, why not phase two
>**bi-directional un-terminated** wires together and use the phase box
>switch the pattern around.
>
>I have modeled this with my MININEC program and it seems to look ok, but
>I don't trust MININEC for beverage modeling.
>
>Anyone done this??
I haven't done this, but one issue is that with unterminated
Beverages, the input impedance is a lot more variable (and usually
reactive) and that could make it harder to get the current magnitudes
equal and the phase to the desired value. There can be a significant
amount of mutual coupling between unterminated Beverages that
complicates the phasing (as in many TX arrays).
I looked at an EZNEC model over "Real/High Accuracy" ground (with 2
perpendicular radials at each of the ground points) and surprisingly
the pattern toward the open end doesn't look too different with or
without terminations (167.6 m long, 25 m stagger, 4 m separation,
1.83 m high). Of course, this is with equal currents and ideal
phasing (85 degrees).
The pattern away from the open end with equal currents but with the
sign of the phase angle reversed doesn't have quite as clean a
pattern, however. The F/B is about 12 dB, vs. 30 dB in the other
direction. Some of this is probably because a single unterminated
Beverage has about 2.5 dB F/B (using the dimensions above). The
unterminated F/B increases with overall length.
Changing the current ratio and phase angle for the "reverse" pattern
can increase the back null significantly, however. With -95 degrees
and current ratio of 1.5:1, I was able to obtain a 30 db F/B.
If you have one of those variable receive phasing boxes, you can
probably find a setting that gives nice patterns by experimentation.
73, Terry N6RY
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