>>Any and all comments appreciated.
73 de Jim Smith VE7FO<<
Rather than analyzing your particular situation (more info needed), here are
some general observations:
1. If you have strong local BC or other signals, some antenna analyzers
indicate wrong values due to overload. Using filters or TX SWR bridge may
overcome
that.
2. Picture resonant dipole, horizontal or vertical. The lowest impedance is
at the maximum current point, which is in the center. As you start moving away
from the center, your impedance increases. That is used to advantage for
matching to feedline. You don't have to feed dipole in the center. K7GCO is big
advocate of feeding Yagis off center, avoiding lossy "matches".
3. If your "dipole" is made of vertical - tower with top loading and part of
it is (who knows what) earth, radials, coax, plate, you might not know where
exactly in the "dipole" you are. By playing around with stuff that is added to
the "other half" of your "dipole" you are changing resonant frequency of the
system and depends at what point you are tapping your feedpoint to it, that's
what impedance you get and see.
4. Want "ideal" single vertical antenna? Try 3/8 electrical resonant length
of radiator (could be top loaded L or T) and elevated 4 radials 1/8 wavelength
long. You will most likely get exactly 50 ohm feedpoint impedance, slightly
more gain, current radiating portion of the antenna is longer, more efficient,
lower angle.
5. With L (single wire) loading or radial, you get some horizontal component.
Might be welcome for local stuff. If you use balanced T type symmetrical
loading or radials (same length, in line, opposite directions) you will cancel
out
the horizontal polarization and high angle component and you will have better
DX antenna.
6. If the "other" half of the antenna is large ground screen, or (steel
frame) wall of the tall building, then usually radiator works as 1/4 wave
vertical
and its resonance is given by that length.
I recently operated as C6AYB using half (quarter wave) sloper that came from
being radiator in balloon supported vertical with four elevated radials. It
was stretched from the 9th floor of the hotel, full length down to a palm tree,
45 deg. angle. It was fed with 50 ohm short piece of coax which had center
conductor clipped to the sloper and shield to the sliding door frame. Perfect
match to 50 ohms at 1830, no trimming or adjustment was necessary. (Using IC706
100W I beat the LP world record in CQ 160m CW:-) This sloper, being fed at the
top, had high current point at the top and was "seeing" salt water further out
with great enhancement to low angle signals.
7. So most likely, when you added radial(s) you changed the "length" of the
"other half" of your "dipole" (also it became better "half" than ground plate
and "took over") which changed the resonant frequency, which changed (moved)
your feed point within the dipole and as result you get different impedance.
Using modeling programs like EZnec helps to understand and optimize the
antenna design for the purpose and your particular situation. It is important
to
know what are you trying to achieve (pattern, gain) and then select the best
solution for particular situation (location, resources, desired performance).
I hope this clears the fog somewhat :-)
Yuri, K3BU
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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