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Re: [TowerTalk] Why did adding a radial make the input Z of myinverted L

To: Towertalk@contesting.com
Subject: Re: [TowerTalk] Why did adding a radial make the input Z of myinverted L go ...
From: K3BU@aol.com
Date: Wed, 17 Mar 2004 09:47:43 EST
List-post: <mailto:towertalk@contesting.com>
>>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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