At 04:49 AM 10/25/2005, you wrote:
>Several have asked for a summary of the responses to my earlier
>posting asking whether adjusting radial length was a way of trimming
>an inverted L for resonance on 160. Many opinions but probably the
>most useful was from K1FZ (who if he reads this may want to add something).
>He said that the radial length is not the right way to go but rather
>shortening (or lengthening) the radiator is correct and that the
>best way to do it is to add a variable capacitator between the
>center conductor of the coax and the radiator or just cutting and
>measuring until a low SWR is achieved. He says by trimming the
>radials you can get some desired results but by so doing you may
>effect take off angle adversely.
>Additional comments or corrections posted on Topband are welcome and
>thanks all for theiresponses and interest on the subject, Pete W2LL.
BTW, Pete--
I disagree with your summation about adversely affecting the
radiation angle when
making an adjustment to the radials that is relatively small.
Also see the work done by K5IU, published in Communications
Quarterly, concerning
the usefulness of deliberately shortening the radials because at a
nominal 1/4 wave
length the rate of change of reactance is at its maximum and it is
the most difficult
point to get the elevated radials to equally share currents since it
is almost guaranteed
that they will be electrically different due to minor changes in
ground conductivity
and height. Dick shows that it is better to make all of the radials
shorter than a
1/4 wave and tie them together and then resonate the entire radial
system with a
single inductor.
In fact, the reason your antenna MAY be low in freq is because the
radials are already
TOO LONG, due to capacitive coupling with the earth. So, maybe shortening your
radials will just raise the resonant frequency to where you want it
and actually make
the radials a true 1/4 wave length.
Another advantage of making the radials a bit short and the vertical
a bit long is to raise
the maximum current point a bit higher in the air, something that
might be useful if the
antenna is surrounding by objects like a house and power lines.
There is nothing magic about the height of the vertical itself --
most AM broadcast
towers are NOT 1/4 wave tall. And the same goes for elevated
radials, there is nothing
magic about any specific length. What you want to do is to resonate
the entire system
by tuning out the reactance so that proper current flows in the entire system.
This can be accomplished in a number of ways -- if the radials are
short then you can
lengthen them with an inductive reactance in the radial system or you
can lengthen the
radiator physically or with an inductance. This is the same as an
off-center fed dipole.
If the antenna is low in frequency you can shorten the radial system
OR the antenna,
AND either by physically shortening either one (or both) OR
electrically shortening either
one (or both) with capacitive reactance.
John W0UN
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