K4SB wrote:
Well, of course the resonant frequency will change if the feedline is
part of the antenna, but under those circumstances, you are no longer
discussing the original antenna.
By posing a simple example which I hoped would be obvious, I was trying
to illustrate just what you said, the feedline will change the resonant
frequency of the antenna in this situation. This same concept applies to
any antenna. If an antenna is unbalanced and you don't fully decouple
the feedline, the feedline will have currents on it. This will change
the resonant point of the antenna, because the feedline will now become
part of the antenna. In the case of an off-center-fed dipole the effect
is extreme, but in the case of a vertical, the effect is usually very
small, but still present. A vertical, or an inverted L are both
unbalanced and with only a few radials the feedline is not very well
decoupled.
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This is the old trick to short the PL259 in the shack, and connect it to
one side of the balun of a tuner. In such a manner, a 80 meter dipole
can be used on 160 meters.
Talk about a way to light up your house. Where do you suppose the other
half of the antenna is?
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And, it's obvious that we have different opinions, so let me try one
more time. Nowhere in the formula for a resonant antenna does "feed
line" length appear.
The feedline doesn't appear in the formula because you left it out. The
formula you are probably refering to does not cover any of the miriad of
second order effects which may be applicable to a particular antenna.
This formula is for a half wave length in free space. It doesn't even
include a feedline, or a wire for that matter. It does not include wire
diameter or height above ground, all of which effect the resonant frequency.
---------------------------
As for the case cited above, the resonate frequency of the original
antenna does not change, however, reactance is introduced which may
cause an "apparent" change.
If you change the reactance of an antenna, by definition you change its
resonant frequency. Remember SWR is not necessarily lowest at the
resonant frequency. For most of the things we do, a low SWR is more
important, and the resonant frequency of the antenna may be unimportant.
-----------------------------
But, if you introduced an equivilent amount of opposite reactance at the
feed point, you will find
the resonant frequency is identical.
If you first increase an antenna reactance by an amount, then decrease
it by the same amount, yes, you are back to where you started. I don't
see what this has to do with the feedline.
--------------------------
I was under he impression we were discussing a 1/4 wave inverted L fed
with coax.
The principles apply to any antenna, including 1/4 wave L.
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And of course, if you do not use a choke balun, the feedline will
radiate, and that radiation will show in both the pattern and the
"apparent resonance" of the antenna.
Not only that, it changes the real resonant point of the antenna because
the "antenna" now includes the feedline.
--------------------------
And you will also find in almost any antenna handbook a statement that "
until the number of radials approaches a certain point, their length has
little to do with the performance of the antenna".
A statement taken out of context, and misquoted. It means you don't get
any additional benefit from making radials very long unless you have
many of them (more than about 60). A quote from the same source: "When
only a few radials are used, there is little point in making them longer
than 0.1 wavelength."
------------------------------
And in fact, the recommended proceedure if you have less than optimum
space is that "more
short radials are more effective than 1 or 2 long ones".
Agreed.
-----------------------------
If you have a modeling program, you can see the resonant point of a
vertical vary as a function of number of radials and also as a function
of the height of those radials. If your modeling program only uses a
mininec ground or a perfect ground, the results are misleading because
the mininec ground assumes perfect ground when calculating impedances
and SWR.
In order to see the feedline effects, you have to know how to model a
real transmission line. The internal transmission line models of these
programs are only mathematical models, not physical ones.
Unfortunately, you don't see some of these things discussed too much in
antenna books, because they come under the heading of "not doing it
right and what happens then". For the resonant frequency changing as a
function of the number of radials, this is usually a trivial parameter
for a buried radial system, so there is not much data on it, but it does
say something about the quality of your radial system. ON4UN's Low Band
DXing book does have a few words to say about it.
Jerry, K4SAV
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