[SECC] ARRL 160 - KU8E

[K4SAV]

K4SB wrote:
I would be more comfortable with Jerry's first statement if he had said 
"If your feed line changes the "apparent" resonate frequency of your 
antenna, you've got a BIG problem. This old wives tale should be put to 
rest once and for all. There is absolutely NO way a change in feed line 
length can affect the
resonate frequency of ANY antenna.

When the feedline appears as part of the antenna, it will change the 
resonant frequency of the antenna. Try feeding an off center fed dipole 
without a choke balun and see what the feedline does to the resonant 
frequency of the antenna.  The same thing happens to a vertical with 
only a few radials.  The feedline is not being decoupled from the 
antenna because of the relatively high impedance of the ground radial 
system. Even if you do decouple the feedline 100%, the resonant 
frequency of the antenna will still move with the addition of more 
ground radials.  The radials are ½ of the antenna.  They are not an 
independent part of the antenna.
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It merely shifts the reactance between the outgoing wave and the 
reflected wave to where the SWR meter is reading a cancellation of 
reactance because of
the phase difference between the 2 waves. Add the line AFTER the SWR 
meter to the Tx, and you will see absolutely no change at all. It's
become a point of WHERE you are measuring it.

That's not the way an SWR meter works.  You may be trying to describe 
the fact that an SWR meter whose internal reference is the same as the 
feedline impedance, will not show a change in SWR when the feedline 
length is changed.  This is a true statement if you have a balanced 
antenna. It is not a true statement when the feedline has common mode 
currents generated by the antenna.  These currents cause the feedline to 
be part of the antenna, and will change the SWR and the resonant frequency.

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Adding radials has absolutely no effect on the resonant frequency of the 
antenna. It MAY appear to do so because you are putting more power into 
the antenna, and again, that dumb SWR god is being chased.

Let's boils this down to something real simple.  A dipole could be 
considered as an antenna with a single radial.  Are trying to say that 
the resonant frequency is not affected by the length of one side? Now 
consider a vertical with elevated radials.  It's the same thing, only 
one side is larger.  Now it's a small step to moving these radials to 
the top of the ground.

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In Jeff's case, he could have as much as 20 to 30 ohms of ground loss, 
and adding radials would cause a drop in SWR because he would be 
approaching the pure resistance of the antenna. Actually, assuming he 
put up about 60 radials, his indicated SWR would go through 1:1 as he 
approached the ground loss figure of 13 ohms, and then begin to rise, 
which would be a great indication that he was making real improvement. 
When he reached a point where the SWR was at 1.35:1, he's got as good as 
you can get.

A true statement, but this doesn't necessarily have anything to do with 
the resonant frequency of the antenna.

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In summary, getting the antenna resonate FIRST is the most important, 
then deal with the reactances you find.

The reactances are what determine the resonant frequency. The antenna is 
resonant when the reactance is zero.  The SWR does not necessarily have 
anything to do with the resonant frequency.

Jerry, K4SAV