An antenna user has to understand that SWR is a measure of more than the
antenna, as it is the system performance at the point of measurement,
but reflects (no pun intended) the combined match from Transmitter to
any matching device, matching device to feed line, and finally, feed
line to the point of the antenna where you are connected.
Also in the picture are the surroundings of the antenna. Let's take a
simple example. For a half wave dipole, the lowest impedance would be
at the center of the wire for a center fed dipole in "free space". But
suppose one half of the real antenna is closer to some conducing object
like a metal roof than the other half?
Now the impedance match is affected because one element has differing
capacitance and inductance near it, than the other. The lowest point of
impedance may shift away from center. And, remember the half wave
dipole is not 50 ohms at center for free space, but closer to 72 ohms or
so, and even this depends on element diameter, ratio of diameter of
element to length, etc.
Thus for a practical antenna the SWR you measure may be a combination of
several things affecting all parts of the "antenna system", which starts
at the rig output connecter and effectively ends somewhere past the ends
of the antenna. In fact, the height of the antenna is of importance to
impedance, and balance of current in each half of the dipole may not be
perfect due to greatly differing RF character of whatever is under one
half of the antenna rather than the other.
Most hams forget that minimum SWR has a finite band of frequencies
associated with it, as a range of low SWR that the rig can load.
In fact, there is only ONE frequency where a given antenna may load with
lowest SWR on its fundamental resonance. To the accuracy of a ham
measuring SWR, that is OK, the small differences are not going to change
the S meter reading at the distant station.
It is like the antenna articles you used to see in the 50's and 60's
where some poor ham sweated to get his antenna "tuned up", and hacked
off bits of wire, then measured, hacked off some more bits until he
"thought" he had the antenna "tuned". But, as soon as he went to the
other end of the band things changed. And, he had overlooked the ease
of simply folding the ends of the wires back if the antenna resonated
too low in the band, and he wanted to shorten the wire. That gives the
advantage of being able to alter the antenna length again if it is moved
to a different location or height. The usual half wave dipole formula
is an approximation, good for some earth under the wire but not perfect
for all.
If you get access to the SWR and antenna analyzers like the MFJ models
or others that have both SWR read out on an analog meter and digital
display of reactance and impedance, you will be able to measure various
antennas under various installation conditions, and grasp the varience
that are real world. Make up a 6m dipole and install it with one half
near a good conducting ground like aluminum foil, and the other half
over dirt. Try moving the foil to be off to one side of the antenna, or
vertical near one end of the dipole. An afternoon of your using an
analyzer and "what if" tests will be very educational. Keep good notes
of what you did, for later reference. You will get a much better
understanding of antenna measurement. And remember, antenna modeling is
not exact either. It depends on your assumptions trying to replicate
the real location where the antenna must perform. After installation
you may have to extend the extra length at the end wires or fold them up
to make the antenna
accept most power from the rig, but that is normal.
Stuart Rohre
K5KVH
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