The basic definition of resonance is a condition of the inductive terms
canceling the capacitive terms. This applies to antennas, or tuned circuits
of lumped components as well.
A half wave dipole satisfies this definition at A single frequency.
A shortened dipole or any other short antenna cannot be Resonant unless it has
added inductive or capacitive elements attached to bring it into resonance.
Now, if you look at the formulas for antennas, they are written without a
height term. That is because they are written for the ideal free space case
(492/f for a dipole), or for the case including end effects of the supporting
lines and insulators capacitance term (468/f). "f" is in MHz in the formulas.
When dipoles are placed sufficiently high of average terrain, these formulas
are still true.
There are many variables including height in the near field of typical ham
antennas. These will affect the impedance of the feedpoint of the antenna,
but the resonance is not changed unless there is inductive coupling or
capacitive coupling of the antenna to something near it, upsetting the balance
of L and C, thus changing the antenna effective capacitive term, for example.
But, this happens in common antenna use as the antenna gets close to the
ground.
This happens with the quarter wave vertical antenna, because unlike the
dipole, it is not made up of two balanced conductors, but requires a ground
plane, or radials, or RF counterpoise, or low RF loss earth to provide the
missing "other half" of the antenna. It capacitively couples to the earth
beneath it, if mounted close to the earth. This is the "ground mounted
vertical". Note that just connecting one side of its feed to an earth rod, is
not the same as providing a Low RF loss earth connection, or counterpoise, or
ground plane. A single earth rod just does not have enough coupling to the
earth to be of much help. "A ground rod" is not an RF ground in most soils.
The ARRL Handbook for years has had a chart showing the variation in feedpoint
impedance with the varying height of the dipole antenna. Impedance is one
concept. A resistor has impedance, but is not usually resonant at any band we
commonly use. Resonance is a different concept. For the moment consider that
the center of a "very high and in the clear" (whatever that takes at a given
location), folded dipole will have an impedance of 300 ohms. A simple dipole
will have an impedance of 72 ohms at the same location. But both are
resonant, and at the same frequency, if their dimensions have been cut to the
formula for a half wave dipole antenna. (And if they are not occupying the
same space at the same time!)
I agree, the matching device at the end of the balanced feedline does permit
the use of the whole antenna system over a wide range of frequencies, but it
does this by impedance transformation at the rig end. It is not actually
changing the Length of the antenna. The key here is detaching the concept of
the resonance of the antenna from the concept of transferring maximum power to
the system to use it on many frequencies. I just think it is easier to
conceptualize, although all the parts are joined in a common system.
The imperfect nature of matching boxes is because of component loss in
matching devices. They are made up of somewhat lossy components, such as
inductors and capacitors but careful construction can minimize these losses as
can proper tuning of the popular Tee network.
"Antenna Tuning" is a semantic useage that is very confusing to many in
electronics. But study the basic equations and texts, and what a transmission
line should do, what a tuned network like an antenna matching device should
do, and where they are in the overall circuit.
Inductors and capacitors on the antenna end of the transmission line are
affecting directly the resonance of the antenna, because they are affecting
the balance of inductance and capacitance of the antenna determined by its
dimensions relative to the frequency. Inductors and capacitors in a network
on the rig end of the transmission line are affecting the impedance match
between that line and the rig.
If the transmission line has become part of the antenna, it is radiating,
which is a lossy condition if the transmission line is not in the clear.
An older antenna used for many years did put inductors and capacitors in
series with the legs of the parallel feed lines, and turned the system into a
tuned feeder which radiated, and this undoubtedly adds to the confusion of
understanding of antennas and transmission lines. Normally, you do not want
your transmission line to radiate, because it disrupts the actual antenna
geographic coverage pattern of the dipole antenna if that is what is attached
to the elevated end of the transmission line.
At frequencies where the antenna has capacitance terms not matching the
inductive terms, it does not meet the basic definition of resonance, yet the
the other end of the feedline can be matched by the Trans Match to the rig,
without the worrying about the non resonance or impedance condition between
the antenna and its end of the transmission line. Power can be transferred
to a resonant or non resonant antenna.
To be totally aware, one must consider that even a resonant antenna may not do
you much good, if there are no sun spots to enhance propagation to the other
end of your desired path! A resonant antenna should help, under poor
conditions, because it is more efficient. But, the total circuit path is not
only the antenna and transmission line, but the ionospheric path as well.
Transmatches used today are really substitutes for the missing tank circuits
found for many years in tube rigs in forms such as the Pi Network. They both
tuned the tube output to resonance and provided reasonably wide impedance
matching to the transmission line. Today, we find the transistor output has a
tank circuit that only matches to 50 ohms, and without the adjustable
features of the old Pi Network. The transmitter matching circuit is useful to
restore that adjustable feature to our rigs and allow use of varying antennas
over wide frequency ranges.
The nice thing about the Ten Tec Antenna Matching circuits that have been
discussed is that they are L networks which have only one matched setting, and
that is the optimum setting unlike the other commercial tuners of the Tee
type. When the capacitor of an L net can be changed from the antenna side to
the transmitter side of the network, you gain extra flexibility of
adjustment.
Stuart K5KVH
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|