On 9/23/97 3:05 PM, rohre at rohre@arlut.utexas.edu wrote:
>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.
Sounds good to me.
>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.
OK. Now, what if you took those elements and squeezed them back through
the transmission line until you got to a box just before the transmitter.
Voila! An antenna tuner! And it produces a resonant antenna system.
>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.
This formula is for half-wavelength dipoles in feet. The reason for the
reduction in length is NOT for end effects of the supporting lines and
insulating capacitance. (You'd get similar figures for a dipole made out
of small diameter pipe or other rigid material with no end supports) It's
for the velocity factor of the conductor. Electricity doesn't flow at
exactly the speed of light, it is deterred by a number of effects, hence
the forshortening of the dipole formula.
>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.
You just stated that resonance is a condition where reactive components
cancel each other. What does this have to do to the length? It is the
electrical length that matters, not the physical length.
If you look at every single point of the feedline from a properly tuned
antenna tuner to a "non-resonant" antenna, you'll find that the reactive
components cancel each other.
> 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.
Efficiency and resonance are two separate concepts, as you indicated. In
any antenna system, if we eliminate loss, we'll have greater efficiency
-- whether it is at resonance or not.
You could have a perfectly tuned dipole fed with 100 feet of coax be less
efficient than a non-resonant antenna fed with 100 feet of open wire,
even including the losses in the antenna tuner.
>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.
I fail to see the distinction. Electrically, there is none (other than
accounting for losses in the transmission line). Lumped or distributed
reactive components work the same whether they are at the antenna, the
rig or somewhere in between.
> 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.
WRONG! As long as the currents in open wire are balanced, it does not
radiate. And even slight unbalance in open wire can be more efficient
than coax, because coax has appreciably more loss than open wire.
A transmission line is part of an antenna SYSTEM. If it is radiating, it
has properly become an antenna. A radiating transmission like is NOT,
necessarily, a lossly situation. Depends on the exact nature of the
system.
>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.
Properly balanced tuned feeders DO NOT radiate. This is a misconception.
>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.
Yes, feedline radiation can disrupt antenna patterns, but it does not
have to be inefficient.
> 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.
Once matched by a transmatch or other antenna tuner, the entire antenna
system has acheived a state of resonance.
>A resonant antenna should help, under poor
>conditions, because it is more efficient.
Please explain how a resonant antenna is somehow more efficient.
Efficiency depends on minimizing losses.
>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.
Please explain how a tube output to an antenna system consisting of
resistive and reactive components is any different from a transistor
output through a fixed 50 ohm tank to an antenna tuner attached to the
same antenna system.
And, I remind you, in the REALLY old days (pre WWII), most transmitters
didn't use a pi network, but instead where typically link-coupled to
balanced feeders.
And they didn't give a DAMN about SWR then, either. (Hint: the secret is
in the feedline)
>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.
Only problem with the L network is it must be reversed to tune both
higher impedance and lower impedance antenna systems.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not in a thousand years will man ever fly!"
-- Wilbur Wright, 1901
--
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
|