[TenTec] Resonance and more
Cook, Thomas A
CookTA@corning.com
Tue, 23 Sep 1997 15:51:41 -0400
You are right on the nose. ANY matching network's job is to transform
the effective resistance and cancel any reactance that would be
presented to a transmitter to (ideally) 50 Ohms resistive. A *System*
may present 50 Ohms resistive only because of a matching network (which
will have its inherent losses too - see the recent tuner reviews in QST)
or a fortuitious combination of antenna geometry, location, material,
length of feedline, etc. None of these address the antenna effiency
(which is heat loss), e.g., a *resonant* vertical will present 37.5 Ohms
resistive impedence to a transmitter (with half-wave xmission lines) IF
AND ONLY IF it is over a perfect ground. Take a look at the ground
system for commercial BC stations. With a less-than-perfect ground, the
resistive losses increase, the Q drops and the effective "usable"
bandwidth goes up. This isn't always a bad compromise for a practical
system. 73, Tom - WA2BPE
> ----------
> From: rohre[SMTP:rohre@arlut.utexas.edu]
> Sent: Tuesday, September 23, 1997 3:05 PM
> To: tentec list
> Subject: [TenTec] Resonance and more
>
> 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
>
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