I am focusing on the issue of power transfer at the junction of the line and
the antenna. Specifically, in a multiband dipole where the figures (all of
them) will vary wildly from band to band. Maximum power transfers at
resonance (oh no, let's not get into a war defining that). But I think it
is safe to say by anyone's definition, a multiband dipole usually not
operated at it's resonant frequencies. If the line is 450 ohm and the
antenna is 2 ohm or 20 ohm or 2000 ohm, there is not resonance.
Intuitively, I have got to think a 200:1 mismatch is significant.

So what is the loss at the antenna/line junction? I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.
--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64

On Wed, 7 Jan 2004 11:43:17 -0500, "Craig Buck" wrote:
[snip]
So what is the loss at the antenna/line junction? I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.

I suggest you download and run TLDetails and see for yourself. It is a
great freeware program which should answer most, if not all, of your
questions.

You can download it at: http://www.qsl.net/ac6la/tldetails.html

73
Danny, K6MHE

Hi Craig,

The 200:1 mismatch is "significant", but it does not directly cause any loss. In
handwaving fashion, this is how things work.

A mismatch allows some fraction of the power to pass through the connection
point, with the remainder reflected. (You can substitute voltage or current for
power. The numbers are different, but the principle is the same.)

Assume the transmitter supplies some level of power, say 100 W, to the
transmission line in a perfectly matched manner. A tuner will generally be
required. For purposes of this discussion, nothing passes from the transmission
line back to the transmitter.

The energy supplied by the transmitter has to go somewhere, and the only two
choices are to the antenna or to losses in the transmission line. If the line is
lossless then all of the energy goes into the antenna.

How does this happen when the junction between the line and the antenna reflects
most of the power?

The power level in the line increases so that even the small percentage
transferred to the antenna equals the same 100 W supplied by the transmitter.
There are typically long and loud arguments in this newsgroup on the exact
mechanism for this buildup in the transmission line, but it does happen within a
few cycles of RF. The resulting voltages and currents will be much higher than
those found in a fully matched system.

So far all is good. The antenna receives the full transmitter output, and there
are no added losses.

The problem comes from the higher losses that occur in even the "lossless"
transmission line when operating at high voltages and currents. In the case of
ladder line these losses may still remain quite small, but in the case of RG-58
they can become quite large. The transmission line may fail at lower power
levels than expected.

Soooo, the mismatch at the antenna junction cannot really be ignored, but its
impact is in the transmission line, not the junction itself. Unless the mismatch
is extreme the ladder line solution takes care of the loss problem.

73,
Gene, W4SZ





Craig Buck wrote:
I am focusing on the issue of power transfer at the junction of the line and
the antenna. Specifically, in a multiband dipole where the figures (all of
them) will vary wildly from band to band. Maximum power transfers at
resonance (oh no, let's not get into a war defining that). But I think it
is safe to say by anyone's definition, a multiband dipole usually not
operated at it's resonant frequencies. If the line is 450 ohm and the
antenna is 2 ohm or 20 ohm or 2000 ohm, there is not resonance.
Intuitively, I have got to think a 200:1 mismatch is significant.

So what is the loss at the antenna/line junction? I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.

Gene Fuller, W4SZ wrote:
"The problem comes from the higher losses that occur in even the
"lossless" transmission line when operating at high voltages and
currents."

That`s right. Gene put quotes around "lossless".

The power output of the transmitter equals thaat taken by the load
inspite of hier indicated forward power. The difference is a power that
continues to circulate, much as baggage on an airport carrousel
continues to circulate until it is taken away.

Best regards, Richard Harrison, KB5WZI

Craig Buck wrote:
If the line is 450 ohm and the
antenna is 2 ohm or 20 ohm or 2000 ohm, there is not resonance.
Intuitively, I have got to think a 200:1 mismatch is significant.

But consider a 9:1 mismatch using 450 ohm feedline. The impedance
at the current maximum point on the feedline is 50 ohms.

So what is the loss at the antenna/line junction? I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.

It's not ignored. It is taken into account by the losses in the feedline.
The power reflected by the antenna is not the antenna's problem. It is the
feedline's problem. A two ohm copper or aluminum antenna is probably very
efficient. It is just hard to feed directly.
--
73, Cecil http://www.qsl.net/w5dxp



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Buck, K4IA wrote:
"If the line is 450 ohm and the antenna is 2 ohm or 20 ohm or 2000 ohm,
there is not resonance."

A resonant dipole has a drivepoint impedance that varies from near zero
ohms at zero elevation above the earth to about 100 ohms at 0.3
wavelength above the earth. For harmonic resonances, the radiation
resistances and the drivepoint resistances are higher than at the first
(1/2-wave) resonance.

The antenna may be devoid of reactance (resonant) and have divergent
drivepoint resistances depending upon which resonance, 1st, 2nd, 3rd,
etc, and placement of the antenna with respect to its surroundings.
Resonance and impedance matching are two different things.

Best regards, Richard Harrison, KB5WZI

On Wed, 7 Jan 2004 11:43:17 -0500, "Craig Buck" wrote:

I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.

Hi Craig,

Because the match performs a complete reflection without Rloss and the
antenna performs a less than complete reflection with Rr. At least in
theory. The Rr eventually claims all the power. If you wish, this
could be thought of a dampened ringing to every cycle of transmission.
If you compare the length of the storage component, the transmission
line, to the Wavelength; then such a dampened wave diminishes to
inconsequence within microseconds. Your ear (or your contact rather)
will never witness the blur of data unless your transmission line is
several milliseconds long (and its loss will certainly snub the effect
which still means no one will ever hear it).

Leaving the purity of theory behind, the match is not without Rloss as
every physical component exhibits some value even if immeasurable by
common instrumentation. Its comparison to the Rr of the antenna,
again, bears on efficiency.

This scenario of the match interface to antenna interface is akin to
the resonant cavity of the Laser whose light bounces between two
mirrors, one perfect, the other partially transmitting, but always
with a significant emission. Your speculated SWR of 200 only
amplifies the loss of the system and is not typically encountered AND
ignored.

A typical SWR of 1.2 or 1.5 or 2 or 3 or 5 could be ignored with
impunity, but the scale of loss is in the ohmic resistance of the
system components. The hazard of high SWR is more to the source and
components (arc over or melt down). If you had a 5 Ohm Rr antenna
with a 45 Ohms of ohmic resistance, the evident SWR of 1:1 would not
guarantee an efficient solution. More folks pay attention to THAT
than a 2:1 for a 25/100 Ohm Rr.

73's
Richard Clark, KB7QHC