Richard Fry wrote in news:908e1320-965a-4e11-b02b-
:

On Jun 11, 6:16*am, Owen Duffy wrote:

... The transmitter output power is probably different ...

Thank you, Owen.

Do your comments apply to a transmitter designed/adjusted for, and
expecting a 50 + j 0 ohm load?

In answer to the question you directly asked, yes. But that says nothing
of whether such a transmitter, in the general case, is well represented
by a Thevenin equivalent circuit with Zeq=50+j0.

As you know, there is a proposition that a transmitter "designed/adjusted
for, and expecting a 50 + j 0 ohm load" can be well represented by a
Thevenin equivalent circuit and naturally has Zeq=50+j0. However, that
proposition is easily proven wrong by valid experiments in the real
world, and those of us who have done such experiments are disinclined to
accept the proposition.


IOW, if the net output power of such a transmitter (which equals that
dissipated in the load) probably is different with such a mismatch, do
you expect the reason for that to be related to "reflected power?"

Average power at the transmitter terminals is given by the average value
of the instantaneous product of v and i over time. There is no need for
"net" in the calculation. The average power delivered by the transmitter
will depend on the load impedance (ie the complex ratio of v/i) at its
load terminals, but that dependence is not well predicted in the general
case by a Thevenin equivalent circuit.

I gave a link to a simple experiment to test Zeq of a tx, and that uses
equipment found in most ham shacks. The article is at
http://vk1od.net/blog/?p=1028 .

Owen

On Wed, 9 Jun 2010 19:00:37 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

If you are designing a tuner, where would you design it to go?

Hi Geoff,

You would put it closer to the antenna.

If you connect a tuner and it is "tuned", none of the power is reflected
back to the transmitter.
Obviously it has to go somewhere.

From the perspective of the transmission line in between, when it hits
the discontinuity of the tuner, it is reflected back to the antenna.
The antenna drains away some of that power (just as it did in the
original pass). The process repeats (the antenna is still partially
reflective) and is further sustained with new cycles of energy.

There are finer details that shift these dynamics.

73's
Richard Clark, KB7QHC

Geoffrey S. Mendelson wrote:
If you have a resonant antenna, supposedly all of the power is radiated.

(SWR 1:1)

If you have a nonresonant antenna, some of the power is reflected back to
the transmitter.

(SWR 1:1)

If you connect a tuner and it is "tuned", none of the power is reflected
back to the transmitter.

(SWR at transmitter 1:1, at antenna still 1:1)

Obviously it has to go somewhere.
Differentiate between an antenna that happens to present the wrong
non-reactive resistance... the tuner is just a transformer (whether done
with linked magnetic fields, or by a narrow band matching network of Ls
and Cs)

and an antenna that presents a reactive feedpoint impedance.
In this case: energy circulates between the tuning network and the antenna.

Say your antenna presents a Z that is inductive and you put a parallel
capacitor across the feed to exactly cancel the "inductance". What's
really happening is that you have the equivalent of an LC tank where the
energy moves back and forth between L and C every cycle. In the classic
LC, the energy moves between the magnetic field of the L and the
electric field of the C. In the antenna case, it's somewhat more
complex: the antenna stores energy in the near field in both electric
and magnetic fields.

That answers the question...
The problem is that nothing is lossless, so as it moves, there's a loss.
If it's a resistive loss, it goes as I^2, so doubling the current
results in 4 times the loss. And, if you have an antenna with high
stored energy (about which more, later), this square of the current
means bad news.




Many antennas don't have an explicit separate matching network, but do
the cancelling by doing it within the antenna structu say by
changing element lengths, etc... so now that "circulating current" is
circulating between different parts of the antenna.

In fact, the ration between that stored energy and the amount flowing
"through" (i.e. radiated away) is related to the directivity of the
antenna: high directivity antennas have high stored energy (large
magnetic and electric fields): the ratio of stored to radiated energy
is "antenna Q" (analogous to the stored energy in a LC circuit leading
to resonant rise).

So, high directivity = high stored energy = high circulating energy =
high I2R losses.

It circulates between tuner and antenna.




Where?

If you are designing a tuner, where would you design it to go?

Thanks in advance

Geoff.

On Jun 9, 5:38*pm, Jim Lux wrote:
It circulates between tuner and antenna.

Just a nit: A certain magnitude of energy circulates between tuner and
antenna. Experiments with TV signal ghosting prove that it is not the
identical energy, just the same magnitude of energy.
--
73, Cecil, w5dxp.com

On Jun 9, 10:38*pm, Jim Lux wrote:

In fact, the ration between that stored energy and the amount flowing
"through" (i.e. radiated away) is related to the directivity of the
antenna: high directivity antennas have high stored energy (large
magnetic and electric fields): *the ratio of stored to radiated energy
is "antenna Q" (analogous to the stored energy in a LC circuit leading
to resonant rise).

So, high directivity = high stored energy = high circulating energy =
high I2R losses.

this is a relationship i haven't heard of before... and would be very
wary of stating so simply. it may be true for a specific type of
antenna, MAYBE Yagi's, MAYBE rhombics or or close coupled wire arrays,
but some of the most directive antennas are parabolic dishes which i
would expect to have very low Q and extremely low losses. you could
also have an antenna with very high Q, very high i^2r losses, but very
low directivity, so i would be careful about drawing a direct link
between the two.

On Jun 11, 10:45*am, K1TTT wrote:
On Jun 9, 10:38*pm, Jim Lux wrote:



In fact, the ration between that stored energy and the amount flowing
"through" (i.e. radiated away) is related to the directivity of the
antenna: high directivity antennas have high stored energy (large
magnetic and electric fields): *the ratio of stored to radiated energy
is "antenna Q" (analogous to the stored energy in a LC circuit leading
to resonant rise).

So, high directivity = high stored energy = high circulating energy =
high I2R losses.

this is a relationship i haven't heard of before... and would be very
wary of stating so simply. *it may be true for a specific type of
antenna, MAYBE Yagi's, MAYBE rhombics or or close coupled wire arrays,
but some of the most directive antennas are parabolic dishes which i
would expect to have very low Q and extremely low losses. *you could
also have an antenna with very high Q, very high i^2r losses, but very
low directivity, so i would be careful about drawing a direct link
between the two.

and yes, this does work for complex loads and multiple stubs and
connections to the line.

this is a reasonable description of the derivation of these
techniques, study especially the Thevenin equivalent impedance
representation on page 2-13 and how it is applied:
http://ee.sharif.edu/~comcir/readings/tran%20line.pdf

K1TTT wrote:
On Jun 9, 10:38 pm, Jim Lux wrote:
In fact, the ration between that stored energy and the amount flowing
"through" (i.e. radiated away) is related to the directivity of the
antenna: high directivity antennas have high stored energy (large
magnetic and electric fields): the ratio of stored to radiated energy
is "antenna Q" (analogous to the stored energy in a LC circuit leading
to resonant rise).

So, high directivity = high stored energy = high circulating energy =
high I2R losses.

this is a relationship i haven't heard of before... and would be very
wary of stating so simply.

I should have used arrows rather than equals signs.
But it's basically a manifestation of Chu's idea combined with practical
materials.

Chu proposed the concept relating directivity and stored energy and
physical size. A passively excited multi element array (like a Yagi) has
to transfer energy from element to element to work, and it follows the
characteristics outlined by Chu. And anything with circulating energy
that gets carried by a conductor is going to have high(er) I2R losses
than something that doesn't.


it may be true for a specific type of
antenna, MAYBE Yagi's, MAYBE rhombics or or close coupled wire arrays,
but some of the most directive antennas are parabolic dishes which i
would expect to have very low Q and extremely low losses.

Interesting case there. Loss isn't all that low (typical parabolic
antennas with their feed have an efficiency of 50-70%), although it IS
low compared to the directivity. And, in fact, there's not much stored
energy (so the Q is low). On the other hand a parabolic antenna is
physically very large compared to a wavelength, so the Chu relationship
holds. I'd have to think about whether one can count the energy in the
wave propagating from feed to reflector surface as "stored", but I think
not. Probably only the E and H fields at the reflector surface.


you could
also have an antenna with very high Q, very high i^2r losses, but very
low directivity, so i would be careful about drawing a direct link
between the two.

Yes.. you're right.. the relations set an upper bound on what's
possible.. That is, for a given directivity, you can get either small
size and large stored energy (the Yagi-Uda or W8JK), or large size and
small stored energy (the parabolic reflector and feed). As you note, a
dummy load has very low directivity.