Transmitter Output Impedance
 

This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)

Transmitter Output Impedance
 

"Sal M. Onella" wrote in message
...
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)

A transmitter output impedance is designed for maximum power transfer at a
specific impedance. Most of the the older tube transmitters impedance was
tunable within a range.

In simple terms the impedance of the transmitter tube is the plate voltage
devided by the current. This impedance is then transformed to the nominal
50 ohms of the antenna system. If the transmitter has the usual tune and
load controls, the exect impedance will not mater as you adjust for maximum
transmitter output.

Most of the transistor transmitters are not adjustable so the output
impedance is usually fixed at 50 ohms for maximum power transfer. If the
impedance of the antenna system is not 50 ohms, then the output power will
be less than the designed output. You can use the antenna tuner to adjust
for a match.

Transmitter Output Impedance
 

Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).
http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm


Grant Bingeman also has words on this:
http://www.km5kg.com/loads.htm

Transmitter Output Impedance
 

On Mon, 25 Apr 2011 17:35:29 -0700 (PDT), "Sal M. Onella"
wrote:

Something I haven't seen is a discussion of the source impedance of
the transmitter.

sigh....

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio.

Hi OM,

Look at the prospective SWR and how much is
lost/reflected/absorbed/what-have-you? More heat comes from a less
than optimal system efficiency than what your computation will reveal.
So much that it will swamp it.

But the trick here is that the reflected "power" (arguments turn on
this word) doesn't always mean heat and it could actually cool -
however hot or cold it may alter the situation, that same "power"
never got out into the air.

Now, as for source impedance, that is a subject fraught with denial in
the face of the obvious: Those fins in the back of your rig are to
help bookend your QSL cards into groups (the heat bears no relation to
efficiency nor match loss).

A standard definition (courtesy of Wikipedia) for Return Loss is:
where Zs is the impedance toward the source and
Zl is the impedance toward the load.
and we find from the values you supply that it is
0.20

Of course, such a definition is utterly useless when the concept of Zs
is replaced with (in most cases) "it ain't 50 Ohms, thet's fur
shure").

If, perchance, some brave soul steps into the breach of NOT 50 Ohms to
suggest what Zs is, then we can give it the acid test of engineering
(an act that I am usually reminded is beyond the understanding of
readers and the province of discussion here). Let's be gentle and go
only by an order of two (which is reasonably available and can be
coaxed out of my TS-440). Return loss for a rig exhibiting an Zs of
25 Ohms into the 75 Ohm line (presuming it is infinite in length)
would give us:
0.50
that doesn't look good, so let's try Zs of 100 Ohms:
0.14
that looks better all 'round. Even intuition agrees.

Let's press intuition to the proximal limit and say that Zs is 74 Ohms
(yes, my thumb is on the scale):
0.01

What does intuition affirm? What is preferable?

73's
Richard Clark, KB7QHC

Transmitter Output Impedance
 

On Apr 25, 7:35*pm, "Sal M. Onella" wrote:
Wisdom in any form would be appreciated. *Thanks.

Have you seen these?

http://www.w2du.com/QEXMayJun01.pdf

http://www.w2du.com/Appendix12.pdf

http://www.w2du.com/r3ch19a.pdf
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

On 26 abr, 02:35, "Sal M. Onella" wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. *My curiosity was piqued today as I took some baby
steps into EZNEC. *A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. *Doesn't seem like it.

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. *My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. *(I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. *Thanks.

"Sal"
(KD6VKW)

Hello Sal,

Make yourself up for a long discussion (as we had such a thread
earlier).

Most amplifiers are designed just to provide the desired amount of
power into a certain load. The actual output impedance of the
amplifier is not important in many cases. Changing the load impedance
(for example in case of a solid state amplifier) may result in less or
more heat generated in the active device(s). You can see this by
changing the load (for example with an external matcher between power
meter and PA) and watching the output power and current consumption.

If you have an amplifier with tunable output section (vacuum tube PA),
you are within the range of the tuner, and you tune it for maximum
output power (given certain drive), the output impedance equals the
load impedance (or conjugated value in case of non-ohmic load). If
you change the drive (so adjust the output power), the output
impedance may change (due to saturation issues). Same is valid for the
load, if you change the load, voltage across and current through the
active device may saturate, hence changing the output impedance.

Non-tunable amplifiers (for example a 3…30 MHz balanced amplifier)
will mostly not present 50 Ohms to your load (unless specially
designed for that using feedback).

Virtually all high-efficient switching amplifiers do not show 50 Ohms
to the load. If you add an external tuner and match to maximum output
power, you will very likely destroy the amplifier in case of no
supervisory circuits present.

You can do some experiments with your own amplifiers. Just change the
load impedance and see what the forward power indicator on your
reflectometer/VSWR meter does. If it doesn't change, your output
impedance is very close to 50 Ohms.

Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.

With kind regards,

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely.

Transmitter Output Impedance
 

Ralph Mowery wrote:
"Sal M. Onella" wrote in message
...
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)

A transmitter output impedance is designed for maximum power transfer at a
specific impedance. Most of the the older tube transmitters impedance was
tunable within a range.

In simple terms the impedance of the transmitter tube is the plate voltage
devided by the current. This impedance is then transformed to the nominal
50 ohms of the antenna system. If the transmitter has the usual tune and
load controls, the exect impedance will not mater as you adjust for maximum
transmitter output.

Most of the transistor transmitters are not adjustable so the output
impedance is usually fixed at 50 ohms for maximum power transfer. If the
impedance of the antenna system is not 50 ohms, then the output power will
be less than the designed output. You can use the antenna tuner to adjust
for a match.


Not exactly..

A "match" provides the optimum power transfer from generator to load,
but that is NOT the maximum load power, nor is it either the maximum or
minimum power dissipated in the source.

Say I have a zero output impedance on my source and I'm putting out 7
Volts RMS into a 50 ohm load. That's about 1 watt into the load. Now..
if I reduce the load impedance to 25 ohms, and since I've got zero
output impedance, I'm now putting out 2 Watts. The source impedance is
zero, so I'm not dissipating any extra power in the source, either.

It is true that a "matched load" to my zero ohm source would, in fact,
be zero ohms, and would have infinite power. Any other load impedance
would have less power into the load, so the Thevenin theorem is satisfied.


Now.. if my generator had a fixed output impedance, it's true that the
load impedance that will get the most power out is the conjugate of the
output Z. For resistive sources/loads, here's an example..

You also have to be careful about looking at Thevenin equivalent sources
(e.g. a ideal voltage in series with a Z, or a ideal current in parallel
with a Z), because just because *the model* has a resistor in series
does NOT mean that you're actually dissipating any power in the source.
(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)


Say my generator is 40 ohms, and I'm putting out 7 Volts into a 40 ohm
load. OK, that means that the imaginary voltage source is putting out
14 V. I'm getting about 1.23 Watts into my load. Now, if I decrease my
load Z to 20 ohms, what do I get? Now, I have 4.67 (=14/3) Volts
instead of 7, and I get 1.1 Watts. Yep, less.. Thevenin works. Let's
try increasing the Z to 60 ohms.. Now the voltage on my load is 8.4 V,
and I'm dissipating 1.18W, again, less than my 1.23.

But here's some weird stuff.. let's look at how much power is dissipated
in that imaginary resistor (i.e. our source *really is* a ideal voltage
source in series with a resistor)

At 40 ohm load, we've got 7 volts on the load and 7 volts across the
resistor, so they both dissipate the same 1.225 Watts. Pload/Pgen = 1

In the 20 ohm load case, we've got 1.1 dissipated in the load and 2.2
dissipated in the generator. Pload/Pgen = 0.5

In the 60 ohm load case, we've got 1.18 dissipated in the load, and 0.78
dissipated in the generator. Pload/Pgen = 1.5 (i.e. we dissipate more
in the load than in the generator... how about that!)

And let's look at "efficiency" of the system (assuming that the total
power in is the sum of what's dissipated in the generator and the load)

20 ohm load, 33%
40 ohm load, 50% (what you'd expect)
60 ohm load, 60% (hey.. it's more efficient, too)

- take home message... a "good match" is sort of an artificial thing
from a power transfer standpoint.. it depends on what you're trying to
optimize for.

- Where you get bitten is when "match" varies with frequency... now,
all of a sudden, you have a system that has a response that varies with
frequency, which is generally undesirable. When you get up into the
microwave region, where a transmission line is often many wavelengths
long, that mismatch can result in remarkably wild fluctuations in gain
with respect to frequency.

Transmitter Output Impedance
 

Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.


ON9CVD's website I linked to has a very simple technique.. 50 ohm dummy
load and a 220 ohm resistor you can switch in.

At 100W (into 50 ohms), the 220 ohms would only dissipate 22W. You
could get some of those non-inductive resistors from Caddock and series
them up to do something like this.

BTW, this is a simplified version of what's called a "load pull" test...
which makes me wonder if one could cobble up a quick test set that could
be controlled by a computer to do automated output Z measurements of an
HF transceiver over a reasonably wide range... One approach would be to
use a RS-232 controlled antenna tuner and, maybe, a antenna relay box
with several different load resistances).

The challenge (having actually looked at doing this with a LDG AT200PC)
is that the Z of the tuner isn't very well defined. It's a pretty big
calibration project in itself.

Maybe, though, one could build a few test dummy loads.. say a 25 ohm and
a 75 or 100 ohm, along with your vanilla 50 ohm, and the antenna switch
(like an RCS-8V). Basically, you're building a "high power resistor
substitution box"

You'd want some sort of nice inline watt meter (like an LP100) to make
the measurements.

Transmitter Output Impedance
 

Jim Lux wrote:
Ralph Mowery wrote:
"Sal M. Onella" wrote in message
...
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)

A transmitter output impedance is designed for maximum power transfer
at a specific impedance. Most of the the older tube transmitters
impedance was tunable within a range.

In simple terms the impedance of the transmitter tube is the plate
voltage devided by the current. This impedance is then transformed to
the nominal 50 ohms of the antenna system. If the transmitter has the
usual tune and load controls, the exect impedance will not mater as
you adjust for maximum transmitter output.

Most of the transistor transmitters are not adjustable so the output
impedance is usually fixed at 50 ohms for maximum power transfer. If
the impedance of the antenna system is not 50 ohms, then the output
power will be less than the designed output. You can use the antenna
tuner to adjust for a match.


Not exactly..

A "match" provides the optimum power transfer from generator to load,
but that is NOT the maximum load power, nor is it either the maximum or
minimum power dissipated in the source.

Say I have a zero output impedance on my source and I'm putting out 7
Volts RMS into a 50 ohm load. That's about 1 watt into the load. Now..
if I reduce the load impedance to 25 ohms, and since I've got zero
output impedance, I'm now putting out 2 Watts. The source impedance is
zero, so I'm not dissipating any extra power in the source, either.

It is true that a "matched load" to my zero ohm source would, in fact,
be zero ohms, and would have infinite power. Any other load impedance
would have less power into the load, so the Thevenin theorem is satisfied.


Now.. if my generator had a fixed output impedance, it's true that the
load impedance that will get the most power out is the conjugate of the
output Z. For resistive sources/loads, here's an example..

You also have to be careful about looking at Thevenin equivalent sources
(e.g. a ideal voltage in series with a Z, or a ideal current in parallel
with a Z), because just because *the model* has a resistor in series
does NOT mean that you're actually dissipating any power in the source.
(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)


Say my generator is 40 ohms, and I'm putting out 7 Volts into a 40 ohm
load. OK, that means that the imaginary voltage source is putting out
14 V. I'm getting about 1.23 Watts into my load. Now, if I decrease my
load Z to 20 ohms, what do I get? Now, I have 4.67 (=14/3) Volts
instead of 7, and I get 1.1 Watts. Yep, less.. Thevenin works. Let's
try increasing the Z to 60 ohms.. Now the voltage on my load is 8.4 V,
and I'm dissipating 1.18W, again, less than my 1.23.

But here's some weird stuff.. let's look at how much power is dissipated
in that imaginary resistor (i.e. our source *really is* a ideal voltage
source in series with a resistor)

At 40 ohm load, we've got 7 volts on the load and 7 volts across the
resistor, so they both dissipate the same 1.225 Watts. Pload/Pgen = 1

In the 20 ohm load case, we've got 1.1 dissipated in the load and 2.2
dissipated in the generator. Pload/Pgen = 0.5

In the 60 ohm load case, we've got 1.18 dissipated in the load, and 0.78
dissipated in the generator. Pload/Pgen = 1.5 (i.e. we dissipate more
in the load than in the generator... how about that!)

And let's look at "efficiency" of the system (assuming that the total
power in is the sum of what's dissipated in the generator and the load)

20 ohm load, 33%
40 ohm load, 50% (what you'd expect)
60 ohm load, 60% (hey.. it's more efficient, too)

- take home message... a "good match" is sort of an artificial thing
from a power transfer standpoint.. it depends on what you're trying to
optimize for.

- Where you get bitten is when "match" varies with frequency... now,
all of a sudden, you have a system that has a response that varies with
frequency, which is generally undesirable. When you get up into the
microwave region, where a transmission line is often many wavelengths
long, that mismatch can result in remarkably wild fluctuations in gain
with respect to frequency.

Vgen Rgen Rload V load I load P load Pgen Pload/Pgen Ptotal Pload/Ptotal
10 40 20 3.33 0.17 0.56 1.11 0.50 1.67 0.33
10 40 40 5.00 0.13 0.63 0.63 1.00 1.25 0.50
10 40 60 6.00 0.10 0.60 0.40 1.50 1.00 0.60

Transmitter Output Impedance
 

On Apr 26, 10:44*am, Wimpie wrote:
The actual output impedance of the
amplifier is not important in many cases.

Yes, consider a source with 70.7 volts at its output terminals
connected to a 50 ohm load through a 50 ohm feedline. Except for
source efficiency, the source impedance simply doesn't matter. Any
70.7 volt source will deliver 100 watts to the feedline no matter what
the source impedance.

There's a simple method called load-pulling. Keeping everything the
same at the source, if the power to the load increases when the load
is changed away from 50 ohms, the source doesn't have a 50 ohm
internal impedance.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK