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)

"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.

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.

On Tue, 26 Apr 2011 13:12:23 -0700, Jim Lux
wrote:

(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)

I can see why this is parenthetical, because it covers a lot of sins
of omission.

First we bang up against the wall of Gain Bandwidth Product. If you
are talking about resistive loads at low power DC, then your statement
is trivially valid.

Second, the ability to "simulate" any arbitrary output (or input for
that matter) impedance for an OP AMP is well defined in the closed
loop gain (which robs from the open loop gain for frequency by
proportion to GBP).

Taking the conventional RF Power Deck of any consumer (Ham) product,
the similarity to an OP AMP is wholly foreign, and for good and
commercial reason. If one were motivated to engineer in the necessary
noise amplifier (a term coined by H.W. Bode who defined this topic of
source Z and applied it to the negative gain or feedback path); then
we would find that the exact same loss is exhibited in the exact same
component(s).

However, by virtue of OP AMP characteristics we would benefit to
vastly better distortion figures, far less spurious content, and
virtually no need for either the conventional impedance transformer,
nor the bandwidth filter that follows the same power deck (provided,
of course, that the drive input is sinusoidal - which it never is,
unfortunately, for this scenario). This novel OP-AMP/Power-Deck
redesign would also confer considerable power supply rejection (that
voltage could sag or rise without appreciable effect) and noise
rejection (the internal noise from other circuitry would not migrate
into the signal output). ALL such benefits are strictly derived from
the amount of negative feedback (not to be confused, as are many
readers to this topic, with the rather ordinary compensation cap in
the last stage).

Why isn't this done as a service to the customer?

Cost.

Again, OP AMP design merits are paid for in lost gain and bandwidth.
The price is found in the amount of negative feedback that goes to
lower the overall amplification. Would you pay for this improved cool
performance to run 10W in the 80M band from a formerly crackly and hot
100W 10-80M band capable source?

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Tue, 26 Apr 2011 13:12:23 -0700, Jim Lux
wrote:

(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)

I can see why this is parenthetical, because it covers a lot of sins
of omission.

yep.. not possible to build such a thing, anymore than one can build a
zero ohm output impedance RF source with any signficant power.
Suggested more as an example that the power dissipation in the source
doesn't necessarily correlate with match, load Z, or anything else in
general.

(You can get pretty darn close at powers less than a watt and HF, though..)


However, by virtue of OP AMP characteristics we would benefit to
vastly better distortion figures, far less spurious content, and
virtually no need for either the conventional impedance transformer,
nor the bandwidth filter that follows the same power deck (provided,
of course, that the drive input is sinusoidal - which it never is,
unfortunately, for this scenario). This novel OP-AMP/Power-Deck
redesign would also confer considerable power supply rejection (that
voltage could sag or rise without appreciable effect) and noise
rejection (the internal noise from other circuitry would not migrate
into the signal output). ALL such benefits are strictly derived from
the amount of negative feedback (not to be confused, as are many
readers to this topic, with the rather ordinary compensation cap in
the last stage).

One can also do a lot of this with various clever schemes if the input
to your PA is coming out of some signal processing. Generically,
predistortion, but it can be so much more.
People have literally spent their lives working out ever more
sophisticated approaches



Why isn't this done as a service to the customer?

Cost.

Like race cars... how fast do you want to go..just bring money

On Tue, 26 Apr 2011 15:27:28 -0700, Jim Lux
wrote:

(If I had a very efficient op amp, I could simulate any arbitrary output
impedance, without dissipating any power in the source)

I can see why this is parenthetical, because it covers a lot of sins
of omission.

yep.. not possible to build such a thing, anymore than one can build a
zero ohm output impedance RF source with any signficant power.
Suggested more as an example that the power dissipation in the source
doesn't necessarily correlate with match, load Z, or anything else in
general.

(You can get pretty darn close at powers less than a watt and HF, though..)

OP AMPs are a constant of my admiration in the possibilities offered.
That and the signal processing you suggest (plus digital oscillators)
"could" change the playing field - if conventional design weren't so
universally fallen back upon.

73's
Richard Clark, KB7QHC

On 26 abr, 02:59, "Ralph Mowery" wrote:
"Sal M. Onella" wrote in ...



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.

Hello Ralph,

The actual output impedance can be anything, but is mostly not 50
Ohms. If you want it close to 50 Ohms, you have to spend money in
components and design time. As 50 Ohms isn't mostly required, one will
not design for that.

Just as an example, take a hard-driven totem pole or push pull stage
with only a series tuned circuit to suppress harmonics (so the LC
circuit shows zero ohms at the carrier frequency). As the active
devices are used a switches, the output impedance of this arrangement
is almost zero (at least far below 50 Ohms).

When you connect a 50 Ohms quarter-wave cable between the output and
the 50 Ohms load, the amplifier-cable combination has very high output
impedance (quarter wave transformer formula).

For power amplifiers, there is no relation between actual output
impedance and efficiency. When an amplifier is designed for 50 Ohms,
it only means that the amplifier will work correctly when terminated
with 50 Ohms. When you deviate from that, output power may decrease
or increase. This may result in more or less stress on the amplifier's
components.

With kind regards,

Wim
PA3DJS
www.tetech.nl

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

On Apr 25, 6:07*pm, Jim Lux wrote:
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

I agree with Jim. While it's true that if a source (transmitter) is
tuned for maximum output, the output impedance must necessarily be the
conjugate of the load impedance, it is NOT generally the case that the
transmitter is tuned for maximum output. Rather, the transmitter is
tuned for an output that won't destroy the output devices and will
result in acceptable distortion (in the case of a linear amplifier).
There are plenty of cases of sources designed to be loaded with an
impedance far different from their output (source) impedance: the AC
power line, audio amplifiers, ... .

A while back, I set up a couple precision high power directional
couplers so I could measure the output impedance of a couple different
ham rigs. In the case of the rig with vacuum tube output stage, if I
operated the output stage with limited grid drive and tuned the plate
tank for maximum output power, indeed the output impedance was 50
ohms, within the tolerance of my ability to adjust the output for
maximum. But if I increased the grid drive for solid class-C
operation and tuned for the rated output power (which is no longer the
maximum possible power), the impedance seen at the output dropped. If
you work through the pi-network transformation back to the vacuum tube
plates, it's apparent that the plates under those operating conditions
represent a considerably higher source impedance than when things are
tuned for maximum available power (as first described).

But coming back to "Sal's" original question, it's always made sense
to me given the availability of inexpensive 75 ohm line with low loss
to go ahead and use it to feed antennas that have a feedpoint
impedance closer to 75 ohms than to 50 ohms. If you need to provide a
bit of matching at the transmitter end so that the transmitter is
operating correctly, it should be straightforward to do that. But
whether the actual source impedance of the transmitter is one value or
another is really of very little importance. The only time I can
think that it would matter is if you're trying to transmit a very
broadband signal and you don't want power that's reflected at the
transmission-line:antenna interface to re-reflect from the
transmitter:transmission-line interface and go back to the antenna,
delayed by enough to cause a "ghost" (in a television picture), for
example. In such a case, you'll be well served by insuring that the
antenna is well matched to the transmission line so there is an
insignificant reflection there anyway.

Cheers,
Tom

On Apr 25, 9:07*pm, Jim Lux wrote:
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

Being aware that this was the nature of my solidstate transceiver I
attempted to use a tuner with one to improve my match to my antenna
system. While I didnt damage my transmitter I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .

Jimmie