On Wed, 27 Apr 2011 09:40:14 -0700 (PDT), JIMMIE
wrote:

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.

Hi Jimmie,

This is somewhat cryptic, you would be expected to need a tuner.

While I didnt damage my transmitter

This transcends cryptic. It's like saying that as you came to a stop
sign, you used your brakes and you didn't damage your car.

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 .

Two different circuits inside, hence two different loads. Two
different loads, hence two different matches. Some receive paths
might be 50 Ohms, but that value is not as essential as for
transmission.

73's
Richard Clark, KB7QHC

On 27 abr, 18:40, JIMMIE wrote:
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

Hello Jimmie,

I noticed this also when I was experimenting with CB equipment and
simple antenna experiments.

For several CB transceivers I could get more output by slightly
mismatching the load as seen by the PA (but in many cases with too
much increase in current consumption).

I tried to use a matcher/tuner (and later a high Q resonator) to
reject other stations close by and then I figured out that when
applying mismatch to the receiver, the S-meter moved further.

The above isn't strange. As PA's are mostly not designed to show 50
Ohms, many receivers are also not designed to show 50 Ohms. I am not
discussing wide band receivers (for example digital or analog video).

Most active devices have lowest noise figure when driven from a source
impedance that is far from the input impedance of the active device.
If you want them to be equal, you need to use feedback and that
complicates the design.

Also filters with significant pass band ripple show, even when
designed for 50 Ohms, significant input reflection when referenced to
50 Ohms

With kind regards,

Wim
PA3DJS
www.tetech.nl

JIMMIE wrote:

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 .


Not surprising at all.

Consider the whole black art of adjusting the input match for lowest
noise figure, which may or may not coincide with the largest output
signal for a given input.

All those theorems about matching always have an asterisk about the
assumption that they're reciprocal linear devices with constant
impedances, etc. Start putting nonideal active devices in the mix, and
life gets interesting.

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

On Apr 26, 1:59*am, Richard Clark wrote:
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

Thanks Richard. Intuition is that the Zs is near 50 ohms for as many
frequencies as the designer can manage. I am on record (including in
this group) of letting intuition lead me down the path to ruin.

I get from you that there's a presupposition that I know the source
impedance or can easily establish it. Hm-m-m ... not so. One big
problem I see is the need to try to measure power delivered in a
non-50-ohm system with my existing instruments that depend on a 50-ohm
system. I don't have a nominal 75-ohm power meter. Won't putting a
50-ohm meter into a 75-ohm circuit not only read wrong but introduce
reflection losses, too?

I think I'd need a collection of non-inductive load resistors and an
accurate rf ammeter. I'd need to connect them and calculate power at
a few points in every band.

Maybe the papers that Cecil cited for me will fill in the gaps or
suggest other approaches.

"Sal"

On Tue, 26 Apr 2011 14:37:58 -0700 (PDT), "Sal M. Onella"
wrote:

Thanks Richard. Intuition is that the Zs is near 50 ohms for as many
frequencies as the designer can manage. I am on record (including in
this group) of letting intuition lead me down the path to ruin.

Well, if you miss the path, you are certain to be reminded where it
is.

I get from you that there's a presupposition that I know the source
impedance or can easily establish it. Hm-m-m ... not so.

It is printed in the specifications. There are other ways to derive
it, of course, and they would merely confirm that number.

One big
problem I see is the need to try to measure power delivered in a
non-50-ohm system with my existing instruments that depend on a 50-ohm
system. I don't have a nominal 75-ohm power meter. Won't putting a
50-ohm meter into a 75-ohm circuit not only read wrong but introduce
reflection losses, too?

It is easier to measure voltage and current and use your calculator.
Now having said that, measuring voltage and current is damned hard at
HF. It is achievable with care, but now we are back into your same
question with many hands pointing at that path to ruin.

As both are difficult (power or voltage times current), you could
trust authority (which confirms what is intuitive), or you could
listen to argument (which at the distance of time and recall becomes
murky and opaque).

I think I'd need a collection of non-inductive load resistors and an
accurate rf ammeter. I'd need to connect them and calculate power at
a few points in every band.

Bravo! This reduces complexity because RF Power resistors of high
accuracy and bandwidth are commercially available. You will need to
practice your skill at mounting to a heat sink, however (another
non-trivial achievement).

Maybe the papers that Cecil cited for me will fill in the gaps or
suggest other approaches.

Walt and I have been corresponding over these matters just these past
two weeks. Skimming that content will once again confirm intuition.

73's
Richard Clark, KB7QHC

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

On Apr 26, 7:20*am, Cecil Moore wrote:
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

Well, I have _now_! 'Twill take just a bit of time to digest them.
Thanks.

"Sal"

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.

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