"Bob Nielsen" wrote in message
...
(Jumping in after this argument has gone on for a long time)

This has evolved somewhat into an "apples and oranges" discussion. What
Richard Fry appears to be concerned with is the reflection of a video
pulse by the load (antenna) and its re-reflection (at a later time,
dependent on the length and Vp of the transmission line) by the source
(transmitter) and subsequent radiation (as a ghost image--the effect
repeats, ad infinitum).

While this is certainly a real phenomenon, its effect is a function of
both the mismatch at the load (S11) and the mismatch looking from the
transmission line back toward the source (S22). It does NOT influence
the VSWR and could, obviously, be significantly reduced by the use of a
circulator at either end of the transmission line (which may not be
practical at the power levels of televison transmitters, to use his
example.)

73,
Bob Nielsen, N7XY

To shed more light on this, when I tried to find information in the
literature about transmitter output impedance, I ran across two cases where
reflections from the transmitter are bad. One was the case of multiple
transmitting antennas on one tower; a particular antenna could pick up
another transmitter, and then re radiate it with some delay. The second was
the case of a wide band spread spectrum transmitter, where it is not
possible to have a good antenna match on all frequencies.

Tam/WB2TT

On Sun, 5 Sep 2004 20:25:31 -0400, "Tam/WB2TT"
wrote:

when I tried to find information in the
literature about transmitter output impedance, I ran across two cases where
reflections from the transmitter are bad.

Hi All,

It doesn't take much research into the matter of matching and Harris
Transmitters to find from their Digital AM stations:

The newest Harris AM Transmitters of 10 KW and higher (up to 1
megawatt and more) are totally solid state and use a completely
different modulation system called Digital AM Modulation. This
scheme is another Harris Patented system, using a large quantity
of plug-in modules, each one generates RF at a different
modulation level; in order for this to function, any incoming
audio (if analog) is broken down into digitized data, then used to
drive the various modules at the varying levels needed to have an
amplitude modulated output. It sounds pretty simple, in fact,
each module has a torroid (coil) which is the load for that
module; all those torroids are lined up and an iron pipe is run
through these torroids so that the combined output of all the
modules is coupled into this pipe. One end of the pipe is at
ground, the other end is the RF output of the transmitter. There
is an output network to match impedance of the pipe, which is
probably only a few Ohms, to the required output impedance which
is usually 50 Ohms, although sometimes high power transmitters are
set up to provide 75 Ohms, once in a while even 300 Ohms, to drive
an open wire type transmission line system.

Hardly a wit difference between Geoff's first FM amplifier design that
was documented for FCC type acceptance and now when it comes to
matching - all very commonplace and not even a hair out of place in
the conventional wisdom found in Terman (one of his references).

As for the specification for output impedance gone missing :-)
MW-10B SPECIFICATIONS
RF OUTPUT IMPEDANCE: 50 ohms, unbalanced.
Other output impedances available on special order.

Harris Platinum Z FM transmitter
100 ohm output impedance (unbalanced)

HARRIS SW-50 A
RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR

Well, instead of reciting their complete catalogue, it is easier to
simply say I could not find any product that did not specify an output
impedance, much less that others were available on special order.

One wonders if it doesn't matter, do they just change the spec page
and the sticker above the connector? ;-)

73's
Richard Clark, KB7QHC

On Mon, 06 Sep 2004 01:23:04 GMT, Richard Clark wrote:

On Sun, 5 Sep 2004 20:25:31 -0400, "Tam/WB2TT"
wrote:

when I tried to find information in the
literature about transmitter output impedance, I ran across two cases where
reflections from the transmitter are bad.

Hi All,

It doesn't take much research into the matter of matching and Harris
Transmitters to find from their Digital AM stations:

The newest Harris AM Transmitters of 10 KW and higher (up to 1
megawatt and more) are totally solid state and use a completely
different modulation system called Digital AM Modulation. This
scheme is another Harris Patented system, using a large quantity
of plug-in modules, each one generates RF at a different
modulation level; in order for this to function, any incoming
audio (if analog) is broken down into digitized data, then used to
drive the various modules at the varying levels needed to have an
amplitude modulated output. It sounds pretty simple, in fact,
each module has a torroid (coil) which is the load for that
module; all those torroids are lined up and an iron pipe is run
through these torroids so that the combined output of all the
modules is coupled into this pipe. One end of the pipe is at
ground, the other end is the RF output of the transmitter. There
is an output network to match impedance of the pipe, which is
probably only a few Ohms, to the required output impedance which
is usually 50 Ohms, although sometimes high power transmitters are
set up to provide 75 Ohms, once in a while even 300 Ohms, to drive
an open wire type transmission line system.

Hardly a wit difference between Geoff's first FM amplifier design that
was documented for FCC type acceptance and now when it comes to
matching - all very commonplace and not even a hair out of place in
the conventional wisdom found in Terman (one of his references).
snip
Well, instead of reciting their complete catalogue, it is easier to
simply say I could not find any product that did not specify an output
impedance, much less that others were available on special order.

One wonders if it doesn't matter, do they just change the spec page
and the sticker above the connector? ;-)

73's
Richard Clark, KB7QHC

To Rich Fry,

Rich, I just now saw Richard Clark's post above. Until reading it I was totally
unaware that high-power BC transmitters used solid-state final amps.

Consequently, my comments were all directed toward tx's using tubes and LC
networks to isolate the non-linear input from the linear output. I'm not at all
familiar with the circuitryused in solid-state amps, please interpret my
comments accordingly.

Walt

"Walter Maxwell" wrote to R. Fry:
I'm not at all familiar with the circuitry used in solid-state
amps, please interpret my comments accordingly.
__________

Understand, and will keep that in mind. However I won't be
reading/responding to anything for the next few days, as I'll be taking a
short "R&R" trip.

RF

"Richard Clark" wrote about the specs for Harris Broadcast Txs:
As for the specification for output impedance gone missing :-)
MW-10B SPECIFICATIONS
RF OUTPUT IMPEDANCE: 50 ohms, unbalanced.
Other output impedances available on special order.
Harris Platinum Z FM transmitter
100 ohm output impedance (unbalanced)
HARRIS SW-50 A
RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR
Well, instead of reciting their complete catalogue, it is easier to
simply say I could not find any product that did not specify an output
impedance, much less that others were available on special order.
_____________

I won't try to soften this: your conclusion above doesn't just _appear_ to
be wrong, it IS wrong. 100% wrong. The impedance published by Harris is the
expected load impedance, not the tx source impedance -- which is the
convention used by all OEMs of broadcast txs (at least).

How do I know? I was the author of all of the brochures and technical data
sheets for Harris' entire FM product line for the ten years before I
retired. That any published value of impedance applied to the load
impedance expected was/is universal across the product lines: AM/FM/TV.

Your post above showing 100 ohms for a Platinum Z FM tx also is wrong. It
is designed for a 50 ohm load. Here is a link to an on-line brochure for
the current 10kW model:
http://www.broadcast.harris.com/prod.../HAR173569.pdf
Observe the statement at the bottom of the right column on the last page
thereof, which I quote below for your convenience:

"All specifications referenced to any single output frequency
(87.5-108MHz),
nominal rated output power, and 50 ohm, isolated, non-reactive load."

No other RF impedance spec is given in the very detailed list of specs
contained in these brochures.

Kindly recant your accusations.

RF

Hi Richard,
I don't have anything to do with broadcasting, but when I first ran across
their DX line of AM transmitters, it really blew my mind. It seems though,
that it is really hard to find detailed info on them. Do you have a link
that describes what happens in some detail. The best I have been able to
come up with was anecdotal data let to from the WLW web site. There must be
some article that was published in some trade magazine. My questions a

The 50 KW transmitter supposedly has 60 some modules of about 1KW each. How
does this jive with being able to get the 200KW PEP that is required for
100% upward modulation? Does the combiner add voltage or power? Seems like
it would be voltage, but then the impedance a module sees would depend on
how many are active at a given time.

What is the digital sampling rate? Seems to me they need a high power
bandpass filter to get rid of the sampling frequency after the combiner.

BTW, I think a lot of people are going to confuse this with Digital AM.

Tam/WB2TT

"Richard Fry" wrote in message
...
"Richard Clark" wrote about the specs for Harris Broadcast Txs:
As for the specification for output impedance gone missing :-)
MW-10B SPECIFICATIONS
RF OUTPUT IMPEDANCE: 50 ohms, unbalanced.
Other output impedances available on special order.
Harris Platinum Z FM transmitter
100 ohm output impedance (unbalanced)
HARRIS SW-50 A
RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR
Well, instead of reciting their complete catalogue, it is easier to
simply say I could not find any product that did not specify an output
impedance, much less that others were available on special order.
_____________

I won't try to soften this: your conclusion above doesn't just _appear_ to
be wrong, it IS wrong. 100% wrong. The impedance published by Harris is
the
expected load impedance, not the tx source impedance -- which is the
convention used by all OEMs of broadcast txs (at least).

How do I know? I was the author of all of the brochures and technical
data
sheets for Harris' entire FM product line for the ten years before I
retired. That any published value of impedance applied to the load
impedance expected was/is universal across the product lines: AM/FM/TV.

Your post above showing 100 ohms for a Platinum Z FM tx also is wrong. It
is designed for a 50 ohm load. Here is a link to an on-line brochure for
the current 10kW model:
http://www.broadcast.harris.com/prod.../HAR173569.pdf
Observe the statement at the bottom of the right column on the last page
thereof, which I quote below for your convenience:

"All specifications referenced to any single output frequency
(87.5-108MHz),
nominal rated output power, and 50 ohm, isolated, non-reactive
load."

No other RF impedance spec is given in the very detailed list of specs
contained in these brochures.

Kindly recant your accusations.

RF

On Mon, 6 Sep 2004 06:20:23 -0500, "Richard Fry"
wrote:
I won't try to soften this

Hi OM,

I kin take it ;-)

: your conclusion above doesn't just _appear_ to
be wrong, it IS wrong. 100% wrong.

Hmmm, in light of what is about to be said by you this continues to be
amusing.

How do I know? I was the author of all of the brochures and technical data
sheets for Harris' entire FM product line for the ten years before I
retired.

Yes, your patter sounds like a publicist rather than a design engineer
- I've snipped your commercial fluff as it is pretty soft still.

It
is designed for a 50 ohm load.

Imagine THAT! Of course I won't hold my breath for your explanations
WHY (or -gasp- how) it is designed for 50/75/100/300 Ohms and not just
slap the output into any load - that would be an engineering issue of
matching which seems to be foreign to your discussion. Glad to see my
inclusions of so much of your copy brought back you memory that
rejected this:
On Sun, 5 Sep 2004 11:25:02 -0500, "Richard Fry"
wrote:
The source impedance of most transmitters is not published even today.
I can accept that perhaps under your hand it was not, but Harris has
returned to the fold of conventional design specifications:
MW-10B SPECIFICATIONS
RF OUTPUT IMPEDANCE: 50 ohms, unbalanced.
Other output impedances available on special order.

Harris Platinum Z FM transmitter
100 ohm output impedance (unbalanced)

HARRIS SW-50 A
RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR
Don't bother rummaging up you own drafts lingering in the musty
corners of the web, there are newer spec sheets available that easily
eclipse you in numbers to the contrary, but typical engineering
regards for specifying the output impedance.

I would suggest you review the works of a real engineer, Geoff
Mendenhall (you know, the guy you dissed) who explicitly offers
formula and specifications that correlate to current Harris equipment
design considerations for matching source Z to load Z. This has been
fairly common material available from Terman (One of Geoff's
references) for 70 years now and even the digital age has not changed
this:

It sounds pretty simple, in fact,
each module has a torroid (coil) which is the load for that
module; all those torroids are lined up and an iron pipe is run
through these torroids so that the combined output of all the
modules is coupled into this pipe. One end of the pipe is at
ground, the other end is the RF output of the transmitter. There
is an output network to match impedance of the pipe, which is
probably only a few Ohms, to the required output impedance which
is usually 50 Ohms, although sometimes high power transmitters are
set up to provide 75 Ohms, once in a while even 300 Ohms, to drive
an open wire type transmission line system.

From the Goatman:
"It was necessary to determine the plate
load impedance (formula) = 1000 Ohms
where Emin min drop across the tube in saturation
I1 ac plate current.

"Since this Zp was to be coupled into a
Z output of 50 Ohm, a impedance transformation
of 20:1 was needed."

Perhaps this is too many words for the publicity department style
sheet, but to engineers there isn't an iota of difference in the
design considerations of the final over the course of 1968-2004. It
goes much further back than this - we will skip that so as to not
appear to be roughing the receiver.

However, to this subject I am quite used to the rebuttal "You are not
going to change my mind." Impeccable logic such as this and
variations offered by you have scant foothold in the sweep of time. I
especially enjoy the sharp swerves to avoid the Goatman's simple
expression above. Reminds me of the Chinese contortionist acts that
used to be the staple of the Ed Sullivan show.

73's
Richard Clark, KB7QHC

On Mon, 6 Sep 2004 06:20:23 -0500, "Richard Fry"
wrote:

How do I know? I was the author of all of the brochures and technical data
sheets for Harris' entire FM product line for the ten years before I
retired. That any published value of impedance applied to the load
impedance expected was/is universal across the product lines: AM/FM/TV.

Hi OM,

If you simply follow the reference trail of those trade publications
(the design guides used by the engineers who did the actual work) you
would find PLENTY of source Z discussion:

from Harris White Paper:
"High Linearity RF Amplifier Design"
referencing
"Advances in AM Modulation Techniques to Improve Digital
Transmission of HD Radio and DRM"

"The poor antenna match will create a significant ripple in the
transfer characteristic of the main power amplifier. If that
amplifier itself has a poor output match to 50 Ohms
[the succeeding discussion gets into the inability of feed
forward correction to accommodate to this condition and the
introduction of an expensive isolator and lowered efficiency]
.... Alternatively, the amplifier itself could be modified to
improve its output match."

From Harris White Paper:
"Improving Efficiency With Split-Level Combining"
referencing
"High-Linearity RF Amplifier Design" (op. cit.)
and
"RF Power Amplifiers for Wireless Communications"
ppg. 31-32, 110
Discussion of load pulling to determine the "source impedance."
pg. 99-103 , Figures 4.21 & 4.23
Discussion of the source transformation from source Z 4 Ohms to
load Z 50 Ohms. Amplifier exhibits an 78% efficiency.
and
"Feed Forward Linear Amplifiers"
"Frequency ripple is one of the principle factors that limit broad
band signal cancellation and hence feed forward performance."
....
"As previously discussed, to prevent loss of power due to
impedance mismatches the input and output impedance of a device
should be equal to the characteristic impedance (e.g. 50 Ohms)
of the source and load to which it is connected."

From Harris White Paper:
"REDUCING FM IBOC TRANSMISSION COSTS WITH THE PROPER
CONFIGURATION AND LINEARIZATION TECHNIQUES"
referencing
"Feed Forward Linear Amplifiers" (op. cit.)
and
"RF Power Amplifiers for Wireless Communications" (op. cit.)
and
So on and so on and so on (no deviance from commonplace
design considerations that have been described for generations).

73's
Richard Clark, KB7QHC