G. White wrote:
"Output Z is irrelevant."

This is an old argument in this newsgroup. I became convinced long ago
that there are cases in which impedance is very important.

"Transmission Lines, Antennas, and Wave Guides" by King, Mimno, and Wing
make a clear and concise case for the princuple of conjugates in
impedance matching on page 43:

"If a dissipationless network is inserted between a constant-voltage
generator of impedance Zg, and a load of impedance ZR such that maximum
power is delivered to the load, at every pair of terminals the impedance
looking in opposite directions are conjugates of each other.

To secure maximum power output from a generator whose emf and internal
impedance are constant the load must have an impedance equal to the
conjugate of the generator`s internal impedance."

Radio transmitters don`t produce significant harmonics. It`s the law.
They are linear power sources. We can and do tune them for all the power
they will produce under their particular operating conditions of drive
and d-c power supply. They operate at more than 50% efficiency which
means that they don`t take power 100% of the time, but are switched-off
during part of the r-f cycle. Output impedance is thus an average over
the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the
tank circuit and other filters. The radio is a proper source. The
impedance added by off-time is called "dissipationless resistance"
because no power is lost in the radio while it is switched-off.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote:
Radio transmitters don`t produce significant harmonics. It`s the law.
They are linear power sources. We can and do tune them for all the power
they will produce under their particular operating conditions of drive
and d-c power supply. They operate at more than 50% efficiency which
means that they don`t take power 100% of the time, but are switched-off
during part of the r-f cycle. Output impedance is thus an average over
the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the
tank circuit and other filters.
_______________

Note that without adjustment, modern, solid-state FM broadcast transmitters
can (and do) provide 80% or better PA efficiency into a 50 ohm load across
20% bandwidth, with no tank circuit or other in-band filter(s).

If this is done in a commercial service, certainly it could be done in
amateur radio devices. Physics is not application-selective.

Posters of various forms of "Absolute Truths" to the contrary might well do
a bit more research.

RF

Richard Fry wrote:
"Physics is not application-selsctive."

True. The laws of physics are inviolable.

The FM amplifier does not need linearity. Amplitude distortion is
irrevelant. Severe clipping to remove amplitude variations is common
practice. Phase/frequency shift is the modulation of interest.

Clipping generates harmonics and FCC rules limit harmonic transmission
in all services. Any manufacturer wants to require the fewest user
adjustments. I`m not surprised that tuned frequency selective circuits
are minimized.

I would be surprised if some final filter were not used to guarantee
compliance with the rules.

Best regards, Richard Harrison, KB5WZI

On Fri, 4 Mar 2005 11:11:32 -0600, (Richard
Harrison) wrote:
"Physics is not application-selsctive."
True. The laws of physics are inviolable.
....
I would be surprised if some final filter were not used to guarantee
compliance with the rules.

Hi Richard,

Yes, that would be the technical marvel of the ages, but just like our
rigs, even the biggest FM transmitters bend to the necessity for
output filtering:
http://www.broadcast.harris.com/prod...%20Bro%2DB.pdf

There is an amusing claim, however, for their power module(s)
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

Not a very good copy editing job is my guess.

Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
• Z2CD: 4.0kW at 2.2kW output power
55%
• Z3.5CD: 6.1kW at 3.75kW output power
61%
• Z5CD: 7.9kW at 5kW output power
63%
• Z7.5CD:11.7kW at 7.5kW output power
64%
• Z10CD: 15.3kW at 10kW output power
65%
• ZD20CD:31kW at 20kW output power
65%

73's
Richard Clark, KB7QHC

"Richard Clark" wrote:
Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
(clip)
_________________

Another case of writing without knowing, I see.

The power consumptions you cite are the TOTAL values for those transmitters,
not of the RF power amplifiers alone. The total value includes the exciter,
driver(s), power supply losses, control circuits, and RF combining losses,
as well as power for the internal cooling fans. The PA modules have 80% or
better efficiency, by themselves.

The reason I know is that I was the author of those specs.

RF

On Fri, 4 Mar 2005 12:42:47 -0600, "Richard Fry"
wrote:

"Richard Clark" wrote:
Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
(clip)
_________________

Another case of writing without knowing, I see.

Hi OM,

Yes, I do recall your claims that contradicted Mendenhall's explicit
efficiency computations. So I see no need to pursue undocumented
claims you offer. Unless you can supply specific references from
Harris about this 80% efficiency, then such comments remain as suspect
as before.

The reason I know is that I was the author of those specs.

I am still wondering about the odd entry of:
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

I notice you passed on discussion to this particular point of
accuracy. 11s can be explained by hitting 1 too many times, or 80 by
hitting an errant 0 too many. One of those things that escape the
notice of a spell-checker.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote:
Unless you can supply specific references from Harris
about this 80% efficiency, then such comments remain
as suspect as before.

You may take what I wrote as being "from Harris," because I was part of
Harris FM Product Management for those transmitters before my retirement in
1999 (after 19 years there). I was responsible for documenting all
performance features and parameters published for the product line, using
numbers generated and approved by Engineering.

If the PAs alone were as (in)efficient as you imply with your calculations,
power consumption for the entire transmitter would be considerably higher.
Common sense should tell you that PA module efficiency would have to be much
higher than the efficiency calculations you posted in order for total power
consumption to be as stated on the Harris spec sheets.

I am still wondering about the odd entry of:
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

Yes, that is a "typo," as you noted. Very good. It should read
"...VSWR of 1.5:1."

RF

Richard Clark wrote:
"There is an amusing claim, however, for their power module(s) "Each
module is conservatively rated to produce 850W of power into a system
VSWR of 1,5:11."
Not a very good copy editing job is my guess."

Richard must be right. I guess a finger was left too long on the no.1
key and nobody caught it in time.

I admire Gates` scheme of paralleling many relatively low powered
amplifiers. If one fails, you can continue almost as if nothing
happened. Very nice.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
The FM amplifier does not need linearity. Amplitude distortion is
irrevelant. Severe clipping to remove amplitude variations
is common practice.

Not so. You confuse receivers with transmitters. Limiting is supplied by
the IF strips of FM receivers to reduce/remove AM components on the incoming
wave, but FM broadcast transmitters are operated well below any
limiting/clipping level, and that is probably true of ham FM txs also.

Synchronous and asynchcronous AM are low in broadcast FM tx RF stages (the
FCC spec is -50dBc), but not because the FM amplifiers are "clipping."
Broadcast FM txs easily can be adjusted over an output power range of ~25%
to 105% or more simply by adjusting drive to the PA (keeping constant PA
volts). This technique often is used for output power control/VSWR
foldback, actually.

Clipping generates harmonics and FCC rules limit harmonic
transmission in all services. .. I would be surprised if some
final filter were not used to guarantee compliance with the rules.

You are confused again. I wrote that no "tank circuit or in-band filter(s)"
were necessary to achieve the high efficiency I described. Your post I was
responding to states that a "tank circuit and other filters" are necessary
for high efficiency -- that is not true.

Harmonics are present at the PA output of an FM transmitter, but "clipping"
is not the process whereby they are generated, as I state above. They are
reduced to legal values using a lowpass/harmonic filter. The FCC
attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB
below the unmodulated carrier.

The lowpass/harmonic filter does not improve efficiency--it has a small
amount of insertion loss in the FM band.

RF

Richard Fry wrote:
"Not so. You confuse receivers and transmitters."

FM transmitters often use Class C amplifiers and frequency multipliers
on the modulated signal. An AM signal can not be amplified by a Class C
amplifier because of severe distortion of the modulated signal. In FM,
amplitude distortion is irrelevant no matter where it occurs, receiver
or transmitter. The recovered audio will sound just fine. It`s one of
the many advantages of FM.

Best regards, Richard Harrison, KB5WZI