Dear gwhite [no call, no location]:
Notwithstanding the clear limitations on making conclusions about what
happens inside of a circuit that has been modeled using Thevenin's theorem,
it is part of Religion that the least important theorem in circuit theory is
applicable.
Debates about Faith are a waste of energy. Avoid the tar-baby.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"gwhite"

"gwhite" wrote in message
...
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps
are
not "impedance matched." Neither are audio power amps for that matter.

My stereo amp has a spec on output impedance. As I recall, it was around
0.16 Ohms. Intended load is 4 - 16 Ohms.

Tam

"gwhite" wrote in message
...
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps
are
not "impedance matched." Neither are audio power amps for that matter.

'OM' is radio ham speak for 'Old man'. 8-)

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller

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

Richard Fry wrote:
"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)."

Well, Richard Fry didn`t say there were no out-of-band filters or traps.

One could have a low-pass filter that cut-off above 108 MHz, but below
176 MHz, and no harmonic would get through the filter.

80% or better efficiency isn`t coming from a Class A amplifier, so maybe
it comes from a Class B amplifier.

One fly in the ointment is found on page 354 of Terman`s 1955 edition:
"The theoretical maximum possible plate efficiency that can be realized
in a Class B amplifier is pi/4 or 78.5 per cent;---."

Best regards, Richard Harrison, KB5WZI

Nug wrote:
"I am building an rf transmitter for a short range data link at 433 MHz
and am almost done, but I would like to understand better exactly what I
am seeing with regard to antenna performance."

Your wavelength is about 0.69 meter or about 2.3 feet. In antennas
everything depends on wavelength. If you use a transmitter housing as
the ground plane for your antenna, it needs to be a sizeable part of a
wavelength or the salient part of your antenna must be longer to
compensate for the small ground plane.

If you had an infinite ground plane, a 1/2-wave wire perpendicular to it
would produce up to 50% more volts per meter field strength than a
1/4-wave wire perpendicular to the ground plane. It`s not something for
nothing. Total radiation is the same in both cases. More of the
radiation is perpendicular to the wire in the 1/2-wave and less goes off
at some other angle to the wire.

50% more field in some particular direction is realy not very
significant in most cases, and there are other consequences of using a
1/2-wave wire instead of a 1/4-wave wire.

An end-driven 1/2-wave wire presents a very high impedance. It is
equivalent to a parallel-resonant circuit. It would match a direct
connection to a parallel resonant tank circuit perhaps.

An end-driven 1/4-wave wire presents a very low impedance when worked
against a ground plane, maybe about 30 ohms.

How well you are able to radiate a signal from a wire is likely to
depend on how well it is matched to the transmitter and less about the
bends in the wire. In any case the complete antenna must be resonant to
eliminate reactance which opposes the signal`s entry into the wire.

For a small transmitter operating at a very short wavelength, the size
of the antenna is not onerous and it would be possible to use a
center-fed 1/2-wave antenna. Each half would be just a little over a
half foot in length. Drivepoint impedance is in the 70-ohm range.
Another possibility is a full-wave loop, about 2.3 feet in perimeter
with a drivepoint impedance of about 120 ohms.

Performance of all the suggestions is probably about the same. You can
find the best by trying them.

Best regards, Richard Harrison, KB5WZI

Thanks for all your responses, good stuff here.

Rich, I am using TX modules from Liapac which specify a 50 ohm load.

So putting it all together (I think) by bending the antenna (both 1/4
and 1/2) I am messing with its impedance and subsequent matching? So I
would be better off with a straight 1/4 over ground plane if I had the
choice.

From reading I've done I think that a bend from vertical to horizontal
(closer to ground) would be decreasing the antenna impedance, does that
sound correct? Would I benefit from adjusting (extending) the antenna
length slightly (I don't think so because it would no longer be at
wavelength, but I am not sure?).

Ken said ... "If I've read what you wrote correctly, the antenna spends
more of its length parallel to the surface of the PCB than it does
running 90 degrees away from it"... Actually it's about half and half
(feed up 15mm , loop around the case, up - {clear of case} 130mm).
Note the horizontal loop does not cross over itself.

Sort of like this (~~ is horizontal loop) side view.

|
|
|
~~~~
|
feed

I understand that more of the radiation is perpendicular to the wire in
the 1/2-wave (than 1/4 wave), broadly speaking how will the bend('s) in
the aerial effect radiation pattern (I understand this is tough to
answer)? I can't fit a full loop inside the case.

I have no rf test equipment so can only use trial and error, thanks
again to all who responded. As I said it actually works fine it's just
I don't like not understanding the reason its working, and would like
to make any small changes that may improve performance.

Thanks Again