[TowerTalk] Steve Best on Mismatch loss, transmitters and tuners
Wed, 22 Jul 1998 15:18:53 -1000
Jim, KH7M had written:
>> Steve, I have a Bird 43 watt meter that says I am sending more power
>> out my transmission line than is coming out of the transmitter!! From
>> where do you suppose that power came.
>"Wait a minute, now you are really confusing me.
>Are you saying that a mismatched feed system creates gain?"
Having not heard anything back from Steve Best, I will go ahead
and answer David.
No, of course not, the feed system has created nothing. What
has occured is that 200 watts of the transmitter's output power
has reflected back down the transmission line at the connection.
where a mismatch occurs, of that line to the antenna terminals.
This power is easily measured simply by turning the "slug" in
the Bird watt meter around so that it now measure reflected
rather than forward power. In my example case, it is reading
200 watts reflected.
When the Bird slug reads the forward power on the line, it is reading
the sum of the transmitter's output power PLUS this 200 watts which
get re-reflected at the output terminals of the antenna tuner between
the transmitter and the transmission line to the antenna. So the
Bird meter reads 1700 watts forward power when the transmitter
is only "outputing" 1500 watts. And 1500 watts is all that ever gets
radiated by the antenna, another 200 watts is constantly reflected
back down the line as long as the transmitter is key down, in CW.
This is precisely why and what an antenna tuner does: it makes
certain that ALL of the power out from our transmitters gets
radiated! It's job certainly is not to "fool" the transmitter, hi!
KH7M also had written:
>> This is ridiculous!! Transmitters get hot, but not because they
>> are dissipating reflected antenna power!! If true, the output
>> inductor of the transmitter would soon melt, as the reflected
>> power keeps on a'coming!
So David asks:
>"Again, are you saying that the output from the transmitter is amplified
>within the feed system (not the antenna, because what it amplifies it
>radiates towards the intended object ... not the transmitter, I hope!)"
No, no amplification, see the above explanation of exactly what
an antenna tuner is placed into the system to accomplish. Also,
my statement about the trasmitter not absorbing reflected power
is only partially correct. With much reflected power, most linears,
or tube output transmitters will never tune-up properly; my
Alpha 87 just shuts down, and a solid state rig will just
reduce it's output power until the amplitude in watts of
the reflected energy is down to something allowed by the
solid state rig design.
>>> " You do not radiate more power
>> [KH7M comment: Of course not.
>> Steve, what happens to those couple hundred extra watts read
>> on my Bird, that did not come out of the transmitter, were they
>> generated in my tuner?? ]
KH7M: Guess I have answerd my own question above. Those
couple hundred watts get radiated, but are immediately replaced
by another 200! And, no they are not generated in the tuner, they
just get reflected back up the line from the tuner ouput terminals.
>Sounds as though there is something fooling the Bird into reading what is
>not there ... unless as above you intend to suggest that mismatches
No, the Bird is reading exactly, within it's tolerance limits,
what it is sensing on the transmission line; you just have to
look in both directions, hi.
>>> "If you feed an antenna with a 20:1 VSWR you will
>>> give up 7.41 dB of radiated power. VSWR results in a mismatch loss
>>> regardless of cable attenuation. "
>> Steve, then why, when using the Smith chart to design such a system
>> as you go on to describe, it is true that the greater the reflection
>> the better the system match is to the antenna system???
David, I am afraid I am guilty of asking a "trick" question in this one.
I just wanted to see if Steve understands what reflection loss is, hi.
HINT: It is not "mismatch loss" as I understand him to be using the
>Is this because once the energy is lost (gone as heat or radiated by the
>pre-antenna components) there is no longer anything stimulating a mismatch
>at the antenna?
No, it is because very high, many, many dB of return loss translates
to nearly a perfect impedance match through out the system carrying
the RF energy. It "means" the reflected power is many, many dB below
the forward power on the line, hence a very high loss is suffered by
the return power, get it?
I can give you the timing of all this if you are interested; consider a
30 wpm CW "dit": lasts about 43 milliseconds. Several reflection/
re-reflection pairs in 100 feet of typical coax transmission line
are all over and done with in only 3 or 4 microseconds! So neither
you nor the guy on the rcv'g end can possibly hear, nor can the
rigs amateurs use, possibly tell anything about this slightly later
radiated energy. The "dit" tone, after all, is 43,000 times
longer in time duration! You will never know that these
reflection phenomena are occuring, nor will your rig.
All perfectly clear now, I'll bet, yeah, right!
73, Jim, KH7M
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