Re: Experiment of reproducible results PART II (gotten longer)

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From: Dave Hottell (hottell@gulftel.com)
Date: Sun Jul 14 2002 - 13:47:31 CDT


George and all,

Although this *is* getting a bit academic, let me note that the only item I
was objecting to in the original post was the use of the word "only". As
in "Remember that the only goal for all this impedance matching". There
are other reasons why we use antenna tuners. I mentioned two, maximum
power out and maximum receive level. I note that all three purposes are
accomplished simultaneously -- when we get a conjugate match on the antenna
tuner, where ever it may be located.

>10. The SWR bridge in the external tuner showing zero reflected power *at
>that point in the system* does not prove or ensure that a 50 ohm resistive
>load has been presented to the final amplifier of the transmitter. It may
>or it may not, depending upon the properties of the bridge and those of the
>rest of the system. It is here that bridge imperfections appear to muddy
>the waters. An indication of 1:1 *at the tuner* may not in fact represent
>the actual SWR at the impedance match-point at the tuner input.
>

I would first note here that, AFAIK, the output network to most
transmitters is not actually a pure 50 ohms resistive. Perhaps Dave
Fifield or another of the designers can chip in here and correct me if I am
wrong. In reviewing the schematics of various transmitters it appears to
me to be very unlikely that the real output impedance is 50ąj0.

So what we are faced with is not matching 50 ohm per se, but rather deliver
maximum power the feedline. If we create a conjugate match using an
antenna tuner, I believe we will do so in both directions. By this I mean
that there will be a match of the impedance of the feedline going to the
antenna, as well as a match on the coax looking toward the xmit. This will
result in a match at the junction of the transmitter/coax just as it
results in a match at the junction of the antenna/feedline.

An antenna tuner doesn't so much transform 50 ohms into the impedance it
sees on the antenna feedline as it transforms whatever impedance it sees on
one side to the impedance it sees on the other side. In doing so, I
believe it creates a conjugate match looking both ways. I don't see how it
could do othewise when properly adjusted.

So once we tune the tuner properly -- for zero reflected energy -- we will
have a conjugate match at all junctions -- where the transmitter feeds the
coax to the tuner, at both sides of the tuner, and at the antenna/feedline
junction. We have matched whatever the output impedance of the transmitter
may be, even if it is not 50 ohms. We now have minumum losses throughtout
the system.

Perhaps this is not correct. I believe it is, but I am certainly open to
correction by someone better versed than I.

And I agree completely with your statements about using more than one
tuner. This can be hazardous to your wealth. By all means, put one in
bypass mode. I don't think it makes much difference which.

And, Yes, adding anything to the feedline -- whether it be a an additional
SWR bridge or something else -- will disturb the impedances on the line to
some degree as well as introduce some loss.

73 es gl,
Dave
ab9ca

At 12:37 PM 7/14/02 -0500, George, W5YR wrote:
>I regret that my recent posting which prompted the responses quoted in part
>below was evidently so poorly worded that it allowed for significant
>misinterpretation and misunderstanding. Perhaps I can be clearer if I try
>again, so bear with me.
>
>1. The original posting dealt *not* with the issue of whether an external
>antenna tuner should be used in a given system, but only with the manner by
>which it should be adjusted; which of two SWR meters in the system should
>be used as the arbiter of correct external tuner adjustment: the one in the
>transmitter or the one in the tuner?
>
<big snip>


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