[TowerTalk] Reflections, Tuners and Antenna Forum Talks
Jim Reid
kh7m@hsa-kauai.net
Wed, 10 May 2000 13:20:06 -1000
Aloha from Kauai to those interested in the subject topic,
others better delete now, but Aloha (much honor and
respect) to you not interested also, hi!
This is LONG.
In Steve's (Dr. Best's) post today, he summarized
well a discussion which has been running for about a
decade now!! I will not further summarize, and do very
much anticipate attending the antenna forum in Dayton,
Friday afternoon, the 19th. I'll hide in the back
somewhere, where Steve can't throw something at
me, and I expect to say not a word!
Steve did a good job of presenting his view of Walt
Maxwell's views on the topic: that is, the behavior
in the line when a tuner is or is not used; and presented
his reasons for objecting to some of Walt's written
material. I have done some measurements today,
following Steve's post and will now quote just a portion
of that post and then my data, which I hope will add
some "real" numbers for the on-going discussion.
> In order for Walt to tie his analysis and math together, he
> developed the concept of total power re-reflection via the
> virtual short or open circuit.
> With any matching device, Walt states that a total
> re-reflection of power occurs "in-phase" with the delivered
> power such that the two add to become the total forward
> traveling power in the transmission line. That brings me
> to point 2.
>
>
> Point 2) Do "in-phase" forward traveling powers add to
> become the total forward traveling power in the transmission
> line?
>
> In one of the examples in his text, Walt presents an example
> where 100 watts of power (steady state) is delivered to the
> input of a tuner. The tuner is used to match an antenna
> with an impedance of 150 ohms. The Steady state
> forward traveling power in the transmission line
> (connecting the tuner and antenna) is 133.33 watts and
> the reflected power in the transmission line is
> 33.33 watts. Walt's hypothesis is that the 33.33 watts
> of reflected power is totally re-reflected at the tuner in
> phase with the 100 watts of delivered power and the two
> add "in-phase" to become the 133.33 watts of total forward
> traveling power.
Steve did show, using voltages and not powers, that if the
voltages, from which the incident and reflected powers
could be calculated, were added "in phase", the resulting
voltage sum would equate to a total forward power of
about 249 watts, not the 133 argued by Walt.
Well, this is what I decided to check out. And the crux of
the matter, as discussed by Steve is to keep track of
the real incident power into the "antenna system" from
the rig source, that is be sure it is really the 100 watts,
and not actually the sum/difference of the "real" incident
power and some reflected power from somewhere (to
really appreciate Steve's point here, and the importance
to his argument of this issue, you really should read his
full paper as published a year or whenever ago in
Communications Quarterly, I seem to have miss-placed
my issue, but I have trouble keeping track of the many
publications I receive, hi!
So here is what I have done, and the results. Keep in
mind I was using Bird 43 wattmeter, accurate to +/-
5% of a given "slugs" max value; I used Bird slugs of
100 watt, 500 watt, 1000 watt and 2500 watt values during
the effort, will say when which was used in the following.
Equipment used in the tests: source, Yaesu FT-1000 set
to 7088 kHz. Henry tube linear amplifier capable of at least
2500 watts output steady state, CW mode. Bird "lab
precision" coaxial load resistor (aka dummy load, hi),
model 8890-300 will dissipate, all day, 3000 watts continuous.
The Palstar AT4K antenna tuner, a C-L-C device. An
MFJ-269B analyzer to determine the correct initial set
up for the tuner, and the presented impedance here
in the shack at the end of the coax transmission line
up to my 40 meter Force 12 EF--140S dipole.
Initial conditions:
Input Z numbers at the shack end of the coax line to
the antenna: R = 17 ohms and X = 11 ohms At 7.088 mHz,
vswr 2.9:1 . Now, please note: these numbers are at
the end of some length of coax line that is NOT connected
to its' characteristic impedance, rather to some unknown
impedance at the input of the Force 12 dipole up on the
tower! But they are the impedance which must be worked
with here in the shack.
I had intended to do these numbers at the 100 watt power
level, but my Henry doesn't like such low power, and
the reflected power could not be measured under all
conditions at such low incident power into the tuner
or dummy load. So I did all set up and measurements
looking at 1000 watts as the reference output number
after the linear amp. Input drive power was kept constant
from the FT-1000; when checked from time to time
it was always 30 to 33 watts.
The tests and data.
1. Set up with 1000 watts out from the Henry into the
Bird 50 ohm dummy load. Did this with the 1000 watt,
full scale "slug" in the Bird 43 meter. 1000 watts into
the load, reflected power, appeared to be, maybe
0.8 watt, read using a 100 watt slug in the reverse
direction (reflection) position.
Henry amp tune and load control settings for this condition
into a "pure" 50 ohm load. Tune = 363; Load = 326,
these are the known 50 ohm load reference set values
then for the amp at 1000 watt output power (to within the
+/- 50 watt accuracy of the Bird meter and slug combination).
2. Connect the feedline directly to the Force 12 40 meter
dipole, without the use of a tuner. That is, with the amp
tuned to drive 1000 watts with the given input power, into
a 50 ohm, pure R, load, what will occur when it is
connected directly to the R=17, X=11, vswr=2.9:1 antenna
"system" load?
Result: Without touching the amplifier tune or load
controls, Power forward reading, using Bird 1000 watt
slug, was 700 watts; rotating slug to the reverse,
reflected power position, 180 watts.
3. Realizing that the amp was no longer tuned for
optimum performance at this new, non-50 ohm pure
R load, I "tweaked" the tune and load controls to
peak the forward power reading. The "tweaked"
control settings were, tune=348, load=520.
Bird readings increased to: power forward = 1400 watts
(using the 2500 watt full scale Bird slug);
power reflected = 360 watts(using 500 watt slug)!
Or 1400 minus the 360 watts reduces to 1040 watts;
or, it seems we are back to 1000 watts output from
the linear amp.
Now this is occurring with the output tank circuit of
the amplifier "re-tuned" to provide a max output
power with given input power to the mismatched
Z at the input end of the transmission line.
4. Now using the Palstar AT4K antenna tuner.
Pre tuned the Palstar using the MFJ-259B, this
gets pretty close to what the amp wants to see,
that is, includes whatever error the 259B has.
Initial conditions of the Palstar C-L-C components
to show the MFJ meter a "match" at the Palstar input
with the mismatched 40 meter dipole transmission
lined connected to the tuner output (moved from the
amp output to the tuner output now).
Tuner component settings:
Input C at about 65% maximum mesh, or presumably
about 0.65 of the 350 pF total available C, or 228 pF.
Inductor tap at 1.6 uHy if the total of 28 uHy inductance
available.
Output C, at 100% full mesh, or 350 pF in series with the
transmission line.
MIFF-259B then reads at the tuner input terminal a "match"
to the antenna system, now including the tuner, or
an R = 50 ohms, X = about 8 ohms (varies a bit as the
wind is blowing the dipole about up there), indicated
vswr = 1.1:1.
Now removed the 259B, and connected the input terminal
of the tuner to the output terminal of the Henry amp.
Reset the Henry amp tune and load controls to the
original settings found when the amp was feeding
its' output power directly into the Bird coaxial load.
Applied the given drive power to the input of the Henry
tube amp.
Power forward, per the Bird 43 into the antenna system
(including the Palstar AT4K set as indicated and in line),
1000 watts; power reflected (using 100 watt slug in the
reversed/reflected position) 8 to 10 watts, varying with
the wind. vswr indicated, 1.17:1.
Again, these measured with the amp set as originally into
the Bird load.
5. Now, disconnect the antenna system and feed the
amp directly to the Bird load again; Power into the
load, 1000 watts, (using the 1000 watt full scale slug),
power reflected from the load, about 0.8 watt (read,
just bare deflection using the 100 watt slug, reverse
position). Again no change to the drive or amp tune/
load controls.
6. Final step and measurement. Move the Bird to
the output of the Palstar tuner to measure the
indicated forward and reverse powers with all
set up and operating as in 4. above. Will the indicated
forward power, less the indicated reflected power
show the 1000 watts, as Walt Maxwell's writings
assert? We will see.
6.1 Forward indicated power on the Bird, using the
2500 watt full scale "slug" shows 1150 to 1200 watts,
again wind varying.
6.2 Reflected power at the output terminals of the AT4K,
about 230 to 300 watts on the 500 watt slug in the reverse
position.
So, within the tolerances of the measuring apparatus,
sure enough, the forward power indicated less the
indicated reflected power, does sum to the 1000 watts
at the output of the amplifier.
This seems to validate Walt Maxwell's view that the
forward power into a line/antenna system on the output
side of an antenna tuner, when used, would be the
sum of the "true" amp output power plus the re-reflected
mismatch power, in this case of some 230 to 360 watts from
the mismatch up at the dipole antenna.
So, do not understand yet Steve's claim that the powers
above should not add/subtract as my tests, and Walt's
text seem to indicate.
It is too bad that Bird does not provide a Model 43 slug
which has a max value, full scale, of 1500 watts! But,
that would require yet another scale on the face of
the meter!
Well, for whatever all of this is worth! And, I still do
not understand Steve's voltage analysis, and why it
seems to differ so wildly from what Walt, W2DU and
my measurements today seem to show. Must keep
a very open mind in Dayton next week, hi.
73, Jim, KH7M
On the Garden Island of Kauai
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