[TowerTalk] Bird ratings, Reflections and Tuners
Jim Reid
kh7m@hsa-kauai.net
Thu, 11 May 2000 08:58:57 -1000
Some on going discussion about my measurements
yesterday, and some new info/data:
Someone wonder about the following,
> > OUTPUT = 1150-1200 watts
> > REFLECTED = 250-300 watts
> > DIFFERENCE = 900 watts
Well, why pick that delta? It is just one of several possibilities
given the Bird slug accuracy specs.
> > BIRD ERROR = +/- 5% or 50 watts @ 1000 watts
Yes, but the power over 1000 watts was measured with
a Bird 2500 watt slug; accuracy, 125 watts!
> The accuracy of a calibrated in-spec Bird is + or - 5% of the full
> scale reading ***anywhere on the scale***.
Ok, lets shed a bit more light on the actual slugs I have.
Numbers below compare readings of a constant power
level on several of the slugs I used to collect my data.
Also have some new data to share.
Conditions: Will set 1000 watts from the Henry into the
Bird 8890-300, 3000 watt dummy load, will do this
using the Bird 1000 watt slug; then see what the
2500 watt slug reads.
1. Set to 1000 watts, as stated.......done.
1.1 Insert 2500 watt slug, read.......940 watts.
1.2 Insert 5000 watt slug, read.......910 watts
1000 watt slug was reinserted each time to insure power
constancy as measured with that slug.
2. Set to 500 watts, using 500 watt slug, full scale.....done
2.1 Insert 1000 watt slug, read........600 watts
2.2 Insert 2500 watt slug, read........600 watts
(Note: each of these readings is taken several times,
as well as rechecking the 500 watt slug reading again, again, etc.)
2.3 Insert 5000 watt slug, read........560 watts
3. Set to 100 watts, using 100 watt slug, full scale, Henry amp
OFF, just using FT-1000 to generate the 100 watts....done
2.1 Insert 500 watt slug, read........108 watts
2.2 Insert 1000 watt slug, read......120 watts
2.3 Insert 2500 watt slug, read......130 watts
2.4 Inset 5000 watt slug, read......110 watts(just more than a
single scale line deflection)
Trust the above serves to illustrate the difficulty of making
accurate power measurements, hi. These comparisons
were all done at the 7.088 mHz test frequency used
throughout this data collection for Steve Best.
Now, yesterday, Steve had asked if it were possible to
supply added info. Purpose of the added info is to
attempt to determine the component settings more
accurately within the tuner; and for me to make sure
I am doing the measurements correctly, and to provide
useful info for Steve's calculations.
The following were Steve's requests, a corrections I discovered
I needed to make in my method, and the data supplied:
First request.
> * Jim - connect a 50 ohm load at the tuner output and measure
> the tuner input impedance.
To be sure things are set up as yesterday, and
with the tuner set up, same component percentages
as yesterday, with the antenna "system" connected
to the tuner output, the MFJ-259B reads R= 44 or 45
ohms , X = 1 and vswr = 1.1 : 1 at the end of an 18 inch
coax jumper cable between the tuner input terminal and
the MFJ meter terminal. Ok.
Now, to your requested measurement:
With the Bird 50 ohm dummy load connected at the tuner
output, the MFJ meter reads: R = 18 ohms, and
X = 6 ohms, and vswr = 2.7 , again there is the 18 inch
jumper coax between the meter and the tuner input
terminal.
Second request.
> Also, with the 50 ohm load connected at the tuner output,
> deliver 1000 watts of forward power to the tuner input and
> measure the reflected power.*
Appears to be about 170 watts reflected from the tuner
input, measured with the Bird 500 watt slug. Bird 1000
watt slug was used to set the input power to the tuner
at 1000 watts for this test with the Bird 50 ohm load
connected to the tuner output. Tuner components remain
set as above. Initial 1000 watt setting was made directly
into the Bird load, tuner bypassed.
Third requested test.
> * Jim - can you measure the forward power delivered to
> the transmission line when the tuner sees a Zo impedance
> (the antenna is replaced with 50 ohms) and when 1000 watts
> of forward power is incident at the tuner input?*
Will use a second Bird 43 meter at the tuner output for this
measurement, keeping the first Bird in the tuner input
line to assure the 1000 watt input. So the second Bird
will be between the tuner output terminal and the 50 ohm
load. Will have to use the 2500 watt Bird slug here, as I
only have the one 1000 watt slug.
Appears to be 650 watts into the dummy load input
with 1000 watts into the tuner terminals, Steve. Don't
forget the +/- 5% allowance on the 2500 watt slug,
or an error window of +/- 125 watts around the measured
650 watts, I guess that is how you apply the Bird
accuracy statements.
Fourth requested measurement.
> * Jim can you terminate the tuner input with Zo and measure
> the impedance seen looking rearward into the tuner output.
Measured at the end of the 18 inch jumper coax again
between tuner output and the meter input connector:
Meter reads: R = 17, X = 7, and vswr = 2.8 !
NOTE: Here I detected an error made yesterday, Steve.
Above is not exactly a conjugate to 52 + j 58! However,
I was measuring yesterday a few feet on down the
coax. Let me now re-measure the antenna "system"
Z directly at the tuner output, well again with the 18
inch piece of coax jumper to the MFJ meter. Bet this
will be different from that 52 + j 58 I got yesterday.
Read: R = 17, X = 11, and vswr 2.9 ! A great deal
different from what I told you yesterday -- I have really
screwed up. I have cost you a lot of time, Steve, as
you used my fouled up measured numbers to do
your calculations -- nuts to me! Only redeeming
feature is the numbers now appear to be closer to
conjugates; unfortunately, the MFJ meter does not
give the sign of the reactive component of Z.
Well, so there we have the results of the items you requested,
Steve. And in the process, I found the error I had made
yesterday. Did not realize that even a few feet would make
such a delta on a coax line not connected to its characteristic
impedance at 40 meter wavelength -- learned my lesson!
Hope these new numbers are of greater use/validity, Steve."
And that is the latest of which I am aware about the topic.
73, Jim, KH7M
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