Aloha again to those interested in the twin line replacing
coax (long, delete if of no interest!),
First, I have rcv'd two comments from more than one of
you: exactly how were the measurements made, and what
is a Matchbox modified tuner, does it have an internal balun,
etc.
First, to quote a bit from the longest query about the measurement
method:
"I don't understand how you are doing these measurements. Measurements
require that you find the frequency of minimum Z ( or maximum Z) for the
line you're measuring (with the far end shorted or open ). Then the
formula in the manual is used to generate the loss. This is dependent
upon the electrical length of line and the frequencies at which it
occurs is rarely in the ham bands (multiples of quarter or half wave
electrically). To find out what it is at the frequency of interest, I
have to generate a plot of db loss versus frequency (using the multiple
points of minimum Z) and interpolate between them. "
I am using the very easy to use, new MFJ-259B antenna and cable
analyzer. The second "standard" mode of operation of this unit is called
Cable Loss. The measurement method has been discussed just
a bit here on the reflector in the last couple of weeks or so. W8JI,
Tom, did a fine job of explanation, so you can look in the reflector
archives for a post from him, or perhaps he will comment to the
group about the workings of the 259B and measuring cable loss.
The unit can measure loss in any circuit component that is 50 ohms
impedance, I guess, and which is "open" circuited at the "far" end
or output side.
To measure my new transmission path set up from the rig to the
input to the Top-Ten switch unit of my vertical antenna "farm",
I input from the linear directly into an old, but modified,
E F Johnson Matchbox antenna tuner.
A note to those interested about the modified Matchbox:
The Matchbox is unlike any US made tuner at the present time. It was
designed 40+ years ago as a full kW tuner for the Johnson AM kW
transmitter of the day, so it had to withstand 100% AM modulation
of a 1kW input transmitter's output power; not sure exactly what that
would be today. However the components within are LARGE.
Judging from plate spacing and inductor sizes, at least 7kV
rated components, IMO.
This unit, as modified, has both a tuned primary series resonant input
circuit, which is tightly and inductively coupled to a parallel resonant
output tank circuit, across which another capacitor acts as an output
RF voltage divider. There is no balun in this tuner, it has by nature a
balanced output for direct connection to the old timers 600 or so
ohm homebrewed ladder line. Details of the Johnson Matchbox
design, and the info for adding the series input capacitors for
tuning out the input reactance are given in super detail by LB
at his web site, W4RNL, under the heading "Link Coupled
Tuner - Tutorials", this is a four or five part piece and well worth
understanding should you wish to modernize an old Johnson
tuner. I would be happy to provide specific details on how
mine was modified to any one seriously interested.
The modified unit is a pleasure to use in comparison to the
original circuit. Being able to tune the input primary circuit
as the first step of QSY makes the secondary resonance and
match adjustment almost simple! And I always do this with
the 259B switched in in place of the rig for tune up of the
antenna system at QSY.
To measure the loss of my twin lead plus tuner and far balun
transmission line system, the tuner must first be adjusted to
bring the entire system to resonance (that is, all the reactance
canceled) with the antenna which serves the frequency of
interest connected to the transmission line system. Once
the tuner is correctly adjusted, the output of the Amidon
hi-power 4:1 balun out in the antenna field at the input to
the Top-Ten switch is opened up by simply unscrewing
the short piece of coax between the balun output and
the Top-Ten switch. The switch and the short coax run
over to a particular Gladiator vertical could also be
measured, or included, if desired. However, my earlier
posted tests using the LMR400 line also stopped at the
input to the TT switch.
With the transmission line system now open circuited, the
MFJ unit immediately reads out the cable loss for whatever
frequency the system had been resonated.
With the LMR line, where neither input tuner nor output
balun was required, you can read the loss at any frequency
just by changing the frequency output from the 259B; this
is not possible without resetting the Matchbox when measuring
the loss of my nearly 200 foot run of twin line.
Incidentally, the 450 ohm [or whatever its' Z might be (hi!)] line is
run high in the air, above the "jungle below; for the most part,
it comes in contact with nothing. I managed this by suspending
the cable from a length of dacron rope hung over the near
end of one of my 600 foot long wire antenna runs! So the
twin line dangles as an inverted-v, sort of from the shack
window all the way back to the TT switch mounting pole.
This is now too long, so I will stop. Hope this answers most
of the questions.
73, Jim, KH7M
:
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