> Figure 7-23 (B) uses a center tapped transformer to match the 680
> ohm lines to the characteristic impedance of the lines at the end
> termination. In this case, he suggests a 688 : 340 match. I am
> using the transformer at the termination end since I'm using 688 ohm
> lines. What has me confused is the 340 ohm target impedance. My
> question has been 'how the heck is this determined?'
Hi Ford,
The 340 ohms is close to the impedance of the antenna over ground.
You can use Tom's method of measuring that impedance or there are
other ways to do it.
You might save yourself some effort and just use 450 ohm twin-lead.
(I purchase mine from WB0W, Inc in St Joseph, MO.) Mount it on 10 foot
PVC pipes, etc. At a height of 10 feet the antenna impedance will
be ~450 ohms and then you can wind a 1:1 REFLECTION
TRANSFORMER for an easy match. However.... :))
You may use any other transmission line of a known impedance, with
the one choice being properly installed, balanced line. (It doesn't
matter
what the impedance of the line is, but wide-spaced high impedance
lines are generally more difficult to match and may not be entirely
uniform along its length.) If you choose to use the "classic" wide
spaced homebrew line anyway, you might want to wind some
good "square" coils on the classic oatmeal box "coil forms" just for
the fun and challenge of it. After all, this is hobby stuff!! :))
Referring to John's book (see note 1below), page 7-22....
The Primary winding of the relection transformer
(T5a - n1) should match the transmission line, and the
secondary should match the "antenna impedance" which will
vary according to height above ground, size of the line,
seasonal conditions, etc. I found that the antenna impedances
shown in Table 7-10, page 7-23, are accurate enough to use for
the soil in my area and may, for practical purposes, be used as
presented. But what the heck, it's fun to do the measurements,
so go ahead and experiment!
Let's say that you use the 450 ohm twin-lead. Take a look at
figure 7.23B in John's book....the center-tapped
side (primary) of the transformer matches the twin-lead in it's
transmission line mode and the secondary matches the twin-lead when
it's in the antenna mode. So what's that about?
A "drawing" and a quick explanation of how the system works might
be helpful:
(Dir A>) NEAR---antenna/2- wire transmission line---FAR (<Dir B)
When a signal arrives from direction A and travels toward B,
equal voltages (or at least nearly so) are induced on BOTH
wires of the twin-lead. The signal is in phase on both wires.
That's the "antenna mode" and that's the reason for
measuring or calculating the antenna impedance of the line with both
wires shorted together at each end.
When the signal arrives at the FAR (B) end, where the REFLECTION
transformer (T5) is connected, equal voltages, in phase, are connected
to the T5 primary (n1), so no current flows in the primary. That
also means that no current is induced in the secondary (n2) by
transformer action, but the signal from the NEAR end (A) is
available at the primary's center tap and that is connected to the
secondary winding. The signal flows through secondary on
its way to ground and induces a current in the primary (n1).
The induced signal is now 180 degrees out of phase at the "ends"
of the primary and that is connected to our 450 ohm transmission
line.
So, now we have the signal on the transmission line, in the
transmission line mode (sometimes called the "push-pull mode)
traveling toward the NEAR end. BTW, the transmission line is
"balanced"; the signal won't "jump off" or radiate (if everything
is installed correctly). So now the fun begins.
When the signal arrives via our twin-lead transmission line at T4 (again,
figure 7-23) it induces a current in the T4 secondary (n2) and that
becomes the signal from the NEAR direction. It has "bounced back"
via the REFLECTION transformer. It can either go to
your receiver or to a termination (more about that later). The
signal cancels at the center tap of T4's primary and therefore
there is no voltage available for T1's primary. So (in theory) all
we can hear is the signal from the NEAR end.
Let's reverse the direction...
Assume there is another signal coming from the FAR end, direction B.
The twin-lead is now in the Antenna Mode again (also called "Push-Push"
mode). The signal first arrives at T4's primary (n1), the signal is in
phase on both wires and, as in the reflection transformer, there is no
current flowing in T4's secondary (n2). So in theory, nothing is
heard in the receiver connected to T4. The FAR signal is available
at T4's center tap and that is connected to T1's primary (n1). Current
flows in the primary winding on it's way to ground and a current
is induced into T1's secondary winding, so now we can hear the
signal from the FAR end.
T4 "matches" our 450 ohm twin lead to the coax line
that is connected to say, a 50 ohm receiver and T1 "matches" the twin
lead in it's antenna mode to the coax which also may connect
to a 50 ohm receiver. The 50 ohms is not a requirement. You
can wind coils to match ~75 ohms if you want to.
The secondary windings of T1 and T4 must always be terminated.
The terminations for the FAR END and the NEAR END is either
a receiver or (normally) a resistor, NOT in the reflection transformer.
If say, the transformers are wound to match a 50 ohm load,
the windings must either connect to a 50 ohm receiver or a
50 ohm resistive load and should not be left "floating" or shorted.
To summarize the coils that you need to wind (in this example):
REFLECTION transformer: 450 ohms CT primary:450 ohms secondary.
FAR transformer: 450 ohms primary:50 ohms secondary.
NEAR transformer: 450 ohms CT primary:50 ohms secondary.
You can use just one coax line to connect the Beverage antenna.
The "near end" box would contain a DPDT relay to select either
direction and terminate the other with a resistor. You can use
the same coax to deliver power to the relay. There was a very
recent article in QST on how to do that.
I hope this helps!
73,
Charlie
N0TT
Note 1, John's book: "ON4UN's Low Band DXing" third edition
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