Chuck Hutton wrote:
...SNIP...
> Second, I'm not understanding your transformers. I have been thinking you
> were using the standard 2 wire system where the far end of wire #1 is open
> and the far end of wire #2 is grounded. The two wires feed into a
> transformer whose secondary impedance is the push - pull impedance of the
> pair and the primary is the receiver impedance. The center tap of the
> secondary feeds the secondary of another transformer whose secondary matches
> the impedance of the 2 wires to ground and primary matches the receiver.
> (Sorry to be so long winded but some people don't have the reference and
> some people have different versions so I didn't want to just give a figure
> number.)
>
> So the first transformer matches 680 Ohms (in your case) to 50 Ohms. The
> second transformer matches 350 Ohms to 50 Ohms. That's why I don't
> understand when you write "680 to 450". 1:1 would be even worse.....
>
>
> Chuck
An explanation is in order. I'm looking at the 3rd edition of John-ON4UN's
book. Figure 7-23 (A)describes the basic system you mention with one wire
grounded at the end and one wire floating. But he also indicates that this
only works when the characteristic impedance of the transmission line pair
match the characteristic impedance of the lines over ground--further suggesting
the use of 450 ohm lines for the wires. At the feedpoint, a center tapped
transformer matching 680 ohms to 50 is used for the first direction. The
second direction uses a transformer (340 : 50) tied to the center tap of the
first transformer.
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?'
So I'm trying to dial this in on my beverage. Tom-W8JI just posted an
excellent procedure for me to dial this in. I'll try this today.
Frankly, I was a bit confused when I made my initial measurements. I was
looking for the characteristic impedance of just one of the wires over ground,
with the second one floating at both ends. My initial 'best guess' was about
450 ohms. I suspect my data was not as pristine due to that other floating
wire. Tom-W8JI has provided some much needed basics on what it is I'm looking
for. The two lines must be tied in parallel and THEN search for the
characteristic impedance of the line(s) over ground. It's very time consuming
and tedious work. Last time I walked almost 3 miles back and forth looking for
the 450 ohm 'best guess.'
Yesterday, a suggestion was made to measure the ground resistance and subtract
it from the 680 ohm line impedance and use that. My ground is about 30 ohms,
so the 1 : 1 transformer was suggested to approximate the 680 : 650 transform
(darn near impossible to wind by the way ; ).
Thanks for the help guys. I want a beverage that works well and I'm a bit
out-of-my-league in figuring out how to make it work well. So what I lack in
the form of brains I need to compensate with labor... In this case, the old
Swedish proverb is appropriate: "When you fail to use your head, you get to use
your feet."
Ford-N0FP
ford@cmgate.com
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