From: "Chuck Hutton"
> Ford:
>
> I'm not sure I understand your test setup - did you have the push-pull and
> single ended transformers in place while you did the measurements? If so,
> the "termination resistor" you added is a spurious element; the push-pull
> transformer should match the 688 Ohms impedance between the two wires and
> the single ended transformer should match the 350 Ohms to ground. No further
> termination is needed.
No transformers--I was trying to figure out WHAT transformers to wind.
>
> If you had the transformers disconnected and were measuring just one wire
> the 450 Ohms you measured would be the transmission line impedance to
> ground, and 450 is a very reasonable value for that.
>
>
> Chuck
From: "Tom Rauch"
> Are you including ground resistance in that termination or
> assuming it is zero? A 6 foot copper pipe runs 50-100 ohms
> here. You might be terminating with even more resistance
> than you think.
>
> My single wire Beverages are all about 500-550 ohms, so a
> 470 ohm termination makes them pretty flat.
Let me respond to these and pose another query.
I was under the impression that the termination transformers were to match the
characteristic impedance of the lines (688 ohms) with the characteristic
impedance of the individual lines over ground. ON4UN suggests this to be in
the neighborhood of 340 ohms. So he recommends a termination transformer that
matches 688 to 340.
Chuck is correct in his interpretation. I terminated each of the lines
individually while the other line just floated at each end. I changed the
value of the resistor at the termination end and swept a band of frequencies
and noted the max Z and min Z values. Each time I changed it, the range of Z
variation began to converge. At roughly 450-470 ohms, the range in Z values
over frequency was around 70 ohms. Any higher or lower resistance at the
termination and the Range was much bigger. (E.g. at 315 ohms the range was
230. At 630 ohms the range was also 230) So I concluded that the lines are in
fact roughly 450-470 ohm impedance over ground with a Z range of about 70 ohms,
the smallest I could get it)
So I wound transformers to match 688 to 450 and installed them at both ends.
So called "T4" matched 50:648 with a 5:18 turns CT transformer. So called "T1"
matched 50:450 with a 5:15 turns tranformer. So called "T5" matched 648:450
with a 5:6 turns CT transformer. This flies in the face of ON4UN's book so I'm
trying to figure out what is correct.
Tonight, I was able to finish the project and installed the ends. The output
is as expected on the transformer that matches 648:50. W1AW is in the
boresight looking east from MN and it was about 2S units lower than the
transmitting vertical, and much less noisy. Looking at the reverse the output
is very low. I can barely detect signal. In fact, that direction the antenna
is all but deaf.
To summarize: Do I match the 688 ohm lines to 340 ohms according to the ON4UN
book or do I match it to the characteristic impedance of my lines over ground?
Maybe if I understood where John derived the 340 ohms at 9' I would understand
this better.
Tom:
I am using 2 - 8' copper clad ground rods spaced 10' apart at each end. Just
for fun and giggles, I looked at each pair of ground rods with the VA1.
Interesting. On 160M, they were about 60 ohms. On 80M, about 90 ohms. On
40M, about 120 ohms. So I figure that they are in series in my measurement
setup. But in the installation, they are in parallel. So am I correct in
assuming that each rod is about 30 ohms on 160M? And in parallel the pair
would be what? 15 ohms?
Thanks for all the help guys. I "owe" all who responded thus far.
Ford-N0FP
ford@cmgate.com
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|