Ford:
I stuck some comments inline.
Chuck Hutton
----- Original Message -----
From: "Ford Peterson" <ford@cmgate.com>
Sent: Monday, August 16, 2004 9:26 PM
Subject: Re: Topband: Beverage Query...
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.
CH: The first transformer matches the 688 Ohms of "push-pull" impedance
btween the two wires to the antenna input of your receiver (assumed to be 50
Ohms). The second transformer marches the impedance of the two wire (not
single wire) system which is about 350 Ohms to the receiver input impedance.
Hence, there's no 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)
CH: Without knowing your grounding resistance or at least a clue via the
soil type, it's hard to know what to make of this. I habitually ground
Beverages into three 6-foot rods in sandy soil and feel lucky to get a
termination less than 200 Ohms. You'll likely be better off, but I agree
with Tom that it's likrly you have a true impedance near 350 Ohms and also
significant additional grounding resistance that leaves you in the mid
400's.
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.
CH: The transformer #'s in my ON4UN (2nd ed) don't match yours, so all I can
say is that there should not be a transformer matching 648:450 as mentioned
above.
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.
CH: Ford - maybe each of the formulas will help:
single wire to ground: Z = 138 log (4h/d)
push-pull: Z = 276 * log (2S/D)
parallel wires to ground: Z = 69 * log [4h/d * sqrt(1 + (2h squared /
S)]
A spreadsheet is available with formulas and charts and other Beverage
calculations (wave tilt, effects of height, effects of soil, etc.) if you
wish - Chuck
Thanks for all the help guys. I "owe" all who responded thus far.
Ford-N0FP
ford@cmgate.com
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