Ford:
First of all, I think ON4UN's values should be treated as approximations.
The formulas he uses are simplified formulas for wires over pefect ground.
There's quite a difference when the more demanding formulas are used that
take into account conductivity and dielectric constant.
An example for a single wire Beverage with height of 9 feet, 1.8 mHz,
conductivity of .001 and dielectric constant of 3:
perfect ground : 501 Ohms
complex ground: 557 Ohms
That's maybe an average case. With lower height, higher dielectric constant
or lower conductivity the differences are much more severe.
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
----- Original Message -----
From: "Ford Peterson" <ford@cmgate.com>
To: "Tom Rauch" <w8ji@contesting.com>; <topband@contesting.com>
Sent: Saturday, August 21, 2004 2:54 PM
Subject: Re: Topband: Two Wire Beverage Query...
<CLIP>
Taking this information one step further, my 680 ohm characteristic
impedance bidirectional beverage wire should be terminated at 680 less 30 or
650 ohms? This is a huge departure from the table in ON4UN's 3rd Ed book,
which suggests 680 ohms to 340 ohms or so.
As it is, I suspected the book's table was amiss and wound the terminations
for 680 to 450. Would this impedance mis-match cause degradation of the
pattern? If so, maybe I'll wind a 1:1 and try this at the termination.
Thanks for the help by the way. It's interesting to figure out new methods
of figuring stuff out. Your suggestion was great.
Ford-N0FP
ford@cmgate.com
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|