>From: "Doherty, Robert" <Robert.Doherty@fmr.com>
To: <topband@contesting.com>
>Date: Fri, 7 Nov 1997 16:04:00 -0500
>
Snip...
>We paralleled some resistors to get about 500 ohms at about 4
>wts.. How critical is selection of the terminating resistor??
>Can it be anything from 400 to 800 ohms ?? Or does have to be 5%
>or less.... in my case using the 75 ohm coax it would come out
>to be 675 ohms.....Is it just nit picking???? or does this
>REALLY matter???? Does it NOTICABLY improve receiving
>performance when the resistor is matched exactly?? Or can it
>only ....as they say....."Be measured with sensitive
>instruments?? tnx Whitey K1VV
>
The only parameter greatly affected by the terminating resistor
value is directivity. If the value is correct, the response from
the back will be minimized. The value required for this is
dependent on nothing but the effective surge impedance of the
antenna when operated as a transmission line (it varies depending
on the exact circumstances of each antenna). I assume that you
arrived at 675 by multiplying 75 times 9. Again, the value
required has nothing to do with feedpoint matching values.
To make a measurement to determine the value, you could generate
a small signal from some distance behind the antenna and vary the
termination resistance to arive at an optimum value (tune for
minimum response). The feedpoint impedance will end up being
very close to the terminating resistance value when the correct
resistance is used. However, since most real antennas turn out
to be between 500 and 800 or so ohms, it won't really be
necessary to change from your 9:1 transformer for matching to the
feedline.
73, Eric N7CL
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