Whitey, K1VV writes:
>We put up a the classic ON4UN 580 ft Beverage Ant..... with the 9-1
>matching toroid exactly as shown in his Low Band DXing handbook,....we
>checked the transformer input with the MFJ259 and a 500 ohm resistor
>on the secondary....it was flat SWR for 1 to 10 mhz... we fed it with 75
>ohm RG59, abt 100 ft.....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 value of termination resistance is certainly not dependant on the
feedpoint transformer or the feedline impedance. If modeling software
can be believed, the termination value is not very critical.
The software shows that your 580-foot Beverage at 10' high has optimum
rejection of skywave signals to the rear when terminated with about 500
ohms when constructed over "poor", "good", or "very good" soil. Over
"very poor" ground, the best value suddenly drops to about 320 ohms.
If the desire is to attenuate low-angle groundwave (man-made) noise to
the rear, then the best termination values are about 200 ohms higher than
the best skywave values.
It's interesting to note that modeling software shows that the
best-performing Beverage is one erected over "very poor" ground, which is
in agreement with textbook literature. Such a Beverage has more gain and
lower-angle reception than those over better soil. Indeed, over "very
good" soil, a Beverage looks quite bad when modeled. "Very poor" soil is
a blessing when viewed in this respect!
Modeling shows an improvement in the patterns of Beverages over "good" to
"very good" soil if they are 7' high (rather than 10'). The best
termination resistance values for 7' high Beverages is shown to be about
50 ohms higher.
73, de Earl, K6SE
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