[Elecraft] 50 Ohm Dummy Load

[Don Wilhelm]

Daniel,
I believe it may be difficult to mount 20 1/2 watt resistors in a practical
configuration that will minimize the reactance.

If you need to use paralleled resistors to create a dummy load, I suggest
that you mount them on a small (2" x 2" or so) piece of copper or PC board
material.  Make a hole to mount a BNC or UHF chassis mount or bulkhead
connector in the center of this board then arrange the resistors like spokes
of a wheel around the connector - one leg of each resistor soldered to the
connector center pin and the other end(s) soldered to the copper ground
plane.  That configuration with 6 3 watt 300 ohm metal oxide film resistors
yields an 18 watt 50 ohm dummy load that has low SWR on all HF bands (flat
through 10 MHz and only 4 ohms inductive at 30 MHz).

Digi-Key www.digikey.com  has the resistors - P300W-3BK-ND for $0.52 each or
$0.448 for quantities from 5 to 100.
To offset the $5 handling charge that DigiKey wants for orders less than
$25, you may want ot order the PC board (PC5-ND - 3 x 4.5 inch @ $3.10 or
PC6-ND -4 x 6 inch @ $4.85) and connector (BNC bulkhead jack A24520-ND @
$2.23 or UHF connector ARF1007-ND @ $4.60) as well as other components at
the same time (or order enough for up to 6 dummy loads, a single 4 x 6 PC
board is big enough for all of them and the components for either 5 or 6 of
them gets you over the $25 hump).

73,
Don W3FPR

----- Original Message -----

> I'm working on the ATU and read ahead last night and discovered that I'll
need
> a 50 Ohm dummy load. Up til now, I've used test equipment at work, but it
seems
> like I really ought to have a QRP dummy load.
>
> Would it work if I built one out of twenty 1k 1/2W resistors in parallel?
Does
> it matter if I place them along a board or build them into a small pack?
Will
> there be a problem with inductance or capacitance? Except for buying a 10W
50
> Ohm Carbon resistor, is there any other way to do this? I know that I
shouldn't
> use the wirewound resistors at Radio Shack.
>