"Brad" wrote in message
...
Hi,

I wish to make a 25 watt (short duty) 50 ohm dummy load for UHF 440Mhz
band. I have a UHF SWR meter and I have made dummy loads before, but they
are
of little use at UHF frequencies due to internal capacitances in the
resistors. The only good dummy load I made that works very well at UHF
is a
2 W 50 ohm straight carbon resistor solder on the inside of a PL259
connector.
I can use this as a 10W (short duty) dummy load as long as I make a brief
transmission.

I thought about putting five 2 watt 10 ohm straight carbon resistors in
series inside a copper tube which is soldered to the back of a PL259
connector. What are your thoughts about this idea?

Thanks in advance, Brad

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My initial thought is that your sums are in error. Although 5 x 10 ohm
resistors in series makes 50 ohms, the power rating remains 2 watts. Use 20
x 1k 2 watt non inductive resistors in parallel. This gives 50 ohms at 40
watts. The way to construct a dummy load for the 70cms band, is to cut two
discs of double sided PCB material, then drill twenty resistor lead-out
sized holes in each, plus a 3mm hole in the centre of one, and a 1.5mm hole
in the centre of the other, then solder the resistors between the two,
soldering to both sides of the pcb material. Take a small paint tin, or
similar, and fix an SO239 socket to the lid. Solder a stiff thick insulated
wire to the centre pin of the socket, and run this down through the large
hole in the top pcb, into the hole in the centre of the lower pcb disc, and
solder it Finally, solder multiple braids from the upper pcb disc to the
paint tin lid, then put the lid back onto the can. You now have a fully
screened pretty much non inductive 50 ohm load. If you want, you can fill
the tin up with mineral oil to increase the power dissipation to probably
100 watts short term, but if you do this, make sure that your SO239 is
either a sealed type, or seal it with some silicon rubber compound. I have
several dummies of this construction in use all the time, and have never had
the slightest trouble with them. Mine are actually built into golden syrup
cans, which solder easily.

Arfa