Home Made UHF 50 Ohm Dummy Load

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

Before you type your password, credit card number, etc., be sure there is no active key logger (spyware) in your PC.

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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

Reply to
Arfa Daily

snipped-for-privacy@verizon.net (Brad) hath wroth:

Nope, not cazapitance. It's the lead inductance that usually causes problems at high frequencies. That's why you normally see dummy loads consisting of parallel combinations of resistors, not series. Look at the photos in the ARRL publications (I'm too lazy to reach over to the shelf and find the book and page number). The common method is to take an SO-239 (yech) or a panel mount N connector receptacle, and attach 4ea 220 ohm resistors from the center pin to each corner.

A 2 watt carbon composition will sorta fit into a PL-259. No easy way to add additional heat sinking, so that's about all it will do. However, if you're sneaky, install the same 2 watt resistor, or the coax connector previous described, on the end of a coax pigtail. Then, shove the pigtail into any type of heat dissipating oil in a metal can. Suspend the load so that it doesn't touch the sides. Think if it as an updated Heathkit Cantenna. Choice of oil is a bit problematic. Real xformer oil stinks. PCB's are still out there so be careful. I've used peanut oil, canola oil, olive oil, engine oil, and hydraulic fluid. Basically, avoid anything that turns rancid, is caustic, smells bad, or conducts electricity.

A small 2 watt carbon composition resistor cannot move enough heat to the case fast enought to handle much power. If you submerge such a resitor in a very large bucket of oil, it will still blow up. My guess is about 15 watts continuous duty and no more.

Not series. Think parallel. Take an RG-8/u or better yet, LMR-400 pigtail with a connector on only one end. The other end gets as many resistors as you can ring around the coax end, connected in parallel, between the center pin and the exposed shield. I figure you can get about 9ea 470 ohm 2watt resistors around the cable.

I just use a camcorder, or digital camera in video mode, to record someone typing in their password. I can then play it back and recover the keystrokes. Beware of friends bearing cameras.

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Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
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Jeff Liebermann

are

is a

connector.

Use about 100 feet of rg58 or 50 feet of rg174 coax. It has enough loss without anything o the end to look like a good dummy load at 400 mhz and above.

I have seen (bought some ) dummy loads at hamfests for about $ 15 that are good for 500 mhz .

Reply to
Ralph Mowery

Good idea. The problem is that RG174 doesn't really have much power handling capeability. There's chart at:

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which claims 60 watts. I think that's rated at 40C (104F) which is rather warm. The heat dissipation is NOT uniform. The bulk of the smoke will be coming from the radio end of the coax cable, while the other end remains fairly cool.

Well, I was trying to encourage the OP to build something instead of buying. However, if you must spend money, MFJ as a 25 watt (continuous duty) load for $40.

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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558            jeffl@comix.santa-cruz.ca.us
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Jeff Liebermann

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