Home Made UHF 50 Ohm Dummy Load

[Brad]

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.

 

[Arfa Daily]

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.

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

 

[Jeff Liebermann]

[email protected] (Brad) hath wroth:

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.

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.

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.

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.

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?

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.

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

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.

 

[Ralph Mowery]

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.

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 .

 

[Jeff Liebermann]

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.

Good idea. The problem is that RG174 doesn't really have much power
handling capeability. There's chart at:
http://www.therfc.com/powerrat.htm
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.

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

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.
http://www.mfjenterprises.com/products.php?prodid=MFJ-260C