Home Made UHF 50 Ohm Dummy Load

Discussion in 'Electronic Repair' started by Brad, Jun 13, 2006.

  1. Brad

    Brad Guest

    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.
     
    Brad, Jun 13, 2006
    #1
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  2. Brad

    Arfa Daily Guest

    "Brad" <> wrote in message
    news:...
    > 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
     
    Arfa Daily, Jun 13, 2006
    #2
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  3. (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.

    --
    Jeff Liebermann -cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 http://802.11junk.com
    Skype: JeffLiebermann AE6KS 831-336-2558
     
    Jeff Liebermann, Jun 13, 2006
    #3
  4. Brad

    Ralph Mowery Guest

    "Brad" <> wrote in message
    news:...
    > 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 .
     
    Ralph Mowery, Jun 13, 2006
    #4
  5. On Tue, 13 Jun 2006 22:54:24 GMT, "Ralph Mowery"
    <> wrote:

    >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


    --
    # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
    # 831-336-2558 -cruz.ca.us
    # http://802.11junk.com
    # http://www.LearnByDestroying.com AE6KS
     
    Jeff Liebermann, Jun 14, 2006
    #5
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