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Author Topic: Dummy load questions  (Read 5573 times)
PA0BLAH
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Posts: 0




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« Reply #15 on: October 02, 2012, 02:24:11 PM »

Any load fed via a tuner can be useful. But there are problems where a proper non-reactive wideband non-radiating load is required to fix them - and strings of lightbulbs etc will radiate.
Light bulbs don't radiate more then unshielded resistors.

What is the purpose of a wide band resistive load, when your real load will be an antenna transformed by a tuner to 50 ohm?
So a water cooker transformed to 50 ohm by a tuner is a more realistic testload then a pure resistance load of 50 ohm over a wide band.
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G3RZP
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Posts: 4625




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« Reply #16 on: October 03, 2012, 04:22:56 AM »

>Light bulbs don't radiate more then unshielded resistors.<

Which is why you don't use unshielded resistors. I'd hate to try to sort out a rig with a failed component in an output low pass filter that led to it going unstable on anything other than a wideband 50+j0 load.

You use a dummy load to give the tx its designed load impedance mainly for sorting out problems. Adding other unknowns complicates that, especially if the impedance is a function of power level - see the article in the RSGB Bulletin in the late 1950s on the subject. If you want to use lightbulbs, then you should use Robertson lamps, which were designed as RF loads.

Plus, when you are beginner, as is the OP, it's best to keep things simple.
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K0IZ
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Posts: 737




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« Reply #17 on: October 03, 2012, 05:48:00 AM »

Another source for a dummy load eBay.  Fairly often I see used Bird dummy loads for sale at very good prices.  Hi wattage capability, usually flat to 1 or 2GHz.  My 500 watt Bird will handle 3KW for several minutes!  Heres one for sale right now:  

http://www.ebay.com/itm/BIRD-TERMALINE-COAXIAL-RESISTOR-82A-DUMMY-LOAD-BARKER-WILLIAMSON-500W-51-5-OHMS-/120992476639?pt=LH_DefaultDomain_0&hash=item1c2bb6addf

Another type on eBay are the 50 or 100 watt loads with heat sinks.  These go for very reasonable prices.  I have a couple and they seem to work very well up into the UHF range.  Here's a couple of examples.  Both are used but I see new ones quite frequently:

http://www.ebay.com/itm/RF-Dummy-Load-N-type-100w-4Ghz-50ohm-58B83464r01-/251160836782?pt=LH_DefaultDomain_0&hash=item3a7a5a3aae

http://www.ebay.com/itm/DECIBEL-PROUCTS-DB4303B-CONTINUOUS-DUTY-100-WATT-100W-RF-DRY-DUMMY-LOAD-N-MALE-/221117414756?pt=US_Radio_Comm_Antennas&hash=item337b9ffd64
« Last Edit: October 03, 2012, 05:54:41 AM by K0IZ » Logged
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