[Elecraft] Dummy load (with light bulb) doesn't work

[David A. Belsley]

Light bulbs make terrible dummy loads.  Not only are they not 50 ohms, 
their impedance changes radically when the filament heats up and, 
hence, the bulb presents a constantly changing load under short bursts.

best wishes,

dave belsley, w1euy


On Apr 10, 2004, at 8:47 PM, Don Wilhelm wrote:

> Jeff,
>
> If you MUST use a light bulb, a higher wattage bulb would present a 
> load
> much closer to 50 ohms than a 40 watt bulb - the one I just measured 
> showed
> only 25 ohms cold resistance, so 2 of them in series would be a bit 
> better.
>
> Be aware that a light bulb will have a lot of inductance - so expect 
> the SWR
> to be high because of that even if the resistance is correct.  Light 
> bulbs
> make very POOR dummy loads at RF frequencies, and they change 
> resistance as
> the element heats.
>
> Bottom line - almost any form of 50 ohm resistor would make a better 
> dummy
> load than a light bulb, but wirewound resistors are not very good 
> either
> because of the inductance.  The Elecraft DL1 is a good investment if 
> you
> need a dummy load, and there are other sources too.  Every ham shack 
> should
> have some type of good dummy load.
>
> If need be, borrow a good 50 ohm resistive dummy load if you can to 
> complete
> your alignment - your K1 (and you) will be happier for it.
>
> 73,
> Don W3FPR
>
> ----- Original Message -----
>
>> I'm ready to align and test a K1's transmit side.  And I need a dummy
>> load to do it so...
>>
>> My first attempt at a homemade dummy load went like this:
>>
>> A light bulb socket (with 40 watt bulb in it) on a plastic stand was
>> connected on the underside to two pieces of about 9" insulated wire 
>> (wire
>> is about 18 gauge, I'm guessing).  The positive wire I soldered to the
>> center of an SO-239 (female) connector, the negative wire I soldered 
>> to
>> the outside (threaded) part of the same connector.  That connector 
>> has a
>> BNC type connector at its other end, and I attached that to the K1
>> antenna jack.  When I turned the rig on, the output wattage said 2.0, 
>> but
>> the BAND button would not give me either band, just the reading P=0 on
>> the screen, which I know means that no power is going out.
>>
>> How can I change the dummy load to make it work?
>>
>
>
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