[2m] plans needed

[ahb@ahb.net]

Hi Chad !

There is no way to make a real cheap version of a dummy load. 
There are two ways I used, that are quite affordable. 

MFJ sells a 50 Ohms Dummy that can handle 150 Watts continuos 
power and up to 1.5kW but only for a few seconds. It comes in a 
small metal housing. I tried this with 1.2kW for a few more 
seconds and it started smelling :-).
The trick to use this dummy for high power for a longer time is 
to help the resistor to get rid of the heat. I suggest to put 
it in a metal bin, like those that are used for painting color 
etc. and fill it with oil. There are special versions of oil 
that are high insulating and can handle several kVolts per mm. 
I got some from a local transformator dealer for free. Normaly 
normal motor oil should do the job as well, but I have never 
tried it. Simply drill some holes in the cover of the bin so 
you can access the dummy's connector. Then fill the bin with 
the oil and your 1.5kW Continuous dummy is ready to go. 
Heathkit sold something like this a couple of years ago but I 
am not sure wether they still do and at what price.

Another way to get rid of VHF/UHF power is to use cheap coax 
cable. If you have a length of 100m say RG58 you will have a 
attentuation (one way) of around 10db at 432 MHz and around 6dB 
at 144. If you simply let the end open, all the energy will be 
reflected there and will travel the way back to the 
transmitter. However if you have 10db on one way the HF will be 
attentuated with 20db when it has traveled back to the 
transmitter. That would be around 1 Watt if you put in 100 
Watts. I would think this should be a SWR of about 1.2 which 
should work for every amp. On 144 this would be 12 db 
attentuation which will result in 6 Watts returning HF, which 
should be still ok. I used this up to 500 Watts on 2m but I 
don't know if the cable can handle more power. Just check it 
with your fingers and if the outer cover starts to get flexible 
it is a good idea to stop for a few minutes :-). This will of 
course not work on lower frequencies unless you plan to place a 
drum with 2kilometers of cable in your backyard :-)

Concerning the test of your DC Power supplies, there is a real 
simple and cheap way. I usually use 12V Halogen spots with 100 
Watts (or whatever you have) and add as much as I need. To test 
a power supply with 50 Amps you would need something of 600 
Watts at 12 Volt, that is 6 100 Watt Lamps or 10 of 60Watts or 
whatever you have. 
You should be aware that these lamps have a rather low cold 
resistance, so it could happen that your power supply will 
"see" something like a sort circuit at the first moment. In 
this case start with a few lamps and add more once the first 
have been lit.

Hope that will help you.

bye and 73's from Germany

   Achim DL3EBB

On 8 May 2002 at 6:31, Chad~~KM5QF~~ wrote:

> Anyone know where I can find plans for making a  vhf/uhf 50ohm dummy
> load that can handle upwards of a few hundred watts? would also like
> to make it pull secondary duty somehow as a resistive load for testing
> dc power supplies up to around 50 amps. Nothing I have puts any real
> load on the supplies. Can't find any data or plans anywhere. Thank you
> for any/all help. 
> 
> chad
> 
> .
> .
> caeli enarrant gloriam Dei.
> .
> .
> .
> 73 73 de KM5QF k k kn
> 
> 
> 
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