Amateur Radio High Power 50 Ohm Dummy Load

1. 

   ElPaso TubeAmps says:

   September 21, 2014 at 8:43 PM

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   Reply
2. 

   WA7PRC says:

   September 21, 2014 at 8:02 PM

   Congratulations, you’ve learned the difference between DCR (DC Resistance)
   and AC Impedance. Even if the load resistors aren’t wirewound, a straight
   piece of wire exhibits series inductive reactance at RF. Likely, you have
   capacitive reactance tuning out the inductive reactance at higher
   frequencies, and not enough capacitive reactance to tune it out on lower
   frequencies.

   Even if you get it to play electrically, you still have the issue of
   getting rid of the heat. This design (as is the root Heath pn HN31
   “Cantenna” design) is inefficient in that regard. I see nothing to remove
   the heat from the container. For continuous operation, consider some form
   of liquid-to-air heat exchanger.

   OTOH, I used four 50 Ohm / 400W flange-mount RF resistors on two heatsinks
   for a DL that can handle 1500W at 100% duty cycle as long as the fan is
   blowing (no liquid involved). Cooling was verified using an inexpensive
   LM35DT temperature measurement IC, and a 40.68 MHZ ISM RF power source. My
   design with short connections resulted in VSWR under 1.5:1 thru 54 MHz.
   Reasonably-priced RF power resistors can be found from various vendors on
   ebay.

   Reply
3. 

   Scubasky says:

   September 21, 2014 at 8:00 PM

   I don’t know much, but the first thing that comes to mind is you got lucky
   and built it to the right size, and on any other frequency band the
   resonance is causing the signal to be reflected?

   Reply
4. 

   adrian mouse says:

   September 21, 2014 at 7:48 PM

   a heating element has an inductance and ther for reactance a dummy load
   should be purely restive 

   Reply
5. 

   Jon's Home Workshop says:

   September 21, 2014 at 7:21 PM

   Can’t answer your question, unfortunately, but I do the odd transceiver
   repair and after watching your previous dummy load video, it did cross my
   mind to make one for this work. Definitely watching with interest.

   Something to do with the oil perhaps? I am assuming it is oil filled.

   Jon.

   Reply
6. 

   TubiCal says:

   September 21, 2014 at 6:25 PM

   Are you talking about this: 20 m Band (14.000 MHz-14.350 MHz)?

   Then we do some little math here:
   You described your Rc to 47muF, ok as for the Xc it is 1/(2*pi*f*c) is
   aprox 50Ohm correct is 50R – 0.000235Xc (minus 90 degres because the
   reflection will be positive or PreFlexions which i call them;) at 14Mhz….
   For the groundig impedance you´ll have to add 0.01Xc reflection resistance
   to the 50Ohms….(same math as above with 684.5pf without Real R because we
   already know it´s 50Ohms)
   So no reflections nither through the cable nor to ground..

   For full detail we still need Xl so you have to measure the L of this
   thing.May its adding some real amount Xl so it will not cancel out on lower
   frequency or it will may produce more Xl than Xc and will add some
   ringing….

   It´s by far more easier to draw it on a Xl/Xc L/C V/F circle diagram to
   work the whole thing out.

   But for the Xc it works out just fine!

   Have fun! :)

   Reply
7. 

   matthew Kubiak says:

   September 21, 2014 at 5:32 PM

   you need to use non inductive resistors…. that’s why…. 

   Reply
8. 

   Joseph Lorentzen says:

   September 21, 2014 at 5:00 PM

   Wow, you lucked out. I tried so called non inductive wire wound resistors
   to make a dummy load and I know of another dummy load that was made up of a
   whole mess of 1k ohm 5 watt resistors. In both cases, the inductance
   introduced was intolerable and it took a huge capacitor in a homebrewed
   tuner to come anywhere near a fair match. 

   Reply
9. 

   ElPaso TubeAmps says:

   September 21, 2014 at 4:43 PM

   After a reasonable amount of work with this dummy load my conclusions are:
   (1) these elements may be rated collectively at 10,500 watts as heater
   elements for water heaters but at 14.2 MHz they can only safely dissipate
   1.5 KW for about 15 minutes before the water (2 gallons) reaches 150 F and
   is boiling around the elements. (2) Obviously, these elements are not made
   for RF and I can only get this dummy load to work satisfactorily, using an
   antenna tuner between the load and the transmitter, on the 20 meter band –
   so… proceed with caution if you build this device and don’t expect it to
   perform like a high quality commercial RF dummy load.

   Reply

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