2
$\begingroup$

I've seen many plans for 50 Ohm dummy loads in paint cans, such as the one here: https://www.nonstopsystems.com/radio/frank_radio_antenna_dummy.htm. I've already assembled a functional dummy load, but have avoided using it at full power because it's currently resistors attached to copper plates in open air.

I'd like to move the entire thing in to a paint can (as in the link), but have a concern. It appears to me that the can itself becomes connected to one side of the dummy load. Couldn't this allow high voltage (relative to the radio) to appear on the can itself? Is there some insulating factor I'm missing? Would another material (such as a plastic project box) be a better choice?

$\endgroup$
  • $\begingroup$ Key questions: Is your radio itself grounded properly? Are you feeding the dummy load with coax? $\endgroup$ – SDsolar Oct 12 '17 at 17:15
  • $\begingroup$ In my case, the radio is grounded, only through the negative side of the power supply. When I switch to a battery, it is not grounded at all. Yes, I am using coax to connect to the dummy load. It appears that the answers below sufficiently answer my question. At the least, I have no more risk of high voltage appearing on the can than I do on the case to my radio. $\endgroup$ – Adam KC0DAD Oct 12 '17 at 19:45
  • $\begingroup$ That is true, to be sure. btw, you will get better results with operating your radio if it is truly grounded. Ground is part of the antenna system as well as being a safety measure. $\endgroup$ – SDsolar Oct 12 '17 at 23:06
7
$\begingroup$

Assuming you are using coaxial cable and connectors, the case of the dummy load is contiguous with the shield of the cable and the usually-metal case of your radio. So touching the dummy load is not any more dangerous (i.e. not at all if everything is working normally) than touching your radio. The shields everywhere in the system remain at the same potential as your power supply ground, and the RF voltage appears only inside the coax and across the load resistor (which is isolated inside the can, of course).

If you were to feed a canned dummy load of the same construction with balanced feed line (window line, ladder line, etc.) then connecting one side of the line to the metal can would cause the can to be energized. I think — I'm not entirely certain I'm thinking about it properly — this would also cause the antenna system to radiate, because it is no longer completely balanced. In this case, you would still want a metal can for shielding (because the interior wiring is not a properly designed transmission line), but both conductors of the feed line would be insulated from the can.

$\endgroup$
  • $\begingroup$ I'll be using a couple feet of RG-58, so no worries there. I figured it was fine as every design does this...but it seemed odd. Thanks. $\endgroup$ – Adam KC0DAD Oct 11 '17 at 14:07
6
$\begingroup$

The metal exterior enclosure is an important component for a dummy load. The metal acts as a shield, containing any electromagnetic energy inside the can. Getting a shock is not an issue since the metal shield is at ground potential, the same as the chassis of your radio.

When I had my Novice license, I used to use a lightbulb as a dummy load. It worked for my purposes until one day, a station in Germany answered me on my lightbulb. A great contact but after that, I bought a proper dummy load in a metal paint can - the venerable Heathkit Cantenna. I also learned about common mode current...

$\endgroup$
  • $\begingroup$ Common mode on the coax shield? Countless hams (me included) had that experience! $\endgroup$ – Mike Waters Oct 10 '17 at 23:16
  • $\begingroup$ You've just made me want to use a lightbulb as an antenna...just because it can be done. $\endgroup$ – Adam KC0DAD Oct 11 '17 at 14:06
  • $\begingroup$ It certainly can. Back in the day, the light bulb dummy load was the best way to tune up the finals in a Heath DX-60 or DX-100 or HW-16. You avoid the false grid current dips by watching the bulb for maximum output power. And in the process you avoid tuning up on-air. Of course, you can do the same with a Cantenna and a wattmeter, but back then we made do with what we had on hand. $\endgroup$ – SDsolar Oct 12 '17 at 17:12
1
$\begingroup$

Usually it is the shield side (assuming coax, but similar for other types of transmission cables) that is connected to the can via the connector. That is how they are usually made. This is necessary to provide a "return" path for the circuit. The shield is usually at "ground" potential (similar to the negative side of a battery, like in your car). This poses no safety hazard to you.

$\endgroup$
  • $\begingroup$ Yep - and as long as the radio itself is grounded your body would not be the path of least resistance for any current on the exterior of the Cantenna. $\endgroup$ – SDsolar Oct 12 '17 at 17:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.