Observations from Scotland March 2006 (2) GM1SXX
Poor man's CANTENNA.
In 1986 when I first got my ticket, it seemed to me that a half decent 'dummy load' would be a very good thing to own. My main interests have always really leaned toward VHF and above so many of the commercial HF loads would probably have been unsuitable for my needs.
I looked at the' Cantenna' type load and decided to try a scaled down version using ordinary resistors. The biggest (non-wirewound) resistors I could find were 5 watt metal film types to I got hold of ten 470R 5W metal film types and made the load shown below. Ideally such a load should be cooled using proper transformer oil, hard to find and very expensive, so I went for the somewhat cheaper option of using ordinary domestic cooking oil instead. The stuff I actually used is called "Spry Crisp 'n Dry" and the oil in the photographs is now 20 years old and still clean and clear, so I guess my choice wasn't that bad!
My original load has served me VERY well over it's 20 year life and can take the full output from a 100W HF rig without becoming very hot. The resistors look like new and have not drifted in value so their 5W rating is adequate. SO, why write this article?
Well, a few weeks ago, I noticed that the original load was weeping traces of oil from the base , a sure sign of corrosion.
Photo of the original rusty (leaky) load alongside the new 'housing'
Today, I did something about it. I went to the local shops and bough a new can of 'Golden Syrup' so I could replace the 'housing'. This cost me the princely sum of 51 Pence.
.... Golden Syrup is truly dangerous stuff. Don't eat it... it rots your teeth. Don't whatever you do, pour it down the sink.... it will block up instantly! Ditto chucking it down the toilet. To be honest, I can't think of a use for the stuff except perhaps as a weapon of some sort.
IMHO, there is only ONE safe way to dispose of this hazardous material... dig a hole and bury the damned stuff. When you have disposed of the syrup (takes ages, so support the can upside-down on two old sticks over the hole you have dug and just leave for an hour or so until empty. Fill in the hole immediately... Remember, this stuff is dangerous!
Armed with the now empty can, wash it out thoroughly with detergent and dry it carefully with some kitchen paper. Leave somewhere warm to get rid of any residual moisture. Make a hole in the lid to take your connector. BNC is best but SO239 can also be used. (NOTE.. some SO-239's are not sealed... use a sealed type).
The cooking oil should NOT fill the can. Its best to arrange the resistors to be immersed in oil with an air-space above them for safety. I use about half and half.
New 'housing' for the load. Note the spotless 20 year old oil.
My 20 year old oil was spotless, so I just dunked the whole lot into the new can.
The resistors should be arranged in a circle. Ideally, there should be a small gap between each resistor and its neighbour to encourage thermal convection. The resistors are paralleled to give a measured 47R. This is close enough to 50 Ohms (actually 52!) to not cause any problems.
You may wonder why I did not parallel 10 times 510R for a better match. Well, these large resistors only come in a limited range of values and 510R wasn't one of them.
Resistor arrangement from below.
Rebuilt dummy load.
So, how does such a simply made load perform? Well the answer is that it works very well, flat from from DC -144Mhz. The SWR is a little higher on 435Mhz but is still acceptable. It will handle 100w for a couple of minutes without distress although it starts to get hot.
Possible improvements include a pressure vent. This could be as simple as a very small hole in the lid with some rubber gasket (or similar) material epoxied to it and (once cured) a VERY small hole made in the material to act as a safety vent. Being lazy, I didn't do this on either the old or the new load. The air space above the resistors seems to be adequate.
One other refinement might be to fit a screw-in feedthrough capacitor (1000pf nominal) with a diode to the top end (RF 'hot' end) of the resistor bundle to monitor the voltage developed (and therefore the TX power). being bone idle, I didn't do that either. Actually that's quite a good idea. My next dummy load will have a power probe :-)
73 Al.
GM1SXX