Homebrew Cantenna

[VE3ELQ]

Lately I've been conducting a lot of RF deck experiments using an old beater Cantenna that measures 46 ohms, leaks oil, smells bad and has very poor heat distribution. So I built a new one.  Its made up of 98 100ohm 2W resistors in 14 parallel strings of 7 in series arranged in a circular bow shape. I bought a new 1 gal paint can, painted it flat black and fitted the lid with an SO239 coax connector and a small brass vent tube. Its filled with the cheapest oil I could find a winter grade chain saw bar oil.  It works great. SWR is flat to about 48 mhz and 1.1 to 1 slightly inductive at 54 mhz. After 20 minutes at 350W RF the top 3/4 of the can gets hot with no leaks or smell and its bang on 50 Ohms.  Not sure how much power it will take but the resistors in free air will handle 196W so in oil should do a lot more. Its an inexpensive alternative for moderate power level HF testing. Pic attached.
73s  Nigel

[WD5JKO]

   Nigel,

  That load seems perfect for your application. If you need to run it longer, then set the can in a bucket of water to add thermal capacity.

   I once needed a similar dummy load. I looked around, and found a shopping bag full of 47K 1/2W CC resistors. I said Hmmmm, and made ten rings of 100 resistors, all strapped in parallel. The JS'd arrangement was also immersed in a 1 gallon can with ATF oil inside. It came out just over 50 ohms, but with considerable capacitive reactance. Each resistor, even if 1 pf each end to end made at least 1000 pf shunt capacitance. The whole ordeal was a bust.  

   An interesting thing about dummy loads. After they heat up, disconnect the coax and measure the dummy load for voltage. I've seen several 10's of millivolts. Measuring the resistance will vary depending on ohm-meter polarity. At work we have a 3KW 30DB attenuator followed by another smaller 30DB attenuator and then feeding a RF power meter. Once the big 30DB attenuator is hot, the RF power meter reads about 100 milliwatts. As the 30 db attenuator cools, the wattmeter reading drops to zero.

Jim
Wd5JKO

[w1vtp]

Very interesting approach Nigel!..  I couldn't tell if there was provision for venting.  The Heathkit cantenna that I have has a hole with a rubber gasket and a metal stopper that pops up if there is anything that wants to get out.

On a lark, I did this 60 watt peanut can load that is good up through 6 meters.  Just installed a SO239 connecter in the middle of the bottom of the can, soldered a buss wire in the center conductor and then proceeded to solder each 2 watt carbon comp resistor radially around the can.  I used a prick punch to make a hole in the side of the can, put the resistor through the hole and then wrapped the other end around the buss.  I kept doing it until I had the number of resistors with the correct value of load Z.

More detail? and thanks, Al

[Opcom]

This one would be nice in oil, having pc board sections to aid cooling. It is good @ <1:1.2 to about 200MHz believe it or not.
110 pcs. 56 Ohm 2W resistors, 11x10. However, it works fine in cobwebs, no need for oil. I rate at 100W to avoid heating the resistors.

[W2PFY]

I have a dummy load called Big Dummy. I installed an industrial thermometer in it by drilling a hole in the top of the lid that goes on the can. One could use an ordinary baking thermometer like the type used to check when the meat gets up to proper temperature for consumption simply by drilling a hole like I did on the top. A rubber grommet will dress it up a bit and prevent oil vapor coating the top of the can. This modification will not only measure the temperature of the oil but will also vent the can. 

[W3GMS]

Very interesting approach Nigel!..  I couldn't tell if there was provision for venting.  The Heathkit cantenna that I have has a hole with a rubber gasket and a metal stopper that pops up if there is anything that wants to get out.

On a lark, I did this 60 watt peanut can load that is good up through 6 meters.  Just installed a SO239 connecter in the middle of the bottom of the can, soldered a buss wire in the center conductor and then proceeded to solder each 2 watt carbon comp resistor radially around the can.  I used a prick punch to make a hole in the side of the can, put the resistor through the hole and then wrapped the other end around the buss.  I kept doing it until I had the number of resistors with the correct value of load Z.

More detail? and thanks, Al

All great ones, but I really like the one you built Al!  Put a small fan on it and on the other side some inlet air holes and it looks like it would easily air cool. 

Joe-GMS

[Opcom]

I hate cantennas because after time there is always an unpleasant oily dust-and-cat-hair film on them. It's due to vapor. The only way to stop it is seal the can. All oils have some vapor pressure it seems. A grommet helps greatly but after a few years, there may still be a mess due to minute leakages.

There is no problem sealing cantennas. A bellows is used with a Bird or Jones, and it need never be messy. Tin cans can usually be soldered to, if prepared, because they are either 'tin' alloy or have some sort of plating.

Metal bellows of brass or copper can be costly but rubber balloons are not. A cheap build could use a large or thick-wall "punching ball' balloon inside a metal tube with these small loads as there would be more than enough volume for diaphragm movement before any annoying amount of pressure could build and cause nasty messes. The idea is the baloon is delfated but with some air in it, so it can move a few CC either way easily. Here is a sketch that explains the suggestion. The proper oil and rubber materials would not be incompatible. Surely someone could refine this idea to make it very practical.

BTW - it is counter intuitive that the balloon held vertically in the oil would contain any volume of air at all, like shown. however, if put under just slight pressure via the vent, to make it hold shape, the lit can be shut and soldered and it will then remain as shown more or less, due to air pressure. I think so anyway.