A Rattle in the Dummy Load?  Say it's not so!

WB0SSB passed to me his Heath Cantenna for which I remain grateful to this day.  But like all gadgets of that vintage, it had some middle age wrinkles.  

The tell was that the SWR changed with the position of the unit.  And with a little power, the SWR would jump around.  What could be the source?

Popping open the can and taking a look in the bottom revealed the root cause immediately.  The silver plated strips serving as the interconnections were cracked in most places and completely broken in others.

What to do?  At the time I discovered this, I had just completed the first rework on the SB200 amp described elsewhere on the web site, and was eager to see what the amp would do.  Testing into an antenna is less optimal because of the QRM it creates as well as the number of variables it introduces.  A nice flat 50 ohm resistive load, independent of frequency, is really what is needed.

Time to improvise...

I needed to clamp onto each end and the only thing in my junkbox that looked like it would work were some hose clamps.  However, I was worried that the steel clamps would have a tough time of evenly distributing the force against the resistor's ceramic-like material.  A interposer of some kind would be needed.

After looking a while, I settled on some #18 copper stranded wire.  Fanning out the strands over the contact area and trimming them made for a nice base on which the hose clamp could contact on the outside - and the resistor would support on the inside.    


Snugging the clamps and tightning the rest of the hardware, the only steps remaining

were to reconnect the outer tube.  And that's where I ran into a bit more trouble. 

The hose clamp fit just fine into the base where the electrical contact was supposed to be made.

However, on the other end, I could expect several hundred volts to be present (depending on how my luck was running with the amp), and some clearance would be needed to give me comfort.

The solution I turned to was simply to releive part of the tube and allow the clamp to be centered in the tube.


That solution worked well.  The load resistance tested close to 50 ohms when I finished the assembly.  And since that time, the load has been abused severly in amp torture testing sessions lasting as long as 30 minutes.

That was nearly 2 years ago and reciently I was able to sweep the load with the VNA.  Nearly zero X and nearly 50 ohms up to 70 Mhz.  Really amazing after all these years!












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