Wednesday, February 16, 2011

Thermal drift and increased SWR

Along with just about everyone else on the planet we've been experiencing seasonably "different" weather for the past few weeks. Texas has been particularly cold, with a lot more freezing than we're used to! This cold snap happened to coincide with the first real heavy use of my Kenwood TS-140S & small transmitting loop antenna.
Well, now that the weather has warmed up, I've discovered why the original builder of my antenna had clamped the capacitive tuning assembly securely together. It seems that while continuously transmitting, the energy running through the antenna is enough to cause a very small amount of heating, this heating is just enough to shift the VERY sensitive tuning elements and de-tune the antenna slightly!
I had left the clamps loose while I moved the tuning of the antenna and since it is located in a low traffic area (the garage) I didn't see a need to tighten them again. The freezing temperatures must have kept any heating to a minimum or at least below a critical value and the tuning remained solid.
The whole antenna is made from copper pipe and has fair coefficient of thermal expansion so it shouldn't come as a surprise that a change in temperature will cause the tuning elements to shift. I'll be interested to see how stable it can remain when I clamp the elements back together again.

More experimenting required!
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