[Elecraft] Power measurement with a 'scope

[John Buck]

On 2/15/02 7:59 PM, John Buck at kh7t@arrl.net wrote:

 >>Do not forget to consider that the old Heath cantenna is probably 
 >>about 40 ohms instead of the original 50.

Bill Coleman, AA4LR, PP-ASEL, wrote:

 >I have a cantenna that is 25 years old, and it is still 50 ohms.
 >Why should it change?

This came up on the antenna reflector a couple years ago.
Several people noted that their Cantennas were 40 to 45 ohms.  Someone 
theorized that interaction of the oil with the element over long time 
led to reduced resistance.  I got mine out and it measured about 42 
ohms.  I do not remember the shack temperature at that time.

After I got your note, I remeasured mine and got 62 ohms.  Temperature 
in the shack was about 60 Degrees.  Removed all coax and switches and 
got the same reading at the Cantenna terminal.

So then I ran 600 watts into it for about a minute and remeasured and 
the resistance was 56 ohms.  Measured a minute later and got 53.8 ohms.

Checked the ohmmeter with a 1% 75 ohm resistor and it read 74 ohms.

Ran 600 watts for 2 minutes more then measured 48 ohms.
5 minutes later it is back up to 53 ohms.

Now I am confused.  I do not know why I got the original 42 ohm reading.

Other than temperature sensitivity I have no clue as to why the earlier 
reading was so low.  I believe that my readings are consistent and the 
connections are good.  It is unlikely that my original reading was made hot.

One thought is that my Cantenna had not been used much at all in the 
years prior to my original 42 ohm reading.  It had been very rarely used 
at power levels of 100 watts or less because I had a dummy load in the 
tuner.  Since that time I started using it with an SB220 to tune up 
after band changes.  So it would get about 10 seconds of 600 watts a few 
times a week.

I think the moral of this story is that the Cantenna is not a precision 
50 ohm load.

Aloha,
John KH7T