In message , Cecil
Moore writes
Ian Jackson wrote:
I'm not an antenna person, Cecil. Are you going to do the
hono(u)rable thing and calculate the dimensions?

I took a look at the Force 12 Sigma-80 and Sigma-40.
Nowhere do they say a Sigma-80 will work on 40m.

The Sigma antennas appear to be center-loaded vertical
dipoles with end hats as in the following fixed font.
FP is the feedpoint.

| |
| coil coil |
+-------------/////FP/////-------------+
| || |
| || |

I estimated the dimensions of the Sigma-80 from pictures
on the Force 12 web page. The center section is about
36 feet long and the end caps are about 22 feet long.
Plugging those values into EZNEC says that simply
shorting out the loading coils will make the antenna
resonant very close to 40m.

That's it then.
Connect a 80m quarterwave S/C stub (preferably twin feeder) across both
coils (ie from the outer end to the outer end). That will leave the
coils O/C on 80m and S/C the coils on 40m.
Should work!
Ian.
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