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

What I am lacking is the feedpoint impedance on 40m.
I'll try to get time to model the antenna.
--
73, Cecil http://www.w5dxp.com

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.
--
73, Cecil http://www.w5dxp.com

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.
--

Ian Jackson wrote:
Cecil Moore writes

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

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.

A somewhat weird looking modification for a center-fed
vertical antenna. I suppose the two stubs could double
as guy wires. I might be tempted to accomplish the switching
with a large open frame relay or two.
--
73, Cecil http://www.w5dxp.com

In message , Cecil Moore
writes
Ian Jackson wrote:
Cecil Moore writes

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

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.

A somewhat weird looking modification for a center-fed
vertical antenna. I suppose the two stubs could double
as guy wires. I might be tempted to accomplish the switching
with a large open frame relay or two.

From the original ASCII diagram, it isn't too clear what the
configuration is. Is it actually a 'single-ended' vertical version of
your diagram above of the horizontal balanced antenna? The two loading
coils (original diagram) would in series on 80m, with the stub shorting
out the lower one on 40m. This would enable freer adjustments of the
resonances of the antenna on the two bands. It would also help explain
the original description that the stub was connected 'across the
feedpoint' (although it isn't).
Ian.
--

Cecil Moore wrote:

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.

I did say modified and similar. I also stated that the top horizontal
portion was now vertical.


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

this one ia asymmetrical feed about 6 ft off the ground which I also
mentioned previously.



| |
| 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.

--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!

Ian Jackson wrote:
From the original ASCII diagram, it isn't too clear what the
configuration is.

As I understand it, the antenna under discussion is like
the Sigma-80 except the top hat is omitted and the upper
element is instead extended upward - like this.

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

So I'm assuming whatever works for the Sigma-80
will work for this antenna.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:

Ian Jackson wrote:
From the original ASCII diagram, it isn't too clear what the
configuration is.

As I understand it, the antenna under discussion is like
the Sigma-80 except the top hat is omitted and the upper
element is instead extended upward - like this.

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

So I'm assuming whatever works for the Sigma-80
will work for this antenna.


it does work and very well!

--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!