Is there a formula to set the gamma match on a Cobwebb type element??
as I have built a Cobwebb ant for the higher shortwave broadcast bands
using normal unmatched dipoles (468/f) but I don`t think it is optimal hence
the need to gamma match!!

Regards.

Lee ...G6ZSG.

Lee,

The "bent dipole" elements used in the CobWebb have a low feedpoint
impedance - around 10-12 ohms. So the elements are formed as "folded
dipoles" to get a 4:1 impedance transformation and raise the
feedpoint impedance close to 50 ohms.

When you use "twin flex" for those elements you have to place the
shorting links in the right place to account for the **differential-
mode** velocity factor of the cable; otherwise you create two low
reactance stubs which shunt the feedpoint and modify the impedance.

Take a look at the detailed description on my web site and PM me if
it's not clear:

http://www.karinya.net/g3txq/folded_dipole/

You can avoid the need for folded dipoles entirely by using single-
wire elements and putting a 1:4 current balun at the feedpoint. Here's
how I did it:

http://www.karinya.net/g3txq/cobweb/

73,
Steve G3TXQ




On Nov 22, 8:28*am, "Lee" wrote:
Is there a formula to set the gamma match on a Cobwebb type element??
as I have built a Cobwebb ant for the higher shortwave broadcast bands
using normal unmatched dipoles (468/f) but I don`t think it is optimal hence
the need to gamma match!!

Regards.

Lee ...G6ZSG.

Thankyou for reply Steve,

I will get back to you via your email address.

Regards.

Lee ... G6ZSG.


"steveeh131047" wrote in message
...
Lee,

The "bent dipole" elements used in the CobWebb have a low feedpoint
impedance - around 10-12 ohms. So the elements are formed as "folded
dipoles" to get a 4:1 impedance transformation and raise the
feedpoint impedance close to 50 ohms.

When you use "twin flex" for those elements you have to place the
shorting links in the right place to account for the **differential-
mode** velocity factor of the cable; otherwise you create two low
reactance stubs which shunt the feedpoint and modify the impedance.

Take a look at the detailed description on my web site and PM me if
it's not clear:

http://www.karinya.net/g3txq/folded_dipole/

You can avoid the need for folded dipoles entirely by using single-
wire elements and putting a 1:4 current balun at the feedpoint. Here's
how I did it:

http://www.karinya.net/g3txq/cobweb/

73,
Steve G3TXQ




On Nov 22, 8:28 am, "Lee" wrote:
Is there a formula to set the gamma match on a Cobwebb type element??
as I have built a Cobwebb ant for the higher shortwave broadcast bands
using normal unmatched dipoles (468/f) but I don`t think it is optimal
hence
the need to gamma match!!

Regards.

Lee ...G6ZSG.

Lee,

My eMails to you keep "bouncing" so I've tried your QRZ HotMail
address instead. If that doesn't work I'll post a response here.

73,
Steve G3TXQ

OK Steve, it must be my firewall but the hotmal one got through fine and you
have a reply ... sorry about that.

Thanks for the info, all understood.

Lee ...G6ZSG.

"steveeh131047" wrote in message
...
Lee,

My eMails to you keep "bouncing" so I've tried your QRZ HotMail
address instead. If that doesn't work I'll post a response here.

73,
Steve G3TXQ