RE: [amsat-bb] Power divider for X-yagis

["Howard Long" <howard@xxxxxxxxxxxxxx>]

Folks

> If you look at the Microwave Handbook Volume 2 of the RSGB edited
> by M.W.Dixon, G3PRF page 13.11 you will find that for a square tube
> with air as dielectric and equal sides with inside dimension (D) and a
> round inner conductor of outside diameter ( d ) the characteristic
impedance
> Zo can be computed using the following formula where (D) and (d) are
> expressed in millimeters.
>
>                              1.178 D
> Zo= 138 log      ( ----------- )
>                      10         d

I was going to wait until I got home to discuss this, but as it's come out
now I believe that there has some disagreement over the validity of this
formula. It's widely quoted in lots of RSGB literature, but the correct
version is, for D/d > 1.5,

Z0 = 138 * log10 ( 1.08 * D / d )

A treatise online on this subject is covered here:

http://fermi.la.asu.edu/w9cf/articles/square

and here

http://hjem.get2net.dk/ole_nykjaer/oz2oe/combiner/combiner.html

True, the differences around 50 ohms are only of the order of 10%, but I
guess if you're scratching your head wondering why things ain't working like
they should be this is one more concern to consider.

73, Howard G6LVB
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