Wes wrote:
A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.

That is a very reasonable strategy: if the impedance comes somewhere
close to a convenient value for matching, then optimize it to exactly
that value.

For example, DK7ZB has developed a range of yagi designs optimized for
28 ohms.

And there he says: "For the VHF-Bands (50-50.5MHz, 144-146MHz,
430-440MHz) a radiation resistance of 25-30Ohm has the best balance for
gain, back- and sidelobes, bandwidth and SWR at tenable losses."

We have come full-circle. [g]

Why, so we have...

At the risk of going round again, I definitely wouldn't agree with that
statement as written. However, it is valuable to point out that 28 ohms
is one of those "convenient" impedances, which might otherwise have been
overlooked.


Also, DK7ZB in describing what is clearly an unbalanced connection of
parallel lengths of coax ("Classic" match) says, "2. This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of
the cable running to the station.", which it is clearly not.

Agreed. However, there is an alternative for 50MHz because the
paralleled quarter-wave sections are a convenient length to be coiled up
to make a small, neat choke.

I had a 3-element 50MHz beam which had been thrown together using
existing gamma match parts, but it was picking up a lot of noise and
crud - on surprising on thinking about it, because the coax shield, the
boom and the mast were all connected together and acting as an antenna
for local noise. Since the feedpoint impedance happened to be about 28
ohms, it was very easy to convert it to a fully balanced feed with a
choke, and all the noise problems went away.


There are links to these and several other designs from:http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek