Mark Keith wrote:
It does make a difference, but I don't think it's anything that would
ruin an antenna.

That's true, in the sense that yagis "want" to work. Even when elements
drop off completely, they will continue to "beam up" in the forward
direction.

However, this isn't an absolute guarantee that you can forget about
correcting for element diameter and boom effects. If all the directors
are too long, it can tip the first (longest) director into behaving as a
reflector, and then the whole beam can reverse direction at the high end
of its frequency range. Now that, you would notice.

When building my 6m yagi, "three element" I copied the eznec NBS specs
exactly as far as element lengths and dia.
....
I gave no real thought about boom size or type, DE type, etc.

On 6m you probably don't have to, because boom corrections on that band
are very small for most reasonable mounting methods. The reason why it
worked is because you *did* scale the element diameter.

As so often, whether something "matters" depends on how much
satisfaction you get from doing the best you can.


Regarding Richard's multiple musings, let's cut to the chase.

Richard, you've thought yourself a long way into this problem, and much
of your thinking is good. But now you're spinning your wheels and
getting no further. What you need now is to DO something!

1. Decide roughly how long you want your beam to be.

2. Go to a good timber merchant where they'll let you select a straight,
knot-free length of square timber, about 35mm square. Pick up a packet
of small staples and a pot of varnish.

3. Go to a farmer's merchant (lots of those in central London) and buy
10 metres of heavy galvanized fencing wire.

4. Varnish the boom. While the varnish is drying, go to www.cebik.com
and pick an OWA (Optimized Wideband Antenna) design that will fit on the
boom, and is designed for a direct 50-ohm feed.

5. Run Cebik's element dimensions through ELE.EXE to re-scale them to
your frequency and your element diameter. If you won't be able to
measure the SWR, make the driven element length half-way between the
lengths of the reflector and the first director.

6. Now switch off your computer, and don't come back until you've
actually BUILT a yagi. You don't need advice for that - you NEED to work
it out for yourself.

(Hints: Watch your thumbs when you staple those elements to the boom.
You'll have to work out a different way of supporting the driven
element, which has been cut in the middle... any Blue Peter[*] graduate
would know how.)

[*] British children's TV show, notorious for its construction projects.
"If your mother allows you to use sharp scissors, then cut a leg off the
kitchen table and... "


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek