Richard wrote:


What has happenned is that I, who know little of the art, am relying on
whoever condescends to make a comment. And comments so far have tended to
highlight how unsatisfactory it is taking a model and bulding a yagi on the
dimensions.

Mine came out nearly perfect. 6m version...

There seems to have been a desire to point out to me how
impossible a task it is to build a decent antenna unless one factors in
alteration of boom size and element diameters. Technically these comments
are correct, you cannot expect to get the same antenna performance if these
things are ignored and I feel I've been concentrating on these details for
fear of not doing enough. But on the other hand, will the difference on
receive amount to much whether elements are insulted from a boom or not .
Probably not eh.

It does make a difference, but I don't think it's anything that would
ruin an antenna. Lets say "perfect" design shows 9.4 dbi. Lets say "not
quite perfect" design shows 8.4 dbi gain. Hardly noticable in the real
world. Most consider 6 db to equal an S unit in theory as an
example...When building my 6m yagi, "three element" I copied the eznec
NBS specs exactly as far as element lengths and dia. The dia was 1/2
inch , which was easy to copy using 1/2 inch copper tubing. I gave no
real thought about boom size or type, DE type, etc. I built the antenna
per specs using welded to the boom elements, and welded to the boom one
piece DE, matched using a simple gamma match. The antenna is all one
piece including the boom. If I had used a split DE, I would have still
used the same lengths...Anyway, I built it and tested it using local
stations as signal sources. The gain and f/b "seemed" to be almost
exactly as modeled, as well as I could measure. IE: the modeled f/b is
appx 10 db. Thats about what I got. The forward gain seemed to be about
right on also. "appx 9.4 dbi".
The easy way out for you, would be to get an existing 2m yagi, and
simply trim the elements. You could rescale in the model program to tell
you how much to take off. Sure, the spacings will change slightly, but
again I bet it won't be noticable. If anything, being the spacing will
actually slightly increase in wl, the gain "could" actually increase
slightly. I bet it doesn't drop much... The modeling program would give
you a good idea of what to expect from it. MK

--
http://web.wt.net/~nm5k

Mark Keith wrote:

The easy way out for you, would be to get an existing 2m yagi, and
simply trim the elements. You could rescale in the model program to tell
you how much to take off.

BTW, if you rescale a 2m antenna, it will also rescale the spacing. So
in order to be accurate, you would need to respace the rescaled shorter
elements to match the old spacings. "After rescaling". Fairly
simple..That would give you a decent idea what to expect from the
modified antenna. MK

--
http://web.wt.net/~nm5k

Mark Keith wrote:

Mark Keith wrote:

The easy way out for you, would be to get an existing 2m yagi, and
simply trim the elements. You could rescale in the model program to tell
you how much to take off.

BTW, if you rescale a 2m antenna, it will also rescale the spacing. So
in order to be accurate, you would need to respace the rescaled shorter
elements to match the old spacings. "After rescaling". Fairly
simple..That would give you a decent idea what to expect from the
modified antenna. MK

As a test I first rescaled the eznec 50 mhz NBS yagi to 146 mhz. I used
6-7-6 as the demo segment numbers. You are limited to 20 in the demo.
This kept the source in the middle of the DE..I ran it through the mill
and got 9.63 dbi gain, and 9.35 f/b. I then rescaled that antenna to 159
mhz. I then set the element spacings back to the 146mhz version specs.
Through the mill, and got 9.68 dbi gain, and 7.73 db f/b. About as I
expected...It would still be a good antenna. f/b is not much of an issue
for your needs in general...Thats more of an HF thing...You want gain
for local VHF. The chances of someone on your backside causing QRM are
fairly low... Find a 3-4-5 el 2m antenna, whack the elements down a tad,
and you'll be done with it. Well, unless you still wanna try building
one. It's easier to make a decent working yagi than you think, I
think...All that worrying about utter perfection will drive you to the
rubber room. :/ You would have to see about a 3 db difference in
performance to start really being noticable. Or my guess anyway on 159
mhz... MK
--
http://web.wt.net/~nm5k

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