I've got a design from Wes Stewart (see "Wideband VHF Yagi - Do I have to
use a folded dipole configuration?" below) for a 6 element yagi covering
156-162 Mhz. I'm not sure what is, I mean NBS, OWA.-whatever. (Wes, I'd like
to know if possible because then I know what I'm dealing with).

Anyway, DE is 5/16" and parasitic elements are 3/16" in this design.

Now not being an antenna buff I have, regarding the element diameters, the
following thoughts arise:

1 Better not think about altering any of the element diameters. Advantages:
Does not run risk of altering design performance. Disadvantages: If I alter
diameters then maybe performance will suffer or I'll have to try to engage
in a bit of designing myself to ensure performance does not suffer from any
changes.Hey I'm not a designer.

2 Stick with diameters because the element diameters given seem to me to
make for a lighter antenna with reduced windload. Why increase element
diameters (if you don't feel you have to) if there is no performance
advantage, all you are doing is creating a bulkier antenna.

3 Alter element diameter(s) because of the limitations imposed by the
materials you can get your hands on. For instance let's say I can buy a
commercial dipole center peice that takes 1/2" tube. So, the reasoning is
alter DE to 1/2", make any adjustments to maintain performance. And what if
you could buy commercial fixings that allow you to fix 3/8" tube, you might
then decide to substiutute 3/8" tube for the 3/16" parasitic elements.

As you can see, all these things go thru my mind after being presented with
a design, and I get a bit flustered.

You might say that the conserative guy, afraid to risk anything just sticks
with the original elemment dimentions no matter what.

The give-it-a-go guy says, don't be afraid, if you feel you want to use 1/2"
for DE, and perhaps 3/8" for the parasiticvs, alter that in the 4nec2
programme and see what happens. Or is that going to lead to disaster! The
changes being too great.

One thing though, I suppose after makling alterations and before
construction, one could pass the new design through the hands of antenna
buffs. Dead simple to email the NEC file.

One time most constructors would just be presented with a design and they
would stick to it no matter what. Nowadays with these antenna programs
people who know little (like me) can seek to mess with the design if for
instance one has some tubing to hand that is not the same diameter as in the
design. Perhaps that can be gotten away with. Maybe.

I think there is something to be said for 2. Might mean items are not
available off the shelf though.

"Richard" wrote in message
...

I think there is something to be said for 2. Might mean items are not
available off the shelf though.

Actually the only issue is the dipole center peice. Getting 3/16"and 5/16"
tubing is no big problem, (although a lot of stockists just have 3/8").

5/16" will fit with sleeving in a dipole center peice meant for 1/2"tube.

I bet I could substitute 5/16" with 3/8" for DE in the design with very
little efect.

That leaves me to mount 3/8" tube in a dipole center peice meant to take
1/2". Simple, sleeving should do.

And then you think, ah, just use 1/2" tubing for DE. Again that won't make
much difference.

From G3SEK site:

IMPORTANT WARNING!
If you are copying an existing Yagi design, you must copy:

all the element lengths
all the element diameters
the element mounting method - why?
That means: copy EXACTLY! If you change any of these important details, you
are building a new design!
-------------
Gosh, just what m I getting myself into here. All I have is an NEC file for
a 156-162 Mhz yagi.That's a long way from having a design is it not.

Richard wrote:
From G3SEK site:

IMPORTANT WARNING!
If you are copying an existing Yagi design, you must copy:

all the element lengths
all the element diameters
the element mounting method - why?
That means: copy EXACTLY! If you change any of these important details, you
are building a new design!

That quote is from my 'VHF/UHF Long Yagi Workshop' page - one click from
the home-page link below.

I've just updated that page, and there is now more information about
changing element diameters and mounting methods - including a small
MS-DOS program to do the boring calculations.

(I'm supposed to be painting window-frames, but this is so much more
interesting...)


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