Kev wrote:
Hi all

I want to build a relatively small & cheap Airband Yagi for receiving
only. I have found the Tape Measure Yagi page:

http://home.att.net/~jleggio/projects/rdf/tape_bm.htm

and that seems like a good starting point so have got the bits I need
for that (tape measure 25mm width, wooden beam and RG58 Coax).

I have also found this YAGI design page:

http://www.k7mem.150m.com/Electronic.../yagi_vhf.html

that looks like it will give me the spacings I need.

[...]

The YAGI I am intending on building is about 6 elements, limited really
by the practical boom length, however 'officially' this is to small for
the calculator above as it is designed for 'long' yagi's - is the
calculator above still ok?

No. The first 6 elements of the DL6WU yagis on that page are
specifically optimized to be the start of a much longer yagi. On their
own, they would not make a good 6-element yagi.

If all you want are 6 elements, look for a 6-element design. There is a
good example at:
http://www.ifwtech.co.uk/g3sek/diy-yagi/dubus/bvo6.htm

Also try searching www.cebik.com for "owa" (Optimized Wideband Array).

You are unlikely to find a design for the particular frequency you want
to use. Modifying designs to a different frequency requires is basically
about building an exact scale model, with all its dimensions scaled to a
different wavelength... but I do mean *all* dimensions. This often isn't
mechanically practical, and if you change a key feature like the element
diameter, the diameter of a metal boom and/or the element mounting
method, then all the element lengths will need to change as well.
There's a program to help you do this on my 'VHF/UHF Long Yagi Workshop'
page:
http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm

Yagi performance falls off quite rapidly above the design frequency, so
if you want to cover the whole band, you should design it for a
frequency towards the high end.

From that same page, you'll also find some practical construction tips
(although I'm not personally familiar with the tape measure
construction).

[...]

I appreciate there are a few questions here, and that it isn't an exact
science, so all comments, suggestions, answers etc welcome.

Yagi design is quite an exact science. The art *building* yagis is to
let the science work :-)


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

Hi Guys

I have to admit I am struggling a bit here. I looked at the scaling
option, but the program you pointed to didn't seem to cover all the
variables required to be scaled.

So, rather than admitting defeat I tried to build a 6 element using the
dimensions from the k7mem link above using tape measure for the
elements and a wooden boom. However I didn't know what to do for the
driven element, both in terms of width (how big a gap in the middle and
how does this alter the overall width of the element) and matching
network. Currently I have about a 1 inch air gap, which makes the
total width about 1" wider but the width of metal is the same (I don't
know which is relevant?). Currently I have no matching network of any
kind, just connected straight to the coax.

This doesn't work, it is probably worse than a rubber duck! So, I'm
after some help please, what should I do for driven element width and
matching, and what should I do about element widths and spacings (I can
redo them if I need to)?

Thanks in advance.
Cheers
Kev