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