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.

However it leaves me with some questions:

What center frequency should i use? Well I guess that depends on the
bandwidth of the YAGI I build, and i have no idea about that, is it
1Mhz, 10Mhz is it balanced +10 and -10 or is it +2 and -10, etc. The
main frequency I want to receive is 122.7Mhz, airband is 108 to 137 I
believe.

I read that for a tape measure yagi I should use an element diameter of
roughly half the tape measure width to put into the calculations - is
that ok?

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?

The Tape Measure Yagi uses a single straight driven element with a
hairpin for matching. The calculator above suggests using a folded
dipole with a piece of coax for the matching network. Can I use a
single straight tape measure driven element with the calculations from
the K7MEM page? If so what do I need to do with the 'hairpin' matching
network, how long does the hairpin need to be?


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

Thanks
Kev

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 kevin,

Since you're interested in a 2 element yagi for reception only,
how about a Moxon antenna ?

You can use aluminum rod and pvc, takes up less space
then the Yagi but almost same performance.
Or try it with the tape measure for elements.
Direct fed with 50 ohm coax.

http://www.cebik.com/moxon/moxbld.html

Here is a calculator to help build a Moxon for a given frequency.

http://www.cebik.com/moxon/moxpage.html

Good luck!

jimbo

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