On Sun, 23 Oct 2005 18:46:51 +1000, "Brad" bradvk2qq AT w6ir.com
said in rec.radio.scanner:

The loss will be considerable. Yagis are quite narrow band. For a few
bucks/pounds/euros you can buy some aluminium tubing from a hardware store
and build your own.

Or look for "broomstick and wire hanger" designs on the web. If you
have a long enough piece of wood for the boom and a few wire hangers
it'll cost you nothing and still work as well. Or you might look into
building a quad - more gain with less antenna.

Hi

I liked the idea of the 'broom stick and coat hanger' antenna, but have
strugled on the practicalties. I went through the design he

http://k7mem.150m.com/Electronic_Not.../yagi_vhf.html

and ended up with a very large (about 48" square) antenna that still
complained it was to short and the elements too thin!

It also doesn't explain the driven element connection details.

Help please

Cheers
Kev

On 24 Oct 2005 04:18:49 -0700, said in
rec.radio.scanner:

I liked the idea of the 'broom stick and coat hanger' antenna, but have
strugled on the practicalties. I went through the design he

http://k7mem.150m.com/Electronic_Not.../yagi_vhf.html

and ended up with a very large (about 48" square) antenna that still
complained it was to short and the elements too thin!

48" square? The antenna is 1" thick at the center (the boom) and
~1/8" thick for the rest (the element thickness). It's about 12' long
and 4' wide at the reflector end. You can shorten the boom by asking
for less gain - the gain is determined by boom length, but you can't
shorten the elements. 1/4 wavelength is fixed by the laws of physics
for any given frequency. (A quad is smaller for the same gain than a
Yagi.)

Try this - it's for 10dbd gain (~10 times the signal strength - signal
doubles for each 3dbs of gain) at the center of the band (122 MHz):

122 MHz, 8 Elements, 10.575 dBd Estimated Gain

Warning: The specified Gain of 10 dBd is too Small. Gain should be
greater than 11.8 dBd.

41.4 Degrees Horizontal Beam Width

44.3 Degrees Vertical Beam Width

N/A Diameter, Non-Metalic Boom with Insulated Elements. N0 Boom
Correction applied.

Electrical Boom Length of 148.9" (12' 4-7/8"). Allow for overhang when
cutting boom to length.

00.125" (0-1/8") Driven Element Diameter.

00.125" (0-1/8") Parasitic Element Diameter.

Cumulative Spacing-Element Name-Element Length

Zero Reflector 47-7/16"
19-3/8" Driven Element 47-1/8"
26-5/8" Director1 44"
44" D2 43-11/16"
64-13/16" D3 43-5/16"
89" D4 42-15/16"
116-1/8" D5 42-9/16"
145-1/8" D6 42-5/16"

It also doesn't explain the driven element connection details.

You can use just about any driven element. The simplest one is to
offset the 2 halves (the elements are all total length, except the
driven element, each half of which is 1" longer than half the total
length. Drive these halves into the boom about 1/4" apart, centered
on the correct mounting point (IOW, 1 piece 1/8" in front of the
12-3/8" point, the other 1/8" behind that point). Connect the cable
center conductor to one half, shield to the other half.

Or you could do this:
http://k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf_feed.htm

Dress the cable along the boom to the balance point, at which point
you'll mount the mast, and dress the cable down the mast.

It probably won't be too great for transmitting, but it should beat
the pants off the rubber ducky for receiving.

On 24 Oct 2005 04:18:49 -0700, wrotF:

Hi

I liked the idea of the 'broom stick and coat hanger' antenna, but have
strugled on the practicalties. I went through the design he

http://k7mem.150m.com/Electronic_Not.../yagi_vhf.html

and ended up with a very large (about 48" square) antenna that still
complained it was to short and the elements too thin!

It also doesn't explain the driven element connection details.

Help please

Cheers
Kev
Take a look here.
http://k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf_feed.html#Page_Top

wrote in 1130004363.276531.327200
@z14g2000cwz.googlegroups.com:

Hi

I would like to improve my airband reception for my local airfield
(Sywell). Currently I can hardly make out what the tower is saying and
struggle with 75% of aircraft as the voice signal levels dissappear
into the noise levels. Most signals do activate the squelch ok, but
the audio noise levels are just to high.

My current setup consist of a dual band (2m & 70cm) vertical fiber
glass antenna (I can't find the exact details at the mo) on a 2.5m
meter pole which takes the top of the aerial just above level with the
neighbouring houses roofs. It then feeds into the loft with about 3 M
of Y[1] cable, through a small lightening discharge unit (which I still
need to put the ground connection in for) and then through 3m meters
more of Y[1] cable. Then inside the loft room it goes into a panel
mount BNC connector and through 1M of RG-59/U cable before going into
the AOR AR900 scanner.

[1] It has no numbers written on it, but it is a 1cm diameter coax
cable
original bought for amateur radio use.

I suspect the loft room is fairly RF noisy as it is my study including
two computers, monitors, ADSL, wireless 802.11b etc. Connecting the
scanner to mains (via it's transformer, or battery makes no difference.

I'm looking for a rough idea of how much the following options would
improve my reception experience against the cost / difficulty of
completing those changes:

1) Change aerial (must also be able to Rx & Tx on 2m and 70cm)
2) Raise aerial by 1m or so, so that it is above neighbours roofs.
3) Change aerial feed cable
4) Ground the lightening arrestor (something I know I need to do
anyway)
5) Change Rx end BNC to BNC cable (is that the right kind of cable?
Impedance mismatch?)
6) Change panel mount BNC connector (does this let noise in due to
short unshielded bit of inner cable or impedance mismatch?)
7) Add a band pass filter for airband at the Rx end
8) Add a pre-amp for airband as close to the aerial as possible

Any other suggestions?

Thanks in advance
Kev


Antenna is undoubtedly important, but have you checked your scanner in other
environments? May be there is something wrong with it, especially if it's bought second
hand?
Al Levy