VHF on a shoestring budget
 

After over 40 years in ham radio I have decided to try my hand at VHF + weak
signal modes. Unfortunately, due to a severely restricted budget and
physical space for antennas, I am trying to put together a small VHF +
station that will provide some fun.

Following a 1999 QST article I have converted two Radio Shack 6 element FM
yagis to 5 element 2 Meter beams. I need to stack these and I am wondering
what the proper procedure and spacing is for this array. The old VHF
handbook says 5/8 wave length if they booms are under 1 wavelength. The ARRL
HB (2003) says for booms 1 wavelength long use a 1 wavelength spacing.
Unfortunately, I cannot space them at 1 wavelength. The rotor I am going to
use won't support that spacing. Can anyone point me in the right direction
on stacking and phasing these two beams?

73 Rich WB3WVC

I had used 3/4 wave spacing for a pair of CushCraft 7 element 2m beams.
Rotor was a U100, but the lower beam was only a few inches above the rotor.
Assuming the antennas are about 50 Ohms impedance, you phase them by running
equal lengths of 75 Ohm coax from each antenna to a T connector. The lengths
of the 75 Ohm coaxes must be an ODD electrical multiple of 1/4 wavelength,
as for instance 3/4 wavelength. As I recall, the calculated stacking gain
for my setup was about 2 db.

Tam/WB2TT
"news.verizon.net" wrote in message
...


After over 40 years in ham radio I have decided to try my hand at VHF +
weak
signal modes. Unfortunately, due to a severely restricted budget and
physical space for antennas, I am trying to put together a small VHF +
station that will provide some fun.

Following a 1999 QST article I have converted two Radio Shack 6 element FM
yagis to 5 element 2 Meter beams. I need to stack these and I am wondering
what the proper procedure and spacing is for this array. The old VHF
handbook says 5/8 wave length if they booms are under 1 wavelength. The
ARRL
HB (2003) says for booms 1 wavelength long use a 1 wavelength spacing.
Unfortunately, I cannot space them at 1 wavelength. The rotor I am going
to
use won't support that spacing. Can anyone point me in the right direction
on stacking and phasing these two beams?

73 Rich WB3WVC

news.verizon.net wrote:


After over 40 years in ham radio I have decided to try my hand at VHF + weak
signal modes. Unfortunately, due to a severely restricted budget and
physical space for antennas, I am trying to put together a small VHF +
station that will provide some fun.

Following a 1999 QST article I have converted two Radio Shack 6 element FM
yagis to 5 element 2 Meter beams. I need to stack these and I am wondering
what the proper procedure and spacing is for this array. The old VHF
handbook says 5/8 wave length if they booms are under 1 wavelength. The ARRL
HB (2003) says for booms 1 wavelength long use a 1 wavelength spacing.
Unfortunately, I cannot space them at 1 wavelength. The rotor I am going to
use won't support that spacing. Can anyone point me in the right direction
on stacking and phasing these two beams?

The "magic numbers" in the old handbooks for optimum stacking distance,
such as 1/2 wl, 5/8 wl and "half the boom length", were all guesses.

The optimum stacking distance actually depends on both the type of beam
and the performance you want. For example, closer stacking can greatly
reduce sidelobe levels at a small cost in forward gain.

There's more background information on my website, but the only real way
to know is to model the antennas. One of the free trial downloads such
as EZNEC will easily handle your two small yagis, and you'll also learn
a new and valuable skill in modelling.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek