Richard wrote:
I don't know whether I've got the know-how or the time to design a homebrew
VHF yagi for the marine band.
So, is anyone out there interested in providing a service to all marine band
DXers/eavesdroppers of designing and producing *full constructional notes*
on a range of marine antennas, and then putting the designs up on a website.
All antennas shall principally be for reception.
All antennas should be described as covering 156-162Mhz.
As far as the yagis are concerned, they should range from 2 to 6 elements.
All yagis shall have 4mm (or 3/16") diameter parasitic elements.
All parasitic elements shall be secured to boom by nylon rivets as per
G3SEK's notes at:
http://www.ifwtech.co.uk/g3sek/diy-yagi/#Construction
All booms shall consist of either 20mm or 15mm square tubing.
The DE for all yagis shall be 8mm (or 5/16").
The DE for all yagis shall be a regular hertz dipole.
There shall be some kind of balun.
Please, please!
There are half a jillion designs already out there. Also in books. All
designs can scaled to any other frequency. IE: any 2 meter VHF antenna
design can be easily scaled to 160 mhz. Why the rigid construction
requirements?
If you are scaling a design to another freq, you should also scale the
element dia. Luckily, there should be no need to change element diameter
for a rescaled 2m antenna, as it's already close enough. I once built a
marine band 4 element yagi from a thin copper tube as a boom, and
coathanger wire soldered to the boom as the elements. Not the greatest
materials, and it eventually rusted, but I could hear stuff that didn't
exist on verticals. I'm in Houston, and was listening to marine traffic
out towards Galveston, and the gulf. 50-60 plus miles easy... If you
have NBS designs, what are you waiting for? Those spacings are quite
good enough. They do lean towards max gain vs f/b, but you don't need
super f/b with what you are doing. So a NBS design should be fine. "the
f/b is still about 10 db" Thats what I used. My 6m beam is also a NBS
yagi. All my homebrew yagi's use a driven element grounded to the boom.
Also, I use gamma or T matches, etc...I rarely use a split driven
element insulated from the boom. It's more work, and I'm lazy...Only my
store bought "I didn't pay for it" HF beam is built that way. MK
--
http://web.wt.net/~nm5k