Scanning Yagi....I Was Wondering......

[k0tbt]

Hello All,

I recently purchased a Pro96 and the 800 Mhz antenna. It all works pretty good, but there seems to be some transmissions that don't quite make it. The solution? Better antenna! A yagi seems the answer and fairly simple to construct.....well, kinda simple. While thinking about cutting the elements and mounting on a suitable boom, I came upon a thought.

I wonder if this would work? For a receiving antenna, I can't see why not. Picture if you will, a yagi. A boom with the elements mounted perpendicular to the boom. Now mentally erase the boom and picture the elements floating in open space. Further image a suitable sized piece of rigid foamcore board into this mental picture. What if the elements were glued to the foamcore? Lightweight, no trying to drill holes and keeping them all square and centered along a centerline. Instead of fastening the elements perpendicularly to a boom, the elements are parallel to the flat surface of the foamcore. Laying out the centerline and element spacing would be so much easier on the foamcore; one centerline drawn on it and perpendicular lines drawn along the length of the centerline at the appropriate spacing. The wire elements would then be glued in position. The finished yagi would be then mounted on edge with the elements in a vertical orientation. I don't have the exact details for the drive element worked out along with attaching the coax, but I've got a couple of thoughts. I'm thinking of a folded dipole and reducing the 300 ohms to 75 ohms.

I simply don't see why this type of construction won't work. The weather forcast for us is a snow storm this weekend. Sounds like a good time to scrape up some bits and pieces I've got laying around and give the idea a try. Some pictures to post would be in order too.

I thought I would share my brainstorming. If it doesn't work, no biggie; it might cost me $2 to build.

Karl

 

[jonny290]

Problems:

-wind load (astronomical relative to the strength of the foam core)
-losses (that foam core board not only will eat a bit of signal, but has a velocity factor - radio waves travel slower through it than through air/vacuum so you have to adjust all your dimensions)

Also, the impedance matching for a yagi isn't quite that simple. Normal dipole elements in yagis have very low direct feed impedances - 5-10 ohms or so most of the time.

You can either:

-gamma/delta match, which means feeding the element a bit from each center - as you move outwards, impedance raises so your goal is to find the new 50 ohm sweet spot)

-run an OWA (optimized wideband) type of Yagi, which is fed directly at 50 ohms but the first director is _extremely_ close and functions partially as an impedance matcher)

-run a folded dipole and build the yagi such that the natural impedance of it drops to near 50 ohms. Remember that a standard yagi's drive element is roughly 10-25% of its natural impedance; if you can design the yagi such that the driven element is about 16% of its original impedance, a 300 ohm folded dipole will be a near-perfect match (and will have good wide bandwidth) to 50 ohm coax.

it's really not that hard to build UHF beams if you can measure millimeter accurate. Don't use PVC - it's heavy, flexy, hard to drill straight and can suck up RF. I am working on Yagis these days using 1x2 furring strips for VHF and 1x1" trim pieces for UHF. Works very well for short yagis and flat cross-section pieces are so much easier to drill accurately, which will affect your pattern and match.

Just as an aside, 'board' yagis do exist and they are often etched on to a single piece of printed circuit board. Usually this is only done above 1-2 GHz or so as the wind load, strength and cost of such large pieces of printed circuit board are prohibitive.

 

[k0tbt]

Good points!

By the way, the antenna would be strictly an indoor antenna. I live in an apartment.

Good point about the velocity factor of the foamcore. I forgot about that one.

I was using the K7MEM website yagi calulator for my dimensions. The figuring is for a 300 ohm folded dipole driver. I do, however, like the idea of a delta/gamma match...no bending to contruct the driver.

The bandwidth would only have to cover the 856 - 858 or so Mhz used by our trunked digital system.

Tinkering is part of the fun!

Karl

 

[jonny290]

that's fine, a yagi will easily give you 2-5% bandwidth at your target freq so you'll easily get that and more.

 

[SkipSanders]

Unless you just want to tinker, or are broke (as so many of us are, eh?) the commercial solution is actually easily aquired:

http://store.saveontelephones.com/osb/itemdetails.cfm?ID=505

A nice Wilson 800 MHz yagi, for $51 plus shipping.

Number of Elements 5
Frequency 800-900 MHz
Impedance 50 ohms
Gain 10 dBi
Polarization Linear / Vertical
VSWR 1.5
3db Horizontal Beamwidth 64°
3db Vertical Beamwidth 56°
Maximum Power 10 watts
Connector N Female
Material Aluminum
Length 14.4 inches / 37 cm (with mount)
Weight 2.9 ounces / 81.8 grams (with mount)
Wind Surface Area 100 cm squared
Brackets Max OD 2 inches