[NLRS] An idea for an NLRS beacon network

[Dr. Gerald N. Johnson]

After MUD last fall I promised Tom who spent the entire weekend 
measuring stuff including my shoebox of items that I'd carve a 2.4 GHz 
beacon antenna. So far he's not decided on what he wants. But I've been 
doing lots of library research.

Most every slotted waveguide antenna works predictably, but without fins 
it has notches in the plane of the guide in the azimuth pattern. Going 
to a low height guide tends to help even up the circularity. For a metal 
antenna at 2.4 I figure on folding up guide about 4" wide and a quarter 
inch high (inside) because that's the thinnest I can bend offsets in my 
box break. If I had a press brake I could fold thinner offsets, or 
alternatively use flat sheets for the faces with thin aluminum bar stock 
as the edges and spacers, then the faces could be really close, less 
than the face metal thickness. This thin height guide means there is a 
requirement for significant impedance matching of the feed.

Four flute end mills tend to make irregular slots because as the cutting 
edge digs into the front end of the slot, the 90 degree edges dig into 
the already cut face because of mill and machine deflection. 2 flute end 
mills make much better slots for that reason. Singly grooved router bits 
cut aluminum just fine at typical milling machine speeds and can cut 
nice slots. They just happen to have a lot more metal on the periphery 
where it means the most for stiffness. The bending strength of a round 
is proportional to the outside diameter to the 4th power. I have cut 
keyhole slots in my skin of my Airstream (2024 AlClad) with a router bit 
run by a variable speed electric drill with good results. In the mill 
the router bit would do better.

Another way to cut slots, but with odd ends would be to use a small 
diameter metal cutting saw blade in the mill, (vertical or horizontal). 
I have a very large supply of those, probably thousands all used, mostly 
3 to 4" diameter which is big for a 5 or 10 GHz slot. Generally .032 to 
.068" thick. I don't know the effect of the outside and inside guide 
slot lengths being different. But then filing square or round with most 
of the slot cut cleanly should not be difficult work.

There is a modern alternative, that's using circuit board as waveguide 
and antenna called SIW (Solid or Surface Integrated Waveguide). Define 
the guide with rows of vias and etch the slots. Makes it thin and I 
think with a good circular pattern. The literature that I've gathered 
and read show good results at least for short (up to 8 slots in line) 
radiators, it takes some severe impedance matching because of the 
dielectric filled guide, and I see no reason for all those vias along 
the edges when the only thing wanted is antenna so it could be sheared 
or sawed to width and the edges closed with soldered on copper foil 
(check Hobby Lobby stained glass area for solderable copper foil tape). 
I haven't read all the articles I've downloaded at the ISU library. 
Etching slots in a chunk of circuit board should be a lot easier than 
milling gummy copper waveguide. The interior surface on extruded 
aluminum rectangular tube may be not great for waveguide propagation.

I often wish for a 10 GHz beacon between Mason City and Albert Lea for 
use when preselecting sites for contest roves in central Iowa. But I 
haven't put together hardware or tried to find a location. For my uses 
it could be temporary, preferable on a building roof, not a tower and 
could be powered by a solar panel and maybe back up battery.

Enough "professional" climbers fall off towers that it does take good 
insurance (and expensive) coverage to get them off the ground. 
Flyswatter antennas might be more practical for microwave band beacons 
though that will tend to make them very directional sometimes negating 
the beacon concept. At least that can make the maintenance of the 
electronics simpler.

73, Jerry, K0CQ

On 4/23/2014 10:35 AM, Doug Reed wrote:
>
>
> I currently have some site access at towers around the area, from
> Rochester to Grand Rapids, from Chippewa Falls to Litchfield. If there
> was beacon equipment to install, I'd have to see what it is and talk
> to the site owner. The usual terms are to help with keeping the site
> looking good (i.e. clearing weeds and brush) and pay for the
> electricity the equipment uses. Considering the cost of coax, the best
> configuration might be an all-weather box mounted on the tower. OTOH,
> consider the cost of trained and insured tower climbers....
>
> And I have a reasonable stock of 6GHz bricks, power amps, and assorted
> WR-137 waveguide parts. We have material and dimensions for making
> 5760 waveguide slot antennas but getting them machined would need a
> CNC mill and lots of one and two flute 3/32" mill cutters. Or you can
> try doing it the old fashioned way with drill bits in a drill press
> and needle files.... Ray Johnson milled one antenna for
> proof-of-concept but it hasn't been tested yet.
>
> 73, Doug Reed, N0NAS.
>