I've been having a few adventures lately with making helical antennas based on Jason Hecker's design (which can be found here.)
Here's a quick look at what's been going on (click pictures for larger images) :
The "exploded" helical
My custom fron end cap to provide better support (NB : the only reason for the "slots" is I didn't make the inner tube length slightly longer and unwound in my original model.)
Close up of the reflector / rear end cap assembly
Exterior of the end cap - note the washer under the 'N' connector
Mmmmm... Aesthetics - freshly sprayed with white enamel paint - so it looks a little less "industrial" on the roof
Helical Construction Notes
Seeing that Hyrax and Mustang (youve been lurking at irc.perthwireless.net:6667 #perthwireless havent you) are planning on playing with helicals this week I thought I had better throw in a few more observations / notes on their construction based on my experiences :
Version 1.0ß 22/10/2002 Most important : Heckers original instructions seem to have a MAJOR typo in them : the pipe you want to use for the centre piece (not the cover) is called 32mm PVC Reticulation Pipe it is about 40mm in size and characteristically white - but it is NOT called 40mm. Do not use 40mm SWV this is grey pipe and used for waste water it is a fair bit more expensive and a slightly different diameter.End caps : finding the flat end caps (32mm) is not as easy as you would think. The standard ones available everywhere have a domed end and are completely useless for mounting flat. So far the only hardware stores I have found them in are the old BBC hardwares (now Bunnings Warehouses) Bunnings is changing the remaining stock over to their suppliers (the useless domed types) so its basically what is left on the shelf. Bunnings in Innaloo had stock last time I was there. Failing that, get on the phone and start calling up some retic suppliers : Mitre 10, Eden, Total Irrigation, Joel, Makit Hardwares are some others I dont know what types these guys carry.
Wire : Just about any electronics supplier keeps suitable winding wire. Jaycar and Altronics are two in Perth.
Back plate : just about any metal will do. Aluminium, copper or brass are the best options. I chose aluminium for minimum corrosion. Bunnings sells large sheets for about $20 (found in their Perspex racks) which I easily cut with one of those electric drill cutting scribes.
Glue : While I am a big Araldyte fan the 24 hour stuff just takes too damn long. Plus I find any Araldyte on PVC is never really that strong anyway. I recommend you use standard uPVC Plumbers Cement (Pipe Glue the smelly ketone stuff that is usually green or blue) use the normal stuff P type for Pressure fittings. Its got a fairly fast drying time but has a good window that allows you to twist / align bits. Its cheap and "one pack" too.
Fixings : Just about any dissimilar metals (brass, copper, steel, aluminium) are going to set up nasty electrolytic (and corrosive) reactions. Try to insulate with plastic / fibre washers where you can (the plumbing section is a great source of little non-metallic washer) and make sure the water keeps out.
Outer tube : the original plans call for 150mm SWV. This stuff, if you can find it, is rather costly not to mention quite weighty. I had much better luck using the much cheaper and lighter 90mm storm water pipe. Sure the reflector is a tad smaller and things are a little tighter on the back plate but I think its a much better product this way. A shootout soon should yield some stats
I also recommend making the centre pipe length an extra 20mm long or so (but not winding this section) this facilitates making an end cap (similar to the one I have pictured) that when fitted on the end of the OUTER tube also holds the INNER tube creating much better support.When fitting the "reflector" end cap I used a few small screws (rather than permanently glue it) for support. The other end cap I just pushed on. Both were then sealed with a few laps of good quality insulation tape. I found this useful in that you can fairly easily disassemble the outer housing on a roof top for inspection purposes.Aesthetics : If you want to paint it just use a simple enamel paint. For a lasting job you can wipe down the exterior with Plumbers Priming Fluid (similar to the glue) or use something like Easy Surface Prep these make sure the PVC surface is well keyed for the paint.
Pigtails, connectors and more : See Hyrax's pages http://www.westnet.com.au/hyrax/wireless/cheap_parts.htmComments? E-mail me or catch me as snachez on the above mentioned IRC channel.
Cheers, Antony.
Disclaimer : Info provided "as-is" and in good faith. If you mess anything / yourself up its your fault not mine. You are a gumby.