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) :

Exploded Helical Image

The "exploded" helical

Custom Front Endcap Image

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.)

Rear Endcap Detail Image

Close up of the reflector / rear end cap assembly

Rear Endcap Image

Exterior of the end cap - note the washer under the 'N' connector

Painted Helical Image

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 (you’ve been lurking at irc.perthwireless.net:6667 #perthwireless haven’t 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 : Hecker’s 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 it’s 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 don’t 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 it’s 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.htm

Comments? 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 it’s your fault – not mine. You are a gumby.