The omni antenna, with a cigarette for scale.
The omni antenna,
with a cigarette for scale.
The omni-directional antenna was built using an N-type chassis mount connector with a short lengths of stiff fencing wire soldered into each corner hole. The driven element (the wire soldered into the centre, or conductor) was a quarter wavelength as was each ground plane (four soldered around the body of the connector). Each groundplane was then bent over at 30 degrees from the horizontal to attempt to match the impedance to 50 Ohms.
A
semi-technical drawing of the antenna - click to enlarge.
We obtained the N-type connector from R.S. at £2.93 (part number 112-2139). On the packaging it had printed: Telegartner (presumably manufacturer) Type: N-Flanschbuchse J01021H1082 Tel +49 (0) 7157/125-0 Fax -120
Prototypes were made with thinner wire (twin and earth from domestic main installations), but they were too flimsy to store or use reliably. So we ended up using the thickest, stiffest fencing wire that would fit in the holes of the N connector body. It was only bendable with two decent sets of pliers which means that in use it remains in place, even with quite a heavy bird on it...
Soldering to the "solder bucket" in the centre of the connector was easy because it has been pre-tinned. The same was not true for soldering to the holes in the body of the connector. The area around these needed a good roughing up with sandpaper, then tinning by applying solder until it flowed, then the wire soldering into the holes until all the solder flowed together, while the whole connector is hot. This means the connector gets really sodding hot (ensure it's in a safe place and well held - a vice, mole-grips), fortunately the dialectric (insulating plastic) did not melt, as has happened on cheaper connectors. All solder connections were tested by "wiggling" when cool - a very telling test.
So far this antenna has not seemed to need weatherproofing. The driven element is shrouded with the outer plastic stripped from wire of the same gauge.
When we have used this antenna it has been sufficient to screw it to the cable and affix the cable securely; this is using URM 67 coaxial cable which is 10 mm outer diameter and resists bending . The antenna is so strong and light the cable seems to provide adequate mounting in itself.
The antenna
mounted on an N-type cable.
What can I say? Bargain at £2.93, assuming you find the fencing wire in the rubbish (or a local fence) and don't really have to pay for your solder. Laughing. No seriously, this antenna is pound for pound gonna piss all over the antenna in your Airport or Airstation or Air-whatever; "it's the kiddy" as they say.
Apart from the fact it works really well, no-one has yet popped on their lab-coat and done any high-brow tests on this "homebrew twig", and of course manufacturers recommend you don't do anything which they don't recommend, or attach non-proprietary stuff to their stuff. Of course.
Author: spacepleb@psand.net
Copyright (c) 2002 Psand Limited. Permission is granted to copy, distributed and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".