Antennas in the real world.
lyle.williams at bigpond.com
Fri Dec 20 22:03:41 EST 2002
Well, I ran around with two HUGE quad helicals experimenting with antenna performance. On paper, the 4x28 turn monster was ~27dBi and the 4x17 turn ~24dBi.
Paper is an exceptionally good propagation medium - I recommend it wholeheartedly. Unfortunately my radio waves had to get through air, rather than paper. Real world gain on the helicals was nothing like the paper gain. The original gain formulas published for helicals were too optomistic, and more modern formulas discount the gain by about 5dB. Less another 3dB if working circular polarisation into linear polarisation.
Implementaion losses probably crept in here too, but I did spend quite a bit of time trying to get the helicals right. Test paths from Gungahlin to Black Mt and Gungahlin to Mt Ainslie came up OK.
I built a pair of 123x123mm biquads with 30mm skirts, as described at http://www.trevormarshall.com/biquad.htm . These perform very well, and given the ease of construction, seem like an excellent choice for starting out. The biquads worked over the paths mentioned above too. They also occupy about 1/250th the volume of the helical arrays.
I am taken by the slotted waveguides described at http://www.trevormarshall.com/waveguides.htm . By flattening the radiation pattern to a narrow area close to the horizon, these antennas give reasonable gain over 180 or 360 degrees (depending upon the design) WA FreeNet seems to be getting excellent results (25km paths between omnis!) out of similar designs. This morning I picked up the parts to make four 16 slot (or 16+16 slot) waveguides. Damage was around $150. The major construction challenge seems to be cutting rectangular holes in the aluminium. Has anyone got any tips for doing this? Alternately, a waveguide design that allows for slots with curved ends would be nice (easier to cut with a router)
All tips and pointers gladly accepted!
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