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The express-pizza antenna
It's possible to build a directional antenna for the 1.2 GHz band, in 15 minutes. Moreover, with olives on your pizza, the cost is almost nil.
- The next time you go shopping, stop by the pizza shop, that is on the street corner. Ask them for an empty 5 liter can. They will be happy to empty their trash can by offering you more. Decline their generosity. If there is no pizza parlor nearby, do not despair; you should be able to get one from another nearby restaurant or local soup kitchen.
- When you get home, carefully wash the can with soap and hot water (especially if it contained tomato sauce). The inside diameter of 5 l cans appears standardized and is about 152 mm ( 6" ). Their overall height is 244 mm.
You are now the happy owner of a circular waveguide, sealed on one end, with a cutoff wavelength for mode TE11 of 259 mm (the pizza store undoubtedly does not know what they gave you!).
- Drill a 10 mm hole, exactly 87 mm from the bottom edge of the can, to allow the mounting of an N connector.
(click on the picture to increase its size)
- Solder a 50 mm long piece of 2.5 mm2 ( #14 or better #13 AWG) solid wire to the center pin of the N connector. For the express-pizza antenna don't use other type of wire (the diameter is significant). Install the N connector by means of 4 screws, with the nuts on the outside, to limit the protusion inside the guide.
- Now, adjust the antenna for minimum SWR at 1260 MHz, by trimming the monopole a little at a time using cutting pliers. The minimum of SWR corresponds to a monopole length of approximately 47 mm. This adjustment must be done while directing the antenna towards an open space, otherwise the SWR measurement may not be accurate.
- Any convenient mounting method may be used. The unit may be made weather-resistant by sealing it inside a high-quality plastic trash bag.
Here's the results (transmitter power 20W, distance 60 km, no amplifier in the antenna cable):
The express-pizza antenna can be used alone, or be used as illuminator of a dish. It can also have directors added to make it more directing (cigar antenna).
Performance of the express-pizza antenna :
nominal impedance: 50 ohms
SWR in the 1240 - 1300 MHz band < 1.4
gain: approximately 4 dBd (or 6 dBi) (in the course of measurement)
directivity in the E and H planes: approximately 80° with - 3dB (in the course of measurement)
Technical comment :
The cut-off frequency for a circular waveguide with a diameter 152 mm for mode TE11 is 1156 MHz. This cut-off frequency is very close to the beginning of the amateur band which is 1240 MHz. The wavelength in the guide is very large: 700 mm for 1240 MHz and 500 mm for 1300 MHz. Such a variation does not make it possible to obtain an SWR better 1.4 across the whole band. Many tests were carried out while modifying length, diameter, position of the monopole. The given dimensions are those which are best compromise and the results are satisfactory for a ham use.
A guide with a 170 mm diameter would make it possible to have an SWR < 1.2 across the band, but it would no longer be the express-pizza antenna.....
translation with the friendly help of Matt Mendenhall, KE6ALM
B5+ et 73 de Jean-François Fourcadier, F4DAY
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