Design of 900MHz antenna's

You might want to Google for "patch antenna."

The simplest antenna to make is the folded dipole. You can make one from

300-Ohm twinaxial cable as follows: (use courier or similar font to view ASCII art diagram)

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+------------------------------------------------------+ short | cut | short +--------------------------+ +-----------------------+ | | | | | | | | | | | | | | 0 0 put transformer here

The top bar of the 'T' and the vertical part of the 'T' are 300-Ohm twinaxial cable. One of the conductors in the horizontal bar is cut in the center, and the two conductors of the vertical section are inserted there. The two ends of the top bar are shorted together. The distance between the shorts is ~0.5 wavelengths. You can play around with it to see what length works best for you.

The input impedance is approximately 300 Ohms. You can use a UHF 75-Ohm to

300-Ohm transformer to convert the input impedance to 75 Ohms. This may be good enough for your purposes. (If you are not transmitting at high power) These transformers are available cheap because they are used for UHF TV antennas. Television doesn't quite go up to 915 MHz, but I imagine that the transformer will still sort of work at that frequency.

Good luck!

--Mac

I read in sci.electronics.design that Mac wrote (in ) about 'Design of 900MHz antenna's', on Sun, 30 Jan 2005:

I don't think you mean 'twinaxial cable', which is a shielded twisted pair. I think you mean '300 ohm unshielded parallel feeder'.

Thanks. I guess I didn't know what "twinaxial" meant. The cable I'm referring to is that 300-Ohm cable which is still sometimes used for UHF TV antennas.

--Mac

The following two links might be helpful

Well, actually "shielded twisted pair" is misleading. Twinaxial actually refers to controlled-impedance shielded pairs, which look very much like coax with two wires inside rather than one.

Shielded twisted pair generally refers to normal insulated pairs simply twisted together and run through a shield. Their impedance is not well controlled, which is why they aren't used for wideband applications that need good waveform fidelity and such.

John Perry