Wednesday, October 10, 2018

Antenna feeding receiver

The I.E.C. dummy antenna will connect to a receiver and feed the signal. You can just radiate a signal and pick it up. The dummy will feed a specific point. You can feed IF into the antenna terminal or the IF stage input. Anyway this has nothing to do with the question asked for info only. Now to the question.
The length of an antenna determines it radiation pattern and Z. The dipole has a figure eight pattern. "Pointing" the antenna in the wrong direction will cause a null in the signal. This was not the problem either just something to remember. Now the real problem.

The impedance along the half-wave antenna is as shown. When you attach a half-wave antenna to a 50 Ohm amp input your amp loads the 2500 Ohm antenna. Making a center tap and feeding it to get a 73 Ohm feed point would better match you amp. It will be directional and have to be pointed in the right direction.
The vertical with the low Z feed point is also omni-directional so it is what you want for the present testing.
Yet Another Book called for but one more point. The propagation speed in the conductor is slower than free space so the antenna will be about 90% of the calculated value at optimum. It will be close enough for testing but in a transmitter we would make it to the calculated value and trim a little at a time till we find the best length or use an antenna tuner. You can read about that when you get the book.

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