[DutchMaster]

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quote:
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Making the lead multiple lengths of 30 inches (30,60,90..)
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Dutchmaster,, why?

Oops needed some explanation, here it is.

It all has to do with the Standing Wave Ratio (SWR).


But first I shall explain what this wave is (some math knowledge required).

The wave in question is a sinuswave (carrierwave) which on the music is modulated (Frequency Modulation=FM). This looks like this:

The analog signal is with radio the music ofcourse going from 20Hz to 15kHz. The carrier wave frequency depends on the radio station (88MHz-108MHz).
Lets pick 100Mhz, when a 15kHz high pitched beep(15khz sinus) is modulated on the 100.000kHz (=100MHz) the frequency is shifted between 99.985kHz and 100.015kHz depending on the positive or negative status of the amplitude of the 15kHz.
By adding stereo and RDS to the analog signal the frequency is shifted forwards and backwards with 75kHz maximal.
You see that the average frequency still is 100MHz. So the modulated carrier wave still looks like a 100MHz sinus if you catched it on a graph.

Now back to this carrier wave. It travels electromagnetical through the air at the speed of sound (340m/s(1115ft/s)).
To catch this wave with an antenna the variable "lambda" is introduced. This is needed to determen the lenght of the antenna.
lambda =v × P = v/f v=speed of sound, P=time/period of one sinus, f is frequency

With 100MHz, lambda would be 3.40m(11.15ft)
This is pure theoretical. In real life much things have effect on lambda (humity, temparture...)

A full wave antenna would than be about 3meters. But at the end of a half wave the sinus hits zero to like in at the end of a full wave. The only differance is the amplitude being negative or positive. The differance is not "seen" by the antenna so a half wave antenna works perfectly too. (There comes the 468trick in)

Like the waves in the sea the received wave travels through the antenna. If the antenna is to short or to long the waves "break" leaving a small amount going into the cable. If the antenna has the correct length the wave travels into the cable without any problem. This applies to the cable too but not that drastical you can use mutiple instances of 1/4lamda(30inches).

Back to the Standing wave ratio. That's the ratio measured with transmitters sending a signal into the antenna and the signal that gets back. You can see it as the hight of a wave hitting a wall and the hight of the wave that's getting back. How lower the wall lesser goes back so you get a good ratio. If there is no "wall"(=no wave break) no signal comes back to the transmitter meaning a ratio of 1:1
Nothing in real life is perfect so with a good antenna installation 1:1.3 is normal. This also complies to receiving antennas.
To get a less "walls"as possible you have to take the wavelenght in consideration and you need to be sure the impedant(ohm) is equally everywhere. Usually a wire antenna has an impedant of 300 and a coax (like you're cableTV lead) 75 Ohm.

Going from a 75 Ohm output to a 300 Ohm antenna means you're making a "wall". By transforming the 75 to 300 with a balun you can make the "wall" almost disappear

Pffew long story ay. Well you wanted an answer, you get it.
Couldn't sleep so took my time
Hope you all understand it. I only covered like 10%, its difficult enough