# Homework Help: VHF radio experiment

1. Jan 19, 2009

### prime-factor

Hi. I have been assigned the task of designing an interesting experiment involving waves of any kind for physics class. I have chosen to focus on radio waves for my experiment and this is the basis of it:

Three antenna's will be tested for point to point transmission (VHF): [ 144 MHz ]

>>3/4 Wave antenna

>>1/4 Wave antenna

>>5/8 Antenna (Best Angle of Radiation Usually)

These antennas will be connected to a VHF radio transceiver, a voice signal will be transmitted and this will be measured with a field strength meter. The purpose of the experiment is to test whether vertical polarization vs horizontal polarization will make a large difference in field strength. (basically testing antenna up straight and then again when horizontally polarized and looking for differences in field strength).

My aim is to produce data which can be graphed illustrating the differences in field strength based on the different antennas as well as the polarization differences. I am hoping that this can lead on to an explanation of how angles of radiation affect directivity and gain.

Also I am wondering whether to build the antenna's professionally (I have the equipment to do so), or to demonstrate the simplicity and to simply use wire. (The reason I have chosen these antennas is because they are the types that will match my transciever impedance)

Any suggestions/help/advice are NEEDED!, I would highly appreciate it.

2. Jan 19, 2009

### nucleus

How much power are you planning to transmit at?

Radio frequencies are assigned differently in different countries. In the case of US and Canada, 114 MHZ is an Amateur radio band of 2 Meters. If you transmit with more power than allowed, you need a Radio licience and you have to have your radio tuned so you don’t interfer with someone else’s radio or TV etc.

If this is a school project you should check with your teacher to make sure you will not violate any radio laws.

3. Jan 19, 2009

### prime-factor

I am a fully licensed ham operator (Advanced Open License) in Australia. I am going to transmit at 25 Watts. In Australia 144 -148 MHz is the amateur VHF band and 25 Watts is well within the limit :) .

4. Jan 22, 2009

### skeptic2

Also make sure there are no multipath effects. I was once testing transmitters (390 MHz) in an open field and there was one spot where I couldn't pick up any signal from them. It turned out to be multipath from a three strand barbed wire fence about 70 ft. from the transmitter.