Thanks very much for that
.
My only question is how do I use the field strength meter to measure all of those factors.
Is there a basic procedure?
Is there an order. e.g.
from field strength -> directivity -> beam width -> angle of radiation?
A field strength meter is often a device with some sort of directivity itself, so the most important thing in using it is to point it the right way. This depends on what antenna is used by the meter. If it uses a simple dipole antenna, then the direction that it works is perpendicular to the line containing the dipole itself. This kind of antenna is also a single polarization, so if you want to measure field strength with vertical polarization then you hold it so that the antenna is oriented vertically, this means that the dipole conductors are vertical. If you want to measure it horizontal, you hold it so that the dipole is in a horizontal line.
If the meter uses another type of antenna, then you have to check what kind of radiation pattern that antenna has in order to know how to point the field strength meter.
Aside from pointing, the next most important thing is to make sure that you have no conductors very close to the antenna of the field strength meter. This includes your body, so you have to hold it away from your body (like at the end of your extended arm if measuring VHF frequencies) and don't hold it against any metal.
Otherwise, you simply read the voltage from whatever meter is provided. This voltage has to be converted to a field strength and a formula is used for that. Again, you have to know which type of antenna that the field strength meter is using. Let's assume it is a half wave dipole.
Field strength uses units of volts/metre and when we measure it we are measuring the RMS value.
If your field strength meter is calibrated to read voltage in absolute terms, then
E = K x Vr
where E is the field strength, in volts/metre
K is the antenna factor in 1/metre
Vr is the voltage measurement.
or in dB:
E (dBuV/m) = K (dB/m) + Vr (dBuV)
we can calculate K for a half wave dipole using:
K = 20 log f - G - 29.8 all values in dB, result in dB/m
for example, at 150 MHz
K = 20 log 150 - 2.14 - 29.8 = 11.58 dB/m
It is unlikely that a field strength meter that you make will be calibrated, so you will not be able to measure absolute field strength. You will be able to measure changes in field strength and using the values you can plot the radiation pattern. The key to remember is that a radiation pattern is a plot of power vs angle, and you are measuring voltage, not power, so you have to square the changes in voltage to find the changes in power. This is because P = V^2/R.
Measure voltage values by walking around the antenna you are testing in a circle and take readings every 10 degrees. The diameter of this circle should be at least 3 wavelengths from the antenna you are testing, and the further the better. There can be no objects of any sort between the field strength meter and the antenna under test, the area must be clear of everything and the ground underneath should be flat. You should keep the antenna under test and your field strenght meter as high as practical. At 150 Mhz, you should put both up at about 7 feet or more.
Once you plot radiation pattern, you graphically determine the directivity and the beamwidth.