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Field strength meter

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prime

New Member
Hi All.

I have a project for physics studies which involves experimenting and reporting on an experiment involving waves.

I have decided to test different antennas:

=>¾ Wave

=>¼ Wave

=>5/8 Wave

to test directivity, readability and field strength and angle of radiation for point to point transmission of a VHF voice signal. Comparisons will be made between vertical and horizontal polarization, i.e up straight vs on the side.

Is there a way I can make a meter or meter(s) to measure these factors?

Please help. I need to get data that can be graphed, tabulated and compared.
 

Leftyretro

New Member
Hi All.

I have a project for physics studies which involves experimenting and reporting on an experiment involving waves.

I have decided to test different antennas:

=>¾ Wave

=>¼ Wave

=>5/8 Wave

to test directivity, readability and field strength and angle of radiation for point to point transmission of a VHF voice signal. Comparisons will be made between vertical and horizontal polarization, i.e up straight vs on the side.

Is there a way I can make a meter or meter(s) to measure these factors?

Please help. I need to get data that can be graphed, tabulated and compared.

Analog Devices makes a great RF log amp detector chip. It's avalible in a 8 pin DIP package. It works from DC to 500mhz and has a 92db measurement range ( I get 87db out of my circuit.) Anyway it's a great chip for this kind of application, a little pricey but will save you tons of cash in the long run.

https://www.electro-tech-online.com/custompdfs/2009/01/AD8307.pdf

Lefty
 

prime

New Member
Thanks. But this is a school project, so it can't look professional.

Also, is it possible to use a field strength meter to measure all of these factors. Could I use proportion, e.g. directivity by walking around the antenna and measuring the point of highest intensity etc etc??
 

RadioRon

Well-Known Member
You can use a field strength meter to measure field strength, and from these measurements you can plot and calculate directivity and also beamwidth. You can determine angle of radiation. You can plot field strength for horizontal and vertical polarization separately. You cannot determine readability since this is not an antenna parameter, but is a function of the signal to noise ratio in a receiver.

To build a field strength meter, you must have a meter, a power detector and an antenna on the meter. I recommend the use of a half wave dipole attached to the power detector as this will deliver the most power. Beware that such an antenna is directive so you must always make your measurements with the meter antenna broadside to the direction of measurement. The power detector may be as simple as a schottky diode AM detector when then connects directly to a very high impedance DC voltmeter set to measure millivolts. This may not be very sensitive, and if you want more sensitivity, you should indeed use the AD8307 integrated circuit to amplify and detect the power from your antenna.

Here are some useful links:
https://www.electro-tech-online.com/threads/simple-field-strength-meter.58/
Field Strength Meter
 
Last edited:

prime

New Member
RadioRon: Thankyou very much. I really appreciate it. And thanks for the links.

'You can use a field strength meter to measure field strength, and from these measurements you can plot and calculate directivity and also beamwidth. You can determine angle of radiation. You can plot field strength for horizontal and vertical polarization separately. You cannot determine readability since this is not an antenna parameter, but is a function of the signal to noise ratio in a receiver.'

Okay. Excellent :D . But I need to incorporate equations and mathematical concepts in with this. Graphed/Tabulated data for all the antennas as well as polarization will provide extensive datas, but in terms of mathematical concepts and equations I am not sure?
 

RadioRon

Well-Known Member
The basic theory that describes the radiation pattern from elements that are 1/4 wave, 5/8 wave long and so on are too complicated to state here. I suggest that you refer to an antenna engineering book to see the equations behind the directivity. One example is "Antenna Engineering Handbook" edited by Johnson, 3rd edition, chapter 2 and specifically table 2-1 as well as page 4-10 and 4-11. Another example is "Antenna Theory" by Balanis, 2nd edition, chapter 4. Another example is "Antennas for All Applications", 3rd edition, by John Kraus, chapter 2. Another key item that you might want to use is the Friis equation. This does not really define the directivity of the antenna, but it shows how the directivity, or more precisely the Gain, is used in engineering a communications link. The Friis equation shows how much power is received at one end of a radio link for a given amount of power sent from the other end.

Friis transmission equation - Wikipedia, the free encyclopedia

Also, be aware of the relationship between directivity and gain in an antenna:
Antenna gain - Wikipedia, the free encyclopedia

And of course, you should be using Decibels as one of your fundamental units for explanations:
Decibel - Wikipedia, the free encyclopedia


Perhaps it might be helpful to explane the concept of polarization in EM waves too:

Polarization - Wikipedia, the free encyclopedia
 

prime

New Member
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?
 

duffy

Well-Known Member
Leftyretro - I appreciate the link to that chip, even if no one else does. Their loss. That thing's a beaut and looks easy to work with.
 

Mikebits

Well-Known Member
Leftyretro - I appreciate the link to that chip, even if no one else does. Their loss. That thing's a beaut and looks easy to work with.

I am surprized you like that part. Most of the specs are in dB or dBm ;)
 

RadioRon

Well-Known Member
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.
 
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