Hello scientists, makers and people with free time. Let's turn our ordinary digital multimeter into an RF field strength meter (FSM). The FSM we are going to make is a very simple one that we will use to detect the relative RF power being transmitted from an antenna and plot its field. Since I am a licensed amateur radio operator this is designed to be used with HAM radios in mind. It may be possible to do the same for a 802.11 WiFi transmitter but because of the low power of WiFi you may need to amplify the signal before the meter.
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Step 1: Part 1: the Meter
- 2 1N60 Diodes
- 1 0.05uF Ceramic Capacitor
- 1 470pF Ceramic Capacitor
- 1 0.01uF Ceramic Capacitor
- 1 50K Potentiometer
- 2 banana plug
- 1 1'-2' Antenna or stiff wire
- 2 1' people of wire 1 Digital MultiMeter
The capacitors can be of close values but the diodes need to be germanium diodes (1N60 in this case) as these have a low voltage drop of about 300mV instead of the usual silicone diode drop of about 700mV.
The potentiometer will allow us to better adjust the sensitivity of the meter as you will see later.
How it works:
RF is an AC wave, as the RF is collected by the antenna it passes through the first capacitor. Capacitors allow the RF (because its AC) to pass through and will block any DC voltage that may exist on the antenna. Next the RF comes into contact with the diodes, these are arranged in a half wave rectifier configuration meaning that all the positive RF voltage will remain and all the negative RF voltage will go to ground. Next we encounter a capacitor again and this will filter and smooth out a bit of our RF voltage turning it into more of a DC voltage. We now get to our pot and this acts as a voltage divider and allows us to control how much voltage gets to our multimeter. Immediately after our pot we encounter one final capacitor that will again filter, smooth out the voltage that is about to head to our multimeter.
Being that this is not a calibrated test equipment the voltage doesn't actually mean that at say 3 feet you are putting 7V on the air but it's more of a relative representation of the RF field you are generating in that area. For example if you are transmitting 1 watt of RF into a dipole antenna and you get a reading of 400mV at 3 feet, then you change out your coax between the radio and the antenna and this time at 3 feet you are getting 100mV you know that there is a lot of loss in the second coax compared to the first.
Step 2: Using the Meter to Plot an Antenna's RF Pattern
For this you will need the following:
- A radio capable of running at least 1 watt. A handheld radio like the Baofeng does really good for this.
- A polar graph (You can generate one here or print the attached PDF)
- Your new FSM
- The antenna you wish to plot
Collecting the numbers:
For this I used my tape measure yagi and my Baofeng radio set at low power. What radio you do use be sure to use it at its lowest power because you will be very close to the antenna and you want to limit the RF that you will be absorbing into your body.
First if the antenna is a omni directional antenna stand far enough away from the antenna that you get about a volt (adjust the sensitivity if you need to). If the antenna is directional/beam antenna you again want to move back till you are reading about 1V on transmit at main lobe. Most multimeters have a MAX function on them. This allows you to take a reading and it will display and hold on the peak voltage that it received.
With the multimeter on MAX voltage and radio and FSM in hand, start transmitting for about 1-2 seconds. Now read and mark down the voltage. Now move around the antenna about 45 degrees from where you started and again transmit for 1-2 seconds and get the voltage from that point. When you move around keep the same distance between the FSM and the feed point of the antenna. Also hold the FSM so that its antenna's broadside is facing the antenna. This because the tips of an antenna don't receive RF and it's only the length of the antenna that does. Repeat this every 45 degrees until you have made it 1/2 way around the antenna.
Step 3: Plotting the Numbers
Now that we have our list of numbers we can use our polar graph and plot the antenna pattern. Attached to this step you will find a printable polar graph paper that you can use. I used http://incompetech.com/graphpaper/polar/ to generate the PDF so if you wish to change the size or details of the paper.
Starting at the front of the antenna I got the following numbers (in Volts):
@90° 1.09 @45° 0.411 @0° 0.113 @315° 0 @270° 0
Because the opposite of 315° is 225°, 0° -> 180°, and 45° -> 135°. And because the antenna is left and right symmetrical we can fill out the remaining numbers like so:
@225° 0 @180° .113 @135° .411
Now you can technically use these numbers as is and make the chart but the numbers are quite small so lets multiply them all by 10.
@90° 10.9 @45° 4.11 @0° 1.13 @315° 0 @270° 0 @225° 0 @180° 1.13 @135° 4.11
Now that's more like it. Numbers big enough to see on paper so let's start plotting. So on the graph paper if you use the attached one you will see going outward from the center 12 rings. The center will be 0 and going outward 1...12. On the outside you will 0° on the right and going round counter clockwise. On my yagi 0° and 180° is the tips of my driven element, 90° is the front of the yagi and 270° is the rear of it (See image). If say you are measuring a dipole then again 0° and 180° will be the tips and 90°,270° will be the broadside.
Now that we are oriented and have our numbers let's finally get to plotting. Starting at 90° for my antenna I am going to put a mark at 10.9 or basically 11 and then continue until I have all my points on the graph. Now that we have all our points plotted we can connect the dots. Since the lines on a polar graph are circular the lines connecting the points will be arcs and not straight lines.
Because we have a sample size of 8 points it's going to give us an acceptable idea of our antenna pattern. More points will give a better accuracy obviously but as you can see this is quite good as it it.
Step 4: Bonus: Testing Coax Quality With Your FSM
Bad for me but good for the reader, I believe that my coax going from my main radio to my main antenna has some water damage. This weekend I started getting reports when talking to friends on 446.5 Mhz that I was not as clear as I once was and I was having issues with receive. When I disconnected the coax from the antenna I seen moisture. Long story short I ordered new coax and it arrived today. So what great timing and practical use of my FSM than to validate my concerns of a bad coax.
Like we did when we were plotting an antenna pattern we set the multimeter to MAX reading and transmit on 1 watt for about 1-2 seconds. First I did this with the suspected bad cable and I got a reading of 22mV. Is that a lot of loss? Well I didn't know until I changed out my coax to the new improved (dry) coax. From the same place at the first reading and using again 1 watt for 1-2 seconds I got my reading of 105mV. Wow no wonder they could not hear me I was hardly putting out any RF before compared to now. I had a quick chat with my friend over the new coax and antenna and when it was mostly static before, now it's basically like talking to him in the same room.