[hamdani yusuf (OP)]

Continuing my effort to understand the behavior of microwave in another thread, here I'd like to share experiments showing how radio wave behaves. I've prepared some experimental equipment, but didn't have enough time to execute the experiments yet, so for now I'll put this well made video from Harvard Natural Sciences Lecture Demonstrations first.

[hamdani yusuf (OP)]

Here is another demonstration from a documentary movie.

Credits: Artifax Production for Channel 4 - 1990

[hamdani yusuf (OP)]

My plan is to repeat those experiments with various shapes and materials of antennae. I hope it can complete our understanding of how radio waves work. In my previous experiments using microwave, we investigated its interactions with various combination of materials and shapes, but the transmitter and reciever antennae have fixed materials and shapes.

[hamdani yusuf (OP)]

I was about to start the experiment using these generic 433 MHz radio transmitter and receiver,


and the circuit diagram (transmitter on the right)

when I realized that their antenna feeds are unbalanced, which are not well suited for dipole antennae. Using monopole antennae is simple and convenient for practical use, but they introduce unwanted complexities which are against the purpose of the experiments. After searching for a while I finally found this source which I think is the best solution for the problem.
http://vk5ajl.com/projects/baluns.php#current

CORE TYPE CURRENT BALUN
Highly recommended. This is a very low loss balun and ideal for use with a tuner.

This balun works by controlling currents. THERE IS NO TRANSFORMER ACTION. The two windings must be in the same sense (dots at the same end). The magnetic fields of opposing balanced working currents will cancel each other out and so present very little impedance (other than the resistance of the wires) to these currents. On the other hand, common mode currents will produce a mutually inductive magnetic field and face a high impedance.

This means the more turns the better, up to a point. In this case, the windings are a transmission line that has losses but these are much lower than the losses transfering energy from one winding to another through a core.

Design considerations are really very minimal. Since the losses of balanced lines are low compared to coax, you aren't losing much except for the resistance of the wires which is very low compared to radiating resistance anyway.

The current balun shown here, wound around a steel bolt, is probably a little crude but why not? Steel or iron is not normally used for RF because there are too many eddys making it too inefficient for transformers. In this application, since there is no magnetic effect for the desired currents, it doesn't matter. For common mode currents on the other hand, inefficiency is an advantage. Not only is a high impedance presented to common mode currents, the energy from them is absorbed by the bolt.

I'd like to hear if someone here has a second opinion.

[hamdani yusuf (OP)]

I was about to start the experiment using these generic 433 MHz radio transmitter and receiver,

Since I only need continuous radio signal, just like in microwave experiment, I simply connected the data pin of the transmitter to vcc 5V, which is supplied by a USB charger. The data pin of the receiver was measured by a Voltmeter. The reading shows only a few milli-Volts. Connecting or disconnecting power supply of the transmitter doesn't change the result.

So, I measured the Voltage of each pin of the transmitter.
I found something strange happened. The vcc, which was connected to USB charger measured above 200V. I checked the voltmeter using other power sources and it was fine. The vcc still read 200V for a few minutes, and then suddenly it went back to normal 5V.

The data pin of the receiver never shown any difference in voltage whether or not the transmitter was powered up. So I think I'll just replace them with another model.

Unfortunately I didn't recorded it in a video. Does anyone have any idea what happened? Maybe I should not directly connected the data pin with Vcc?

[Bored chemist]

I'd like to hear if someone here has a second opinion.

The bolt will be a very poor core for a radio frequency signal.
The eddy current losses will be huge.

But using a bolt to form the two coils is a neat idea.
If you cover the outside of the coils with tape to hold them in place and then "unscrew" the bolt from them, leaving an air cored transformer, that will work better

[hamdani yusuf (OP)]

The data pin of the receiver never shown any difference in voltage whether or not the transmitter was powered up. So I think I'll just replace them with another model.

So I just bought another package, which includes :

1pcs STX882 433MHz ASK Wireless RF Transmitter Module

1pcs SRX882 433MHz ASK Wireless RF Receiver Module)

2pcs SW433-TH32DN (433MHz Nickel Plated Spring Antenna)


STX882 is a ASK transmitter module with small size, ultra high power, low harmonics. With high stability, it can be achieved at 50mW power when the voltage is 3.6V, it is the maximum
ransmitter power module under the the same voltage in the market. Data port of the module can be connected to the microcontroller directly, make wireless product development and production more convenient.

Features:
- Frequency Range: 433/315 MHZ
- ASK modulation mode
- Small volume low self radiation
- High-power long-range
- Frequency stability
- Various international testing standards


SRX882 ASK Wireless Remote Control
----------------------------------------------------
Superheterodyne Receiver Module
SRX882 is a superheterodyne receiver with micro power and strong driving force, is supplementary STX882 / STX888 module. Data port of the module can be connected to the microcontroller directly, make wireless product development and production more convenient.

Features:
- Frequency Range: 433/315 MHZ
- ASK /OOK modulation mode
- Super anti-power interference
- Low-power sleep less than 1uA
- Low self radiation
- Small size
- Frequency stability and reliability
- Various international testing standards

[hamdani yusuf (OP)]

I connected the power supplies, a USB power bank for transmitter and a USB charger for receiver. Both are around 5.1 Volts DC. The data pin of the transmitter is connected to Vcc through 1kOhm resistor. The data pin of the receiver is measured using a Voltmeter relative to ground.

No significant voltage was measured, either in DC nor AC mode. The Voltmeter only shows significant value when the transmitter was turned on or off, but then fell back to near 0. This is not what I'm looking for.

So I checked the datasheet of the receiver chip.
https://html.alldatasheet.com/html-pdf/1161402/PTC/PT4303/61/1/PT4303.html


It looks like I can exclude the OOK/ASK demodulator by measuring output of buffer amplifier at pin #5 instead of data pin #11. What I want to measure is the raw radio signal amplitude.

[hamdani yusuf (OP)]

My initial test shows that the Voltmeter showed voltage of 1.7 VDC on pin #5 when only the receiver was turned on. When the transmitter was also turned on, the voltage increased to around 1.9 VDC.

The transmitter was moved away from receiver, and the voltage decreased accordingly. I knew I was going in the right direction.

I want the voltmeter shows 0 when the transmitter is off. This can be done by changing the voltage reference from ground to 1.7V, which can be achieved using a potentiometer as voltage divider. The measurement would then have a range from 0 to around 200 mV.

I measured the received radio signal with various transmitter position and distance from receiver. I also tried to block the transmission using aluminum plate I used in microwave experiment. I'll upload the video when I'm done.

[hamdani yusuf (OP)]

For initial tests, I used as much original components as possible, just to see if the devices are working as intended. I used spring antennae provided in the package for both transmitter and receiver. The signal was measured for various position and distances between transmitter and receiver. It was also measured when an aluminum plate was introduced as additional obstruction. I finished taking the video clips, and now moving on to editing phase.

Unlike my previous experiments using laser and microwave which have directed transmission, the radio transmitter and receiver used omnidirectional antennae. They make it susceptible to interference from reflection by surrounding environment. The spring antennae add more complexities in making prediction of the results.
In the next video I'll use dipole antenna to simplify the radio transmission pattern, and try to control the reflective environment.

[hamdani yusuf (OP)]

I want the voltmeter shows 0 when the transmitter is off. This can be done by changing the voltage reference from ground to 1.7V, which can be achieved using a potentiometer as voltage divider. The measurement would then have a range from 0 to around 200 mV.

It turns out that it's hard to stabilize the voltage divider using potentiometer alone. A slight movement can change the voltage significantly. So I put additional resistors to reduce the slope of V/R curve around the desired value.

[hamdani yusuf (OP)]

The latest tests I've done using spring antennae are exploring the effect of waveguide using aluminum pipe. Compared to waveguide in microwave, this time it seems more leaky. Perhaps it's because the size of the pipe is much smaller than the wavelength of the radio wave.

I also tested the transmission of radio wave when the transmitter and the antenna are wrapped by several layers of aluminum foil. Of course, the circuitry including antenna has been wrapped first using clear tape to prevent short circuit. It greatly reduces the wave transmission at long distances, but when the transmitter and receiver antennae are very close (a few cm away), the transmission can still be pretty strong. It looks like the aluminum foil wrap can't act as a good Faraday cage for this frequency.

[Bored chemist]

Did you wrap the power supply / battery in aluminium?

[hamdani yusuf (OP)]

Did you wrap the power supply / battery in aluminium?

No. There was USB cable coming in from a power bank to the transmitter.

The reciever can be seen on top right corner.

[hamdani yusuf (OP)]

My latest update on this matter. I covered the whole parts of the transmitter, including power cable and battery. The cable was folded, stuck to the battery.  No signal was detected by the receiver.
Some part of the battery was exposed, and still no detection.
It seems like the cable acted as antenna. As long as it's covered, the radio can't propagate properly.

[hamdani yusuf (OP)]

Here is my first video investigating behavior of radio wave. It shows the preparation phase by constructing radio transmitter and receiver, also the obstacles I met and how to overcome them.

[hamdani yusuf (OP)]

I hope I can investigate the difference between near field and far field EM wave soon.

[hamdani yusuf (OP)]

Here is the functional test of the transmitter and receiver pair.

Before going for more sophisticated setup, I want to make sure that the system works as intended. So I'll go first with the original package, besides those modifications I mentioned previously.

[hamdani yusuf (OP)]

Here is the test using an aluminum plate as obstruction. The size of the plate is less than the wavelength of the radiowave. With frequency or 433 MHz, the wavelength is almost 70 cm, while the side of the plate is just around 20 cm. So, it's expected that the blockage is only partial.

The result looks normal as expected until 1:00.
But if we compare to what happened in previous experiments using partial reflector in microwave transmission, the result is still consistent.

[hamdani yusuf (OP)]

It's been so long since the last time I experimented with radio frequency. I have one that hasn't been edited and narrated. There I measured the signal strength around a long antenna, which is more than one wavelength. I hope to finish it soon so I can share it here.