433 MHz VS. Manhole Cover

Question

I'm working on a sensor which purpose is to measure the water level of a sinkhole without getting in touch with the wastewater. The Sensor is mounted beneath the manhole cover --> The underside is formed like a rhomb and the sensor fits perfectly in the center bulge --> something like this --> similar Manhole Cover. The whole system is attached to the Manhole cover with strong magnets. The heart of the sensor is an ESP8266-12F. The whole system runs on 3,6 Volt.

An ESP8266-12F measures the distance between manhole cover and waterline via ultrasound (HC-SR04) uploads the measurement to Thingspeak if its able to locate a known WiFi nearby and additionally transmits the measurement via 433 MHz to a Receiving Unit. The receiving Unit uses and Arduino micro, a RTC modul and a 433 MHz receiving unit.

433 MHz transmitting and receiving unit are equipped with those antennas AntennaP Picture This are the 433 MHz Receiving and transmitting Units: 433 MHz Superheteodyn I want to achieve approximately 10 Meters transmitting distance from the manhole cover to the receiving unit.

I tested the system in my flat and on numerous distances without any problems and was really confident that an obstacle like the Manhole Cover + 10 Meters Air wouldn't be any problems. I knew that getting the signal out of the sink would be the tricky part because of all concrete and steel and I've got pretty much no experience with radio frequencies and the way they behave.

Today I was able to test the whole system --> The upload to ThingsPeak works like a charm. The WiFi has to be within 10 Meters from the manhole cover so that the ESP8266-12F is able to establish the connection. The 433MHz connection however fails. I need to be within 2 Meters of the manhole cover to get it to work.

Question: What could be the reason for this bad 433 MHz result. Why is the 2,4GHz Signal so much better than the 433 MHz signal ---> I thought when it comes to obstacles a lower frequency is better? Are the antennas bad? Can I improve them? Would it be better to use a long Cable with 17 cm? (Lambda/4) Would it work better with higher VCC than 3,6 V. As far as I know the transmitting unit isn't supposed to go over 3,8 V. The receiving unit runs on 5 Volt from the arduino micro?

Answer 1

The antennas that you are using for the 433 MHz link are very marginal. While there may be regulatory issues at play, if you can increase the gain of both antennas, you would see far better performance on the 433 MHz link.

At the very least, build a 15 dBi gain yagi antenna for the receiver and point it at the transmitter.

Answer 2

There will be gaps around the manhole cover, either air due to sloppy fit, or in the concrete/bricks. The 2.4GHz exploits those gaps with its 100cm/(11 * 4) = 100cm/44 = 2.2cm quarter-wavelength to leak out.

The 433MHz, 6 times longer in wavelength, has exponentially tougher problem in "leaking out".

EDIT Here is link to prior answer on shielding, explaining the exponential behavior. Why are many IR receivers in metal cages?

Answer 3

You might be able to make this work by using the manhole cover as the antenna. A 434 MHz, the wavelength is 690 mm or 27 inches. If the manhole is about half that in diameter, you might be able to resonate the cover in one direction.

This won't be a very efficient antenna, and the impedance will likely be much less than the transmitter wants to see, but you might get enough signal to the top side to be useful. 10 m reception range is probably too optimistic if you want to stay to legal transmitter power levels. However, holding a receiver while standing next to the manhole, or with a receiver on the underside of a truck, might just work.