Total posts : 45366
Bruce, congrats on your progress with your project. It is very good of you to share the blow by blow steps as this is not only interesting but informative about how projects progress.
You have a good memory regarding my work with the 900 MHz modules since it has been a while since I reported on this. Since you mentioned this, I will share an update. My project was to wirelessly link a Doppler radar speed unit with the link between the Gunn diode head and the speed display unit. The Gunn unit produces an audio signal in the tens of microvolts range with a frequency span of 250 to 3 kHz. It took a lot of work to get the signal conditioned and amplified before it is sent into the transmitter input. One big problem was signal to noise ratio but I managed to design an amplifier which works well with the intent of maximizing the input to the transmitter rather than applying gain at the receiver. This is so noise in the link doesn’t get amplified.
I am able to measure the spectrum and signal to noise ratio of the received signal and it worked out very well with a S/N of 45 dB comparing the tone to the white noise. Doppler radar audio contains a lot of noise due to artifacts such as thermal air currents, neighbors’ sprinklers, trees moving with wind, and birds. I clocked birds here at 20 MPH.
When I interrupt the audio signal into the transmitter the link receiver quieting is 60 dB which is excellent. I have not found the maximum range yet but the received signal is excellent even when the transmitter is 100 feet away in my yard and the receiver is in my basement underground with the signal having to penetrate stucco siding with a wire mesh behind it and the intermediate sheathing with foil. Usually my other toys’ (rain gauge, outdoor temp. sender, and motion detector) 433 MHz signals have difficulty penetrating which limits their usable range to about 10 feet outside my house.
I am using home brew quarter wave ground plane antennas for transmit and receive. These are tiny due to the high frequency, measuring about 8 cm. length on the elements. I had intended to build Yagis but didn’t need to since the link range is adequate for what I am doing so I don’t know what the capabilities of this would be. The manufacturer cites a typical range with simple antennas of 1000 feet which is believable based on what I have experienced. A real range test is on my list of things to do and since it has taken me 9 years so far to get to where I am with this it may be a while before I do the range check.
So, the upshot of this long report is to relate my positive experience using 900 MHz part 15 links. The particular units I have are no longer made but there are many other similar ones available in the $30 per unit range.
Neil
