From 0411bbd5b8d4305e780a077db025fb214d0f2f4b Mon Sep 17 00:00:00 2001 From: Daniel H Date: Mon, 1 May 2023 20:32:33 +0200 Subject: [PATCH] Save the FM and 2m amateur bands from harmonics --- README.md | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/README.md b/README.md index b5ed433..0b8434e 100644 --- a/README.md +++ b/README.md @@ -15,6 +15,19 @@ My antenna for this is barely large enough for half of my room to have reception **Also**, this project will overclock your pi. However, 200MHz is fine for 99.9% of picos and is SUPER unlikely to damage it unless you leave it running for a very long time (days or something in a closed environment). +### Most important disclaimer + +Legal restrictions likely aren't enough for some people to not do this. Including me actually (tho minisenders with low power like +this are actually allowed where i live, but they have to be certified). However, there is one **EXTREMELY** importatnt thing to add: + +This uses square waves, so there are GOING to be harmonics. One of those is near 99.6MHz, in the FM band. You will hear the sender there +as well in some cases as FM receivers *can usually* receive AM, its just gonna be really noisy. Your neighbors, sadly, might also hear it +there, as 99MHz FM is is in the FM radio bands. MAKE SURE YOUR ANTENNA IS SMALL!!! + +There is also a MUCH STRONGER harmonic at ~155.726MHz FM, in the 2m amateur radio band. PLEASE, PLEASE, PLEASE don't use a large +enough antenna for it to be able to send that very far. And this signal will actually reach FURTHER than the AM one because it +is a shorter wavelength and therefore sends better using a short antenna. ***__PLEASE add the low pass filter I described.__*** + ## How it works The Pi generates a 1557kHz PWM signal, which will be the carrier wave. The frequency is not changed, only the pulse width. @@ -37,6 +50,11 @@ It has the following jobs: - **An extra resistor of about 50-100 ohms may be added from the AI+ point to the AI- point to load the signal.** This is optional, but *can* clean up a little bit of noise if you have long cables. +**ERRATA :warning:**
+If you aren't an asshole, ***PLEASE*** add the low pass filter as mentioned in the Disclaimer: +- Add a 100ohm resistor between pico and antenna +- Wire a 2nF (2000pF) capacitor between the new start of the antenna and ground + ## How to use it - Flash the ino file using the Arduino IDE