Ham Radio/Is Ham radio the same as Crystal radio?
QUESTION: Hi Chris,
Thank you for volunteering on this site. I have developed an interest in Crystal radio, but I do not know much about and was wondering if you could help me. I am not sure if Ham radio and Crystal radio are the same, but I have read that Crystal radio can be batteryless - that the radio "leeches" its power from the incoming electromagnetic wave.
I am working on a project where I want to send a simple signal from a transmitter to a batteryless receiver. Can you tell me what the receiver would need to contain? I do not want to produce a sound, just use the radio signal to turn a device on and off. Could something like this be batteryless do you think?
Please excuse my ignorance on this topic. I really appreciate this web site.
Thank you and best regards,
PS I also teach in Japan. Domo arigato
ANSWER: Eddie sensai,
Crystal radio and Ham radio used to be pretty much the same. All early radios were variations on the theme. Hams did a lot of experimentation with them and advanced the technology considerably. Early on, radio was thought to be a novelty with no valid commercial use.
A crystal radio is VERY simple. A long wire antenna is connected to a metallic crystal (usually galena). A "cat's whisker" - a piece of spring wire - could be moved to touch various nooks and crannies on the galena until results were achieved. A wire coil was used to help tune the frequency. When I was a kid, the common way to wind a coil was around a round empty Quaker Oats oatmeal box. Smaller coils used the tube inside toilet paper rolls.
The signal for the crystal "set" needs to be pretty strong and nearby. A 50,000 watt station about 3 miles from my home in Los Angeles was my best bet. One of the problems with crystal sets is that they are very non-selective. Sometimes I could hear 2 or 3 stations at the same time. I could also hear the spark plugs in my dad's car when he drove into the garage.
Your description of what you would like to do sounds difficult. The lack of selectivity in a very simple receiver could mean that a far away lightning strike, or the static discharge from petting the cat could trigger the device. You would need some sort of encoding/decoding to insure that the signal you intended is the only one that is acted on. The extremely low-level output might mean you would need some source of power for a circuit to "turn a device on and off".
Getting very small and almost battery-free sounds like an automatic garage door opener. The circuits are very small and simple and the signals are encoded short bursts. The tiny button battery in my garage door opener transmitter has lasted for 4 years.
Hope you are enjoying Tokyo. I thought it was pretty interesting but I much prefer the rural areas. My favorite place was a ryokan in Amanohashidate.
---------- FOLLOW-UP ----------
QUESTION: Thanks very much,
Your answer was very thorough and, importantly, easy for me to understand!
Actually my device does have a battery which turns it on and off, but I am wondering if any power needs to be devoted to the device "listening" for a signal. Would it be possible to receive the signal without using a battery in the receiver?
Again, here I reveal the extent of my ignorance, but would the encoding and decoding of the signal draw any power from the battery, or would this be a feature of the actual physical design?
Turning the device on and off would use the battery.
Would receiving the signal use the battery?
Would processing/decoding the signal use the battery?
Tokyo was fun when I came here, but I am getting homesick now! I loved Kyoto. Never had the chance to go to Amanohashidate unfortunately.
Sorry if I have expressed my questions clumsily, I am still unsure of a lot of things.
Thanks again, and best regards,
Well, Eddie, a no-battery receiver is not very realistic, but a very low battery receiver is very doable. Traditionally, sending a radio signal is very power intensive but receiving a radio signal is not. Look at cell phones, they may have two weeks of listen (standby) time but only 100 minutes of talk time.
Just awaiting a received signal can take very small amounts of power. Once a signal is detected, decoding it takes a little bit of power, but only for a split second. Once decoded, the turning on takes a little power but only briefly if the "switch" is a latching relay which is like a light switch. If the relay is not a latching relay it takes a little power for as long as the switch is on, like a spring loaded push button.
Currently, the need for small amounts of power are often met with small solar cells with a rechargeable backup battery. With these, you can do almost anything, anywhere, anytime.
If you haven't already visited, I would suggest a trip to the Tokyo fish market. Starting at about 4AM until about 7AM. Amazing.
You have been very articulate. It was a pleasure meeting you Eddie.