Hi everyone!

I'm attempting to make a simple RF transmitter and receiver. So the first thing I was going to make was something to receive AM RF. I was looking in to the antenna I would need and given the lowest frequency(taken from wiki, I don't know if any radio stations actually broadcast that low) I can expect is 150k Hz. That gives me a wave length of 2000 meters.

Honestly I know nothing about antenna design besides that antenna that are \(\lambda\) or half \(\lambda\) long seem to be better. I don't have a 1000 meter antenna much less the room for that. So I was wondering how much it matters if I had the antenna at about 3 meters.

In the transmitter and receiver circuit I would like to build, I don't want much more distance than about 5 - 10 meters. Do I really need to worry about the antenna or can it just be a 3 meter long copper wire?

Another quick question slightly related. I would really like to have a frequency of 20k Hz for various equipment reasons. Is this too low(assuming the above characteristics)?

Just to be clear, I don't care at all about the practicality, I'm just doing it for fun.

Thank you!

 

Hello,

Take a look at the links from this page:
http://ac6v.com/antprojects.htm

There are both theoretical as practical links available.

Bertus

 

A random length of wire will work. The question remains: how well? If you want a resonant antenna (desirable but not required), the formula for a 1/2 wave dipole (in feet) is 468 / f-in-MHz. This is not likely to be practical for VLF frequencies. The compromise solution is a whip (length whatever is convenient) with a loading coil in series. The loading coil can have a ferrite core, which makes the device much easier to build. Some portions of the VLF band are set aside for experimentation, so it might be wise to pick a frequency in that neighborhood. In all cases, be aware that FCC and international regulations apply to ALL radio transmissions in any band, so it is a good idea not to violate any of those R&R.

 

Thanks for the link Bertus. I'm sure I'll find something there.

@K7GUH

If I understand the load coil correctly, I essentially want the impedance to be purely real? I really don't know the technical names for this but I want the angle to be as close to 0 as possible on the impedance plane?

 

Thanks for the link Bertus. I'm sure I'll find something there.

@K7GUH

If I understand the load coil correctly, I essentially want the impedance to be purely real? I really don't know the technical names for this but I want the angle to be as close to 0 as possible on the impedance plane?

You might also want to look-up [loopstick antenna], as used on AM-broadcast receivers; it's a coil of fine wire on a ferrite bar, so is mostly magnetic (rather than electric). Also, there are many designs for (very-low-power, Part-15) transmitters in the AM band. Outside the AM band (i.e., below about 530kHz), you want to look closely at the rules for power limits and keep-aways. There are submarine comm's down in the VLF (maybe including 150 and 20kHz), and you probably _don't_ want to be getting visits from the Navy about interfering with them. Hams are about to get some patch of spectrum in the VLF, out of the latest World Radio Conference, so there should be quite a bit of activity (and designs) about, Real Soon Now!

 

If you plan on having one receiver and a transmitter on each remote sensor, you may (will) have problems coordinating the transmission and reception. i.e. How do you keep all the sensors from transmitting at once?

There are two ways that I know to do this. One would be for each sensor to also have a receiver, and only send data when "polled" by the central data logger/collector/processor. Another would be to have each sensor on a different frequency/channel, but then the complexity (cost) of the receiver grows.

There are a number of RF components and technologies available, they all have different advantages/disadvantages. There are also a number of other threads in the sandbox, basic stamp and SX forums. Even if you don't plan on using an SX (or STAMP), the discussions on how to deal with RF may apply to your problem.!

 

If you plan on having one receiver and a transmitter on each remote sensor, you may (will) have problems coordinating the transmission and reception. i.e. How do you keep all the sensors from transmitting at once?

There are two ways that I know to do this. One would be for each sensor to also have a receiver, and only send data when "polled" by the central data logger/collector/processor. Another would be to have each sensor on a different frequency/channel, but then the complexity (cost) of the receiver grows.

I didn't see anything indicating he was doing anything other than a single transmitter and single receiver for fun. Be that as it may, at some point he will hopefully get far enough along in his tinkering to run into the MAC (medium access control) issue. In addition to the two you dewscribe, there is also various collision avoidance protocols. Channelizing the radios is conceptually the cleanest, but in addition to making the receiver more complicated, there is the management issue on the transmitters since each one has to be configured differently. If there are a lot of sensors, it is really nice if each one could be truly identical (with the possible exception of some kind of ID code). Perhaps the easiest way to do this is to use a real simple ARQ (automatic repeat request) protocol in which a transmitter continues to periodically retransmit each message until it receives an acknowledgement from the receiver. Another way to deal with the MAC (media access control) issue is to use a concurrent code which allows identical transmitters (using identical encoding and transmission parameters) to transmit in the blind without worrying about stomping on each other (up to a point, of course). If, say, five transmissions overlap at the receiver, the receiver can still pull out all of the messages. The transmitters are pretty simple, and so is the receiver hardware, but the receiver has a considerable computational burden placed on it.