AM Antenna Suggestions

[nathanpc]

I'm starting to get into AM circuits. For now I'm just using a lengthy (1m) piece of wire as a very basic antenna for testing (it works for now). Soon I'm going to make a PCB and I don't want to continue using the large piece of wire as an antenna, so I started searching on DigiKey's RF Antenna section and I realized choosing an antenna for my project is a lot more difficult than I thought. Any suggestions on what I should look for in a antenna that will be used for AM?

Another question that I have is how I can increase the signal just a little bit? (maybe signal that is powerful enough to be received within a meter of my transmitter?)

Right now I'm basically using Afrotechmod's circuit.

[AG6QR]

What frequency are you working in?  The AM broadcast band?  That's around 0.540 to 1.600 MHz, or a wavelength of roughly 200 to 500 meters.  Long wires need to be really long to work well there -- even a quarter-wave antenna is at least 50 meters long at the shortest end of the band!   What works much better, while maintaining a reasonable size, is a ferrite bar with a coil around it.  If you can scavenge one from an old transistor radio, that's ideal.  If you google "ferrite loop antenna" or "ferrite rod antenna", you'll get a lot more info.

[Zero999]

Ideally is needs to be ¹/₄ of the wavelength of the radio frequency but that's very long at 1MHz.

Your best bet is a ferrite rod with the appropriate number of turns and tuning capacitor.
http://www.rapidonline.com/Electrical-Power/100mm-Ferrite-Rod-Aerial-88-3098/?sid=b9e1b402-6a58-44ca-882c-25281ed82fe6

[PA4TIM]

Are you serious, if you really think 1 meter is a long antenna then switch over to 3 meter. There you only need 75 cm 

AM is not a frequency but a modulation. I asume you mean 550-1600 KHz. AM is for instance also used around 130 MHz for airplane communication, or at 80 meters  some HAM use old army gear in AM.

But one meter must not be a problem with an antenna that is one meter. You can make the tank with a ferite rod and let that outside the cabinet. I builded something like that with tubes and managed to receive it over about 6 meter (buit that used 4 tubes and ran on 350 VDC

[vk3yedotcom]

A full sized antenna for 530kHz - 1600kHz is several hundred metres long.

Failing that a wire between two poles at least 20m long plus an earth connection will work.

Or with less room a 50cm - 1m square tuned loop will do very well and will be directional as well.

Some details in the links below.

[JackOfVA]

A short wire (1 m) and a long wire (tens of meters or even 100m) in the AM broadcast band actually have about the same induced voltage for any given field intensity.*

The problem is a short wire has a much higher impedance -- hundreds or thousands of ohms of capacitive reactance.

If the receiver has a high input impedance, such as a source follower, the short wire will delivery plenty of signal at the source follower output. If the receiver has a low input impedance, such as 50 ohms, then the short wire won't put much voltage into the receiver input, but the longer wire will.

*think about the typical automobile AM radio receiver. The antenna is a vertical whip 1 or 2 meters in length and yet the AM radio works well. This is because the automobile receiver is designed with a high input impedance, quite often a source follower.

[PA0PBZ]

A short wire (1 m) and a long wire (tens of meters or even 100m) in the AM broadcast band actually have about the same induced voltage for any given field intensity.*

I totally agree, but it looks like OP is talking about transmitting, not receiving. A tuned loop is probably the best solution if we are talking ~ 1MHz and the impossibility to go quarter or half wave.

[PA4TIM]

You are right for receiving but he was talking about a transmitter and then you want a lot of current in your antenna. The loop will be a way to increase radiation but it will take up more space as a 1 meter whip and he wants it very small.

[JackOfVA]

I misunderstood the question then as he mentioned both transmitting and receiving.

For medium wave transmitting in a small area I agree a loop is the best answer.

[nathanpc]

Yes, I'm transmitting, not receiving. It's just a little project to experiment with AM so I'm not worried too much about the signal distance. I just want a bit more signal so I don't have to almost touch the receiver antenna.

About the wire loop antenna, any suggestions of where to get it or how to build it?

[PA0PBZ]

About the wire loop antenna, any suggestions of where to get it or how to build it?

http://www.angelfire.com/mb/amandx/loop.html

[madires]

The problem is a short wire has a much higher impedance -- hundreds or thousands of ohms of capacitive reactance.

A 1:9 or 1:16 Unun could help.