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# How do you limit a FM transmitter to broadcast a max of 5ft

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#### captmonkey4777

##### New Member
This is my first first post, what a great site.

I'm new to electronics, but have a project I'd like to start, but need help.

I'd like to build a transmitter & receiver that only work within 5-10ft of each other. (Very similiar to fox-hunting, but on a smaller scale.)

The rough specs are that the transmitter must be under 10 grams and can only run on 3v or less. (Cost: no more than $15) The receiver should weigh no more than 3lbs. and ultimately have a direction finder. (Cost: no more than$75)

I've found many schematics, but I'm not sure how to limit the range to my desired specs.

Any help and links to sites would be apprecaited. Thanks!

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#### Hero999

##### Banned
Why use FM?

Why not continous wave?

The detector could emit a beep when it detects the object.

I would use a small 27M.XXXXHz crystal oscillator for the transmitter (both legal and safe).

For the receiver I'd use a super heterodyne with an crystal IF oscillator, just above or below the transmitter frequency and a tone decoder circuit to latch on to the beat frequency and indicate when the transmitter has been detected.

#### Hero999

##### Banned

My suggestion would have been well and truly overkill, just for 5ft a simple tuned receiver with sufficient amplification to turn on a transistor and make a piezo buzzer go beep will do.

##### Well-Known Member
I recommend that you consider using a much lower frequency. When you go down to around 2 MHz the likelihood of signals bouncing around is much reduced and so it is easier to control your range. The circuit for the transmitter has to be shielded, and I recommend using a short monopole for the transmitter, where you roughly tune the length of the antenna to get the shorter range that you want.

the receiver does not need to be fancy or extremely sensitive.

#### Hero999

##### Banned
What about going all the way down to LF/VLF and using magnetic coupling?

A couple of AM radio loopstick aerials could be used.

#### MikeMl

##### Well-Known Member
What about going all the way down to LF/VLF and using magnetic coupling?

A couple of AM radio loopstick aerials could be used.

Loopsticks have a deep-null when at right angles to each other. That would provide a simplistic RDF, but might be undesirable, depending on the OP's use model. Example is the avalanche rescue system called Peeps. Peeps works on very VLF.

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##### Well-Known Member
What about going all the way down to LF/VLF and using magnetic coupling?

A couple of AM radio loopstick aerials could be used.

Yes indeed. Magnetic coupling can be very effective at 2 MHz as well, but all the way down to 100Khz is feasible. It is very easy to make a transmitter at those frequencies too. But using a loop antenna will make the transmitter quite directional which may work against the goals of the op.

#### mneary

##### New Member
Is this for construction or use by licensed hams (amateur radio operators)? If so, look on ham sites for information on fox hunting.

#### Blatman Bond

##### New Member
No, no FM transmitter coverage could be fix limited, because it is depend on power, ambient noise, topology, receiver performance, and many more.
If a range detector needed, it must be a dual band ultrasonic or something.

#### captmonkey4777

##### New Member
Thank you all for your responses!

Magnetic coupling sounds very interesting, but I'm not able to find any schematics of sample circuits anywhere. (I'm sure they are out there though.)

Can someone help?

Thanks again!!

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##### Well-Known Member
Magnetic coupling is acheived by picking an antenna that has strong emission of magnetic field and weak emissions of electric fields. The classic first choice for this is the loop antenna. The literature often divides this category into SMALL loop antenna and LARGE loop antenna, where small means that the total diameter is much less than one wavelength (for example, less than .15 wavelengths) at your frequency of operation. So, start by googling the keywords "small loop antenna", since you won't want to use a large loop antenna at these lower frequencies. The rest of the transmitter is no different than you might use with any other antenna.

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