Question about specification for RF locator

Hi all,

I'm doing a little project - an RF transmitter and a receiver. The receiver lights up an LED to indicate what direction the transmitter is in.

I'm thinking of using the 900 MHz band (specifically 912 MHz) and using a couple of small antennas.

My problem: I don't know the faintest thing about antenna electronics,
and most of the sites I find regarding it are much more large scale (e.g. how to build a ham radio antenna tower). So my questions:

• 
  * How much power does an average low-power antenna want? What voltage?
  * How is the information transmitted? Do you just adjust the power the antenna is given to adjust the output?
  * Can a standard RF antenna transmitting help tell you where it is?

Thanks for your help. I'm really a newbie wrt antennas, so I appreciate any answers you may have.

-- Josh

shielded loop antennas theory

If you geographic range is short a few mW will work.

If you have plenty of funds available, you could have GPS at both ends and compute the direction and range from that.

Another approach is psudodoppler. A set of antennas in a circle are commutated to the receiver and the phase shift is measured.

If you are allowed to mechanically rotate the antenna, you could use a yagi or other directional antenna.

Re: RF locator?

* How much power does an average low-power antenna want? What voltage?
* How is the information transmitted? Do you just adjust the power the antenna is given to adjust the output?
* Can a standard RF antenna transmitting help tell you where it is?

Click to expand...

An antenna is just an impedance matching device between your transmitter and free space. You are trying to match the output of your transmitter to free space so the maximum amount of electromagnetic energy is transferred from the wired circuit to the atmosphere.

The amount of power or voltage is not important. You can transmit microwatts or megawatts of energy. The only effect of increased power is to increase range. To calculate the length of a whip antenna for your transmitter, there are a number of online resources you can find with a search engine. One of them is at: http://www.csgnetwork.com/antennagenericfreqlencalc.html

For your 912Mhz project, a half wave antenna would be about 6 inches long - you can even use a quarter wave antenna at about 3 inches length if you are willing to lose some efficiency (range).

To make your receiver directional, you could use a shielded loop antenna. A loop antenna will get the maximum signal when it is broadside to the source; it gets the least signal when it is end-on to the source. This is the principle by which marine direction finder receivers work. If you shield one side of the loop, it will only get max signal when broadside on the unshielded side. Fine direction sensing is done by moving the antenna side to side and noting the peaks and nodes. You can find more on the web by looking for "radio direction finding", "shielded loop antennas", "RDF", etc.

I would suggest getting a copy of the ARRL Handbook for good, simple explanations of antenna theory and construction. There are practical projects that have been built by radio amatures to illustrate the theory. You don't need the most recent edition to get benefit from the book. You could pick up an old copy at a used book store that would do just fine for what you need for your project.

Re: RF locator?

Thanks for your quick and informative response!

An antenna is just an impedance matching device between your transmitter and free space. You are trying to match the output of your transmitter to free space so the maximum amount of electromagnetic energy is transferred from the wired circuit to the atmosphere.

The amount of power or voltage is not important. You can transmit microwatts or megawatts of energy. The only effect of increased power is to increase range. To calculate the length of a whip antenna for your transmitter, there are a number of online resources you can find with a search engine. One of them is at: h**p://www.csgnetwork.com/antennagenericfreqlencalc.html

For your 912Mhz project, a half wave antenna would be about 6 inches long - you can even use a quarter wave antenna at about 3 inches length if you are willing to lose some efficiency (range).

Click to expand...

I just found Digi-Key part # ANT-916-JJB-ST-ND. It's about 2 cm long, 912 MHz. I'm not looking for long range here, maybe 100 ft if I'm lucky. Would this be likely to work?

What kind of power source would I need? I'm thinking it might be able to run off a 3V lithium battery. It would probably generate about 1 mW. Would I need something bigger?

To make your receiver directional, you could use a shielded loop antenna. A loop antenna will get the maximum signal when it is broadside to the source; it gets the least signal when it is end-on to the source. This is the principle by which marine direction finder receivers work. If you shield one side of the loop, it will only get max signal when broadside on the unshielded side. Fine direction sensing is done by moving the antenna side to side and noting the peaks and nodes. You can find more on the web by looking for "radio direction finding", "shielded loop antennas", "RDF", etc.

Click to expand...

Great.

I would suggest getting a copy of the ARRL Handbook for good, simple explanations of antenna theory and construction. There are practical projects that have been built by radio amatures to illustrate the theory. You don't need the most recent edition to get benefit from the book. You could pick up an old copy at a used book store that would do just fine for what you need for your project.

Click to expand...

Will do.

Thanks for your help!
If you know off hand of any starting points for algorithmic information about direction finding, please say so. If not, that's fine. I really appreciate your help.

Thanks again!

-- Josh

Re: RF locator?

just found Digi-Key part # ANT-916-JJB-ST-ND. It's about 2 cm long, 912 MHz. I'm not looking for long range here, maybe 100 ft if I'm lucky. Would this be likely to work?

Click to expand...

Yes - the part you cite is an eighth wavelength stub. It doesn't have the gain of a longer antenna; however, it should work OK.

What kind of power source would I need? I'm thinking it might be able to run off a 3V lithium battery. It would probably generate about 1 mW. Would I need something bigger?

Click to expand...

That's a pretty small signal - I've seen micro power transmitters on lower frequencies that had 100mW into the final amp and radiated 40microwatts at the fractional wavelength antenna. The range of that transmitter was about 80ft with the available receiver.

The challenge will be to make your receiver sensitive enough to pick out your signal from the natural background noise. You haven't mentioned what kind of signal you plan to transmit; however, I would assume you plan to "chirp" modulate the RF, or use some sort of pulsed signal. A pulse is easier to pick out of the noise than a steady RF carrier. Additionally, FM is more likely to work for you than AM simply because of the abundance of AM noise around us in the world.

1mW is a pretty challenging power level. You're just going to have to experiment to see if you can make it work. The key to locating a low power transmitter in the range you propose is in the sensitivity of the receiver.

Re: RF locator?

What kind of power source would I need? I'm thinking it might be able to run off a 3V lithium battery. It would probably generate about 1 mW. Would I need something bigger?

Click to expand...

That's a pretty small signal - I've seen micro power transmitters on lower frequencies that had 100mW into the final amp and radiated 40microwatts at the fractional wavelength antenna. The range of that transmitter was about 80ft with the available receiver.

The challenge will be to make your receiver sensitive enough to pick out your signal from the natural background noise. You haven't mentioned what kind of signal you plan to transmit; however, I would assume you plan to "chirp" modulate the RF, or use some sort of pulsed signal. A pulse is easier to pick out of the noise than a steady RF carrier. Additionally, FM is more likely to work for you than AM simply because of the abundance of AM noise around us in the world.

1mW is a pretty challenging power level. You're just going to have to experiment to see if you can make it work. The key to locating a low power transmitter in the range you propose is in the sensitivity of the receiver.

Click to expand...

[/quote]
To make the receiver more sensitive, would you need to have a longer antenna, some sort of amplifier, or both?

As far as the transmit signal goes, I'm thinking of something roughly like this:
[100 ms high] [some unique 8-bit value - 10 ms per bit] [100 ms high]
every few seconds. Presumably, AM would be easier, considering that this is a fixed-frequency antenna. Any thoughts on this?

Though it would be cool (and very long-lasting) to make it work off 1 mW, that is probably impossible. A few hundred mW from a AA or AAA battery might be better.

Thanks for your insights!

-- Josh

Re: RF locator?

To make the receiver more sensitive, would you need to have a longer antenna, some sort of amplifier, or both?

Click to expand...

Each will make the receiver more sensitive. A longer antenna gives you signal gain, as does the RF preamp. Together, you get more signal into the receiver.

Presumably, AM would be easier, considering that this is a fixed-frequency antenna. Any thoughts on this?

Click to expand...

AM is easier; however, every automobile ignition system, electric motor, lightning storm, neon sign, welding machine, etc. is going to add interference to the signal you are trying to detect. Each pulse from one of those AM noise sources will mask one or more of your pulses. You can overcome this problem somewhat in AM with pulse patterns that repeat at irregular intervals. The receiver decoding gets more complicated, but the reliability goes up.

The same antenna can be used for PWM, PSM, FM, AM, etc. You choose the antenna length at the center of your operating frequency. FM and PWM transmitters and receivers can be very simple. Radio control model airplanes use PWM. Some examples of FM (not all are in your frequency range, but the basics are the same):
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http://www.jbgizmo.com/page22.html
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Re: RF locator?

There is a fundamental difference between transmitting and receiving antennas -

A transmitting antenna will radiate all the power you feed it with, minus the losses. So if you can keep the losses small, a transmitting antenna can be made (arbitrarily) small.

A receiving antenna has to physically intercept the waves in order to capture them. So it has to be physically large in order to be efficient.