Rocket Locator Radio Beacon

[dk-info]

I am looking for a FM transmitter circuit that transmits a tone that can be controlled by a simple microcontroller. For example, I would like to transmit a 1kHz tone for 500 mSec every 3 seconds, then transmit a Morse code pattern like a call sign every 10 minutes. I can handle the digital design but radio is still magic for me.
I need a stable design that will not drift over temperature or shock, and the output stage needs to be designed so poor antenna orientation will still yield acceptable range in the bush.
All the designs I see on line are of the "FM bug" or "FM witeless mic" varieties. They work after a fashion but are impractical (read drift too much) to be of use with modern digital FM recievers.
Is there a crystal or PLL controlled circuit that can be driven by a TTL output that has a reasonable deviation to minimize splatter? As long as I am asking, where can I purchase inductor forms with ferrite cores and shields of a size to match the 88 - 108 MHz band?
I am willing to layout a SMT version of a working circuit and place it in the public domain.
Please forgive my ignorance, any help getting started with a practical circuit (for 2012) would be appreciated.
David W,
Melbourne, Flordia.

 

[ClydeCrashKop]

In Nuts and Volts magazine there is a Bi-monthly Near Space column which covers this new realm of exploration - space for the backyard scientist. Learn to construct and launch vehicles with a focus on the instrumentation and experiments that reveal interesting features of our world. Beginner to intermediate. Started in February 2004.
https://www.nutsvolts.com/index.php?/magazine/column/near_space

They have a lot of interesting articles including logging position data and using it to plot the path in Google Earth 3-D. I know they had a project to build a locating device to recover their balloons. It might be what you are looking for.

 

[MikeMl]

How much can it weigh?
How far do you need to track it?
What frequencies can you legally use?

 

[dk-info]

I actually have to put 32oz in the nose to keep the bird from going too high. Practically anything on the order of 2" x 4" and a 9V power source will work, that all works out to roughly 8oz.

The launch area we fly in leaves the bird at most 300m from the available path. In some places it has 20' tall trees, some places only waist high grass.

I would like to work in the FM band (88-108MHz) to keep the receiver useful, I think 250mW would be the maximum power I could practically use.

David

 

[alec_t]

Working in the FM band almost certainly won't be legal. Unless you're in the middle of nowhere you will also incur the wrath of neighbours trying to listen to their radios .

 

[dk-info]

Working in the FM band almost certainly won't be legal. Unless you're in the middle of nowhere you will also incur the wrath of neighbours trying to listen to their radios .

By necessity we launch in the middle of no-where. It is at least 4km from the nearest residence. I need a stable transmitter to insure I do not "splash" into used frequencies. The trick is to select a frequency that is not used in that area, and limit the output power to 250 mW or less.

The FCC in America allows for licensed operators to use the FM band with the stipulation that it does not interfere with ongoing broadcasts. The only way to insure that is to limit power and have a "tight" circuit. Difficult to do when the temperature changes from 30°C to 55°C, direct sun to shade, and the shock/vibration of the launch, parachute deployment and landing.

I think this will require surface mount parts, and I would like to investigate printed circuit board inductors (traces shaped to be inductors) for the tank and PLL feedback.

In theory I should be able to to "print" the tank inductor on one side of the PCB and on the other, directly under it, the PLL feedback inductor. Theory is nice, but does anyone have a rule of thumb design for printing inductors on PCB boards?

Thanks as always for the great comments.

David

 

[RichTheDude]

By necessity we launch in the middle of no-where. It is at least 4km from the nearest residence. I need a stable transmitter to insure I do not "splash" into used frequencies. The trick is to select a frequency that is not used in that area, and limit the output power to 250 mW or less.

The FCC in America allows for licensed operators to use the FM band with the stipulation that it does not interfere with ongoing broadcasts. The only way to insure that is to limit power and have a "tight" circuit. Difficult to do when the temperature changes from 30°C to 55°C, direct sun to shade, and the shock/vibration of the launch, parachute deployment and landing.

I think this will require surface mount parts, and I would like to investigate printed circuit board inductors (traces shaped to be inductors) for the tank and PLL feedback.
In theory I should be able to to "print" the tank inductor on one side of the PCB and on the other, directly under it, the PLL feedback inductor. Theory is nice, but does anyone have a rule of thumb design for printing inductors on PCB boards?

Thanks as always for the great comments.

David

Are you sure about this? I have a class A UK radio "ham" license and I can not transmit on what is known as band 2 (FM). Ofcom (UK) and the FCC have even recalled car MP3 to FM transmitters for interference. How would you go going about getting license for transmitting on the "FM" band? A quarter of a Watt at that frequency would propagate quite far in line of sight conditions!

Would be much wiser in my opinion to use a license free band. A low power transmitter, and a whistling dipole DF setup would do all you wanted

 

[cowboybob]

There are a lot of very low power FM, 88 - 108 MHz band xmitters out there (as you have seen) for very little money.

One I have used and was quite pleased with (leatherback turtle hatching monitors):

https://www.canakit.com/mini-wireless-fm-transmitter-kit-ck105-uk105.html

Their output level is legal (per Part 15 FCC regs) so long as you do not alter the antenna.

The provided antenna could be loosely wound within the rocket body (the xmitter will easily fit as well) and would give you, I believe, an adequate propagation pattern for the range and orientation you describe. Or the antenna could be simply trailed behind the rocket, giving a better range. Its drag, however, might affect total payload weight allowance.

The drifting problen can be settled by replacing the LC circuit, that sets the freq, with a crystal. You would, of course, want to choose a freq to so the output is within a "dead-zone" for your area, for neighborliness, if nothing else.

They usually come with a mic modulator attached. If you use the TTL sig to drive a small oscillator (1kHz, or whatever), its output could be used to drive the modulation of the xmitter.

Now that I think about it, you might be able to get by with a simple 10 second digital recorder/playback device (like in a birthday card where you record your own message) set up with a timer to continuously loop. Feed the output directly into the xmitter.

These little cheapos can run for days (up to a week) off of one AAA.

There might be a range issue, but I would think that if your visually tracking the rocket, you could head in the direction of landfall and, even if heavy foliage, eventually pick up the signal up to locate the rocket.

All the while staying Part 15 compliant.

And I would suggest putting the real money into a very good receiver/antenna rig.

 

[dk-info]

Are you sure about this? I have a class A UK radio "ham" license and I can not transmit on what is known as band 2 (FM). Ofcom (UK) and the FCC have even recalled car MP3 to FM transmitters for interference. How would you go going about getting license for transmitting on the "FM" band? A quarter of a Watt at that frequency would propagate quite far in line of sight conditions!...

You are correct, the Technician license does not allow transmission in the FM band:


But Part 15 of the FCC regulations speak to "unlicensed low power FM transmitters" -

Unlicensed broadcasts on the TV broadcast bands are prohibited, except for certain medical telemetry devices, wireless microphones, and other 'low power auxiliary' stations with an output of 50 mW or less. 87.5 to 88.0 MHz is considered part of the VHF TV low band (channel 6 analog audio is on 87.75), though it shows up on most FM tuners. For TV, 15.241 and 15.242 deal with high VHF (channels 7 to 13), 15.242 also deals with UHF (band IV and band V).

Your point is well taken, 50 mW or less will not yield any significant range, due to the access paths I need at a minimum 100 meters in dense shrubs and light trees. (google map: **broken link removed**) The image is two years old, there is 50% more foliage!

I guess this means something in the 2m range, a Yagi antenna and special receiver... I was trying to do this on the cheap.

 

[dk-info]

I have ordered one of these kits, and will give it a go, just to say I did try. I put one together years ago without much success, probably was my poor soldering skills!

 

[cowboybob]

I would suggest you get one that is pre-assembled. It's well worth the price..

 

[MikeMl]

The legality of what you are proposing is covered in this FCC publication. See the 88-108 MHz section of the Part 15 low-power transmitter frequency table, where the allowed field-strength for Periodic Transmissions is only 150uV/m@3m. Using the FCC's formula on page 29, the allowed unlicensed power output from your transmitter is P=0.3*E^2, or 0.3*150e-6*150e-6 = 0.00675W = ~7mW. If you stay legal, the power output of a FCC compliant transmitter is not likely to be very useful for locating the rocket after it lands, and is quite marginal even for tracking while the rocket is up. I would recommend a power output of at least ~100mW. If you choose to bootleg it, you are subjecting yourself to a $10,000 fine.

Good news, you can do everything you want to do and more if you get a US Amateur Radio License. Better yet, get know some local hams and get them to help you... Use the two-meter ham band. Sensitive receivers are cheap. Small transmitters are available in kit form. You can even use GPS with APRS to track the flight and final location on www.aprs.fi (recent flight)

For example, look at what hams are doing for ANSR.