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Low power 147 MHz transmitter

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MikeMl

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I need to make a ~50mW 147Mhz beacon for high altitude balloon payload recovery. We have been using APRS beacons, but their reliability (especially their antennas) is not good. I want a dirt-simple, rugged transmitter which will run for several hours on a primary battery, similar to an aircraft Emergency Locator Transmitter on 121.5MHz, although mine could be modulated using nbfm, or pulsed.

I have been looking at the custom-programmed oscillators offered by DigiKey like this, where I can order a CMOS oscillator right on my choosen frequency.

I'm also looking at the MiniCircuits GVA-60 amplifier module because it looks like it is 50Ω In/Out, has 20db gain and a max power output of +20dbm, runs on 5V at less than 100mA.

The antenna would be a 38" center-fed dipole formed into a semi-circle, so I can build a suitable matching network. I can make this antenna very rugged because it will inside a PVC ring used to hold the parachute "open".

Questions:
Since the oscillator is "digital" and since the amp is untuned, seems like I will need a tuned network between the oscillator and the amp, and possibly another between the amp and the antenna to suppress harmonics.

Another idea is to order an oscillator at half the operating frequency, and then use a tuned doubler transistor ahead of the PA. That would make for a lower power oscillator, and suppress harmonics due to the tuned tank in the collector of the doubler.

What is the best battery for this. During the flight, outside temps get to -60C, but it'll warm to +25C as soon as the parachute descends. I dont want the battery to be damaged by the low temperature. Power draw will be ~125mA key down, duty cycle could be < 30%.

Any ideas on where to get such a transmitter ready made?
 
While I understand your desire to keep things as light and as simple as possible, I can't help but think that you may be over simplifying things to the point where you have to start adding complexity again to get the required functionality. (I think that makes sense?:confused:)

Have you considered something like this:
https://www.amazon.com/BaoFeng-UV-5R-136-174-400-480-Dual-Band/dp/B007H4VT7A
Obviously there is a lot more hardware there than you need to send up in the balloon.
To reduce weight you could strip off:
The case
The display
The keyboard
The microphone
The speaker
etc....

Addressing your original points:

Since the oscillator is "digital" and since the amp is untuned, seems like I will need a tuned network between the oscillator and the amp, and possibly another between the amp and the antenna to suppress harmonics.
The use of an oscillator with a squarewave output sounds a bit rough but you can probably just feed it straight through to the amplifier module. (You may need to attenuate the signal a bit).
On the output of the amplifier you will need a bit more that "a tuned circuit" to give sensible harmonic supression. I would suggest a low pass filter, two or three sections of a Pi configuration L-C filter should be plenty.

Another idea is to order an oscillator at half the operating frequency, and then use a tuned doubler transistor ahead of the PA. That would make for a lower power oscillator, and suppress harmonics due to the tuned tank in the collector of the doubler.
Possible, but one problem you will have is to get sufficient deviation on your NBFM when the oscillator is crystal controlled at Fc or Fc/2.
Which is why I suggested stripping a complete radio which uses a synthesiser and has correct NBFM already.

What is the best battery for this. During the flight, outside temps get to -60C, but it'll warm to +25C as soon as the parachute descends. I dont want the battery to be damaged by the low temperature. Power draw will be ~125mA key down, duty cycle could be < 30%.
Not only the battery, but the electronics may struggle at such low temperatures.

Any ideas on where to get such a transmitter ready made? .
Sorry, I have no suggestions.

JimB
 
Can't you just use a 147 MHz xtal and a couple of transistors to make a transmitter at 5v 100mA power level? There's not much to it. Then modulate it with a PIC to make a distinctive sound pattern.
 
A Colpitts osc. with a doubler should work. You may need a PA.
50mw isn't very much once it touches down. Lithium ion batteries are a good choice for cold weather. Some people use a cell phone with GPS enabled.
With a styrofoam box the transmitter should produce enough heat to keep it working.
I've seen some dog tracking collars in that band and in the 219 mhz band.
 
I need to make a ~50mW 147Mhz beacon for high altitude balloon payload recovery. We have been using APRS beacons, but their reliability (especially their antennas) is not good. I want a dirt-simple, rugged transmitter which will run for several hours on a primary battery, similar to an aircraft Emergency Locator Transmitter on 121.5MHz, although mine could be modulated using nbfm, or pulsed.

I have been looking at the custom-programmed oscillators offered by DigiKey like this, where I can order a CMOS oscillator right on my choosen frequency.

I'm also looking at the MiniCircuits GVA-60 amplifier module because it looks like it is 50Ω In/Out, has 20db gain and a max power output of +20dbm, runs on 5V at less than 100mA.

The antenna would be a 38" center-fed dipole formed into a semi-circle, so I can build a suitable matching network. I can make this antenna very rugged because it will inside a PVC ring used to hold the parachute "open".

Questions:
Since the oscillator is "digital" and since the amp is untuned, seems like I will need a tuned network between the oscillator and the amp, and possibly another between the amp and the antenna to suppress harmonics.

Another idea is to order an oscillator at half the operating frequency, and then use a tuned doubler transistor ahead of the PA. That would make for a lower power oscillator, and suppress harmonics due to the tuned tank in the collector of the doubler.

What is the best battery for this. During the flight, outside temps get to -60C, but it'll warm to +25C as soon as the parachute descends. I dont want the battery to be damaged by the low temperature. Power draw will be ~125mA key down, duty cycle could be < 30%.

Any ideas on where to get such a transmitter ready made?
Mike sorry for the late response. I have a small circuit board with most of the parts mounted on it for a 2m low power transmitter. I used one of the same boards to build a 2m fox transmitter. It uses a MC2833 IC and requires no more semiconductors and does nbfm modulation. If you are still interested let me know and I will get out the module and see what it needs for parts.
Ned
 
Everything I've read says that Li-Ion has the worst temperature range. It's Ni-Cd, then Ni-MH, then Li-Ion. Are you using an insulated payload box? If so you probably need not worry about it getting too cold in there.

Check out these Yahoo groups, the people on there have done many launches and know what's what:

**broken link removed**
 
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This is still an active project. I do not want to put this in an insulated box. I actually dont care if the transmitter doesn't transmit during the flight; in fact I may put a timer on it so that it doesn't wake up until the flight is over. Reason is that on some flights we are lofting a cross-band repeater with a 2m input, and I don't want the low-power beacon to desense the 2m receiver.

The goal is utmost simplicity/reliability. The balloons are equiped with aprs, but the reliability of the aprs beacons is poor, and frequently the GPS loses satellite lock after landing, or there is no packet path from the bottom of the canyon that the payload landed in.

In the event that the aprs tracking is lost, I want a simple beacon that I can DF from my airplane, and then once I localize it, a ground party can do the final location using conventional DF technique.

I dont need to draw current from the battery while it is frozen, I just would like it work when it thaws out... ;)


Is a MC2833 still available for purchase? OnSemi doesn't list it.
 
Not sure if you want to make something, but I just found out that Maxim makes a 45MHz-650MHz VCO chip. MAX2606. Only $2 from Mouser. Six-pin SMD. Doesn't even need an external crystal, just an inductor. Hmm...I'm going to have to get one of these to play with.
 
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