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?
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?