In the first installment I discussed getting set up for packet reception. In this article I'll describe the transmitting side of packet radio.
Most packet operators use their base station 2 meter mobile FM transceiver as the radio. Many of these newer radios have built in data connections, which makes things much easier. Else you take over the mic connection.
Unless you are handy with fine, unforgiving soldering, buying custom cables from a reputable source can save a lot of aggravation.
If you insist on attempting to solder your own, number #1 tip is to plug the connector you are soldering on, to a mating connector. This will keep the pins straight when the plastic gets soft.
Tip #2 small amounts of liquid flux are your friends - particularly when tinning wires and also when making connections. Alcohol cleans up.
The next question is what kind of TNC modem am I going to use? There are two paths here, hardware or software.
Hardware would be a 1200 baud TNC with radio Tx, Rx and PTT, along with DC power and the serial data connection. Since most newer computers do not have async serial ports, you will need a Keyspan serial to USB interface.
Don't forget the serial cables and possibly adapters.
While most use computers running a serial terminal program to talk to the TNC, I've seen the truly nerdy use RS232 to Bluetooth adapters and communicate via tablet or fone.
Software TNC setups use a radio to computer interface. Besides RF isolating and handling the transmit and receive audio, they usually incorporate a VOX type PTT to key the radio when there's data to send.
I personally use and recommend the Signalink USB external soundcard modem. Others prefer the Rig Runner, altho I am not familiar with it.
The most important thing to remember is you are going to be transmitting and like good Hams, we care about our signal. Besides frequency, there are two things under your control, power and deviation (loudness).
Power is simple. Don't use more than you need to get the job done. Accessing a local node using your outdoor antenna will likely work fine at 5 Watts. Save the 50 Watts for when you need it.
This would be a good place to point out many packet stations with a nearby node use Hand Held radios with an external antenna. Trying to use the rubber duck antenna often leads to all sorts of weird RFI issues.
The packet transmit deviation adjustment is best with a radio service monitor, altho, the "by ear method" seems to work well in most cases for us mere mortals.
Like I said earlier, deviation is how loudly the audio data signal is driving the transmitter can be either adjusted internally on most TNCs or on the front panel of the Signalink USB adapter.
The "by ear method" requires a second radio to listen to your transmitted signal while you adjust the volume. The TNCs all have a CALibrate command to key the transmitter with alternating 1200 and 2200 Hz tones.
Software TNCs usually require the packet program provide a calibrate function. In either case, choose an unused simplex frequency or, better yet, use a dummy load to transmit into while you are adjusting.
The idea is to turn up the deviation adjustment to the transmitter until the signal heard at the receiver doesn't get any louder (without distorting), then lower it until the signal is just noticeably quieter.
The way this works is most radios will limit deviation at about 4.5 kHz and your goal is 3 kHz.
Once you are set, it's time to try it out with the KB0MNU packet node. It's on 145.670 MHz simplex and responds to it's alias WILMAR. (Note the spelling - we're limited to 6 chars).
When you are connected, type help or ? to get a cheat sheet on using the node.
I'm including this link to an old, but very comprehensive packet radio primer. It's broken down in to sections, most of the networking stuff is not necessary and apt to be confusing.
https://choisser.com/packet/
73
Mike N0QBH
N0QBH Mike Berg