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Training the hand and mind since 1982.

Saturday, February 12, 2011

NBEMS Cable interface

NBEMS is Narrow Band Emergency Message System. Think back to your last public service event, drill, or deployment. You probably passed a lot of traffic best suited for voice communications but...What if you had been asked to pass Roster of evacuees, Required prescription medications, Directions to a disaster scene. The needs of our Served Agencies have changed. They still need voice communications but...There's an increasing need for data communications. We need to be able to provide more than just voice communications from a ham with an HT. *1*

As seen in June 2009's QST Howard “Skip” Teller, KH6TY "A Sound Card Interface for FM Transceivers". There are a couple of differences with my design, my version will work with isolated microphone ground common on HF radios. And my sound card plug is integrated with the unit so you don't loose it. Software to use NBEMS is at www.w1hkj.com/NBEMS/

How does it work? When you have sound out on your sound card the audio goes through a audio transformer for isolation and impedance matching to the microphone input of the radio. Next the audio gets amplified though Q1, after Q1 the audio goes though a bypass cap C3 and get rectified via D1/D2 and charges C4. C4 value changes the PTT hang time with a 10uF it's approximately 2 second after the audio stops. When C4 reaches .7 volts or more Q2 turns on and conducts PTT line to ground. See below for schematic.

I've made a small production run of two versions, one version uses a common ground for the PTT and microphone, used with most Yaesu 2 Meter radios. The second version has two addition 1uF caps that isolate the mic ground from the PTT ground used with most Kenwood 2 Meter radios.

These projects where sold for charity to the Genesis Amateur Radio Club Repeater Fund. Please e-mail me if you would like the Gerber files for this project.

Tools I used for this project:
T-Tech PCB Milling Machine QC5000
Traxmaker 2000
Soldering Iron
Milling machine

Citations of other works
1. www.portal.state.pa.us/...pt/.../nbems_presentation_power_point_pdf