Tuesday, January 3, 2012

Propeller 30m QRSS Spectrum

I had reluctantly attached my Propeller Beacon and new Low Pass Filter to an older 30m 1/4 wave vertical antenna near my Ham Shack. I know the Beacon output is very low power (3mW), but I did not know if the low pass filter was working as designed. I did not want my Beacon to disrupt others. But, it was attached, anyway.

Low Pass Filter
Yesterday, my friend Jeff - KO7M came over to help install a proper longwire/dipole antenna (see previous post), and I had plan to attached the Beacon to it. Jeff brought along his Spectrum Analyzer, so that we could take a look at the Propeller Beacon's output.

As expected, without the Low Pass Filter, the raw Propeller output fed into terminated 50 ohm load was full of trash and harmonics, which is VERY undesirable for a Amateur Radio Transmitter.

Raw Output
Without Low Pass Filter
After attaching the Low Pass Filter, the trash and harmonics were still there, but at much low levels, more than 40db lower, and unseen at the same scale. Now, I felt good about attaching the Beacon and Filter to the Newly Installed Longwire Antenna.

With Low Pass Filter Attached
The Single Spike Is the Fundamental Frequency
This is what a Transmitter Output should look like
Yet, when we looked at the spectrum via an open probe in the room, all of the trash and harmonics were still very much present.

Jeff got the idea that it was probably radiated from the processor itself. We wrapped the Propeller, Battery, and Low Pass Filter in insulating paper and then in aluminum foil attached to ground. Now, only the radiated power of the fundamental Frequency passing through the Low Pass Filter and leaking from the Ladder Line could be seen with an open probe. The open probe received signal was clean, the same as when the beacon was attached directly to the 50 ohm load at the Analyzer.

My Local View of My Beacon's Signal
My hope was to see my little 3mW Beacon on KK7CC's Grabber in Las Veges (900miles). But so far, no joy.

I have a previously built Homebrew Power Amplifier that I may try, it should boost the signal to 30mW, maybe enough to be received in Las Vegas.

My conclusion

I think the Propeller, with a Low Pass Filter, can be used for experiments with Amateur Radio Beacons. But as always, more experiments and data is needed to know how useful it will be. A report from a remote QRSS Grabber will help the investigation.


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