PA47 Now Ready for On The Air Tests

Thus far I have NOT connected my PA47 Amplifier to an antenna, because I was not sure of it output harmonic content (see previous posts).

Last night at our monthly P&C group meeting, Wayne - NB6M presented a demonstration of his new DSA815 Rigol Spectrum Analyzer with one of his oscillators and the effects of a output filter devices. His filter completely eliminated (down into the noise floor) any harmonic content.

As a second example and demonstration, I provided the PA47 Amplifier and a LowPass filter. A 50 Ohm load and 40db tap was used to reduce the input to the Analyzer. Actually, both the 1 Watt and the 15 Watt versions of the PA47 were checked, the results were about the same.

The 1 Watt and 15 Watt Version of PA47

The output filter that I used, is a simple Chebyshev 5 Pole Filter (as previously posted).

30m LowPass Filter as Designed with LtSpice

The filter was originally designed via an Online Filter Design program, and then available multiple 330pF 1206 SMD caps were used to obtain the nearest values, 3 caps for each end, and 4 for the center. The cores were wound as necessary by measuring the results on an AADE LC meter.

30m LowPass Filter as Implemented

The Results

Wayne declared the PA47 fit for Amateur Radio Service. The second harmonic was down 51db and the third was down 67db from the fundamental.

PA47 Connected to
DSA815 Rigol Spectrum Analyzer

I will soon connect the 1 Watt version of the PA47 to an Antenna, and use if for QRSS Beacon Service on 10.140050MHz (+-50).

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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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