14 Feb 2012

How little power do you need to communicate?

Last night G6ALB and I did a test on 2m to see how little power was needed to communicate between the two of us. Andrew is 3km from me. We both use V2000 triband vertical antennas. We started the test with Andrew using about 0.5mW pep from a simple DSB rig. We then switched to CW (from a decent low leakage signal generator) and G6ALB reduced power in 10dB steps. The lowest level I could still copy his signal was -55dBm which is around 1.8nW. The screenshot above, taken from Spectran, shows his signal at that level.

This sort of power level could be generated by power harvesting, even rectified RF from the HF antenna. Now there's a thought - a free power TX.


David Mills said...

A nice result there Roger.
I did some tests with a local station on 2m late last year. Managed a perfectly good SSB QSO with me transmitting about 500uW. I ran out of in-line attenuators at that point.

I really should fire up my 2m QRSS MEPT again too.

David G7UVW

Roger said...

It is a shame there is not more horizontal SSB/CW activity these days on 2m as it is a wonderful band to carry out this sort of QRPpp experiment.

Anonymous said...

Dont see the point in power harvesting
when a battery would last for years
permenantly on?!
yeah i know its the principle,but..

Dave g4dal

David Mills said...

I need to fix my rotator on the roof when the weather improves, then I'll be active on 2m SSB & data again.

Located in JO01BN, so not a vast distance from Cambs, should be possible to try some QRPp tests in the near future.

David - G7UVW

Michael, AA1TJ said...

Vy FB and tnx for posting your results, Roger.

Along the same lines, I operated today on 40m CW with crystal-controlled Pierce oscillator (a 2N78 made by General Electric in September 1956). The RF output was 6 milliWatts. My receiver used a 1950's vintage 1N34a diode as the heterodyne mixer (product detector) followed by a single stage of audio amplification (a 2N107 made in January 1957).

I only made one QSO (N1PVP @ 293km) but when I checked the RBN log I found that my signal had been logged 93 times today! Some of the capture SNRs were unbelievable (up to 38dB!). My signal was captured by five different receivers (dozens of times by both W3LPL @ 639km and WZ7I @ 451km). A nice QRPp day!

Roger said...

Dave G4DAL said, "Don't see the point in power harvesting....etc"

Well Dave, surely amateur radio is about experimentation, finding out what is or is not possible.

Michael - you continue to inspire us all by your boundary pushing experiments and well done on the 6mW rig and reports.

Bert, PA1B said...

Hello Roger, it's great fun to use low power. Sometimes I can reduce the power to 8 mW or less in CW, when the propagation is extremely good. When the absorbing layer is gone, there is so little attenuation in the path.
73, Bert