zondag 19 februari 2012

QRPp How is it possible

When I started as a HAM, 20 years ago, I used my homebrew transceiver HM7, from Solid State Design, page 214...218, with an Inverted Vee, to work many different DXCC countries in Europe in CW. Because my power was only 500 mW, I had to look for stations, that I received with 599 or more, so the other station could hear my signal.
During daylight hours my RST would be 559, or even lower.
Please notice that a power of 500 mW is only four S-points weaker than 100 Watts.
See the PA1B QRPp page.

Soon I discovered, that when darkness fell, the signals from the south of Europe would be very loud. So this made it easy, to make QSO's over a distance of 1000 km or more.
At that time, I did not realize, that my own QRPp signal, could also be an earsplitting 599, until I got genuine report from Italy, that my signal was S9, while using just 500 mW.

During daylight hours, the signals are attenuated by the D-layer, which is absorbing.

When the D-layer disappears during the night, the signal can reach the the E-layer without attenuation in the D-layer. The only attenuation of this sky wave, is (only) 6 dB for a doubling of the distance.
So the signal will be very strong.

I was wondering, what the lowest possible power would be, if I would reduce my power in that situation with this extreme propagation.

If I reduced my power, so my RST would be 559, the difference is 4 S-point. This is a reduction in power of 4 x 6 dB = 24 dB But also 4x4x4x4 = 256.
So I could reduce the the power from 500 mW to about 2 milliwatts.

I was surprised by the low value, that I calculated, but this value is very realistic.
From 2001 on, I made many CW contest QSO's over more than1000 Miles with 2.5 mW and 5 mW,
using homebrew attenuators.

I discovered that the frequency must be absolutely clear, when you use very low power.

1. Bert,