[trh01uk]

Yes, its possible to do a fairly back-of-an-envelope calculation to get a rough idea of the path loss over this path, to see whether ordinary propagation is possible, given the antennas and power you were running. I have used the ITU-R paper "Propagation by diffraction" because we think there are some significant mountains on the path in the Lake District.

I will take the path from your transmitter to his location on Slieve Croob, since you say you had to the lower transmit power. So here the link parameters:

Frequency: 145.425MHz

Wavelength: 2.06m

Transmit power: +33dBm (20W)

Transmit antenna gain: 5db (I note you have one 5' long)

Transmit antenna height: 301m asl


Receive antenna gain: 5dB (guesstimate)

Receive antenna height: 530m asl (including mast at 50' agl)

Feeder loss: 2dB

Receiver sensitivity: 0.12uV across 50 ohms = -125dBm

Transmit location: 54.473573,-0.865698 (lat/long)

Receiver location: 54.340088,-5.973671

Path length: 332km (great circle)



The first step is to calculate the free space path loss over this path length. Using an online calculator I get 118dB. This is the total loss from the output of your transmitter to the input of his receiver, including antenna gains, and cable losses if both stations were in outer space, transmitting with nothing in the way at all.

The maximum path loss you can stand while still communicating = 33 + 5 + 5 -2 - - 125 = 166dB, so at the moment we have plenty of signal in hand, on this theoretical free space path. And this is our worst case link budget.

Now we add in the extra loss due to the curvature of the earth, and the fact this path is beyond the radio horizon. This makes adjustments for the distance of the link, and the heights of the two antennas above the earth plane.

Using Fig.3 in the ITU-R paper I get another 145dB of loss due to the distance. We then need to offset this by looking up the height gains of the two antennas

Transmit antenna height gain = 19dB

Receive antenna height gain = 34dB

So our total excess loss for over the horizon operation = 145 – 19 -34 = 92dB

The path loss due to free space loss, plus the earth curvature loss = 118 + 92 = 210dB.

This loss is far higher than our link budget of 166dB, so we can say the path is not viable for ordinary across the earth propagation. And in fact the total losses are even higher than this, because we have not yet added the excess loss due to the mountains en route.

I conclude you must have had help from some tropo conditions, which effectively allowed the signal to bend way over the horizon, and skip the mountains too.


Richard