Picture the scene: there's a juicy DX station on that you need for a new country. They're actually a good signal with you, S9 in fact, but the whole world is trying to work them. You've tried to break the pile-up for the last 1/2 hour, but no joy. You start to think, "I give up! I'm only running 100W to a vertical. I'll be really weak with them. I don't stand a chance".

If you find yourself thinking this way, carrying on reading!

The principle of reciprocity

A general communication link is shown below:

Where P_T is the transmitter power, G_A is the antenna gain, N_R is the receiver noise/interference power, and PL is the propagation path loss.

It doesn't matter whether we're talking VHF line of sight here or HF multi-hop via the ionosphere, the path loss is the same in both directions. Additionally, the medium is linear, so there is no change in the path loss with signal power.

Imagine TRX1 is the DX station, and he's running 1kW with a 10dBi antenna, giving an ERP of +70dBm. The path loss is 143dB. At your end (TRX2), your antenna is only 0dBi, so you receive a level of 70-143=-73dBm. This is actually equivalent to 50uV rms in 50 Ohms, which is conventionally S9.

Now let's work it back the other way. You transmit 100W, so your ERP is +50dBm, and your signal is 50-143=-93dBm at his location. But his antenna has the same gain on receive as it did on transmit, so the signal appearing at his receiver input is -83dBm. That's just the differences in transmit power. And hey, that's still S7!

Reciprocity says that the DX station will receive you at exactly the same signal strength as you receive the DX station if the transmit power is the same at each end, regardless of the antennas at each end.

Now here's the rub: If all of the competition are using 1KW to antennas with 10dBi gain, you will be 20dB weaker than they are and will have a hard time making the QSO. Fortunately, the majority of your competition will be using 100W to simple wire antennas, so it's probably worth hanging in there.

I've experienced the situation where a DX station has been very loud with me (S9+10dB). There is no pile-up and he has CQd in my face. According to reciprocity, even with 100W I should have been S9 with him. In this situation the only explanation is a very high noise level (N_R) at his end. This is actually not an uncommon situation working Caribbean stations during the Summer months on the LF bands owing to the very high QRN levels.

It's also worth bearing in mind that a lot of DXpeditions run only 100W. Ignoring the pile-up situation for now, this means that they probably can hear your 100W because their exotic QTH is likely to have at least as low electrical noise.

Whilst I can't find a reference to it, I'm sure I read that G6XN believed that those with beam antennas were more successful DXers in part because they believed their signal was good enough. Those without beams tend to give up earlier. Hopefully this article will help you to persevere!