Let's talk about reciprosity...

Foundations of Amateur Radio

All antennas have a radiation pattern that charts on a sphere where it radiates more and where it radiates less than the theoretical isotropic radiator. This comparison is expressed as dBi antenna gain.

There is a fundamental concept in antenna design called "reciprocity". Essentially it means that transmit and receive behaviour of an antenna is identical. In other words, the radiation pattern of an antenna applies for both transmitting and receiving of signals.

Unfortunately, this does not mean that if two stations are communicating and one can hear the other, the reverse is also true. Let me explain why.

Let's set the scene. Imagine two stations, me, VK6FLAB at Lake Monger, in Perth, Western Australia and Charles NK8O in the Lake of the Ozarks state park within the Ozark Mountains in central Missouri. We're both on the 10m HF band and in this story I've finally managed to learn Morse code and I'm "talking" to Charles, mind you, Charles apparently does have a microphone, so perhaps this might actually happen one day.

To simplify things, we both have the same antenna, the same radio, the same power level, we both love low power or QRP operation, and while we're keeping it simple, we have the same ground conductivity and we're both experiencing the same very low noise levels. While we're constructing this fantasy, the communication paths for both our stations are identical. Note that I said paths, more on that shortly.

In that situation, both Charles and I have the same experience. We can hear each other at the same level, our S-meter has the same reading, and apart from my current inability to actually use Morse code, our readability is identical. You might think this is "reciprocity", but it's not as simple as that.

I'm parked near a lake in the middle of a city and often other vehicles come and go. One new arrival has a solar panel on the roof with a noisy inverter and suddenly the local noise in my location jumps from S0 to S6.

The vehicle arrives whilst I'm transmitting, so at first nothing happens. Charles continues to hear my signal at the same level and at my end I'm blissfully unaware of any change, until I stop transmitting, when I hear the noise. Meanwhile, Charles starts his transmission and I cannot hear him because the local noise in my location is louder than his wanted signal. At this point, Charles still has the ability to hear me, but I can no longer hear him, even though our equipment is identical. The only change is the local noise floor at my location which interferes with my ability to receive the signals coming from Charles.

This means that I can still send "again, again, local QRM" and I can do so as often as I want. Charles will hear this without any issue, but I won't hear his reply until the local noise stops.

What this highlights is that two-way communication between two stations involves two signal paths. They might, or might not, follow the same journey through the ionosphere and be between two identical antennas, but the experience for either station can be, and almost always is, completely different.

Because the ability to transmit isn't affected by local noise at the transmitter, it doesn't figure into the total path loss when you're calculating it for the receiving station. However, when the roles are reversed, it does. So when you're receiving, you need to take into account your local noise, but when you're transmitting, you don't.

So, when Charles is transmitting to me, I need to take into account my local noise and ignore his, and when I'm transmitting to Charles, he needs to take into account his local noise, but not mine.

This is how you can have so-called "alligator" stations, all mouth, no ears. The station is likely using high power with a high gain antenna in a noisy environment. This means that everyone can hear them, but because their local noise is so high, they can often only hear other alligators, but not the rest of the world who can perfectly hear them. If you encounter a station on-air that keeps calling CQ, regardless of how many people are calling back, that's an "alligator".

So, the takeaway is that even if you can hear a station, it doesn't mean that they can hear you and the reverse is also true. You can be transmitting and heard all over the place, but if you're in a high noise environment, you might not be able to hear them. It's one reason that QRP stations prefer to work in low noise environments where they can hear many more stations.

It reminds me of a funny story told by Wally VK6YS, now SK. In his early amateur radio days he operated from Cockatoo Island, an island off the north coast of Western Australia, near Yampi Sound, which is where his callsign came from. With a new radio and transverter for 6m, Wally had been calling CQ for weeks, but nobody would talk to him. Occasionally he'd hear a faint voice in the background. Meanwhile it transpired that amateurs across Japan were getting upset that he wasn't responding to their 20 and 40 over 9 signal reports. His transmission was getting out just fine, his receiver wasn't working nearly as well. Turns out that during manufacturing, a pin on the back of his transverter hadn't been soldered correctly. Once he fixed that, he worked 150 Japanese stations on the first day and a lifelong love of the 6m band was born.

In other words, just because someone can hear you, doesn't mean that you can hear them, sometimes it's noise and sometimes its a faulty connector.

I'm Onno VK6FLAB