# How accurate is the antenna reciprocity principle in practice?

The Antenna Theory book by Balanis goes through a theoretical explanation. In my real antennas (I'm an Amateur Radio operator) there are other things like transmission lines and matching circuits. I'd think that as long as this extra stuff stays linear between high transmission power and low reception power, reciprocity still holds.

My main antenna is a 1/4 wavelength vertical (monopole), but the ground plane is far from perfect - it is a combination of the earth and radial wires, and there are a lot of inefficiencies in the ground plane half. Still, it seems reciprocity could hold.

In Amateur radio, there are often contacts between stations with massive tower mounted high-gain antennas and stations with little omni-directional antennas (as mine). But assuming the rest of the stations are the same (including transmit power and local background noise) it seems that, due to reciprocity, both operators should hear (more or less) the same receive signal quality. How accurate is this?

This is accurate in itself, you will get the same attenuation of the signal in both directions, even with different antennas at different stations talking to each other.

What is not the same in regard to receiving the signal, this is the noise. In order to reliably receive a signal, you need some minimum signal to noise ratio. One of the stations may be in a quiet place and be only limited from its own circuit noise floor, the other may have a powerful noise source nearby (e.g. a thunderstorm if we talk 10m-1800m bands) that can completely drown the same signal.

This is where directional antennas help - they may be directed to pick more signal and less noise, for whatever noise type we talk about (QRM or QRN).

• Thanks for this response. And it is a very good point that a directional antenna has the added bonus of reducing local noise. Commented Mar 4 at 5:28

Local background noise is hardly ever the same. Even not-so-local background noise is often different, since each station in a long-distance contact "sees" a different part of the ionosphere. I know you asked to discount that, but it's worth keeping in mind as a real-world source of "one way propagation". Sometimes, if you're lucky, you can use it to your advantage by getting on at a time when the band is open in the direction you want to work, but closed in the direction that provides the most QRM.

And advanced stations frequently have dedicated receiving antennas, which are designed for high directivity even at the cost of very low efficiency (negative absolute gain).

Aside from that, reciprocity works pretty well. There is a widely-reported phenomenon where the ionosphere is actually non-reciprocal, treating eastbound paths differently from westbound paths. The explanation, which is a bit over my head, involves the Earth's magnetic field, and it happens mainly at MF frequencies (160m and the AM broadcast band) because those are similar to the electron gyrofrequency in the ionosphere.

• Thanks for the useful response including emphasizing the impact of local noise. I am certain my home station has more local noise than a good contest station. Once I was able to operate for a short time while there was a local power outage, and it was remarkable how much better I could read weak signals. Commented Apr 10 at 20:50