I know the answer is going to very over a wide range, so let's take a "hopefully" fairly narrow example.

You have an old AM radio that battery powered and has a telescoping antenna that is roughly three feet (or whatever is the most common length).

What kind of signal power is present at the interface between the antenna and the rest of the circuitry? Or, what kind of a range of signals would such a radio likely be designed to work with?

What kind of impedance are we commonly looking at there, at the interface?

On a related note, if we were to plot, as a function of frequency/wavelength, the minimum signal power out of the antenna that was useable for narrowband reception, what would the rough shape of that plot look like?

 

Tens of microvolts is a typical signal level on an AM radio. this signal may hove originated on a 50 kW Clear chanel AM station a thousand miles away or more. Signal power is often measured in dBm which is decibels with respect to 1 milliwatt. It is not uncommon for a modern receiver to handle signals down to -150 dBm

 

Thanks a lot!

What about at the other end? What if that same radio is tuned to a Clear Channel station whose tower is just a few miles away? If we applied an r^2 rule and went from 1 miles to 1000 miles, we'd have a million times the power or a thousand times the voltage. Does is seem plausible that being near a strong station could produce tens of millivolts out of the antenna?

 

Thanks a lot!

What about at the other end? What if that same radio is tuned to a Clear Channel station whose tower is just a few miles away? If we applied an r^2 rule and went from 1 miles to 1000 miles, we'd have a million times the power or a thousand times the voltage. Does is seem plausible that being near a strong station could produce tens of millivolts out of the antenna?

A receiver in the vicinity of a very strong signal becomes "deaf" across a wide spectrum even if it is not tuned anywhere near that strong signal. The strong signal has the effect of raising the receivers noise floor so high that weak signals can no longer be detected. This is referred to as "desensing". Most receivers also have protection circuits to attenuate very strong nearby signals.

If you start with a signal at -125 dBm, and add 60 dB, then you are still at only -65 dBm which is still a small fraction of milliwatt. A signal at -65 dBm is more than sufficient to copy a message accurately.

We're talking about almost imperceptible amounts of RF energy that modern receivers can pickup and decode.

 

I'm aware of the desensing phenomenon, but I have no feel for where it starts to come into play. I regularly drive by the KOA tower, a station that is regularly heard in Europe, Australia, and Asia, and I'd say I get within five to ten miles of it. I've never had it overpower nearby stations. I keep intending to drive as close to the tower as I can get, but I'm not sure that would tell me much since I don't know if, at ground level, I would be an a relative null of the propagation pattern. When flying, I've orbited the tower at a distance of a thousand feet or so which would have been a great time to do a little experiment. Unfortunately, at the time (1) I had no AM radio with me and, (2) I was preoccupied because the reason I was holding at the tower was because there were severe thunderstorms on the north half of the runway I was trying to land on.*

What amazes me is not only the tiny amount of power in the signal, but the fact that a crystal radio can make an intelligible signal that a human can hear using only the power in the signal (and I suspect the conversion efficiency isn't all that high, either).

* That was the actual broadcast from the control tower -- "severe thunderstom north half of runway" -- as I was on short final landing from the south. One afternoon the weather went from hit and bumpy to widespread thunderstorm cells within minutes and I had already been diverted from my planned flight path trying to get back to my home airport(and wasn't alone as they were diverting airliners from Stapleton to Colorado Springs and Wyoming). I was just trying to get on the ground anywhere safely and was trying to get into Centennial Airport during a lull in the weather activity. It looked like I was going to make it when the tower made that call and then advised me that the crosswind component at the approach end had just jumped from 10kts to 50kts almost instantly. That made my decision real easy and I elected to abort the landing and go orbit the tower for about ten minutes. All in all, it was one of just three times in my life when I found myself up there wishing I were down here instead of the other way around!

 

I appreciate the hair raiser from a fellow pilot. I have a few of my own -- glad you're still with us.

Former owner of Citabria N53952