Mick and Smithy Talk Antennas
Chapter 15
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
Back from the “summer” break, Mick and Smithy continue their discussions about antennas - but as always it’s a bit one sided!
Smithy was already sipping his tea when Mick arrived at the club. “It seems ages ago when I promised to tell you more about the zones around a transmitting antenna” began Smithy. Mick looked a bit blank, it had indeed been a long time since they had last met up at the club and Mick couldn’t quite remember what it was that they had last talked about. Sensing this, Smithy briefly reiterated as follows:
“In the immediate vicinity of the transmitting antenna, electrostatic and magnetic induction fields predominate, but as we move away from the antenna true radiation becomes more significant. We say that a practical antenna has three distinct zones around it - they are the space defined by concentric boundary spheres surrounding the antenna.
The near zone - in which the induction fields predominate - taken to be within a sphere of radius L/2pi centred on the antenna.
The intermediate zone - between the near and far zones, where all the fields are strong relative to each other
The far zone - in which only the radiation field is sufficiently strong to be of importance. We may assume this zone to be outside a sphere of radius 3L around the antenna, L being the wavelength of the RF.”
“You should note though” continued Smithy “that the intermediate zone has sometimes to take account of the maximum linear dimension of the antenna.”
“Ah yes” Mick said “I remember now Smithy, I was going to ask you about something. Some time ago there were adverts in the magazine for a nearfield receiver. Is this something to do with what you’re talking about now?” Smithy pondered this for a while then offered the following” well, I can’t imagine quite what use this would be to anyone operating a normal amateur radio station. For a start, it would be quite difficult to produce a receiver that ONLY responded to near field, especially over a wide range of frequencies, as you will see when I show you some typical values in a minute. But my guess, and it is only that, that the receiver in this case has only a very limited sensitivity such that it would be entirely useless for monitoring communication signals.”
“Mick, you can see what I mean if you look at this graph” said Smithy, pulling a large A4 folder out of his sports bag. Smithy had plotted the extent of the three zones over the frequency range of 1 - 1000MHz, but with the two scales arranged logarithmically, so that the plots were in fact straight lines. But Mick spotted a table under the graph that he was able to understand more easily.
| FREQUENCY | APPROX OUTER LIMIT OF NEAR ZONE | APPROX INNER LIMIT OF FAR ZONE |
| 2MHz | 24 metres | 400 metres |
| 10MHz | 4.5 metres | 90 metres |
| 100MHz | 0.45 metres | 9 metres |
| 500 MHz | 0.1 metres | 2 metres |
“Ah, I see what you mean, so near field only exists in those inner zones and anyway they vary across the frequency range” said Mick. “Exactly” replied Smithy “Now there’s some interesting things that happen in the two inner zones. First of all, as you might imagine from what I’ve already said, that it is useless to make field strength measurements in the inner zones, they will not measure the radiation field. So you can ignore photos that show a fluorescent tube lit by the “RF” when it is held close to one of those magical small antennas - all it is showing you is that there is an intense induction field, it is no measure of the radiating capabilities of the antenna. “Phew!” said Mick.
“There are a couple of other interesting points about the zones” continued Smithy “ for instance, if you wanted to erect two antennas on one site, then they should be separated by at least the two outer zones of both antennas, otherwise there is going to be considerable interaction between the two antennas, whatever their operating condition is.”
“Another interesting property of antennas” continued Smithy” is that whatever the directivity of the antenna is, it is not fully developed until the far zone is reached. This applies to both transmitting and receiving antennas. A long time ago at club, I showed this by setting up a loop Tx in the high HF band, then approaching it through the zones with an Rx having a loop antenna. In the inner zones the loops have to be in parallel for maximum signal, because the field is inductive, as for instance like the coils in a transformer.”
“Just a minute” said Mick” when we do our mini DF hunts out here, you can walk right up to the Tx without any change to the bearing shown by the Rx alignment. That doesn’t agree with what you’ve just said.”
“Quite” smirked Smithy” but that DF Tx has a rather short antenna, so short in fact, that on the basis of its length in wavelength terms, it can be regarded as a point source. But if you were to lengthen the antenna, so that it was an appreciable fraction of a wavelength, you would notice a change in the indicated bearing if you approached it through the far zone and then into the near zone. I know you would Mick, because I’ve done it, and I suspect lots of other people playing around testing DF receivers have probably done likewise and been mystified by an apparently incorrect bearing taken when they were within the near zone of a top band wire antenna, after all in this case the zone extends out to 24 metres.”
With that, the pair move back to the tea area, and Smithy reveals that for the moment the discussion on transmitting antennas should end. However, he indicates that if Mick is willing, he will undertake an explanation of receiving antennas, in terms of RF field behaviour. “And as you will see Mick, those tricky little zones actually exist around receiving antennas as well.”
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
Episode 5.
Episode 6.
Episode 7.
Episode 8.
Episode 9.
Episode 10.
Episode 11.
Episode 12.
Episode 13.
Episode 14.
next episode (Chapter 16).