Mick and Smithy Talk Antennas
Chapter 21

By a club member who (currently) wishes to remain anonymous

If you are new to our saga, click here to start at episode 1

Mick and Smithy meet up again at their local radio club - this will be for the last time as Mick is leaving the district to work elsewhere. Smithy had promised to explain the relationship between properties of an antenna used for transmitting and receiving.

Smithy was drinking tea as Mick entered the clubroom. “Mick” said Smithy between mouthfuls of tea “I’ve always found that one of the fascinating things about radio theory is how the transmitting and receiving parameters of an antenna are inextricably linked. Now, if you remember our discussions on the fields around transmitting and receiving antennas, you will appreciate that these fields are different in the two cases. However, if we consider the behaviour of the antenna as measured at its terminals, there are remarkable similarities between the two modes. For instance, the radiation resistance presented by the antenna in transmit mode also appears as a loss resistance when the antenna is used for receiving. (Remember though, Mick, that this is a fictitious resistance assigned to account for the power lost through radiation, and unfortunately the receiving antenna also loses part of the power it intercepts from a surrounding field by re-radiating.) Also the radiation patterns for transmitting and receiving are identical - even though the fields around the antenna are different in the two cases.”

Smithy paused to finish his tea before continuing, “the best way to appreciate the duality of transmit and receive functions of an antenna is to consider what is called’reciprocity’. Imagine a simple radio system set up as a one-way path between a transmitter and a receiver and that you measured the performance of this system by monitoring the signal strength at the receiver. If you then interchanged the antennas between the transmitter and receiver (but made sure that the same electrical power was supplied to the newly assigned transmit antenna) you would find that the same performance was achieved at the receiver. This is called the reciprocity theorem and exists because of the similarities I told you about. There are some conditions under which this theorem breaks down and you can sometimes come across this in amateur radio operation. If two stations are in contact, both using their antennas for transmit and receive, then signal reports both ways ought to be similar, allowing for discrepancies in transmitter power and receiver sensitivity. This is usually most evident at VHF and above where ionosphere conditions are not involved - and of course, local noise can be different at the two stations.”

Smithy continued “even when different parameters are used to describe either a transmitting antenna or a receiving one, we find that the parameters are in fact just a different way of describing the same basic property of the antenna. For instance, power gain is used to evaluate a transmitting antenna, whilst the aperture or capture area is the important factor for receiving antennas. However the aperture in this case is proportional to the receiving gain.

Although most amateur radio stations use the same antenna for transmit and receive, there may be cases where it is an advantage to use different antennas. One such case is where a station working in the low frequency HF bands has a high local noise level. In such a case it may be found that a loop antenna with active amplification is an improvement on the main antenna in terms of receiving performance.

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Sometimes this duality of transmitting and receiving can be useful in assessing some particular antenna. Some years ago a new concept in antennas was proposed called “the crossed field antenna”. The idea was based on the fact that a radio wave can be described as composed of travelling electric and magnetic fields at right angles. The idea was to produce such a wave by generating these fields separately. Now aside from the fact that both the fields produced would have been mainly stationary induction fields, and both fields would have had both electric and magnetic components, a simple consideration of whether such an antenna would work in the receiving mode immediately shows the concept to be seriously flawed.”

And so the technical discussions between Mick and Smithy come to an end. But before Mick disappears, Smithy is anxious to remind Mick of some of the more important items they’ve discussed:

• SWR - Standing Wave Ratio. This whole subject is difficult to understand. First off, when we talk about standing waves - there aren’t actually any! The “so-called standing wave” is not a physical reality. It is a mathematical summation of forward and reflected currents that are flowing in either a transmission line or an antenna wire. Don’t be mislead by text book diagrams of the current distribution on half-wave dipoles, or any other antenna for that matter, for all that is happening is that currents are flowing in the antenna and being reflected, then being re-reflected ad infinitum! Such diagrams just show what the current distribution “appears” to be. Even worse, radio engineers have characterised antennas as being either standing wave antennas (usually the sort we use) or travelling wave antennas. But of course, whatever the antenna, the current is travelling but it’s just that in the so-called standing wave antenna, these currents are also being reflected.
  
  
• A really important thing to realise is that the SWR on a feeder is NOT altered by anything you do at the transmitter end of the feeder. At this end your SWR meter will be reading the power being transferred from the transmitter to the feeder and antenna via the Antenna Tuning Unit (ATU). The ATU is set to return reflected power back to the antenna (where part of it will be radiated) and also to match the transmitter output to the input impedance seen at the transmitter end of the feeder. Eventually virtually all reflected power gets radiated (which is why graphs of feeder SWR against radiated power show it to be a less than serious problem - it’s the protection of the transmitter PA against dissipating excessive power that’s important.)
  
  
• A very misleading statement, often seen in adverts, is that “such and such an antenna only has an SWR of say 2:1 across the whole band or over several bands.” What they should say of course is that this is the SWR on a matched feeder line to the antenna; for if our ”standing wave antenna” is going to work properly, currents need to be rushing along and back in the wire and in this case the higher the SWR ACTUALLY ON THE ANTENNA - the better!
  
  
• To understand antennas in their proper context you must get away from regarding them as something that only needs to be characterised by their input impedance. Such thinking can sometimes produce inefficient antennas - antennas that waste power but merely satisfy the terminal impedance requirements. Perhaps the best example of this is the “end-fed” where the missing half of the dipole has to be provided by a ground or ground-plane. Attention to such ground connections is equally as important as what you do on the antenna itself.

What can we say - had Mick understood everything - had he become less “gadget-obsessed”? And what of Smithy - well we can say that he did get something out of his conversations with Mick, for he had at last realised that Madonna and Maradona were two entirely different people.

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And so ends our saga. 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.
Episode 15.
Episode 16.
Episode 17.
Episode 18.
Episode 19.
Episode 20.