(In defense of GB3SD’s ‘Full Wave Dipole! www.g3sds.org.uk)
Yes, of course WATTS is a name, it’s my name as it happens but if in conversation I say to to a Radio Amateur ‘running 100 watts here old man’ what am I saying to them? I am not talking about a large number of the Watts family running about surely and just a minute what’s this ‘old man’ business? Rather rude don’t you think to be calling somebody who I might never have met before an ‘old man’ even if he is!
Well, if you dear reader are a Radio Amateur you will know that the 100 watts I am refering to is in fact (these days) the power output of my transmitter and of course ‘old man’ is an often used term of endearment, a friendly greeting to a fellow ‘Ham’ so no problem there at all! Hey, hang on though, what’s this ‘fellow ‘Ham’ ‘ business? Oh, dear this is getting to be complicated! But you all know that this is just the way we speak. Most of the time we understand each other and that’s all that matters is’nt it? Well, actually no it’s not! To judge from some of the conversations I hear on our local repeater it causes consternation!
Lets consider just one the examples I have already made.You might have noticed that when I explained about ‘100 watts’ I said that it meant transmitter output ‘these days’. That’s because in the last decade or so the way we measure our transmitter power has changed. Before reasonably accurate power meters were readily available the power we measured was ‘input’ not ‘output’. In fact, the licence allowed us to use ‘100 watts of power input’ to the transmitter output stage. So when we said we were running 100 watts we were not talking about power output at all.
The important thing about all this is that although in general conversation we might think we understand each other the potential for confusion is enormous!
WHAT IS A DIPOLE ?
Here we go again! ‘I am running a dipole here old man’ would be interpreted as ‘the aerial I am using is a half wave dipole’, that’s the convention. However, you can’t work backwards from this convention. Just because most of the time the dipole we are talking about is a half wavelength long that does not mean the a DIPOLE is a ‘half wave dipole aerial’! The Concise Oxford Dictionary says:-
1 Physics – two equal and oppositely charged or magnetized poles separated by a distance.
2 Chem. a molecule in which a concentration of positive charges is separated from a concentration of negative charges.
3 an aerial consisting of a horizontal metal rod with a connecting wire at its centre.
The only one that fits our meaning is obviously 3) but note that nothing is said about wavelength!
I was surprised to find that the second edition of the Amateur Radio Handbook published by the RSGB in 1943 makes no metion of the term ‘dipole’ or for that matter ‘doublet’ in it’s very extensive chapter on ‘aerials’.
Probably the best explanation of the derivation of the tern ‘dipole’ is to be found in Foundations of Wireless published in 1942. Here, the characteristics of a tuned circuit consisting of a simple strait wire are described in detail and a resonate radiator of this kind is termed a dipole. It is also observed that the resulting waves are almost exactly double the length of the wire and here the arrangement is called a ‘half wave dipole’.
WHAT ABOUT THE DOUBLET
OK, it’s tempting to say the word ‘doublet’ is just another name for a dipole aerial and in truth that is what it is! However, once again by current convention (you know the same one that wrongly says a ‘dipole’ is a half wave dipole aerial’) the word is generally used to describe any centre fed dipole aerial that is not naturally resonant on the band in use. In other words any old length of wire cut in the middle and fed with some kind of twin feeder. By the way don’t bother to look up ‘doublet’ in the dictionary, nothing fits this time, or does it? Well just for the sake of completeness then:-
1 either of a pair of similar things, esp. either of two words of the same derivation but different sense (e.g. fashion and faction, cloak and clock).
2 hist. a man’s short close-fitting jacket, with or without sleeves.
Well, perhaps the first one almsot fits, anyway that’s enough of that!
THE FULL WAVE DIPOLE
OK that’s what this is all about, “If it’s not a half wave then it can’t be a dipole” say the pundits. The aerial that GB3SD uses on receive is described as ‘a full wave dipole’. Nothing wrong with that surely? It’s a dipole, one wavelength long on the operating frequency OK ? Well yes it’s also a half wave dipole on another frequency but so what, you know what I mean don’t you? !!
Finally, to quote chapter and verse in my defense; page 396 of the Third Edition of the RSGB Amateur Radio Handbook published in 1961 describes the type of aerial used by GB3SD, a ‘Full Wave Dipole’ with closed stub matching and on page 401 (Table 4) is shown the ‘Resonant Lengths of Full-wave Dipoles’. The arrangement of two stacked ‘collinear’ half wave dipoles is also described as ‘Full wave dipoles’. Even the well known ‘G5RV’ is described as ‘The G5RV 102ft Dipole’
Confused?, I rest my case! 73 de Geoff, G0EVW – August 2001