It is said,  and I'm quite sure it's true,  "that any old bit of wire will radiate"

    but obviously some bits radiate better than others, so if you don't have much space for that ultimate DX antenna,   then let us take a look at designs that will make the best use of the space available,  and since we are talking about the amateur with limited room,   I shall only consider  single spans of wire.

     If we have only space (or permission) for one wire, then we will want it to work on as many bands as possible,   this of course is where the problems start.    The first sacred cow that has to be sacrificed with a multiband antenna  is that it should be resonant,   the very best you can hope for is resonance on two,   or at the most three bands,   and that will involve using traps or only using bands related by an odd number of harmonics,   so if it can't be resonant on all bands,   why bother trying to get it resonant on any?.

"but surely an antenna works better if it's resonant doesn't it ?"

    not always true!   being resonant simply means there will be a nicely convenient low impedance point at which to feed the antenna,   but on other bands we are back where we started.

    So how long  should we make this multiband wire antenna ?,   "I've heard there are lengths that give better  results than others like the G5RV",

    The so called 'optimum' lengths are very little to do with radiating a signal,    but more to do with achieving a manageable feed impedance on one or more bands.

    Three very relevant points have to be made on the subject of radiation patterns and angles.

    First the vertical radiation pattern  (angle of radiation)  of a single wire,   is almost entirely a function of the height of the wire above ground,    and the nature of the ground itself.
 
    secondly the horizontal radiation pattern becomes almost circular when the height of the wire is a small fraction of a wavelength,   which will be the case at the lowest frequencies of operation.

    Thirdly when the length of the wire is one wavelength or more (as will be the case at the higher frequencies of operation) the pattern becomes multi-lobed.

   The truth is that there are no 'magic' lengths,   what we should aim for is a length that is at least a quarter wavelength at the lowest frequency of interest,  and preferably three eighths or more,   so go out and measure how much space you have,   unless you live on a farm,  chances are,  that is the length you should use.

    Well now you have a length,  how high?   As high as practicable!   but don't worry if you can only manage sixty feet of wire at twenty five feet or even less,   the point is, for the average urban location the best wire is the longest you can fit in as high as possible,  there are no magic lengths,  don't stick up a fifty one foot half size G5RV if you can put up sixty feet.

    Now how are we going to feed this wire ?   There are very good reasons for feeding at the centre,   end fed wires,  offset feeds,  windoms etc.  are all beset by problems such as increased local static,  feeder radiation,   RF in the shack and the risk of RFI,  so if at all possible feed in the centre.

    Next we must consider the feeder,   coax is really out for a multiband antenna,    "why ?"    Coax is an ideal feeder for low impedance systems with a low to moderate SWR,    our multiband wire is likely to have some pretty wild impedance values on some,  if not all the bands,   so open wire feeder it has to be,   either home made out of copper wire and spacers,   300 Ohm twin or 450 Ohm commercial ladder line.

    "How about a length of twin feeder,   then the rest of the way with coax like the G5RV?"

    The G5RV antenna was designed to be resonant on 14Mhz and a random length doublet with a manageable SWR  on other bands,  but as Louis Varney has said,  open wire feeder  all the way to the antenna matching unit is still the preferred option,   remember if we are going to make the best use of the space we have available for antennas,  then it is unlikely that the length is going to be the same as someone else's published design.

    Assume we now have our unspecified length antenna,  centre fed with twin feeder just long enough to reach from the antenna feed point to the antenna matching unit.     This is where the problems of multiband antennas have to be solved,   and  if we get it wrong here,  all our efforts to produce the best antenna in the space available will be spoiled by the problems of matching to the transceiver.

    As far as I know,   no one makes a genuine balanced antenna matching unit any more.   So unless you are lucky enough to find a good second hand example,  or prepared to build your own from one of the designs that can be found in older books on antenna matching, there are several options.

         1.  a balun between the feeder and the antenna matching  unit,
         2.  a balun between the antenna matching unit and the transceiver
         3.  just connect the twin feeder to the output of the antenna matching unit.

we will examine each option in turn.

    The use of a balun (balanced to unbalanced transformer) to connect the balanced twin feeder from the antenna to the unbalanced antenna matching unit is the most popular option, there are a number of commercial A.T.U.s that employ this method,  but a word of warning,  there are many types of balun and not all are suitable for this application.

    Firstly,   at the bottom end of the feeder there is likely to be a reactive and totally unknown impedance.  Most baluns are designed for a specific impedance, some types of balun can overheat and even explode if subjected to the excessive voltages or currents which can be found when operating into highly reactive loads.

    However some designs of balun,   particularly current baluns of transmission line construction,   can withstand the stress of this type of operation and be surprisingly efficient,    these can be of :-   1 : 1,   1 : 4, or even 1 : 9 construction, (the impedance ratio is the square of the turns ratio)  the higher impedance side is usually the balanced,  and the 1 : 4 type  the most commonly used.

    Placing the balun between the transceiver and the antenna matching unit is a fairly recent idea,     the balun can be of the 1 : 1 type,  and since the impedance after the antenna matching unit is going to be 50 Ohms when matched,    the demands on the balun are far less severe than when placed between the antenna matching unit and the feeder.

    However the down side  is the ground of the antenna matching unit has to be left floating,   since it is connected to one side of a balanced feeder, if you intend to run high power  this can result in the case of the antenna matching unit being 'hot' with RF, bringing the risk of excessive RF in the shack, RF burns from the controls on the antenna matching unit and other problems,  clearly the effectiveness of this option will depend upon the type of antenna matching unit used,   one where the case can be disconnected from the antenna ground and earthed would make the best choice.

    Operation without a balun is perfectly feasible,  but the antenna matching unit would need to be effectively earthed to avoid the problems caused by RF in the shack,   this is not easy to achieve unless your shack is at ground level.   The unbalance caused to the feeder may or may not give rise to difficulties,  this option is probably best avoided if running significant power,   or if local static interference on receive is likely to be a problem.

    It is almost certain that when the antenna is up and running it will appear to match better on some bands than others.  On the lowest band of operation the wire is likely to be on the short side,    this means that the feed point will have a fairly high capacitive reactance in series with a rather low resistance,    this however will be transformed by the feeder,    and the impedance seen at the bottom of the feeder will depend on the feeder length.

    In the case of the feeder being around 1/8th wave long then the impedance is likely to be quite low and at some lengths the reactive component will disappear,   if the feeder is around 1/4tr wave long the reactance will become inductive,   and at around 3/8ths wave the impedance and reactance will become pretty high.

    It is obviously going to be easier to get a match if you can avoid the high impedance points on the twin feeder,   lengthening or shortening the feeder a few feet can be a useful way of preventing these 'hot spots' occurring at the antenna matching unit,   take my word for it,  "RF burns are no fun",  the problem is that they will be in different places on different bands so what improves one band may cause problems on another.

    If you can model your wire antenna and feeder on a computer,  using one of the now commonly available antenna modeling programs,   then you can avoid these problems at the design stage,   however not everyone has access to a computer or the programs,   and antenna programs do not always model transmission lines accurately,  I had to write my own to get the results I needed.

    But all is not lost,    you can in the true spirit of amateur radio,  do it the empirical way.   If you make the feeder a fair bit longer than strictly required,   load up on each band in turn,    then using a neon indicator lamp with its leads insulated,   and shaped like a letter U a little wider than the width of your twin feeder,   go looking for the 'hot spots' on each band,  and mark them with pvc tape,   using a different colour for each band.

    When you have found them on all the bands you are going to use, select a point away from any 'hot spot',   to be the end where you will cut the feeder and attach to the balun or antenna matching unit,    if you have to compromise (and you will)  then remember the lowest frequency bands have the 'hottest', 'hot spots'.

    So far I have only mentioned straight wires,  so what happens if we add some bent or dangly bits to the ends.  As a general rule,  extra wire hanging down adds reactance to the antenna,  which can help bring down the capacitive reactance in series with the feed point on the lower bands,  but adds little to the radiation resistance of the antenna,  and if folded back can by cancellation actually reduce the radiation resistance. However this is a great area for experimentation, so don’t be put off trying out your own ideas,  but remember check for hot spots after making changes involving adding or removing wire