The starting point for any dipole is usually a simple calculation stated as '142/divided by the working frequency in megahertz'.   This gives a close approximation of the length (in metres) of a dipole for a particular frequency in Megahertz.

This is an commonly used  approximation and  gives the length of an electrical half-wave at the frequency concerned.

 I won't argue with 142/F, but with most aerials, it's usual to cut them a few percent (5% typically) oversize so they can be trimmed to resonance. You don't actually need to snip the surplus wire off... just fold it tightly back on itself and use ty-wraps to hold the extra wire in place (use black ones because they are UV resistant while white ones are not). This allows you to be able to make future adjustments. 

In general, it's best to place a dipole as high as is possible and away from structures that can detune it.  Most amateur radio dipoles are too low to be very effective for DX working, although of course they will still work for relatively local contacts.

The example above is fed via coax cable and a 1:1 BALUN.

The dipole, and it's derivatives, are balanced aerials, and should be fed from a balanced feed-line, or BALUN  transformer,  and not directly by 50ohm co-ax. (more later)     The dipole is generally considered to have an impedance of around 75 ohms although you should be aware that this figure varies according to the height of the aerial and other factors.  A BALUN is a balanced to unbalanced transformer ....  more of which later.

The inverted Vee shown above is fed from balanced feeder and needs a balanced Aerial Matching Unit ... or AMU (more usually incorrectly titled ATU!), at the shack end.  It could alternatively be fed via co-ax and a balun as with the straight dipole.   In the US, aerial matching units tend to be called 'Antenna Tuners'... but more on that later.

The 'inverted vee' is a dipole whose elements are bent downwards and supported by insulators relatively near ground level so that the complete assembly looks like an upside-down letter 'V'. By relatively, I mean high enough that members of the public cannot touch the ends.  DO BE AWARE though that when transmitting at even fairly modest power levels, there can be many kilovolts present on the ends of a dipole.

Bending the the ends of a dipole downwards in this way reduces the impedance. It also affects the radiation pattern somewhat.

The inverted vee is very useful for portable operation using just a cheap fibreglass fishing-pole (don't use a carbon fibre one!) to support the dipole centre with thin wires ... and long guy lines that double as insulators.  Use a balanced feeder ... as above.

For a permanent Inverted-Vee setup, use real insulators at both ends.

An interesting fact about the Inverted Vee is that it can be made to  be somewhat directive by not having the elements stretched straight out but instead being formed into a V shape.  (You might need a guy line on the opposite side to balance it).

  Another way to make a multiband aerial is to use the wire for the lowest frequency band to support other radiating elements suspended below it. You are simply using a strong dipole to hold up some other dipoles suspended beneath it. The dipole wires all share a common feed-point. This is not something that's seen much in commercially made aerials but it's easy to make and effective.  The wires should be kept apart from each other by several inches, so a number of lightweight 'spreaders' can be used for this job. The spreaders can be any sort of insulating material that will stand up to the weather and not absorb water of disintegrate under the effects of strong sunlight (UV).  PVC water waste is one possible material for this purpose.  It will last many years outdoors.  If you use bare wire for your dipole elements, the spreaders can be kept in place by drilling holes to take the wires then placing a short loop of wire across the element at the spreader and quickly soldering the ends in place. Use a balanced feeder ... as above.  Alternatively, a BALUN may be substituted at the feed-point should you wish to feed it with co-ax.

The multiband aerial above can also be configured as an inverted Vee.

The 'sloper' is a dipole variant that is suspended from a high support at one end and taken to near ground level (not closer than a few metres!) at the other.  Of course, using it in this way, changes it's behaviour somewhat, but slopers do work.  With one end near ground level and the other in the air, the sloper is not a balanced aerial.  It can be fed via co-ax.

 Take care to keep the lower end away from members of the public!

This consists of two or more inverted vee's supported by a common centre support pole and having the dipole elements disposed around it. A simple relay box at the feed-point allows the user to select which inverted-vee is in use.  This allows the direction of fire to be altered remotely, simply by switching a relay. Both LA2QAA and myself have used this method on the old Radio-Sport satellite frequencies around 29.5Mhz. Switched slopers can also be used in this way. Take care to keep the lower ends away from members of the public!

Coaxial cable is very convenient but it is an unbalanced feed-line. Ladder-line on the other hand (and also open-wire feeder) is a balanced line.   Your dipole, or dipole variant, is a balanced aerial and what's more, it's actual impedance is unlikely to exactly match that of the feed-line, whether it be co-axial cable or ladder-line.  Everyone is familiar with co-axial cable. Your TV uses it, but TV co-ax is 75 ohm coaxial line.  The co-axial cable used by almost all amateur radio gear is of the 50 Ohm variety (actually 52R but usually called 50!).  Ladder-line commonly comes in 300 and 450R varieties but other types exist.   So, how to get the various types of lines to work 1) with your radio, and 2) to work with your dipole!

The common answer is quite simple really, use a BALUN transformer! You match the line to your radio, and at the other end, you match the line to the aerial!  That's how it should be done.  In reality, many, if not most,  people opt for the simple solution..... just use 50R coax from the radio to the aerial and connect the far end directly to the dipole halves.

How not to do it!

For a variety of reasons, this is not a good solution.  A somewhat better way is to take the co-ax to the aerial's feedpoint and use a BALUN transformer at the fedpoint to match the unbalanced co-ax to the balanced dipole.

A better solution still, is to use ladder-line or open-wire feeder rather than coax and use a balanced aerial matcher at the  shack end. The balanced tuner matches the 50R unbalanced co-ax to the balanced feeder AND provides the means to add inductance or capacitance to produce an acceptable match to the transceiver.

A word on "Balanced Matchers" or "balanced ATU's"

Genuine balanced matchers use air cored inductors and high quality variable capacitors. A good example of this type is the long-defunct KW EZee-Match.  Many modern matchers are simply unbalanced types with a ferrite balun stuck on the output. These are not true balanced matchers. Compared to types using air-cored transformers, they can be very lossy indeed. 

Almost invariably (and there *are* exceptions), dipoles are made of thin wire. A wire dipole has a high Q-factor making it an essentially narrowband aerial. As you go lower in frequency, the problem becomes even  worse.  So, a common problem is how to make a dipole work over a range of frequencies?  Well, that's the job of the aerial matching unit or AMU.  As you go away from resonance, the impedance that the feeder 'sees', goes from being purely resistive to having some degree of capacitive (aerial too long) or inductive reactance (aerial too short) ... for the frequency in use. 

The AMU (more usually incorrectly titled ATU!) has the job of restoring the match to being resistive. It does this by adding a controlled amount of capacitance or inductance to the system so that the MATCH  as seen by the radio is resistive.  That's all it does.

So, to recap, the 'ATU' doesn't 'Tune' anything, despite it's common name. It's sole purpose in life is to fool the radio into thinking it is seeing a 52R resistor! It does this by adding controlled amounts of capacitance or inductance. It's important to realise that although the aerial WILL work 'off frequency' , it is NOT RESONANT, so the overall system efficiency does suffer.  In short, like in many other areas of life, it's a compromise.

The dipole is a balanced aerial, so it should ideally be fed from a balanced feeder and not co-axial cable. Balanced feeder has lower losses than co-ax and can be connected directly to a dipole via a delta match.  This in turn means that the dipole elements don't need to to be cut for a centre insulator.

Alternatively, you can use a conventional dipole fed via a BALUN.

Wot's a BALUN?

A BAL-UN is simply a balanced to unbalanced transformer. When used as an Unbalanced to Balanced transformer... it's called an UNBAL. Simple.  They have a million and one uses in the real world in uses from computer networks, video systems and telecommunications and are a basic building block for many of those systems.

In amateur radio, a BALUN  or is commonly used to match a balanced aerial (dipole) to an unbalanced feeder(co-ax).  Because the dipole aerial (and it's derivatives) are inherently balanced radiators, you shouldn't feed them directly from Co-axial cable which of course is not a balanced feeder.    Since a dipole has a nominal impedance of 75R, a 1:1 ratio balun can be used to connect the aerial to the 52R coaxial cable with only a small mismatch.

If you use open-wire line or ladder line to feed your dipole, you don't even need to use a centre insulator. Instead, you can use a Delta-match (see below).

*In reality, the impedance of a dipole depends on a number of factors... height above ground, proximity to nearby objects etc.*

For those who choose to do the sensible thing and feed their dipole from balanced line, the Delta match is an excellent way to go.  A big advantage of the Delta match s that it allows you to use an unbroken length of wire for your dipole (no centre insulator).  The part of the line that feeds the centre of the aerial is simply stretched out and attached to the antenna wire at two points equidistant from the centre. It looks like a Letter 'Y'.  You have to find the best tapping point by experimentation. 

The clever guys use a balanced impedance matcher unit and ladder line (or open wire line) to feed their dipoles via a simple delta match.   Why? In a single word... 'efficiency'.  BALUNs using magnetic cores are fairly inefficient so some of your valuable RF ends up warming up the core.  If instead, you use an unbroken dipole element fed with ladder line and matched to the aerial with a delta match, the losses will be far lower. Not only that, but if you use a proper balanced aerial matcher (not an unbalanced one with a built-in BALUN!), you should be able to maintain a high level of efficiency *throughout* the system. This rather elegantly explains why absolutely ancient KW balanced AMU's such as the 'EE-Zee match' fetch silly prices on EBay!

Finding suitable insulators for aerials is not always easy. The ceramic 'egg' isulators are good, but not always obtainable so a cheap substitute is PVC conduit material.  Just drill a hole ... about an inch in from each end.. right through both sides of an 8 or 10 inch piece of conduit and thread your wires through each end of it, bending the ends  back and typing them around themselves..  It's strong and fairly unobtrusive.

A dipole centre can be made from a scrap IKEA chopping board. This is a strong nylon material that's easily cut with hand tools.  Plexiglas (Perspex) is another material suitable for making insulators.

Strips cut from an old IKEA chopping board and drilled at 4 inch intervals also make excellent 'spacers' for multi-band dipoles.

For those with small gardens, too small to take a dipole of their choice, you should be aware that the ends of a dipole can be bent . You can let ten feet or so at each end hang downwards if that lets it fit the space available.  You can even zig-zag the ends. DO BE AWARE though that when transmitting at even fairly modest power levels, there can be many kilovolts present on the ends of a dipole. It's vital therefore to ensure that no people or animals can come close to the ends of the dipole. If you do 'droop' the ends of a dipole, it's a very good idea to place a weight on the wire end.... hang it from a black ty-wrap made into a loop, so it can't blow around in the wind.

Do also remember that if space is available and perhaps a couple of friendly neighbours,  a dipole can use a chimney as it's centre support with the two halves tied off to a couple of convenient buildings or trees on each side  A dipole can be bent out of straight and still work OK  and both ends don't have to be at the same height. It does have an effect on the pattern, but surely a bent dipole beats none at all.  Be inventive!

 Given that the purpose of an AMU (ATU) is to provide an acceptable match to the radio, if your system used co-ax, it should be blindingly obvious that the 'best' place to locate it is at or close to the aerial feed-point where it can do some good!    A look around the vast majority of amateur radio shacks will suggest otherwise.  So, who's wrong? If the AMU/ATU is in the shack, and the match at the aerial is less than perfect, what happens on the feedline? 

Short Answer.... losses!

Commercial radio operators almost always place their matching device at or close to the aerial feed-point. 

Sometimes the narrowband nature of the simple wire dipole can be a problem and one solution to providing greater bandwidth (by making the aerial have a lower Q) is to use 'fat' elements. By fat, I mean they have to be a sizeable percentage of the wavelength in question.  The easiest and most common way to do this is to build a caged dipole. The two halves of the radiator are built as a pair of 'cones' or 'tubes' of wires by means of separator hoops. In this way, a 'fat' structure can be created that will cover a fairly large bandwidth. They are a bit 'ugly' for amateur use in a typical suburban environment but government embassies with plenty of roof space seem to greatly favour this type of aerial system :-)