The Short Dipole
Using Coils (Inductance)
There is very popular method of making a short dipole ("physically shorter than normal" half-wave dipole) resonate at a given frequency. This type of antenna is suitable for single band, narrow bandwidth use.
By replacing part of the normal dipole length "electrically" with inductance in the form of loading coils we can make a shorter half-wave dipole become the "electrical" equivalent of a full size half wavelength dipole.
However, the resulting "loaded" short dipole becomes a set of compromises. Fortunately, careful design can make these compromises "acceptable". What compromises or trade-offs?
When using a "loaded" dipole, you are trading some performance for the capability of being able to install a dipole in less space than normal.
The loading coils introduce resistive and reactive components which will cause a portion of the RF power - fed to or received by the "loaded" antenna - to be lost in the form of heat instead of being radiated out as useful signal!
The resulting loss of a portion of the RF energy may not be critical when in transmit mode, but, in some cases, it certainly can be significant on receive!
Some Factors To Take Into Account
Here are some of the factors one must take into consideration when designing and building - or buying - a "loaded" short dipole.
The positioning of the loading coils along the wire is critical. Ideally, greatest efficiency is obtained with coils near the ends of the dipole ... but then coils would have to be impossibly large (infinite size!) in terms of inductance! The loading coils of a short dipole must therefore be positioned away from the ends but still be closer to the ends than to the center feed point.
Close winding of coils should be avoided!
The wire size (gauge) used for the coil is a factor. The smaller the wire gauge, the more resistance is introduced - because resistance dissipates RF energy as heat, not signal! Coils using small diameter copper tubing would be best, but would introduce mechanical problems in the construction of the antenna.
The quality of the wire is another factor. Solid copper is ideal. Copper-clad steel wire is less desirable.
Air-wound coils would be best but are often not mechanically feasible in this type of application. So coils must be wound on a form. The quality of the form material - its insulating properties - is a factor. Coils wound on glass or porcelain heat up much less (let more RF energy be radiated from the antenna) than coils wound on plastic material.
Another consideration is weather proofing. It is relatively easy to weather proof against rain. But think of sticky snow or even freezing rain! Your "loaded" short dipole will resonate off frequency - and its feed point impedance will be different - when used with a coating of ice on the coils. If you feed enough power to the antenna - and with time - coils will heat up and melt the ice. But, be careful. Even an automatic antenna system tuner will have trouble keeping up with a good match as the frequency and impedance drift while the antenna is melting the ice!
Finally, coils are the "hi-Q" components of the loaded dipole. They are designed for a given frequency. This means that when you are using a "loaded" dipole at a frequency other than that for which it was designed for - other than its resonant frequency - the efficiency and effectiveness of the antenna drop dramatically! The shorter the antenna (physically) with respect to the wavelength - the larger the inductance of the coils - the worse things get! Loaded dipoles are very narrow band antennas, best suited for use on a narrow portion of a frequency band.
The ideal (most efficient) coil is:
air wound (not wound or supported on a form),
as large as it is long (length to diameter ratio of one),
made of solid copper wire. Minimum wire size (gauge) must be chosen for the maximum current that is expected to flow through it.
The "ideal" antenna radiates 100% of the RF energy fed into it.
The "ideal" antenna does not exist!
However, there are a few manufacturers who do make a very acceptable "loaded" short dipole. Their design and craftsmanship reduce the trade-offs to the most acceptable levels possible ... while remaining effective and affordable!
The Short Dipole For Multi-Band Use
By the way, some dipoles are designed to be used on more than one band of frequencies. Multi-band dipoles use "traps". Do not confuse coils and "traps". They may look alike but they do not have the same electrical characteristics. Let me just point out that "traps" are even more "lossy" than simple loading coils!
But, again, quality of design and construction make some trap dipoles much less "lossy" than others!
Return to Antenna Selecting Section
From This Article on the Short Dipole