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Every antenna is at resonance according to length to which it is cut ! They are usually cutto resonate at a particular band of frequencies, ie, Vhf Television band etc.If no resonance then no signal is present and in such a case antenna is merely a randomchunk of metal.
The magnetic field that an antenna puts out will produce an electric current on any conductingsurface that it strikes, however if that surface has a characteristic length the induced current willbe much stronger on the object. For example, when a Citizens Band signal travels through theair, it completes a cycle in approximately 36 feet. If the object that the magnetic wave strikes is18 feet long (1/2 wave length), 9 feet long (1/4 wavelength) or 36 feet long (1 full wavelength),then the induced current will be
much
higher than if the signal struck a metal object that was notsome appreciable fraction of the wavelength of the signal. A resonant antenna is so much more efficient at converting (receiving or transmitting) currentbetween the field and the antenna's feed-point than a non-resonant antenna that much effort isput into configuring resonance. A non-resonant antenna still works as an antenna but simplyrequires a more sensitive receiver or more powerful transmitter.If you have ever heard people say they want to "tune" their antenna, they usually mean theymechanically change lengths in relation to the frequency / wavelength they are trying to match. Itis also possible to change electrical properties to match frequency, which is more handy for matching multiple frequencies with a single antenna.
An
inverted vee antenna
is a type of antenna in which the two side of the dipole are perpendicular to eachother instead of parallel. It is typically used in areas of limited space as it can significantly reduce theground foot print of the antenna without significantly impacting performance. Viewed from the side, it lookslike the English letter "V" turned upside down, hence the name. Inverted vee antennas are commonly usedby amateur radio stations, and aboard sailing vessels requiring better HF performance than available with ashort whip antenna.Inverted vee antennas are horizontally polarizedand they are almost omnidirectional ascompared to a traditional dipole in which they have a deep null off the ends.
Typical amateur radio inverted vee installed on roof. This multiband antenna allows transmissions on the 40/20/15/10meter bands. Center point is held up with masting and ends are secured to roof. Two VHF verticals are also shown.
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Use
Typically, the inverted vee antenna requires only a single, tall support at the center, and the ends can beinsulated and secured to anchors near ground level or near the roof if mounted on a house. This simplifiedarrangement has several advantages, including a shorter ground distance between the ends. For example,a dipole antenna for the 80 meter band requires a ground length of about 140 feet (43 m) from end to end. An inverted vee with a 40-foot (12 m) apex elevation requires only 115 feet (35 m). For radioamateurs living on small parcels of property, such savings can make it possible to use the lower frequencyamateur bands.