What Is A Quarterwave Antenna?
Probably the most commonly thought of Quarterwave antenna is the quarter-wave monopole, a whip type antenna. Dipoles are not real practical for use on a vehicle, so quarter-wave antennas were developed. They only work if they have a suitable ground plane below the antenna. That's because your quarter-wave antenna's element becomes one element of the dipole and the ground plane becomes the other element.
However, technology has moved on and now quarterwave antennas come in a stubby antenna version. Stubby quarterwave antennas are ideal for handheld radios that are worn on the belt or for Mobile phones because the short antenna is non-obtrusive. Modern mobile telephones often use quarterwave type antennas in addition to, or as replacement for, longer halfwave type antennas. Quarterwave antennas have the advantage of a compact and robust size, particularly if the antenna is realized as ahelix antenna. A feature of quarterwave antennas is that they generate electric currents in and/or on the mobile telephone chassis. Since the chassis has conductive properties, for practical reasons the chassis is produced from metal or metallizedplastics, such chassis currents themselves generate electromagnetic fields, wherein the antenna efficiency is reduced because of the interaction with the user, as described above.
As you lengthen your quarter-wave vertical, you'll find a few more critical lengths where the antenna is resonant. Some of those resonant lengths are half-wave, 5/8's wave, and 3/4 wave. You can even stack resonant lengths end to end. (Note that each of those lengths typically require some sort of matching transformer in order to bring the feedpoint impedance in line with the 50 ohms that your antenna is expecting.) Each time you hit one of those resonant lengths, you'll find that your antenna has more gain.
When quarter-wave antennas are constructed over a near perfect radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over a less than optimal radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced onto the feedline.
When a quarter-wave antenna is mistuned or operated without a proper ground plane, the outside of the coax cable's shield becomes "hot" with RF current. So will the DC power lead to the radio, and many other wires in your harness. This does more than produce poor radio performance. It can also result in transmitter energy being coupled into your avionics, fuel injection or ignition computer, or other places where it can cause trouble. It also increases pickup of pulse type ignition noise, the kind you get from your ignition wires. Misrouted RF can, in extreme cases, even scramble the radio's own microprocessor, randomly altering the frequency on which you're transmitting, or cause loud feedback squeals on transmit.
The full-sized dipole will work nearly as well as a quarter-wave vertical, but if you can't find the room for a quarter-wave whip with a ground plane made from wire, do you really have room for a full-sized half-wave? In some cases the answer will be yes, especially if you're trying to 'glas it into a window pillar, or hide it in some other narrow, vertical space. It might be more useful as a Nav antenna, where horizontal orientation is desirable.
However, technology has moved on and now quarterwave antennas come in a stubby antenna version. Stubby quarterwave antennas are ideal for handheld radios that are worn on the belt or for Mobile phones because the short antenna is non-obtrusive. Modern mobile telephones often use quarterwave type antennas in addition to, or as replacement for, longer halfwave type antennas. Quarterwave antennas have the advantage of a compact and robust size, particularly if the antenna is realized as ahelix antenna. A feature of quarterwave antennas is that they generate electric currents in and/or on the mobile telephone chassis. Since the chassis has conductive properties, for practical reasons the chassis is produced from metal or metallizedplastics, such chassis currents themselves generate electromagnetic fields, wherein the antenna efficiency is reduced because of the interaction with the user, as described above.
As you lengthen your quarter-wave vertical, you'll find a few more critical lengths where the antenna is resonant. Some of those resonant lengths are half-wave, 5/8's wave, and 3/4 wave. You can even stack resonant lengths end to end. (Note that each of those lengths typically require some sort of matching transformer in order to bring the feedpoint impedance in line with the 50 ohms that your antenna is expecting.) Each time you hit one of those resonant lengths, you'll find that your antenna has more gain.
When quarter-wave antennas are constructed over a near perfect radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over a less than optimal radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced onto the feedline.
When a quarter-wave antenna is mistuned or operated without a proper ground plane, the outside of the coax cable's shield becomes "hot" with RF current. So will the DC power lead to the radio, and many other wires in your harness. This does more than produce poor radio performance. It can also result in transmitter energy being coupled into your avionics, fuel injection or ignition computer, or other places where it can cause trouble. It also increases pickup of pulse type ignition noise, the kind you get from your ignition wires. Misrouted RF can, in extreme cases, even scramble the radio's own microprocessor, randomly altering the frequency on which you're transmitting, or cause loud feedback squeals on transmit.
The full-sized dipole will work nearly as well as a quarter-wave vertical, but if you can't find the room for a quarter-wave whip with a ground plane made from wire, do you really have room for a full-sized half-wave? In some cases the answer will be yes, especially if you're trying to 'glas it into a window pillar, or hide it in some other narrow, vertical space. It might be more useful as a Nav antenna, where horizontal orientation is desirable.