Introduction to Omnidirectional Antennas
Omnidirectional antenna classification is not based on any specific antenna design structure but it is classified on the basis of the radiation pattern it creates. Based on the above introduction, one might expect the magnitude of gain in the desired directions would be perfectly uniform but that may not be truly possible with all types of omnidirectional antennas. There might be some deviation and hence depending on the application, a very nearly omnidirectional radiation pattern may also be acceptable. This brings us to a term Gain Ripple. This is a term that sets the limit of acceptability of gain variation in different directions where one would theoretically expect a perfect uniformity. Usually, a 2-3 dB gain ripple is often acceptable, however, this limit maybe a little less or a little more depending on the communication application. Let us examine and introduce ourselves to several types of typical omnidirectional antennas which are often used in various radio communication applications.
Marconi 1/4λ (Monopole) antenna and its Derivatives
The Marconi monopole antenna is also popularly known as the Vertical Grounplane antenna. The radiating element is typically 1/4λ in length and is vertically oriented. It uses the ground as a counterpoise and a return path for the electric field produced on the radiating element when driven by a transmitter. An example of a practical antenna of this kind is what we find being used by medium frequency (MF) radio broadcast stations. Being medium-range, local broadcast stations, they need to have an omnidirectional service coverage along the azimuth plane. Moreover, the additional advantage of a vertical monopole antenna is that since it produces a vertically polarized radiation pattern, it offers a far better surface-wave (ground-wave) coverage compared to any horizontally polarized antenna.
It is important for his type of antenna to have good soil properties (conductivity and dielectric constant) of the ground on which it is erected. This may not always be possible. Hence, most monopole ground plane antennas use artificial ground enhancements to improve efficiency and performance. Typically, almost all installations use a set of wire redials or a metallic mesh structure laid out around the antenna. These radials may be laid on the ground surface or maybe buried a little below the soil. This acts as an artificial ground plane and normally has a radius of 1/4λ around the base of the monopole.
Amateur radio operators often use the vertical ground plane antenna very effectively on HF bands as well as VHF/UHF for omnidirectional terrestrial radio communication. This antenna is simple, easy to build, and does not need an antenna rotator, it is quite popular among amateur radio operators. This antenna can efficiently use an artificial ground plane comprising of radial conductors on these bands without an absolute need for the earth’s ground. Therefore, need not be installed near the physical ground.
A monopole antenna works well when mounted anywhere or at any height above ground as long as it has an artificial ground radial system integrated with it. An HF version can be installed on the roof of a building with the radials laid out on the rooftop, while a VHF/UHF version may be mounted on a mast with radial rods attached to the base near the vertical element. The radiation pattern of this antenna is truly omnidirectional in azimuth while it provides a broad elevation coverage in elevation with a deep null at 90° elevation.
There are several variants of this antenna. some of them have different radiating element length from that of a 1/4λ version. One of the very popular versions is a vertical monopole with 5/8λ length radiator. This may be structurally a bit too tall for normal HF amateur radio use but it is perfectly manageable on 6m, 2m, 70cm bands and beyond. The advantage of the 5/8λ vertical antenna is that it has higher gain with an omnidirectional azimuth pattern which is more compressed in the elevation plane making it better for DX operation with low take-off angle radiation. Both these antennas variants are resonant monoband standing=wave type antennas. They can, of course, be turned into multi-band antennas by proper use of traps.
Some of the other prominent omnidirectional antennas for terrestrial communication having omnidirectional azimuth patterns with varying degree of gain and elevation lobe compression are briefly described below.
- Vertical Dipole – A horizontal dipole is perhaps one the most common type of antennas. It is a 1/2λ length of wire driven at the center. It is an efficient resonant antenna which produces a horizontally polarized bi-directional radiation pattern in a horizontal configuration. However, the dipole may also be vertically oriented. This results in an omnidirectional azimuth pattern and is perfectly suitable for terrestrial communication. Although a vertical dipole may be structurally rather tall for practical deployment by an amateur radio operator on most of the HF bands, its physical size may be perfectly fine for deployment on VHF/UHF bands. The polarization of a vertical dipole is vertical.
- Normal mode Helix – Helical antennas are quite often used in various VHF/UHF applications. Depending on the physical dimensions of the Helix, it may be either made to radiate along its axis or orthogonal (broadside) to the axis. These two different modes (dependant on design) are respectively known as Axial Mode and Normal Mode. A normal mode Helix deployed with the axis of the helix in the vertical direction works as an omnidirectional antenna. The upside of this antenna is that it is physically smaller in length than a monopole antenna for equivalent gain, or else, the length of the helix may be increased by having more number of turns and thus achieving higher gain.
- J-Pole antenna – This is essentially a half-wave dipole antenna which is vertically oriented and driven at the bottom end instead of the middle as is done in case of a normal dipole. The problem of driving a dipole at the end is that the feed-point impedance at the end is extremely high and hence it is very difficult to achieve impedance matching for efficient power transfer. The J-Pole antenna uses a clever technique to circumvent the problem. It adds another 1/4λ section at the bottom end of the dipole with a parallel conductor in close vicinity of the first section. This lower section now acts like a 1/4λ transmission line. The bottom ends of the lower section are shorted together. This makes the 1/4λ bottom section present a perfect high matching impedance at the end of the upper dipole section. The total vertical length of a J-Pole is therefore 3/4λ. The feeder from the transmitter is connected at a suitable point, a little above the bottom of the structure where a good impedance match is achieved. This antenna produces a good gain with low take-off angle suitable for DX.
- Slim-Jim antenna – The Slim-Jim is a variant of the J-Pole. The length is identical and so is the gain and omnidirectional radiation pattern. The Slim-Jim can be viewed as a folded dipole (instead of a simple dipole) being driven at its high impedance end using the same method (1/4λ shorted TL section) as is done in the case of the J-Pole. The physical structure of the Slim-Jim allows it to be constructed even using a piece of an open-wire transmission line or a 300Ω or 450Ω transmission line section, thus making it absolutely light-weight and portable.
- Vertical Collinear – This is technically a stacked antenna array. It is like placing one or more 1/2λ length vertically oriented dipole(s) cascaded in-line above a normal 1/4λ vertical monopole antenna. The top-end of the lower monopole drives the bottom-end of the next upper dipole. However, there needs to be a 180° phase shift introduced between the junction points of the monopole and the upper dipole. This can be achieved in several ways, however, the simplest method is to use a 1/4λ length of a transmission line acting as a parallel stub that is shorted at the far end. This serves as the required phase shifter. This is an excellent antenna even with a single dipole section above the monopole. It has an excellent omnidirectional pattern in azimuth with great gain and a low take-off angle for very good DX. Adding more than one dipole on top of the first dipole section produces very high gain but may not be practically feasible for HF. However, multi-section Vertical Collinear antennas are used often for VHF and especially for UHF where the size becomes practical.
- Open Sleeve multiband vertical – An Open Sleeve Vertical antenna is a multi-band antenna that cleverly uses the property of mutual induction between closely spaced conductors. Structurally, this antenna has a normal vertical radiator whose length sized to resonate at the lowest frequency band of interest. This is the primary driven element which is fed at the base along with a ground system or a set of radials, similar to a classic vertical monopole. Several other elements sized to resonate on various higher frequency bands are placed parallel to the driven element in very close proximity for tight induction coupling. As the frequency band of excitation is changed to a higher frequency, the main driven element is no more resonant and the feed-point impedance magnitude rises and it becomes reactive. However, one of the secondary parallel elements which are designed to be resonant on this other band loads the driven element through induction coupling and acquires the TX energy for radiating. as a consequence, the effective parallel loading impedance of the inductively coupled element now determines the feed-point impedance and reflects resonant condition.
- Discone antenna – This antenna has several variants including a dual-Discone and an Inverted Discone. Due to its shape and structure, it performs as a broad-band antenna with the bandwidth spanning over several octaves. Due to its physical size constraints, a Discone antenna is practical only on VHF/UHF or higher frequencies. However, they have also been deployed on upper and middle HF bands too on various occasions. On HF bands, the cone structure is not solid but is made of closely spaced wires forming a wire cage structure.
Omnidirectional LEO Satellite Earth Station Antennas
A fixed omnidirectional antenna for working LEO satellites have a few unique additional requirements in comparison to what we have discussed so far for terrestrial communication applications. Apart from the necessary omnidirectional azimuth coverage, these antennas must have a very broad coverage along the elevation plane in all directions without a significant dip or null at 80° elevation. Secondly, it is preferable that these antennas must produce a circularly polarized radiation pattern and not a linear polarization. This is to account for the polarization mismatch between the earth station antenna and the onboard satellite antennas.
Barring a few, most LEO satellites do not have any kind of orbital attitude stabilization they often use linear polarized antennas, especially on 2m links. However, the antennas for the 70cm links very often use “Canted Turnstile” antennas which are circularly polarized. A continuous variation in the polarization of the satellite antennas is caused both due to the phenomenon of Ionospheric Faraday Rotation as well as the fact that the satellite continuously rolls, pitches, and tumbles as it moves through the orbit.
Some of the popular omnidirectional antennas commonly used by amateur radio operators are briefly described below.
- Quadrifilar Helical (QFH) antenna – This is a robust omnidirectional antenna often used for working LEO satellites. It is a compact structure that can easily be enclosed in a cylindrical tubular hosing made of suitable non-conductive plastic material with low dielectric loss at the operating frequency. Thus it can be a weatherproof antenna that may conveniently be mounted on a mast. It does not require and counterpoise radials or a ground system to function efficiently. The radiation pattern of a QFH antenna is circularly polarized and is truely omnidirectional in the azimuth plane, while it is nearly omnidirectional in the elevation plane. The overall coverage for LEO satellite work is excellent.
- Turnstile antenna – Typically, there are two variants of this antenna. The Normal Mode which is horizontally polarized and the Axial Mode which is circularly polarized. For LEO satellite work, the circularly polarized axial mode turnstile antenna is used. It consists of two dipoles which are oriented at 90%deg; with respect to one another in a crossed dipole configuration. Both dipoles are simultaneously driven but with 90° phase difference usually achieved using a 1/4λ phasing harness. This structure is placed above an artificial ground plane comprising of two crossed conductors of proper length. The resulting antenna is mast mountable and produces an omnidirectional pattern in azimuth as well as in elevation above ground. Another variant of the typical turnstile antenna is the canted-turnstile which is quite commonly used as a circularly polarized omnidirectional antenna onboard many LEO satellites.
- Egg-beater antenna – Intead of using crossed-dipoles as in the case of a turnstile antenna, this antenna uses crossed full-wave loops which resembles a typical kitchen eggbeater and hence its name. The Eggbeater antenna loops may be circular or rectangular. It also requires a crossed conductor ground plane like the turnstile. Due to the full-wave active loops, the gain of an eggbeater is higher than that of a turnstile antenna. The Eggbeater antenna is pretty common and used extensively as a LEO satellite ground station antenna for omnidirectional coverage.
- Lindenblad antenna – This antenna derives its name from its place of origin in the city of Lindenblad in Germany. The Lindenblad antenna was created well before World War II and has a circularly polarized omnidirectional pattern. The original antenna comprised of three folded dipoles inclined with respect to one another and placed at 120° from each other in a circle. All the three folded dipoles were driven elements fed through a complex phasing harness to achieve the desired phase relationship. The feed point impedance transformation to match the transmission line was also relatively complex. However, subsequently, a variant of the classic Lindenblad antenna using three parasitic dipoles and a centrally placed vertical driven dipole element was later developed. This parasitic Lindenblad variant is rather easy to build and provides a good impedance match to a coaxial cable transmission line.