Antenna FAQs:

What does an antenna do?

The antenna is a device that converts Electrical energy (from the Radio Transmitter) into Electromagnetic energy in the form of Radio Waves.

What is a dipole?

A dipole is the basic form of base station antenna and is normally half of a wavelength long, it needs careful construction to ensure efficient conversion of the Electrical energy into Electromagnetic radio waves. The radiation pattern produced is approximately 360° in plan and some 80° in elevation.

What is gain & what is the difference between dBi and dBd?

Gain is the effect produced when either more dipoles are used in an antenna (such as a Collinear) or if extra elements are added (as in a Yagi). What happens is that the shape of the pattern is altered to send more radiation in a particular direction and less in others.

The term dBi refers to gain over an Isotropic source and is really a theoretical term. It would give a radiation pattern in the form of a perfect sphere emanating from this source.

The expression dBd refers to the gain produced by a half wave dipole, which as stated above gives a 360° shaped pattern. The difference between them is 2.14 dB, which the dipole produces above the Isotropic source. An increase of 3dB means a doubling in gain, therefore a 3dBd gain antenna has twice the gain of a half wave dipole and a 6dBd antenna has twice the gain of a 3dBd antenna.

What is a ‘Collinear’ and what is a ‘Yagi’?

‘Collinear’ literally means ‘one above the other’ and usually refers to several dipoles mounted above another and held within a non-conductive tube of Glass-Fibre or similar. The pattern produced is still 360° in the horizontal but much narrower in the elevation depending upon the number of dipoles used. This results in increased gain, typically 3dB or 6dB above that given by a single dipole.

‘Yagi’ antenna is a technique of fixing a passive element alongside a dipole to act as a reflector and then also attaching a number of parasitic elements to the other side, each slightly shorter than its predecessor. The effect is to concentrate the radiation pattern into a cone shaped beam, concentrating the RF Power into one direction and giving a much higher gain figure.

What range will my antenna give?

This is commonly asked but is not easy to state definitely because so many different factors are involved, mainly these are: the height above ground the antenna is installed, the transmit power used (less any losses in the system taken up by feeder cables, filters etc.) and the gain of the antenna used. What should be kept in mind however is that all VHF and UHF radio propagation is essentially ‘line of sight’ that is if the antenna site is at all visible to the outlying mobile unit, then the signal will usually be strong enough. This can vary between one or two kilometers to some 30 km (20 miles) or more.

What sort of feeder cable should I use?

This is another question that has different answers, We offers a good range varying from RG213 which is similar to that fitted to most antennas in short lengths, to the Low Loss, Foam Dielectric types of ½”, 7/8″ and 1 ¼” sizes that will reduce losses. Basically the larger diameter cable the lower the losses, also losses increase the higher in frequency you go so the need for larger cables also increases.

Are the connectors waterproof? if not how do I waterproof them?

Nominally all connectors fitted to antennas and feeder cables are supposed to be ‘weatherproof’ but it is a brave installer who would trust to this. Normal practices today is to first use self-amalgamating tape across the whole of the connector joint and ensure it leaves no voids or sharp edges, then follow this with two layers of good quality electrical insulating tape overlapping by 50% with each turn. This in itself is good enough for most.

What is wavelength and how does it relate to frequency?

Without going too far back into the physics lessons you should have remembered from school, a wavelength (λ) is expressed in terms of metres and is related directly to frequency (f) expressed in Megahertz (MHz). A simple formula gives the conversion: λ (m) = 300/f (MHz)

How antenna beam width and heights are determined and how should one choose this?

An antenna plot tells you where the radiation is concentrated. Patterns are usually referenced to the outer edge of the plot, which is the maximum gain of the antenna. This makes it easy to determine other important antenna characteristics directly from the plot.

Most antenna users are interested in the directivity or beamwidth of the antenna. This is usually referred to as the “half-power” or 3 dB beamwidth, the points between which half the power is radiated or concentrated, and specified in degrees. As an example, the typical half-power beamwidths of a 3, 6 and 10 element Yagi are 60°, 40° and 30° respectively.

Beam width is a function of design, which has to incorporate all the relevant (and related!) factors to achieve the optimal result: gain, VSWR ( voltage standing wave ratio), front-to-back ratio, operating frequency, & bandwidth. Antenna Experts antennas have excellent operating characteristics in that they are broadband, have a VSWR of 1.5:1 or less, a high front-to-back, and very consistent gain across the operating frequency.

If you require more coverage, choose an antenna with more beamwidth. However, more beamwidth can imply a lower nominal gain at the same frequency than an antenna with a narrow beamwidth.

How to determine the gain of Yagi Antenna? What kind of radiation pattern is desired?

This is the first major delineator of antenna selection. The only way to increase gain is to concentrate power in a narrower beamwidth. The narrower the beamwidth, the greater the gain of the antenna.

A Yagi antenna is basically a standard half-wavelength antenna with additional elements placed in front of it to focus the energy for transmission in one direction. The more directive elements, the narrower the beamwidth and the greater the gain. In other words, gain is simply how you focus the radiated energy at the transmitter and how you focus the ‘ear’ of the receiver.

Above 400 MHz, a 3-element yagi will typically have 6 dBd gain, depending on the physical size of the elements, the boom, and other design characteristics. Adding additional elements will increase the gain. (Adding 3 dB is doubling the gain.) So the 9 dBd. gain yagi has twice the gain of the 6 dBd. yagi.

How to determine the gain of omni-directional antennas? How gain is actually affected since they do not have elements that can be added?

Omni antennas radiate transmit power (the signal) in all directions (360°) and listen for incoming messages from all directions. Omni antennas, therefore, do not send a signal as far as a yagi antenna of the same gain.

A vertical omni directional antenna is often used for line-of-sight communications with mobile stations spread out in various directions usually restricted to the horizon. If greater performance is required, using a collinear type of omni that decreases the vertical beamwidth and hence concentrates more power on the horizon where it will be most beneficial can increase the antenna gain. Increasing the length of the antenna (adding vertical elements) will increase the gain of an omni antenna.

How to determine the wind load of antenna for mounting requirements?

WIND LOAD = 1/2 x AIR DENSITY x (WIND VELOCITY)² x (CROSS SECTIONAL AREA)

If the antenna is attached to a tower or mast, what is the diameter of the pipe or mast?

All Antenna Experts antenna mounts are designed to support and hold the antenna from 32mm to 52mm outer diameter pipe. The entire Grid parabolic antennas mounts are suitable to mount the antenna on 75 to 115 mm outer diameters pipe.

What kind of materials used for mounting hardware? Are the mounting hardware standard accessories of antenna?

All the materials used for mounting hardware are made of either stainless steel or galvanized mild steel with powder coating. All fasteners supplied with the antenna are made of high quality stainless steel. Normally all the antennas are supplied with stainless steel mounting hardware, unless if there is any special or custom mounting required by the customer.

What kind of grounding is required that should be taken into account for proper antenna grounding to reduce the noise and system protection?

Lightning protection must be examined from four distinct directions. First off, the place where the antenna is mounted (such as on a tower) is important. Then there must be input protection from the lightning strike itself, typically in the form of a huge and rapid build up of voltage and current at the input to the radio. Thirdly, a proper ground system must be employed to rapidly conduct the lightning bolt energy away from the radio.

If at all possible, don’t mount your antenna on the highest building or tower. Place it a few feet lower and hopefully the fickle lightning bolt, if it generates a direct hit, will not discharge through the antenna. Furthermore, the boom or mast should be grounded to the mast or tower. Don’t forget to ground guy wires that are used on stabilize towers. They are just as likely to be hit since they extend over a wide area around the tower.

The most important lightning protection is good low impedance Earth/ground connection to the associated equipment. The Earth ground connection should be a copper plated rod preferably at least 3-4 meters in length driven into the ground. This ground rod should be located as close to the equipment as possible, typically just outside of a building at the entry point of the antenna feed-lines.