Technical Questions & Answers (FAQs)
Q. What does an antenna do ?
A. The antenna is a device that converts Electrical energy (from the Radio Transmitter) into Electromagnetic energy in the form of Radio Waves.
Q. What is a dipole ?
A. 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 in the shape of a ‘doughnut’ i.e. 360o in plan and some 80o in elevation.
Q. What is gain & what is the difference between dBi and dBd ?
A. Gain is the effect produced when either more dipoles are used in an antenna (such as a Colinear) 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 ‘doughnut’ shaped pattern. The difference between them is 2.15 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.
Q. What is a ‘Colinear’ and what is a ‘Yagi’?
A. Colinear 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 360o 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’ is named after Mr Hidetsugu Yagi from Japan who invented the technique (in 1928) of fixing a passive element alongside a dipole to act as a reflector and then also attaching a number of parasitic elements 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. The most commonly seen examples today of the Yagi antenna is the Television Antenna used for UHF frequency bands.
Q. What range will my antenna give ?
A. This is commonly asked but is not easy to state definitely because so many different factors are involved, chiefly 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 kilometres to some 30 km (20miles) or more.
Q. What sort of feeder cable should I use ?
A. This is another question that has different answers, Skymasts 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.
See the chart we publish showing the losses of the various cables we supply against the popular frequency bands. If you have any doubts we are only too pleased to advise.
Q. Are the connectors waterproof? if not how do I waterproof them ?
A. 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 practice today is to first use self-amalgamating (PIB) 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 installations but some installers also add a layer of ‘DENSO’ tape on top of the insulation tape for good measure, this is not strictly necessary however and must not be used on top of the ‘self-amalgamating’ tape as it degrades it.
Q. What is a wavelength and how does it relate to a frequency ?
A. 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)
300/150MHz = 2.0m
This formula is useful to calculate things such as the distance to space antennas from another when this is usually expressed in wavelengths (l)
