Bredhurst Receiving and Transmitting Society
5. Feeder and Antennas
|Is it an Antenna or an
Aerial? The words antenna and aerial means the same and the two
words can be used completely interchangeably!
5. Feeder and Antenna
5a Feeder requirements
5a.1 Recall the correct cable to use for RF signals and that coaxial cable is most widely used because of its screening qualities.
In order for the 'RF' Signal to reach the Aerial from the Transceiver, we use what is known as a 'FEEDER'.
The goal is to carry the signal from the Transceiver to the aerial with as little loss as possible and with none or very little radiated signal.
I am not sure what you mean can you explain a little more. Ok.
The Transceiver in its transmitter section creates a signal and presents it at its RF connector. This signal has to be transported to your aerial and it is transported by what is called a feeder. Any reduction (loss) in the amount of that signal delivered to the aerial due to it passing along the feeder must be kept as low as possible - else you could end up with nothing at the aerial! As there is usually only one RF connection to the Transceiver the feeder is used for both the transmitted signal and the received signal - where a transceiver has more than one RF connection it is usually for the ability to connect antennas covering different band but going into more detail is outside the scope required for the Foundation Licence Examination.
The feeder must also not radiate any of the signal (or as little as possible) else that radiated signal would not reach the aerial and could also cause problems by being radiated in the wrong place which will be explained more to you in the section on EMC.
So as I was saying :-
The feeder, comes in several forms, the most popular cable for this purpose, amongst Foundation Licence holders, is :- COAXIAL CABLE. It is easy to install and its construction is simple to understand.
COAXIAL FEEDERS is an UNBALANCED FEEDER, and comes in a variety of diameters. It consist of a single or multi-stranded centre wire, which is covered by an insulating material of flexible plastic or polythene, with a braided wire sheath wrapped around it. This braided wire sheath is called the screen.
The outer braided wire (screen) is used to retain the signal within the cable. The screen must be continuous through the plugs and sockets, it is often soldered to ensure good connection.
The following is of interest only and not for the exam
The Impedance of the coaxial cable is determined as the relationship of the distance between the inner wire, and the outer braided screen. The impedance of the coaxial cable is also determined by the diameter of the inner wire, and by the type of the dielectric insulator material between the inner wire conductor, and the outer screen braided wire conductor. Though coaxial cable comes in many different diameters, two common diameters of approx 3/16" and 5/16" are more often used in amateur radio, with the larger of the two being preferred due to its 'LOWER LOSS' characteristics for VHF use.
The following is of interest only and not for the exam
Other feeders are RIBBON FEEDERS is a 'BALANCED FEEDER' consist of two insulated wires running parallel to each other and separated uniformly by spacers. Unlike a coaxial feeder this cannot be connected directly to a transceiver as it does not have a plug fitting suitable so it would be connected to and ATU. However as with the Coaxial feeder one wire will through the ATU be connected to the centre of the coax plug and the other to the ground side of the plug.
The "IMPEDANCE" of the ribbon feeder is determined by the diameter of the wire used, and the distance between the two wires in the ribbon feeder. This type of feeder is called 'BALANCED FEEDER', Normal 300 OHM ribbon feeder has a distance of approximately 10 mm between to the wires, which are separated with a flexible plastic type of insulator, in a 'Ladder' style pattern.
The following is of interest only and not for the exam
The Cables which are mainly used by Radio Amateurs are 'OPEN WIRE' Feeder, (450 OHM, 300 OHM and 75 OHM), and 'COAXIAL' Feeder (75 OHM and 50 OHM).
5a.2 Recall that the plugs and sockets for RF should be of the correct type and that the braid of coaxial cable must be correctly connected to minimise RF signals getting into or out of the cable.
Identify BNC and PL259 plugs.
The two connectors shown above are those which you must be able to recognize. The PL259 is the connector most often associated with HF and VHF coaxial feeder and the BNC with UHF coaxial feeder (but it is also used at VHF). The inner part of the coaxial cable it connected to the centre of the connector and the braid is connected to the outer part of the connector with no short to the centre connector. When the connector is used to connect the feeder to the antenna, the inner part connects with the centre where the signal is coming from and the outer part is connected to the chassis of the Transceiver which is usually at ground potential.
What is to be understood by HF VHF and UHF ? For Amateur Radio purposes HF is any frequency from 1MHz to 30 MHZ, VHF is 144MHZ to 146MHz UHF is above 430MHz.
The point to understand here is that the connectors are of different quality and whilst this might not matter at all at HF it does at UHF and to a lesser extent at VHF. At UHF the PL259 would be said to be a "lossy" connector, as it absorbs some of the power trying to reach the aerial. The better constructed, and usually constant impedance of the, BNC is therefore to be preferred at the higher frequencies.
In each of these connectors the centre and the screen must be properly connected at both ends of the cable and there must NOT be a short circuit between the centre and the braid (screen) else the cable will not function properly.
You will want to carry out a resistance check on any cables that you make up to ensure that there is not a dead short between outer and inner conductors (This is assuming no connection to the antenna as you could have an inductive connection which given you a false reading- in this case you need to know that the outer is connected and you may have no other way than by piercing the outer so that the meter probe can be put in contact with the braid and the other probe placed on the outer of the connector, make the continuity reading and when satisfied all is well tape up the small hole in the cable outer).
5b Types of antenna
5b.1 Recall that the purpose of an antenna is to convert electrical signals into radio waves, and vice-versa and that these are polarised according to the orientation of the antenna, e.g. a horizontally orientated antenna will radiate horizontally polarised waves.
Purpose of the antenna
The electrical signal, or electrical energy that comes from the 'RADIO TRANSMITTER' to the AERIAL via the coaxial feeder is actually a TUNED RADIO FREQUENCY Signal.
In order for this signal to leave the coaxial feeder and radiate into the air, we need what is called an AERIAL.
This Aerial needs to be made for the band of operation, tuned to the frequency that the transmitter is operating on else the amount of radiated signal will be much reduced and the other station may not even hear you.
With a properly tuned aerial on the end of the coax, the electrical energy, or to use the correct term, the Radio Frequency Signal (RF signal), will now radiate into the atmosphere (and at HF into the ionosphere and be reflected back but more about that in the section on Propagation) so that the person that you want to talking to can hear you.
The same rule applies with the received signal, in that the correctly tuned aerial must be used to receive a signal properly. Normally in Amateur Radio use, we use the same aerial for receiving as we do for transmitting on for that same given frequency.
The polarization of the antenna is dependent upon how the radiating elements are orientated. Vertically gives vertical polarization horizontally gives horizontal polarization.
To obtain the best reception from a vertically polarised signal transmitted signal, your antenna need to also be vertically polarised.
obtain the best reception from a horizontally polarised
signal transmitted signal, your antenna need to also be
In amateur radio it is usual for FM transmissions to be from a vertically polarised antenna such as the 1/4 wave 5/8 wave.
In amateur radio it is usual for SSB transmission to be usually from horizontally polarised antennas.
Polarization is particularly important at VHF and UHF because if cross polarization exists with one station using an antenna vertically polarised and the other station using and an antenna horizontally polarised it will result in poor or even no reception where as had both stations been using the same polarisation of antennas reception would have been sufficient for communication.
At HF the importance of the polarisation of the antennas is much less as the radio signals will change polarization during their travel from the distant transmitting station to your reception and similarly on the reverse route.
5b.2 Identify the half wave dipole, /4 wave ground plane, yagi, end-fed wire and 5/8 antennas.
Understand that the sizes of HF and VHF antennas are different because they are related to wavelength, though they operate on the same basic principles.
Aerials come in many types and shapes. At this stage, we will only be dealing with five types of aerial, these are:-
Each of these aerials can be used on most bands and operate on the same basic principles, the deciding factors are dependent :-
The size of any given aerial is also governed by the FREQUENCY, or WAVELENGTH on which the aerial is designed to operate on. The lower the frequency, the longer the Wavelength, and so, the longer or bigger the physical size of the aerial. Conversely, the higher the frequency, the smaller the physical size or length.
Below are diagrams of the dipole, 1/4 wave ground plane, yagi, end-fed wire and 5/8 wave antenna. None of the diagrams are to scale as the size is dependent upon frequency of operation. In any antenna its size is frequency dependent.
Understand that the /2 dipole has a physical length approximately equal to a half wavelength of the correct signal.
Below the drawing on the left explains the dipole where as that on the right is the symbol you could have in the written assessment (this idea of antenna and symbol is similar in the next few drawings).
This is where the fundamental link between the size of the aerial and it wavelength is established. The overall length of both the legs of the 1/2 wave dipole ( /2 dipole) measure about the same length as the conversion of the frequency into the wavelength/2 with the answer given in metres. As a generality 14MHz has a wavelength of 20m. Thus the overall length of a 14 MHz half wave dipole is 10 metres so each leg would be about 5 metres.
The half wave dipole is the most basic of all antennas and is the antenna against which all others can be judged. The dipole can be used vertically or horizontally. The diagram show the antenna in the horizontal position and would be said to be horizontally polarised.
A DIPOLE aerial can be mounted Vertically or Horizontally. Normally for VHF & UHF working, a dipole is used in Vertical Polarisation. When a Dipole aerial is used vertically polarised, it is OMNI DIRECTIONAL. This means that it transmits in all directions around its element. However if a DIPOLE Aerial is used Horizontally Polarised, it only radiates as a outwards from the elements and no signal is from the end, and thus can have some directional element in its use.
1/4 wave ground plane
Note: that the radiating vertical element and the horizontal ground planes are all 1/4 wave long. The Ground plane antenna is always used vertically.
The yagi is said to have gain as it focuses the radio waves into a generally single direction and is not therefore wasting power radiated in directions where it is not required. The Yagi can be used vertically or horizontally. The diagram shows the antenna in the vertical position.
end fed wire
The end fed wire is simply a random length of wire attached to the centre of a coax feeder or more usually linked directly onto the rear of a suitable ATU that can take single wire. This is a poor antenna as it is not tuned to any particular frequency and thus generally performs badly relative to a dipole.
What is a long wire ? It is usually a random length of wire which is often connected directly to the terminal of an ATU which can accept long wire as well as coax feed and ladder wire fed antennas. The likely minimum length of the wire will be 80 feet but is often much longer.
The 5/8 wave
Note :- the 5/8 wave has a slightly better gain over the 1/4 wave antenna shown above. Also used vertically, it differs from the 1/4 wave in that there is a loading coil at the base of the antenna.
5c Antenna basics
5c.1 Understand that the 1/2 wave dipole (mounted vertically), ground planes and 5/8 antenna are omni-directional.
The 1/4 wave GROUND PLANE AERIAL is always used as a vertical and as such has an omni-directional wave form.
THIS PICTURE IS A REPRESENTATION
The 5/8 WAVE AERIAL has 'similar' properties to the 1/4 Wave Ground Plane Aerial. The only difference's being, the 5/8 Wave Aerial is BIGGER, and has a slight 'GAIN' of signal to its output compared to a 1/2 Wave dipole, or a 1/4 Wave Ground Plane but is also OMNI-DIRECTIONAL.
5c.2 Recall that a yagi antenna is directional and has a gain because of its focussing ability.
The YAGI focuses the RF into a beam sending it in a particular direction, in which the beam is pointing, so avoiding radiating the transmissions in directions other than towards the direction required to maximize the received signal for the station being worked.
A YAGI BEAM Aerial is a DIRECTIONAL Aerial, with higher gain than the aerials previously discussed.. This is achieved by the REFLECTOR on the back of the aerial, which forces the signal forward to the DIRECTORS.
The directors focus the RF Signal energy forward like the light beam of a car's headlight or a torch beam. A yagi (beam) aerial can be used vertically polarised or horizontally polarised. Due to the large physically size of a yagi one designed for HF is normally horizontally polarised.
5c.3 Recall that ERP is the product of the power to the antenna and its gain.
ERP = EFFECTIVE RADIATED POWER
Most manufacturers inform you of the gain of their antenna by using the scientific notation dB which stands for decibels. Whilst this may appear more complex you will be meeting it again in the Intermediate and Advanced courses.
Power leaves your transceiver and travels up to the antenna. If you are using an antenna which has what is called "GAIN" then effectively you will be getting more out of the antenna than you are putting in. This is only possible because of the antenna construction.
So what is this EFFECTIVE power. As the power is being radiated we called it EFFECTIVE RADIATED POWER (erp) and this is given by this formula :-
ERP = power fed to antenna from the rig x antenna gain
using linear units and no allowance for feeder loss.
So if you have a transceiver which has power output of 10 watts and the antenna has a gain of 10 the ERP = 10 watts (output) x 10 (gain) = 100 Watts EFFECTIVE RADIATED POWER
BUT consider this .....
This means that if the licence conditions state that the maximum ERP is 10 Watts, and the radio in use only gives output 1 watt of RF Power, then an aerial with a 10 times gain will produce the highest legal power for that frequency. Also, if for the same frequency, the radio in use has a maximum RF Power output of 5 watts, and the aerial in use has a gain of 2 times, then the ERP will be 10 Watts.
You will need to be able to manipulate the equation ERP = power fed to antenna from the rig x antenna gain just as you did V = I x R and P = V x I
So construct another magic triangle for your self. If you have problems with it chat to your club's tutor.
5c.4 Recall that the antenna system must be suitable for the frequency of the transmitted signal.
Recall that if an antenna is not correctly designed for the frequency it will not match the transmitter and will not work effectively.
There are various types of antenna that can be used with a transmitter. Whilst an antenna is designed to work on a single frequency, some of the designs can be used as a practical type of antenna for a wide range of single frequencies whilst other cannot and this is all down to physical size constraints. Thus in the 144MHz and 430 MHz band all antennas designs are practical, but when it comes to the HF bands it is a different matter due to their much bigger size.
The aerial has to be the correct physically size for the frequency in use, other wise the radio transmitter will be damaged due to a high SWR, or Standing Wave Ratio Mismatch, and thus will not operate efficiently.
Note it is the transmitter which could be damaged NOT the antenna. The damage to the transmitter occurs because some of the power is reflected back down the feeder by the antenna to the transmitter.
Elsewhere you may have seen the Frequency to Wavelength conversion chart and this is where it comes into use. For all bands you can think of the most basic antenna as the half wave dipole. This half a wave length is the total overall length of the antenna and thus it has legs each of a quarter wave long. By reference to the chart you will be able to assess what is the full wave length and then divide by 2 to give you a guide as to the over all length of the antenna.
If the antenna is not designed for the particular frequency being the transmitted frequency, then not only will the signal not radiate well but damage could occur to the transmitter and possibly cause EMC problems to next door's TV / radio.
A note of caution.
If you decide to use an antenna analyser to check your antenna do make sure that there is not a near by station transmitting as this will cause false readings on the analyser even though the station is not transmitting necessarily on the same frequency band as the antenna you are making !!!
5c.5 Recall that at HF, where an antenna has not been designed for the particular frequency, an ATU (antenna tuning unit) improves the ability of the antenna to accept power from the transmitter.
Whilst from the above you have learned that an antenna is designed for only a single frequency if you want to work a particular band then it is best to make the antenna resonant on the centre frequency for that band.
Recall that, when an antenna is not well matched to a transmitter, a matching unit, commonly known as an ATU (antenna tuning unit), is used to ensure that the transmitter can supply energy to the antenna without damage to the transmitter.
However by the use of an Antenna Tuning Unit the transceiver can be fooled into thinking that the antenna is the right one for it, and not be damaged and radiate much of the output power from the transceiver.
Let's look at the words Antenna Tuning Unit. Whilst you might think that the antenna is being tuned the only way to tune and antenna is to physically alter it construction. All the Antenna Tuning Unit is doing is changing the impedance of the antenna to appear to be 50 ohms which is required by most modern rigs. It is better to think of and ATU as an antenna matching unit.
5d Balanced antennas
5d.1 Understand the difference between balanced and unbalanced antennas and that a balun should be used when feeding a H.F. dipole with coaxial cable (which is unbalanced).
A balanced antenna is a centre fed dipole, equal length legs symmetrically either side of the centre connector (hence balanced), whilst a quarter wave vertical and five eights wave vertical are unbalanced as they are not symmetrical.
If you look at a dipole you would see that it is made up of two identical length "legs" which are linked at the centre by some form of insulated joint which keeps each leg apart from the other and allows you to link it to the feeder. Often the feeder used on a dipole is the open wire feeder as it too is a balanced feeder but many amateur prefer to use a co-axial feeder which is an unbalanced feeder but a "Balun" is used to link the BAL anced antenna to the UN balanced coaxial feeder.
The choke balun can simply be several turns of the coaxial feeder (say about 6 turns of the coax of 150mm diameter) or more complex by the use of ferrite ring or ferrite bar. A choke balun as it is there to choke off / stop any RF that might try to pass down the braiding rather than the centre of the coax.
Suffice to say what you need to know for the written assessment is that :- a balun should be used when feeding an H.F. dipole with coaxial cable.
5e Meaning of Standing Wave Ratio (SWR)
5e.1 Recall that an SWR meter shows whether an antenna presents the correct match to the transmitter and is reflecting minimum power back to the transmitter
"S" "W" "R" stands for Standing Wave Ratio. The picture shows an SWR meter/power meter. Note that this unit has two needles. This is not always the case but here the forward and reflected power are shown simultaneously. Other meters require the operator to switch between forward and reflected power and compare reading to know relatively how much RF is going in each direction. With a high forward power level and a low reflected power level the antenna could be said to be well matched to the operating frequency but is said to be "mis-matched" if the forward is high and the reflected is high. A high SWR can cause damage to your rig, see more in the next section.
5e.2 Recall that a high SWR (measured at the transmitter) is an indication of a fault in the antenna or feeder (and not the transmitter).(Relate this to item to 4b.5 which says "4b.5 Recall that the RF power amplifier output must be connected to a correctly matched antenna to work properly and that use of the wrong antenna can result in damage to the transmitter.")
It is the SWR meter which is used to measure the SWR (Standing Wave Meter) on the feeder line.
If the aerial is not correctly matched to the transmitter frequency then when a signal travels up the feeder to the aerial it is reflected back to the transmitter and the system is inefficient.
If the ratio of forward to reflected power or "SWR" measured at the transmitter is high then much of the power is being reflected back to the transmitter. This could be the fault of the antenna or some other problem with the feeder such as a broken or incorrectly tightened connector. Thus the fault lies any where but with the transmitter.
With a correctly matched antenna and good feeder only a very small amount if any of the forward power will be reflected back.
A high SWR would also occur if you unwittingly failed to plug the aerial into the transmitter and pressed the PTT if operating AM FM and also SSB and spoke into the microphone.
5f Use of a dummy load
5f.1 Recall that a "dummy load" is a screened resistor connected instead of an antenna to allow the transmitter to be operated without radiating a signal.
A DUMMY LOAD is an artificial aerial, used for test purposes.
The dummy loads shown above represent a range - 100watt on the left 5 watts in the centre and 15 watts on the right.
The Dummy Load MUST be made from CARBON RESISTOR(s) with short connecting wires.
As can be seen from the centre image the dummy load is several carbon resistors in parallel making up 50 OHMS, or a single large CARBON RESISTOR built into a heat sink with the correct connectors on it so that the Dummy Load can be connected to the radio TRANSMITTER / RECEIVER or ATU / SWR Meter for test purpose's without radiating a signal. Tests such as, looking for power loss in feeder, or to test for faults in the feeder or the aerial can be done by putting the Dummy load at the point where the antenna would attach.
The reason that a 50 ohm CARBON resistor is used, is because 50 ohms is the correct impedance value of the aerial system into which the transceiver transmits . The Dummy Load MUST be made from CARBON RESISTOR.
A WIRE WOUND resistor is effectively an inductor ( a coil of wire ) and because the dummy load must not have any inductive properties the wire wound resistor, although of wattage capability and easier to obtain cannot be used. Inductive properties in a dummy load could lead to problems such as radiating a signal.
The origin of some of the text on this page is from the RSGB with additions by the web master