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Bredhurst Receiving and Transmitting Society |
Part 1 |
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5. Feeder and Antenna 5a Feeder basics 5a.1 Be able to identify and recall the use of coaxial and twin feeders. Coaxial feeder is an unbalanced feeder Coaxial feeder or coax as it is know for short is a cable which as a generalization looks on the outside like any other round cable. Whilst the basic construction of the cable remains the same it size can vary from a few mm diameter up to say 50mm depending upon the frequency it is to be used. It is only when you open it up that you see the construction.
There is an inner core which carries the signal, then a layer of polythene or air spaced dielectric, on top of that is an outer braid which for good quality coax is woven to a tight weave so that there is minimum leakage, and lastly an outer jacket which provides waterproofing and resists damage of the internal parts. The outer jacket must be intact else any ingress of water / moisture and the cable will be damaged. Twin feeder on the other hand is a much simpler type of cable being merely two conductors equally spaced along their length. The conductors carry the signal in equal and opposite signals. The spacing of the conductors is a deliberate part of the construction. The wires can be thin say 1mm or up to say 5mm depending upon the conditions that they are to work in.
Uses of the feeders Twin feeders also called balanced feeders Twin feeders can be used for feeding dipoles and are most usually used for frequencies up to 30MHz. Coaxial feeders Coaxial feeders can be used with frequencies up to the 1GHz but then the feeder is usually a wave guide. Coaxial feeders can be used to feed dipoles with baluns, yagis and vertical. 5a.2 Understand equal and opposite currents flowing in a balanced feeder cause equal and opposite fields around the two conductors. Understand that these fields cancel out, but that nearby objects can cause an imbalance which makes the feeder radiate RF energy.
Balanced feeder, or twin feeder as it can also be called, neither of the conductors is screened so each could radiate. As any radiation from the conductors is equal and opposite they cancel each other out.
There is a proviso here in that if conducting objects are close to the feeder and especially if much closer to one conductor than the other then the radiation would not be in balance nor symmetrical and radiation is not cancelled out and stray radiation could occur. Thus there is a need to keep twin feeder away from walls and other objects.
5a.3 Recall that in coaxial cable the RF field only exists within the cable and is not affected by objects outside the cable. Coaxial cable on the other hand does not suffer from the problems of changing radiation pattern due to the proximity of local objects as the screen wrapped over the dielectric prevents the inner signal carrying conductor from radiating and may be run along walls or even buried under ground.
5a.4 Recall that feeders cause loss of signal strength on both transmit and receive. The longer the cable, the greater the loss. All types of feeder do not deliver at the end of the feeder run the same amount of power that was put in at the beginning. Such reductions in transmitted power of the signal at the antenna and reductions in signal strength on the S meter on receive are called "feeder losses". There are several factors associated with feeder losses:-
5a.5 Recall that twin feeder usually has lower loss that coaxial cable. FACT - Twin feeder losses are much less than those for coax. In very high power broad cast transmitting station long twin feeder runs are mounted on poles similar to telegraph poles. Thus using twin feeder for amateur use makes sense because of its lower losses. 5a.6 Recall that loss is measured in dB. Be able to calculate the power delivered to an antenna for a given RF output and given feeder loss (in multiples of 3 dB).
What you have the understand that a 3dB loss represents a half of power reaching the antenna and a 6dB loss represents only a quarter of the power reaching the antenna. Example 1 If you have a 50 watt output power and a 3db loss in the feeder what power is expected at the antenna? 3db loss is a half of the output power at the antenna so 50 / 2 = 25 watts at the antenna. Example 2 If you have a 40 watt output power and a 6db loss in the feeder what power is expected at the antenna? 6db loss is a quarter of the output power at the antenna so 40 / 4 = 10 watts at the antenna. 5b Feeder characteristic impedance 5b.1 Understand that the term 'impedance' can be thought of as a form of AC resistance. In the electrical theory section we have given you an insight into resistance and associated calculation. There is another type of resistance called impedance when associated with AC circuits. The term impedance comes from the word impede meaning to resist and is used with the AC circuit to differentiate from the resistance used in a DC circuit.
5b.2 Recall that feeders have a characteristic impedance
which depends upon the diameter and spacing of the conductors. Recall that
for amateur use, 50 The aim of this section of the syllabus is to ensure that you are aware that different feeders exist and that using the correct one matters. The characteristic impedance of a feeder is dependent upon :-
For this level you do not have to know how this relationship works. With regard to coax feeder the impedance is determined by the ratio of the diameters of the diameters of the centre and the screen. Thus a coax feeder that has bigger diameters in the same proportion will still have the same impedance but the advantage is that the larger cable will have lower loss.
5c Antenna impedance 5c.1 Recall that the feed point impedance of an antenna is related to the dimensions of the antenna and the applied signal. Recall that the current flowing in an antenna is related to the feed point impedance and the potential difference of the applied signal.
Recall that an antenna will only present the correct
impedance when fed with the frequency for which it is designed, and that
a half-wave dipole has a feed point impedance of approximately
50 Recall that if the impedance of the antenna does not match the impedance of the feeder, energy will be reflected back down the feeder; the proportion reflected will depend upon the degree of mismatch. The feed point impedance of a dipole is APPROXIMATELY 50 ohms when fed with the frequency that it is designed to operate. An antenna's feed point impedance is determined by the relative RF potential difference at that point and the current at that point. However the impedance of the antenna also depends upon its dimensions related to the wavelength of the transmitted signal. The antenna will only present the correct impedance if it is fed with the frequency for which it was designed. If the antenna impedance and that of the feeder do not match then some energy will be reflected back down the feeder. The amount of that fed back down will be dependent upon what is called the amount of "mismatch".
5d Standing waves 5d.1 Understand that the signal reflected back down the feeder is not lost but will combine with the waves travelling up the feeder from the transmitter leading to the formation of standing waves. Recall that the presence of a standing wave indicates that there is a mismatch between the feeder and the transmitter. A concept difficult to grasp is that the signal that is sent back down the feeder due to a mismatch is not lost but will combine with the waves travelling up the feeder from the transmitter. This action is the formation of what are called "standing waves". However the standing waves affect the impedance "seen" by the transmitter and if the impedance mismatch to 50 ohms is too great, a modern rig will shut down due to the in built protection. However older equipment may well be damaged due to incorrect impedance. The correct idea here is to ensure that you check the impedance mismatch with your SWR (standing wave meter) on low power. 5e Antenna tuning unit 5e.1 Recall that a transmitter is designed to transfer energy into a specific impedance. Recall that an antenna tuner unit (ATU) can change the impedance that a feeder presents to the transmitter, may also reduce harmonic radiation. A transmitter is designed to "see" a specific impedance at its aerial socket. If it does not "see" this impedance then the PA (Power amplifier) cannot operate efficiently. So if you have made a dipole antenna cut to what you calculate is the correct size of the frequency you wish to operate but then find you have a Standing Wave Ratio or SWR greater than say 1.5:1 you would need an ATU to match the antenna to the rig NOT TUNE THE ANTENNA. An ATU matches is does not tune !! There is a conflict of words when discussing ATU (antenna tuner unit) and antennas. An antenna can only be "tuned" by cutting it to the correct length. Thus the term ATU should really be an AMU or Antenna Matching Unit. By adjusting the ATU it alters the impedance seen by the transmitter to as near 50 ohms impedance as possible . The ATU is then said "tune out" the mismatch or put is another way tune out the standing waves on the feeder. Resistive Load Further the main use of the ATU is to present a resistive load rather than an inductive load, at the correct impedance, to the transmitter. Change frequency / band and re-tune of the ATU is required The tuning out of the mismatch must be carried out for possibly any frequency change but most especially when there is a band change. The reason for this is that there will be a charge in the impedance of the antenna with a change of frequency. Have your tutor demonstrate such a "tuning" up at your club's field day / weekend (if it has such events) -the BRATS hold several such weekend during the year.
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Some students still
confuse balanced and unbalanced feeders. If you have any doubt which is which
start from the top of this page again.
So whilst losses
do exist on reception they are more easily understood as affecting transmissions
as they can be more readily measured. If you place a suitable power meter
at the far end of the cable and having the meter terminated in a suitable
Dummy load the amount of power reaching the end of the cable can be read
of the meter's scale.
Feeder loss are quoted in dB where dB stands for "decibel". For this part
of the syllabus you have to be able to calculate the power delivered to an
antenna for a given output from the transmitter and for given feeder losses.
The calculations are not complex and are limited to multiplies of 3dB.
coaxial feeder is normally used. Coaxial cable for TV and satellite receivers
has a different impedance and that balanced feeder is commonly available
from 75 to 600
