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Syllabus Section 5
Feeders and Antennas.

The part of the Intermediate Licence the syllabus covered on this page:

5. Feeders and Antennas (part 1)

Sections 5d - 5g are on separate pages, see the Intermediate Licence Index.

Syllabus as supplied by the
Radio Communications Foundation (issue 5a)
Syllabus Copyright RCF.

5a Feeder Basics

5a.1 Identify and recall the use of coaxial and twin feeders.

5a.3 Recall that in a correctly connected coaxial cable the RF field only exists within the cable and is not affected by objects outside the cable.
Note that correctly connected means screen continuity through any plug and socket and connected to a balun or unbalanced load, not necessarily of 50Ω

At the Foundation level we were required to learn about coax. At this level we are required to learn about other types of feeder as well.

The radio frequency (RF) signal needs to travel between the radio and the antenna and the Feeder is the cable that allows this to happen. This feeder could be any piece of wire but there are problems with using any piece of wire that falls to hand!

The ideal Feeder will carry the RF signal from the transmitter to the antenna with as little loss as is possible and without radiating the RF signal itself. The transmitter creates a signal at the RF connector which has to be transported to your antenna. Any loss (reduction) in the amount of that signal delivered to the antenna due to it passing along the feeder must be kept as low as possible - otherwise you could end up with no signal at the antenna. The opposite is also true, if yor feeder cannot supply a good signal to the radio then you won't be able to receive weaker stations, you simply will not hear them!

When transmitting the Feeder must not radiate any of the signal (or as little as possible). If it did the radiated amount would not reach the antenna and could also cause problems by being radiated in the wrong place.

The Feeder comes in several forms and possibly the most popular feeder cable is Coaxial Cable (coax). It is easy to install and its' construction is simple to understand.

Coaxial cable structure

Coax is an UNBALANCED feeder, and comes in a variety of diameters. It consists of a single or multi-stranded insulated centre wire, with a braided wire screen wrapped around it, sometimes there is a foil screen as well. This braid is then covered with a waterproof insulation layer of a flexible plastic. The inner insulation can be a flexible plastic, polythene or foam, the outer sheath is usually a form of UV stable plastic. It is called Coaxial because the centre core, insulator, the outer shield and finally the outer insulator are all formed around a common axis (the centre of the cable).

The RF signals travel along the centre conductor and the braid shields this signal and keeps it from escaping (mostly, it depends on the type and quality as to how much signal can escape) and supplies the ground return, the RF fields exist only within the cable. Because of this feature coax can be used close to metallic objects without the problems that can occur with balanced feeders.
Note: The braid can only perform its' shielding if it is properly connected through all plugs, sockets and connections with continuity through all connections.

The characteristic impedance of the coaxial cable is determined by the relationship of the distance between the inner wire, and the outer braided screen, the diameter of the inner wire, 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 5.8 mm (3/16") and 10.3 mm (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 and UHF use.

You can read more about coax on the Coax Explained and Coax In Depth pages if you wish to further your knowledge on this subject. Only the above information is part of the exam.

5a.2 Understand that 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 that makes the feeder radiate RF energy.

Other types of feeder are Ribbon, Ladder and Twin Lead Feeder. These types of feeder are BALANCED feeders. They consist of two insulated wires attached to each other, and running parallel to each other. There are 3 distinct types of balanced feeder:

Ribbon feeder: The insulating separator is solid along the length of the wire:

solid ribbon feeder

Ladder feeder (also called "Slotted" or "Windowed" feeder): The insulating separator is broken by openings along the length of the wire:

ladder feeder

Twin Lead feeder: The wires are side by side:

Twin Lead feeder

Strictly speaking they could all be called twin lead feeders as a single conductor has its' twin with it.

The characteristic impedance of unbalanced feeder is determined by the diameter of the wire used, the material the dielectric is made from and the distance between the two wires in the ribbon feeder.

The slotted types have less dielectric signal loss and also, due to the openings, less wind resistance, when out in the open, than the ribbon types.

The RF signals do not "escape" from twin lead feeders because the currents in the 2 wires are equal and opposite each other. This set up electromagnetic fields around the wires that are also equal and opposite and, therefore, cancel each other out (hence balanced):

diagram showing how the equal and opposite fields occur in a balanced feeder

However, if a balanced feeder is run next to an object, say a wall or (especially) any metal object, the electromagnetic field can become disrupted or distorted and could no longer be equal and opposite to the field in the other wire. This would be likely to cause interference.

5a.4 Recall that feeders cause loss of signal strength on both transmit and receive. The longer the cable, the greater the loss.
Recall that twin feeder usually has lower loss than coaxial cable.

No matter how good or what type your feeder is it will have an inherent loss. This loss affects both transmission and reception. The loss for a particular feeder should be available from that cables' manufacturer.

The longer the cable is the greater the loss. Manufacturers often quote loss as "Attenuation dB per 100 feet" or similar. The loss is also affected by the frequency used as well as the length of cable, the higher the frequency the greater the loss.

Don't rely on figures for a particular type of cable (ie RG213) being the same for all manufacturers as different materials and quality of construction etc. all affect the loss (3 different manufacturers checked state: 4.3, 4.8 and 5.5 dB per 100 feet attenuation at 400MHz for their RG213).

Generally balanced feeder has less loss than coaxial cable but it is a more difficult prospect to run balanced feeder, especially if feeding an antenna on a tower.

5a.5 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 and 10dB).

You will need to be able to estimate the losses that you will have in your feeder. For this yo will need to be able to calculate in deci-Bels (dB). As mentioned above cable manufacturers will state the attenuation (loss) in dB per length at different frequencies. You will need to be able to apply the data to your transmitter power to estimate the power that is actually reaching the antenna.

As difficult as it seems it is quite easy as we do not need to go into the complex formula at this level of learning, you only need to remember that:

Decibels can be added together (or subtracted) so you could calculate a 13dB loss as 10dB then 3dB loss applied to that answer:
40W with a 13dB loss, 40W loss 10dB = 4W (3dB left to calculate)
4W los 3dB = 2W (total power at 13dB loss for 40W = 2W).

You could also work it out the other way around:
40W with 3dB loss = 20W (10dB left to calculate)
20W with 10dB loss = 2W

Personally I found it easier to remember that every 3dB halved the figure (starting with 50W, 3dB loss is 25W, 6dB loss is 12.5W, 9dB loss is 6.25W) and that 10dB loss moved the decimal point one place to the left (50W with 10dB loss is 5.0W)

This is important as your licence conditions quote maximum power permitted and this is the power at the antenna (usually). If your coax loss was 3dB per 100 feet at 144MHz and you had 100 feet connecting your radio to the antenna and wanted your legal maximum power (50W) at the antenna you can calculate that your transmitter would need an output of 100W to get the 50W at the antenna: 100W with 3dB loss (one half) = 50W.

For further reading on dB see the Decibels page elsewhere on this site.

Back to section 5a Syllabus or Intermediate Licence Index

5b Feeder characteristic impedance

5b.1 Recall that feeders have a characteristic impedance which depends upon the diameter and spacing of the conductors.
Recall that this impedance determines the ratio of the RF RMS potential difference to the RF RMS current in a correctly terminated feeder.
Recall that for amateur use, 50Ω coaxial feeder is normally used; that coaxial cable for TV and satellite receivers has a different impedance, and that balanced feeder is commonly available from 75 to 600Ω.
Note that correctly terminated means correctly connected with a resistive load equal to the cable characteristic impedance.

As mentioned in 5a.1 the characteristic impedance of the coaxial cable is determined by the size and spacing of the conductors and the material used for the seperating insulator. Dont confuse the characteristic impedance with resistance, if you test a piece of coax with an ohm meter the resistance for the centre conductor should be virtually 0Ω as should the braid. Checking between the centre and the braid should be infinity (or close to it). This test assumes access to just the coax with nothing connected to either end.

Similarly in 5a.2 we see that the characteristic impedance of unbalanced feeder is determined by the diameter and spacing of the conductors. Tests with an ohm meter should have similar results.

But what is impedance then? Well impedance is resistantce 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. The impedance of a feeder determines the ratio of the RF RMS potential difference to the RF RMS current in a correctly terminated feeder. You do not have to understand how or why this is at this level, just be able to recall it.

Most usually in amateur radio you will be using 50Ω coax but do be careful when you purchase it as TV and Satellite TV coax is 75Ω and will not give a good performance nor match.

Twin feeder is commonly available in impedances of between 75 and 600Ω.

Correctly terminated means correctly connected with a resistive load equal to the cable characteristic impedance, whether an antenna, dummy load, impedance transformer or balun.

Back to section 5a Syllabus or Intermediate Licence Index

5c Antenna impedance

5c.1 Recall that the feed point impedance of an antenna is related to the dimensions of the antenna and the wavelength of the applied signal.
Recall that the current flowing into 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 feed point impedance when fed with the frequency for which it is designed, and that a half-wave dipole has a feed point impedance of approximately 50Ω when used at its designed frequency.
Recall that if the feed point impedance of the antenna does not match that of the feeder, energy will be reflected back down the feeder; the proportion reflected depending upon the degree of mismatch.

When we look at an antenna, what is it that we are actually looking at? If we consider a dipole then the antenna consists of 2 elements, which are conductors, seperated by air. This is basically a capacitor! Other antenna designs may have coils, impedance transformers etc. in their design but the same basic theory holds true.

All antennas have an impedence at their feedpoint, this impedance is dependant on many factors in the design of the antenna and the frequency that it is working on. A dipole of the correct length for the frequency being used has a feedpoint impedance of almost 50Ω and is, therefore, a good impedance match to most transceivers on the market when fed with 50Ω coax (50Ω - 50Ω - 50Ω).

However the antenna feedpoint impedance is not a fixed value, it is related to the physical dimensions of the antenna and the wavelength being fed to it. In the real world the antenna (except for a few special designs) is a fixed length. As you alter the frequency (wavelength) fed to the antenna so the feedpoint impedance changes.

If the feedpoint impedance does not match the transmitters' output impedance then we cannot expect maximum radiation from the antenna. Ohms' Law for AC circuits (and your transmitter, feeder and antenna are an AC circuit) tells us that the current flowing in the circuit is related to the potential difference and the impedance. A change in the impedance anywhere in the circuit will impede (resist) the current flow. As you can see feedpoint impedance is quite important!

Modern transmitters are designed with an output impedance of 50Ω so, as mentioned above, they are well matched to a correctly cut dipole, other antenna types will have to be matched to the transmitter in some way or some of the transmitters energy will be reflected back from the antenna, down the feeder toward the transmitter causing standing waves. The amount of reflected energy is proportional to the degree of mismatch.

That's it for Section 5 Feeders and Antennas subsections a to c. Any questions about this section, please see your Course Tutor. failing that, email the editor of this page: instructor2@highfields-arc.co.uk.

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