Bredhurst Receiving and Transmitting Society

Syllabus Sections:-

5a Feeder basics.

5a.1 Understand that the velocity factor of a feeder is the ratio of the velocity of radio waves in the feeder to that in free space and that the velocity factor is always less than unity. Calculate physical feeder lengths given the frequency and velocity factor. Recall that the velocity factor for coaxial feeder with a polythene dielectric is approximately 0·67 or 2/3.

Velocity factor what is it?

From the syllabus we learn that :-

We know that the velocity of radio wave in free space is the same as the speed of light. From the syllabus statement we learn that the velocity factor is less than unity ( which is numerically 1 ) and for this to be true the velocity of the radio waves in the feeder must be slower.

Whilst we have spoken about velocity factor in relation to feeders the same goes for velocity factor in cables.

Why is this important ?

Well if is because when making up feeder and you want it to be certain multiplies of say 1/4 wave length then if you merely cut the feeder without reference to the velocity factor your feeder would be too long.

The same goes for antennas. If you merely cut the half wave dipole to the calculated length it too will be too long.

So the physical length of cable or antenna is always less than its calculated electrical length due to the velocity factor.

So if we have a frequency of say 3.600Mhz what would the length be of a 1/4 wave stub?

You have to be able to remember that the velocity factor for coaxial feeder with a polythene dielectric is approximately 0·67 or 2/3

So to calculate the wave length in metres we use the equation from the intermediate course click here to check back

v = f x or c = f x

Note that sometimes v is used for the speed of light and at other times c.

The velocity of light = the frequency x wavelength

As we are dealing in MHz we can use the speed of light as 300m/sec

so 300 / 3.6 = wavelength =83.33

so quarter wave = 83.33 / 4 = 20.83 metres

now apply the velocity factor 20.83 x 0.67 =13.9 metres

This is simple mathematics and you should be able to easily solve the problems.

Recall that feeder loss increases with increasing frequency and that lower loss feeders may be desirable at VHF, UHF and above.

As the frequency of operation increases feeder losses also increase and thus to have much the same signal coming out of the end as going is at the beginning you have to use better quality feeders - or as it is known lower loss feeders.

5a.2 Understand that a quarter-wave length of feeder can be used as an impedance transformer. Apply simple examples of the formula Z₀² = Z_in x Z_out .

The formulae for calculating the impedance of a matching section  will be given to you in the exam, if a question on this topic comes up so the important points to know are what do the mathematical notations represent.

Z₀² = Z_in x Z_out

In the formula Z₀ = the impedance of the quarter-wave length of feeder matching line Z_in is the impedance of the antenna Z_out the impedance of the feed line to which the antenna is to be matched. Note that the Z₀ is "squared" so to find the answer as to the impedance of the matching section you would have to use :-

all that we have done is "take the square root" of both sides, so as long as you understand such manipulation the result can be easily found.

Example:- What would be the matching section's impedance to match a 75 ohms dipole to 50 ohms coax. 

Z₀ = square root of( 75 x 50 ) = 61.3 ohms.

Now it is not very easy to find such a piece of coaxial feeder so what other feed line could be better used ?

Twin feeder of course, but for the exam you do not have to go into how you would achieve the physical dimensions of the matching section just be able to calculate it - as a diagram of coaxial feeder is easier to remember that it why we have used it.

5a.3 Recall the basic construction and use of waveguides.

The pictures above and below show wave guides. These take the place of other feeders with which you are familiar for HF and VHF and UHF (coaxial and open wire / balanced) when using Microwave frequencies and if properly made with good joint scan be low loss. The guide is usually a rectangular "tube" with flanges attached to join one piece to another and as shown below can be complicated arrangements or as shown above link to coaxial feeder via an "N" type connector.

Baluns

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