4D1 Recall the equation for calculating half-wavelengths and be able to apply 'end factor correction' when calculating the approximate physical lengths of simple dipoles and end fed antennas.
Maths formula also most certain to be used in this question !!!
As a memory jogger let's show you some of the information you have learned in previous courses.
From the Foundation Licence Course came the Frequency to wavelength chart
From the Intermediate Licence Course (link here)
came the formula v = f
relating
speed of light v to frequency f and wavelength
v = f
and you know that the over all length of a dipole is 1/2
long, so
so the EQUATION for calculating the overall length of a half wave dipole is derived from the one above
( as it is a 1/2 long ):-
Example. What is the length of a dipole for 3.6MHz ?
( 300/3.6 )/2 = 41.66m from end to end !!
But that is not the end of the story as this Advanced syllabus introduces 'end factor correction'.
The equation above would be correct for an aerial in free space, however as we are operating from near to the ground the half wavelength aerial will not be exactly equal to the half wavelength depending upon several factors:-
These items are all taken into account in what is called the "K" Factor which the aerial length calculated above must be multiplied by to give you a "better" approximation as to the length of the aerial.
for wire aerials of wavelengths up to 30MHz the K factor can be taken as 0.95
From above length of aerial is 41.66m
Apply the K factor 41.66 x 0.95 = 39.57m Overall length.
This can then be your starting point to prune the aerial to resonance at the frequency of operation.
So in addition to knowing the formula v = f x and rearranging
to the aerial calculation as shown above you must know the "K"
factor of 0.95 for wire aerials.
4D2 Recall the current and voltage distribution
on the centre fed dipole and 
/4
ground plane antennas
When RF energy is passed via the feeder line to a dipole antenna it develops both voltage and current distribution as shown below.
This indicates that there is a high voltage point at the end of the dipole but zero volts at the centre.
 |
The ground plane
antenna is derived from the half the dipole and has
the same characteristic distribution of voltage and
current from one end of the dipole to the centre. In the ground plane the centre of the dipole is at the bottom so voltage is zero and current at a maximum. |
Recall the feedpoint impedances of half-wave dipoles,
quarter-wave and loaded 5/8
vertical, folded dipoles, full-wave loops and end fed /4 and /2 antennas.
| λ/2 dipole |  |
50 to 75 ohms |
| λ/4 vertical |  |
about 37 - 50 ohms |
| 5λ/8 loaded vertical |  |
50 ohms |
| folded dipole |  |
300 ohms |
| full-wave loop |  |
approx 100 ohms |
| λ/4 end fed |  |
Low Impedance |
| λ/2 end fed |
 |
High Impedance |
Recall the effect of passive antenna elements on feed point impedance and the use of folded dipoles in Yagi antennas.
In the yagi only one element is driven the other elements are called passive elements as they only have a passive effect on the performance rather than the active of the driven element.
The addition of the passive elements, directors and reflector, to the driven element, which is usually a dipole, is to lower the feed impedance. To bring the impedance back up to a level where it can be fed with coax a folded dipole is used as the driven element. If you look to the diagram above you can see that the folded dipole has a higher feed impedance than the dipole.
The origin of some of the text on this page is from the RSGB with additions by the web master