Gain of parabolic antennas: a practical chart

 

Which gain can we expect of a parabolic antenna ?

This one depends of course on the diameter of the antenna and the frequency used. But it also depends on the way in which is carried out the illumination of the paraboloid and this point is often neglected. If a very directing source is used, only a small part of the paraboloid will be touched by the energy resulting from the source and the "useful" diameter of the antenna will be lower than its real diameter. Conversely, if the source produces a too broad beam, part of energy will be radiated backwards of the antenna, which will decrease the useful gain and will degrade the front-to-back ratio. Moreover, one will observe that the source has its own radiation diagram and that it thus does not illuminate the paraboloid in a uniform manner. To take account of all that, one defines a coefficient of efficiency k, accounting for the efficiency of the antenna. For optimal results, one generally seeks by construction, i.e. the choice of the focal distance, the diameter and the directivity of the source, a k coefficient ranging between 0.5 and 0.7. This corresponds coarsely to a level received in periphery of the paraboloid 10 dB to 15 dB lower compared to that received in its center.

When one wishes to obtain a "clean" radiation pattern, i.e. with secondary lobes of low amplitude, one will work on the source quality and one will limit the level in periphery of the paraboloid to -15 dB compared to that present in his center. The k factor will be then close to 0.5. It is the case of the commercial antennas used in telecommunications which generally have a low k value, near of 0.55. To maximize the gain, one will seek a k coefficient close to 0.7 which one will obtain while carrying the level with - 10 dB in periphery.

To summarize, one should to manage a compromise. A good adequacy of the source and paraboloid are essential for optimal results. A serious work requires the preliminary measurement of the radiation pattern of the source alone, or at least its evaluation on a theoretical basis.

The relationship between the data is :

where :

G_dB  is the gain compared to the isotropic radiator
          (substract 2.1 dB to obtain the gain compared to the dipole)

l is the radiation wavelength, in meters

S is the surface of the paraboloid, in square meters

k is the efficiency coefficient

A practical chart :

To avoid tiresome calculations, one will find below the gain values calculated. for various amateur bands, various antenna diameters and a K coefficient of 0.55.

parabolic antenna gains

values in dB_i,   k = 0.55

diameter (m)	frequencies (MHz)
	435	1250	2350	5700	10250	24000	47000
0.2	-	5.8	11.2	18.9	24.0	31.4	37.3
0.3	-	9.3	14.8	22.5	27.6	34.9	40.8
0.4	-	11.8	17.3	25.0	30.1	37.4	43.3
0.6	6.1	15.3	20.8	28.5	33.6	41.0	46.8
0.8	8.6	17.8	23.3	31.0	36.1	43.5	49.3
1.2	12.2	21.3	26.8	34.5	39.6	47.0	52.8
1.6	14.7	23.8	29.3	37.0	42.1	49.5	55.3
2.4	18.2	27.3	32.8	40.5	45.6	53.0	58.8
3.2	20.7	29.8	35.3	43.0	48.1	55.5	61.3
4.8	24.2	33.4	38.8	46.5	51.6	59.0	64.9

 

These values are valid only in certain conditions, in particular:

- a suitable optical quality of the reflector (geometry, asperities...)

- a good illumination (source placed at the focus point, radiation pattern of the source...)

 

B5+ et 73 de Jean-François Fourcadier, F4DAY

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