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Optimal antenna height

How high should an antenna be?

Why sometimes lower antenna is better?

What is optimal antenna height, or is it " the higher the better" ?

For every sky wave radio contact there is an optimal signal take off angle. The angle depends on height of

reflecting layer, distance between stations and earth dimensions.

With horizontal antenna TOA can be adjusted to optimum by changing antenna height.

I wanted to see the relationship between station distance and optimal antenna height.

To find out I used the TOA calculator from my project 26. I assumed the F2-layer to be in 250 or 400km height

and Es-layer 120km. Flat landscape around the antenna was assumed.

I got the following results:

These curves show that station distance and optimal antenna height have a non-linear relationship.

Every additional kilometer in distance requires more incremental height than previous km, until we come to a point,

where single hop distance cannot be increased any more. With 250km layer height that distance is about

3300km. In such a case TOA would be 1degree and optimum antenna height 14.6 λ.

Max distance with 400km layer is about 4100km.

I would like to emphasize that these calculations focus on the vertical main lobe of horizontal antenna.

An antenna more than 1/2 λ above the ground has vertical side lobes, which can be used for good contacts as well.

Therefore optimal height calculated here is not the only possible height for a certain distance.

This analysis is valid also for multi hop contacts, when it is applied to the individual hops.

One more minor condition: MUF shall be high enough to make the contact!

Pekka

30.5.2012