[TowerTalk] Is "The Truth about Trees and Antenna Gain" the whole truth?

[Roger Parsons]

I had been expecting a discussion here on this recent QST article, but there has been very little. So I thought I would jump in. Answering my own question, I do not feel that the article does present the whole truth.

It seems to me that there are two self evident cases where an object placed close to an antenna does not cause loss: 


(1) Where the object is perfectly conducting, it may change the radiation pattern, but as it has no resistance there can be no losses;
(2) Where the object is perfectly insulating, it may affect the characteristics of the antenna (by changing the dielectric) but as it can pass no current, there can be no losses.

In all other cases a loss may occur, and I have no reason to doubt the general methodology described in the article.

However. The NEC based analysis is based on an antenna and a broadly resonant tree in free space. A tree in free space is considerably less likely than an antenna being there! (Actually, as there is currently an expensive motor car in orbit perhaps I am wrong...) The analytical simulation considers an infinitely long tree next to an antenna, again in free space.


Perhaps a right circular cylinder is an accurate representation of some particular tree, but it doesn't seem to fit the generalised case. Trees are ground mounted and have a ground system which probably has higher conductivity than their trunks and foliage - and which actively seeks out water. They also have top loading of almost infinite variety. The cedar tree that I can see from my window has very complex and spread out branches and foliage, whereas a palm tree (which I can unfortunately not see) appears to be quite close to a monopole with a some top loading.

Because a tree is lossy it will have a very broad resonance, but it seems to be stretching credibiity to suggest that a 5m high tree would significantly influence a 1.8MHz vertical. Or that a 50m high tree would have significant coupling to a 28MHz vertical. In each case the tree is likely to be very far from resonance.


I could go on, but my feeling is that although the conclusions reached in the article are reasonable for the model adopted, they are likely to greatly overstate losses in the real world.

73 Roger
VE3ZI


ps Perhaps there has been discussion on another reflector?