[TowerTalk] Yagi gain vs rotary dipole.

[Jim Lux]

On 11/25/14, 12:18 PM, Doug Turnbull wrote:

>
>       I know boom length is important but are you saying that given a long
> enough boom that a three element mono-band Yagi would outperform a six
> element Monobander on a fifty foot boom.    The additional elements surely
> do add some forward gain - some.    I know it may only be one to three dB
> and hard to recognize in the QSB.

Spacing elements farther than a half wavelength apart isn't likely to 
buy you much. Even 1/4 lambda is about as far as you want to go.
In particular, for a Yagi-Uda, which has only one element that is fed, 
you depend on the interelement coupling to get the current into the 
other elements, and when you start getting >1/2 lambda away, the 
coupling starts getting pretty small.

So a 50 foot boom (= 15 meters) for a 3 element 10 meter antenna isn't 
likely to be very useful: 7.5 meters is way more than 1/2 lambda.


The extra elements *might* add some gain, if they suppress a back or 
sidelobe. Superdirective arrays (which most yagis are) achieve their 
directivity by suppressing the back and side lobes more than by 
enhancing the forward lobe.


Note well, though, that I wrote directivity.  Highly directive antennas 
often have a lot of stored energy in the antenna (= high Q.. Q is stored 
energy/radiated power) which in turn often means high currents, so you 
can have a lot of loss. You might be highly directive, but the gain is low.

For a lot of applications this is ok: you're more interested in 
suppressing interfering signals from the wrong direction than you are in 
enhancing signals from the right direction.  Especially, since there's a 
limit on how much "enhancing" is possible.

The other thing multiple elements (potentially) can do is widen the 
bandwidth.

And, more elements give you more variables to adjust when optimizing: a 
3 element yagi has only 5 parameters: the 3 element lengths and the 2 
spacings.  A 6 element yagi has 6 lengths and 5 spacings, so you've got 
twice as many things to adjust to develop that "optimum compromise".