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".
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|