Good morning.
One of my rare monents to make a comment these days - hope the e-mail system
works!
The number of elements in a Yagi is an excellent question. Without a long bit
of
writing, for a monobander, adding more elements than the "miminum" is used to
broaden out the operational bandwidth. To use one with I am quite familiar,
let's use a 44' boom 20 mtr monobander (our MAGNUM 620). A 44' boom 20 mtr Yagi
can have the same peak gain with 4 elements as it can have with 6. However, the
additional 2 elements will maintain the gain and F/B almost level across the
band, whereas the 4 element will not. The F/B is often the aspect that varies
the most. A 4 element might be able to maintain the gain across 20 mtrs within
a
range of 9.7-10.3dBi and the F/B from 12-19dB on a 4 element Yagi designed for
full band coverage. There are a lot of variables, so these are only for
"talking
purposes" and it can be tweeked for certain needs. The 6 element will have gain
running from 9.9-10dBi across the band and the F/B will be almost constant at
20dB. The VSWR bandwidth of the 6 will also be quite flat. This makes the "over
populated" Yagi more effective over the entire band.
Knowing how many elements are "active" in a multiband Yagi is always a great
question! If we think about it, all the elements are "active", as they are all
in the near field, all carry current and all will contribute in some way (both
good and not good). This goes for close-spaced stacks, as well.
One different example is our latest tribander, the C-31XR, where there are
truly
3 distinct monoband Yagis on the same boom and overlaying the same space. Each
uses about 25-26' of boom space (31' overall). The 3el 20 and 4el 15 have about
the same gain, with the 15 a bit ahead, so the 20 is maybe a very good 3 and
the
15 is perhaps a moderate 4 element. On 10 mtrs, there are 7 elements used;
however, some are performing a sleeve function, which is to pass 10 mtr energy
"through" elements that are unfavorable to 10 mtrs. As such, the gain on 10
mtrs
is not that of a wide spaced 7 element, but more like a good wideband 6. So, if
one were to take the minimum elements aside and make separate antennas, you
could get similar gain using 3 on 20, 4 on 15 and (maybe) 5 on 10; however, to
accomplish the same performance as these individual monobanders on a single
boom
requires the additional elements.
Now that everything is absolutely clear...............!
Hope everyone is remaining cool with all this heat. Had a high of 114 here last
week, but it does cool down to 60 at night.
Have a great day & 73,
Tom Schiller, N6BT
President, Force 12, Inc.
Amateur Antennas, Commercial Towers & Antennas
P.O. Box 1349 Paso Robles, CA 93447
Phone: 805.227.1680 FAX 805.227.1684
Web Site: http://www.QTH.com/force12
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