N4KG comments below -
On Mon, 05 Jan 1998 05:45:10 +0000 Mike <no6x@inreach.com> writes:
>I am thinking of putting 3 monoband yagis on one boom, how can I find
>out if they are going to fight with each other before i do it. I have
tried a
>couple of shareware programs, but it only allows for single band
>stuff.
>
>Here is what I want to do:
>5 elements on 10
>5 elements on 15
>4 elements on 20
>all on a 26 foot boom
>For the final director in each antenna It was going to be a trapped
>element that would work as a director for all 3 bands ( old director
from a
>Telrex Yagi)
>
>According to my reading and books I have as single monobanders all
>will work ok, Now I want to go and screw that up and try to put them all
>together. Any Ideas, comments, or tutorials???
>
>TNX
>Mike...NO6X
>
>Dumb kid with lots of Ideas...Wait till
>you hear the next one!
>
Hi Mike -
Bottom line - DON'T DO IT.
There are several problems with interlaced Yagi's.
Several years ago I built a 3L15 on the 24 ft boom of my Telrex 3L20.
It worked but was not very effective. Analysis showed it to be 2 dB
down
from the same 3 elements if they were installed without the 20M elements.
Basically, the 20M director was acting as an inefficient reflector, out
in front
of the 15M Yagi. Placing another director out in front restores 1 dB
or gain.
I verified the latter effect by modeling my TH6 as a 4L10 with no other
elements
in place, then added the 15M director between the two 10M directors.
Reflectors can be lined up because the 10M reflector is closest and the
wider spaced lower frequency reflectors do not interfere. Lower
frequency
Directors are the problem. Using a trapped director is a good idea.
Cushcraft has two new antennas that utilize these principles. The X7
has 3 monoband driven elements with appropriately spaced monoband
reflectors and a single trapped director on a 16 or 18 ft boom. The X9
has
an additional 10M director and a second trapped director on a 28 ft boom.
Force 12 recognized the interlacing problem and has introduced a series
of "foreward staggered" tri-band Yagi's where most of the 15M elements
are in front of the 20M elements, and most of the 10M elements are in
front
of the 15M elements. The 10 element C3XL uses a 33 ft boom for 3 bands.
The W2PV Antenna Book shows gain vs boom length and element number.
Basically, gain is more dependent on boom length than number of elements,
given sufficient coupling. The gain curve is NOT a straight line. It
increases
rapidly between .20 WL and .40 WL, SLOWLY rises between .40 and .60 WL,
increases between .60 and .80 WL and flattens out between .80 and 1.1 WL.
F/B peaks with booms that are an odd number of quarter wavelengths (WL).
See page 2-23 in the W2PV Yagi Antenna Design book for the gain curve.
(A 24 ft boom makes a good 3 or 4L20, 4L15, and 4 or 5L 10.)
W6NL (formerly W6QHS) and others have shown that closely stacking
monobanders on a single mast also results in reduced gain for the
higher band Yagi's. THERE IS NO FREE LUNCH !
Also, NO SINGLE HEIGHT will cover ALL of the angles supported
by the ionosphere. During daytime conditions, LOW antennas (40 ft)
often outperform higher antennas. You need BOTH high and low
antennas for full coverage.
YOU CAN NEVER HAVE ENOUGH ANTENNAS !
Stacked antennas (including tribanders) offer both gain, directional,
and waveangle diversity. This is a POWERFUL combination.
de TOM N4KG
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