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[TowerTalk] Two towers-7 bands-best config

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Subject: [TowerTalk] Two towers-7 bands-best config
From: (
Date: Tue, 27 May 1997 07:00:35 -0700
Good morning, Bill.

Am glad this request came over a long weekend, so I had the time to
generate and share more detailed information.

There are surely many who will respond with various computer models for
your various two tower possibilities. Most everyone will have models for
the 3 monobanders (204BA, 155CA, 105CA), so those models are accurate. The
key is the 12/17 duobander.

Most people will not have a model of the DB1217, particularly the open
sleeve driver cell. We do have a model in our archives, since it is
essentially a copy of the original Force 12 N1217 from 1993. For a couple
of reasons, we extended the boom in late '94 to 18', which is the current
N1217 production model.  The original model (shorter boom) is used for
your questions. The 12/17 is a pair of 3 element monobanders actually
overlaying each other, with the 17 mtr reflector and director on the
outside of the 12 mtr reflector and director. As mentioned, the driver is
an open sleeve two band cell.

These two bands (12 and 17) are quite difficult to deal with in a
vertically stacked environment, particularly with 15 and 10 mtr Yagis. The
main reason is that (see also notes below on fluffed patterns) one should
not have a lower frequency Yagi above a higher frequency one. The elements
on the lower frequecy one act as poorly tuned reflectors. If a 12/17 is
desired to mix with a 10 and a 15 on a single mast, there is no
combination available that does not violate this principle. This is the
very reason we developed the Force 12 4BA to provide high performance on
all 4 bands (17- 15-12-10), utilizing two duobanders for 17/15 and 12/10
carefully stacked horizontally (our "forward stagger" term) on a single

You are fortunate to have two towers available. Suggested configurations
are to have the 15 and 10 on one tower. The 20 and 12/17 should be on the
other. Stacking 10 and 15 on a single mast is more a matter of deciding
which spacing you can safely manage in accordance with the trade-offs.

Placement of a 2 element 40 is best done above the 20. Loaded 40 mtr
elements should be able to co-exist within 4-5' of 20 mtr elements. If you
want to test this, the 20 can be temporarily mounted on a reasonable
support (6' ladder). Yes, this is too low to actually test the 20, but the
purpose is to test for interaction. Carefully secure the 20 so it cannot
move and attach your measuring device, hopefully a sensitive one. A noise
bridge with a VU meter can be used to see (not hear, as in usual use of a
noise bridge) very slight interactions. After the 20 is stable, move a 40
mtr element into proximity with the various 20 mtr ones. Be sure you and
helpers are always in the same locations. The 40 does not have to be
mounted on the boom. It can be hung underneath. Chart the closest it can
be without disturbing your 20 mtr measurements and this will give you a
safe region of co-location. As an example, our EF-140 elements (44.5') are
specifically designed to be mounted withn 48" of a 20 mtr element, which
is why we offer many 20/40 combinations. Also, our EF-140S (37') element
was developed to co-exist on the boom of the C-3 within a couple feet of
the 15 mtr reflector and adjacent to the enhanced open sleeve driver cell.
I have tested other loaded 40 mtr elements and they will all co-exist
within several feet of 20 mtr elements. You just need to be sure how close
(or how far)!

There are three sections below, the first is the 15 and 10 mtr stack. The
second is the 20 and 12/17 mtr stack. The 40 is assumed to be located on
the 20/1217 mast, probably about 3-4' above the 20, which will place the
40 mtr elements at least 4' from any 20 element. The third section is the
20 & 12/17 stack in free space.

A) gains are given in dBi at actual heights above real ground, as in the
typical installation, so they include ground reflection gain;
B) a few gain figures are given in dBd for comparison (approximately
2.15dB less than the dBi numbers);
C) a few gains are computed in free space for comparison;
D) the comment "fluffed" in regards to the pattern means that energy is
being pushed upwards. The front lobes are not as well defined and the
nulls between lobes are not as deep, either. The rear lobe(s) are also
usually affected and are not as deep (poorer F/B). Occasionally, the F/B
is better with a "fluffed" pattern. A fluffed pattern is due to the
elements on the lower antenna (lower frequency) acting like poorly tuned
reflectors in relation to the upper antenna(s) (higher frequency). This is
also why a stack of monobanders should not be installed with the lower
frequency ones above ("inverted stack"), as the higher frequency antennas
are effectively shielded. In the late 70's, I spent two years putting up
various stacks, making careful measurements and operating checks to test.
This was about a decade before the computer model helped answer the
question without all that effort!
E) A potentially serious condition needs to be checked. The 12/17 is fed
directly through 50 ohm coax. This coax now becomes a stub of some type
across the 17 mtr driver in the open sleeve cell. Depending upon its
length and whether or not the far end is open or closed when operating on
20 (i.e. goes to a remote switch where feedlines not selected are
shorted), the 17 mtr driver has the possibility of being some kind of
element that can affect the 20 mtr antenna. To check, watch for
fluctuations in the 20 mtr VSWR while opening and shorting the far end of
the 12/17 mtr feedline. This is not foolproof, but it is a simple check.
F) The term "alone" means the antenna by itself.

I.  Tower one, 15 and 10 mtr stack.
15/10      in dBi including ground reflection (max 6dB)
70'/80' (15 is at 70', 10 is at 80')
        15> 14.68dBi vs 14.83 alone
        10> 15.55    vs 16.01 alone    15 F/B worse by 6dB & whole
                                       pattern fluffed

70'/84' (15 is at 70', 10 is at 84')
        15> 14.60dBi vs 14.77 alone
        10> 15.85    vs 16.03 alone    Whole pattern still fluffed,
                                       but not too badly. Probably 
The approx. 0.5dB and 0.3dB loss on 10mtrs (70'/80' & 70'/84') should be
compared to the peak gain of 8.22dBd on the band (about 7.9dBd average for
28-29MHz). The approx. 0.15dB loss on 15 should be compared to the peak
gain of 7.49dBd on the band (about 7.2dBd average).

II. Tower two, 20 and 12/17 stack (40 not addressed in this model)
20 & 12/17 in dBi including ground reflection (max 6dB, ave about 5.5)

Be sure to note the variance in the 12/17 feedpoint (this might not be the
exact value of the installed tapered cell, but indicates significant

70'/80' (20 is at 70', 12/17 is at 80')
        20> 13.17dBi  vs 13.32 alone   F/B -3dB & fluffed 60 degs on back
        17> 11.15       vs 13.26 alone   F/B -12dB Very fluffed
             64 ohms    vs 42 ohms alone
        12> 12.37       vs 13.18 alone   Ptn very fluffed
             71 ohms    vs 56 ohms alone

70'/86' (20 is at 70', 12/17 is at 86')
        20> 12.89dBi  vs 13.35 alone   Ptn worse than 70/80 or 70/84
        17> 12.22       vs 13.34 alone   F/B -7dB Still very fluffed
              46 ohms    vs 42 ohms alone
        12> 13.54       vs 13.37 alone
              64 ohms    vs 56 ohms alone
70'/86' in dBd, including ground reflection gain (max 6dB):
        17> 10.07 dBd  vs 11.11 dBd alone (F/B 24dB alone)
        12> 11.39 dBd  vs 11.13 dBd alone (F/B 24dB alone)

III. Free Space Figures
These are included to illustrate the magnitude of interaction between the
20 and the 12/17. This can be seen by the maximum forward gain on 12 and
17 not being at 0 degrees. As you would look at the free space pattern,
the anticipated balloon of energy is does not peak at the center. Instead,
the center is tucked in and the maximum energy appears above and below the
0 degree line (the center horizontal line).

20 & 12/17 with 10' spacing:
       20> 5.80dBd
       17> 3.37dBd @ 0 degs (5.70dBd @ 44 degs) vs 5.16dBd alone
             63 ohms                              vs 42 ohms
       12> 4.70dBd @ 0 degs (5.73dBd @ 30 degs) vs 5.32dBd alone
             71 ohms                            vs 56 ohms
20 & 12/17 with 16' spacing
       20> 5.42dBd
       17> 4.46dBd @ 0 degs (6.01dBd @ 32 degs) vs 5.16dBd alone
             45 ohms                              vs 42 ohms
       12> 5.74dBd @ 0 degs (6.23dBd @ 16 degs) vs 5.32dBd alone
             64 ohms                              vs 56 ohms

The conclusion is that the 12/17 needs be as far from the 20 as possible,
with the 12/17 above the 20. The same applies to 10 and 15, with the 10
above the 15. Perhaps that was intuitive and the long way around the
block, but now you can be sure.

Hope this helps your installation.

Have a good day and 73,
                        Tom, N6BT
                        Force 12 Antennas and Systems
                        (Home Page )

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