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Forwarded message:
From: force12@interserv.com
To: K7LXC@aol.com
To: <towertalk@contesting.com>
Date: 97-01-14 13:45:05 EST
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Steve - the following is the reply I sent to Mr. Soper, who asked about the
C-3
vs. the rest. Thought you might find it interesting. I am not sure if all my
replies end up at everyone's Tower Reflector mailbox or not!
Hi, Pete.
Steve already mentioned in his reply that there is a detailed account of the
basic design process of the C-3 contained in ARRAY OF LIGHT. So, not to
re-write that section of the book, I'll only touch on a few important
itms.
Firstly, all the elements in the C-3 are full size. There are no traps or
loading coils in any elements from Force 12. If we thought traps of any
kind (coil/can, KLM) were a valid method for providing performance antennas,
we would have used them. We were starting with a clean slate.
To be sure nobody nit-picks this, one place we do use small inductors
(coils) is in the 80 and 160 mtr switch boxes, which are located at the
center of the element(s). They are relay-switched and enable excursions
within the band (cw to phone, plus sections within the cw and phone
portions). These boxes are available for all three lengths (54, 68,
84') 80/75 mtr dipoles and Yagis and the switch box is standard for the
rotatable 160 mtr dipole. We also make a switch box for the 80/75 and 160
verticals.
The basic design goals for the C-3 (the name came later) were to provide a
low profile, strong, light weight Yagi antenna for 20-15-10 with at least as
much gain as trapped antennas. It also needed to have a single feedline. The
process took 2 1/2 years. The easiest part was determining how much gain was
needed. The most difficult was devloping the 3-band feed system, which
actually turned out to excite the array on 5 bands (17 and 12).
The gain needed was variable. 20 mtrs was easily matched or exceeded by a 2
element Yagi. Most (not all) trapped antennas were usually best on 15. 10
mtrs had the best on the KT-34XA, which should not be a surprise. It has
several full size elements (within the traps) on a fairly long boom. It has
good gain on 10, but no where near the claims. Actually, nobody's claimed
numbers for trapped antennas are close and you don't even need to measure
them - just run some monoband Yagi models yourself. Better get back on
the subject...so, it was decided to incorporate a trio of 2 element Yagis on
the same boom in such a way that they would not destructively interact. In
fact, they are arranged so that they enhance each other. All the elements
carry current regardless of the frequency of excitation (20-10), so the
question of "how many elements are active on a band" is not really relevant.
That is a carry-over from an older generation. The real question is how much
gain does it have?
The answer is simple - model it or test it - or both. Any way you like, the
numbers are the same - nominally a good 2 elemenet Yagi...and...before
someone is quick to say something like, "That can't possibly our perform my
X-element tribander", let's recall how we got here. We tested the antennas
first, remember?! It is not how many elements the antenna has, it is not
necessarily how long the boom is (often appropriate, though), it is how
efficient the antenna is. That word "efficient" is the key. What one should
want is gain - real gain, not claimed numbers.
You can depend upon the Force 12 numbers. They have been modeled on NEC2,
tested and validated, plus some have now been run on NEC4. Same results:
accurate specs. Always have been. No other way to be as far as I'm
concerned. By the way, we make over 120 antennas - from 160 mtrs to 6 GHz.
Which triband antenna would I use? If my tower weren't so populated with 80
mtr stuff, I would use the 33' boom 5BA (20-17-15-12-10), because I like 17
mtrs. As it is, I use a C-3. If I want more gain, a simple stack of two of
them spaced about 30' is a real winner.
I'll conclude with a short personal note (from me, not the company) to
someone who thinks that chosing a C-3 over an "XA" would be smoking
something, since I have done my fair share of contesting and DXpeditions:
any day take a C-3 - smaller, lighter, less tower strain, no 500
parts to assemble and maintain, no power limit, maybe not always
through before an XA on 10 mtrs, but 20 is where the horses run.
- other numbers might look good, but they are only numbers.
Hope you enjoyed this, but the real purpose is to advance our knowledge of
antennas. Part of that is realizing accurate specs exist at least in one
place. Another part is perhaps, "unlearning" things we have collectively
been "taught" through certain literature.
73, Tom, N6BT
Force 12 Antennas and Systems
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