[CQ-Contest] Re: [TowerTalk] yagi comments

force12 at interserv.com force12 at interserv.com
Tue Jun 17 20:07:26 EDT 1997

Good afternoon, folks.

There are some issues in Mr. Tyler Stewart's recent discourse that need to 
be clarified. To clarify them with hopefully good descriptions, I have had 
to delve into a fair amount of detail. If someone has a question in regards 
to anything about Force 12 antenna construction and specifications, please 
just ask!

The so-called "California Antenna Syndrome" doesn't apply here.

I am originally from Minnesota, where I did a lot of antenna building. If I 
recall correctly, it has been known to get cold, wet and windy there. My QTH 
was on the highest hill south of Minneapolis and the 105' of AB-105 was 
fully exposed to everything. It would have been taller, but we were too 
close to an airport (no - didn't have lights on it). The two shorter towers 
were not so difficult to manage. In the Bay Area here, it usually doesn't 
get icy, but the winds are at times tremendous, particularly the 134mph 
storm back 18 months ago (Dec '95).

We actively solict input from everyone on ways to make things meet their 
weather situations (as well as new product interests). Some of the results 
of this are the "H" models, which are rated at 120 mph. These are the 
strongest antennas I have ever seen - anywhere. As one moves up in wind 
pressure, there are points where things must change dramatically as a 
particular construction envelope is reached. We like to provide as much 
strength as someone needs, while not having everyone pay for more than they 
need. This is why there are three wind ratings for amateur antennas: 80, 100 
and 120 mph.

Other enhancements made during the last few years include:
-->more drop-in tilt cradle mounts for booms of 2.5" and even 2". These are 
standard for 3", 4", 5" and 6.5" booms.
-->the fiberglass spreaders on various linear loaded antennas are now 
sleeved with aluminum tubing, and the 4' wide ones are also built 
-->the standard Easy-On mount was made 1/3 thicker
-->most booms are standardly 20% stronger than the elements, but they are 
now even stronger, due to using thicker walls on most booms (we use our own 
extrusion dies)
-->the 40 mtr linear loaded trunk sections are larger diameter
-->the 180B 80/75 mtr element was lengthened slightly, the trunk was made 
larger (for both strength and larger average cross section in the high 
current area)
-->the linear loading wire was changed twice, from stranded copperweld, to 
stranded, tinned copperweld, to Alum-O-Weld, which is excellent (the 
copperweld did not hold up).
-->sidemounts and rotating side arms have been built (one at a time)

Regarding radial ice, please refer to The Physical Design of Yagi Antennas 
by Dr. David Leeson, page 2-22 for some basic information, as follows:
EIA-222-D provides that for ..."0.5" of ice in the absence of other data, 
and the statement that, because of low probability that an extreme ice load 
will occur simultaneously with an extreme wind load, the wind load can be 
reduced 25% (equivalent to 87% of basic wind speed) when considered to occur 
simultaneously with ice load." 

About linear loading - Force 12 does not use aluminum clad stranded wire. 
The wire is #12 Alum-O-Weld, which is aluminum clad steel, solid wire. It 
has a tensile strength of >1,000 pounds. The studs that attach the wire to 
the tubing are machined from 1/2" diameter 6061-T6 round stock. The 
restraining hardware is 304 stainless. The tuning jumper's (either 
Alum-O-Weld or solid rod) are standardly clamped with 1/8" cable clamps. An 
option is Ilsco #6 aluminum split bolts, which are about $2-2.50 each. A 
single element uses 4 of these.

The element-to-boom brackets are made on NC machines. The aluminum plate or 
sheet is of varying thicknesses and sizes to match the application. They 
range in size from mounting on 2" booms to 5" booms. The 80/75 mtr element 
brackets are detachable weldments, so that the elements can be completely 
assembled and tuned without requiring attachment to the boom. This means the 
elements can be attached to the boom after the boom is airborne, if 
necessary. The 160 mtr rotary dipole element-to-mast attachment is a 
multiple plate and support tube assembly that also allows the element to be 
assembled and lifted into position, then attached to the mast.

If someone would like to do an analysis using some kind of software like 
YagiStress, that is fine. We have used it since it first came out. Every 
Force 12 antenna is based on this structural analysis. The element tapers to 
eliminate vibration is also a product of our collective work with Kurt 
(author of YagiStress). We have also collaborated with Dave Leeson, who is 
the one who worked out our element-to-boom bracket design.

The comment about an "obvious wide-gap in physical construction..." requires 
all of us to take a careful look to see if it includes all of the actual 
costs involved (either one-off, or production oriented):
The quoted $2,500 for aluminum tubing on a full size 3el 40 is only a 
beginning, not the end. What is not listed is design time, both electrical 
and mechanical, setting up for multiple production (not one-off), initial 
assembly and test, adjustments, making of more of the same design to 
validate everything, final test, documenting and packaging.

One can't make a claim of tubing cost and think that is all there is to it. 
To do so would lead one wide astray, especially if someone wants to build 
their own. It is a fun thing to do, but it takes a lot more effort than 
simply buying aluminum.

Building for a one-off is much different than designing to meet the customer 
needs and dreams, while maintaining something that is economically feasible. 
If someone wants a one-off and cost is not an issue, let us know! We are 
presently building two antennas for the amateur bands rated at 120mph, using 
4" booms (a 6el 20 and a 15/10 duoband Yagi) and just finished two 
commercial antennas rated at 140mph. 

The Force 12 product line is more than 120 antennas, including commercial, 
ranging from verticals to dipoles to monoband Yagis to multi-monoband Yagis 
from 160 mtrs to 6GHz. This doesn't happen overnight, or on a lark! 

If we can help you, give us a shout! The web site contains a lot of 
information, too, and is almost done. Have some drawings to get converted 
and a couple of new short sections never seen before!

Have a good day and 73,
			Tom, N6BT
			Force 12 Antennas and Systems
			(Home Page   http://www.QTH.com/force12 )

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