Station Design for DX - Part II
October 1966 QST
Part I of this article appeared in last month's
(September) edition. It introduced concepts in antenna types and siting. This second
part talks about cost tradeoffs for various aspects of a DX setup. Author Paul Rockwell
does a nice job of providing graphs of cost versus performance increases for transmitter
power, antenna gain, tower height and constructions, etc. He uses prices typical of the
mid 1960s, but even without knowing the equivalent modern day equipment prices, the shapes
of the curves are good indicators of where the point of diminishing returns exists.
October 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Paul Rockwell wrote a 4-part series on station design for long distance (DX) communications
that covered antenna selection and siting (Part I), economics
and construction (Part II), station
configuration and receiver topics (Part III), and
propagation quirks and operating tips (Part IV).
Station Design for DX - Part II
Part II - Economics of Station Design and Construction
By Paul D. Rockwell, W3AFM
In the pursuit of amateur radio, dollar limitations are always present. What is the most
practical allocation of available funds? Let us first illustrate an analytical - approach
to this question from the standpoint of effective DX-radiated power (DX e.r.p.). Assume
20-meter operation, flat terrain, no voice modulator, and optimum radiation angle of
1°. As a frame of reference, 0-db. will be taken for 100 watts c.w. input, 30-foot
tower height, and 10-foot Yagi boom-length.
Fig. 3 - Transmitter power costs.
Transmitter power costs run about
as shown on Fig. 3. The costs include driver, but exclude v.f.o. The curve would have
to be shaded upward for first-class features, and downward for some home constructors.
Make your own curve, if you prefer. What is being shown on this and the following curves
is a design technique - not a universally applicable set of data. What is important to
note, in this example, is that the last db. (from 780 to 1000 watts) costs $100.
Now consider antenna costs. Fig. 4 presents these for Yagis. The db. gain values
are relative to a half-wave dipole, same height and foreground. The next db. beyond 30-foot
boom length costs $200. Stacking two beams, which gives 3-db. gain at the expense of
40-foot additional tower height is attractive beyond about 30-foot boom length. However,
this introduces the problem of rotating both beams without interfering with guys. The
Telrex Big Bertha solves this by rotating a self-supporting tower. Such a tower, 112
feet high, equipped with antennas and accessories, costs over $15,000 in place.
Now tower height. Fig. 5 presents costs, based on $10 per foot for ordinary lattice
tower, guys, anchors and foundations. Erection costs are added, beyond 40-foot height,
up to $400 for the 150-foot height. No allowance is included for rotator, indicator,
insurance, etc. Gains are related to the assumed ideal of 1° takeoff angle by use
of the image-antenna geometric construction. Analysis by the indicated technique shows
that, to a close approximation, DX e.r.p. at 1° increases as the square of tower
height. That is, each time the tower height is doubled, 6-db. improvement is appreciated.
Because DX signals often arrive (and should be transmitted) at angles considerably above
1°, this figure must be weighted downward. Fig. 5 has been constructed on the basis
of linear relation between e.r.p. and tower height - 3-db. improvement for each doubling
of height. This agrees fairly well with Utlaut's results for very high effective heights.
The concept being developed is: Cost per db. for the last db. of improvement which can
be handled economically. Suppose we can afford
Fig. 4 - Antenna costs.
Fig. 5 - Tower Costs.
$200 for the last db. By examination
of the curves, we see immediately we should run 1-kw. input, for in this department the
last db. costs only $100. We choose from Figure 4a boom length of 40 feet. Tower height
per Figure 5 is 75 feet. Total cost, adding the corresponding ordinates of Figures 3,
4, and 5 is $1980.00.
Perhaps this cost exceeds our means. Maybe we can afford
only $50 for the last db. in each of the three departments principally affecting DX effective
radiated power (DX e.r.p.). On this basis, we choose 150 watts, 23-foot boom, and 40-foot
tower height. Total cost is $350.00.
Once the concept is understood, curves may
be developed to fit the individual situation, and to take in to account all sorts of
other variables: cable losses, fixed costs for auxiliaries, commercial increments of
sizes, nested-rhombics-plus-real-estate versus Yagis, etc.
An important consideration,
so far excluded in order to simplify the discussion, is the fact that antenna db. work
both ways: send and receive. Appraisal of antenna and tower costs for DX e.r.p. should
therefore be weighted, so as to allocate a share of these costs to the receiving advantage.
A reasonable factor is one-half. That is, the dollar values of ordinates of Figs. 4 and
5 can be cut in half, for economic optimization of design with respect to DX e.r.p. only.
On this basis, Fig. 6 shows optimum combinations as a function of funds available. The
figure is constructed by assuming various dollars-per-last-db. values, and connecting
the resultant values by curves. Of course the optimization differs somewhat from the
$50/db. and 200/db. examples above, because the receiving components of costs have been
broken out separately.
For example, suppose $500 are available for the relevant
parts of the station. Refer to "Total Cost" on Fig. 6 at $500. Draw a line straight up.
Parameters are: Power, 150 watts; Boom, 27 feet; Tower, 50 feet. Gain relative to the
reference installation is 5.3 db.
If $2000 are available, parameters are: Power,
1 kw.; Boom, 40 feet; Tower, 75 feet. Gain relative to the reference installation is
Fig. 7 presents compatible equipment complements with regard only for
DX e.r.p. - no allowance for concurrent receiving advantages. This represents a more
conventionally accepted approach. In effect, transmitter power is given greater initial
emphasis. These db. are cheap and more convenient than antenna/tower db., but do not
bring corresponding receiving advantages. After the legal power limit is reached, optimization
proceeds much as on Figure 6. For $500, read off: Power, 275 watt; Boom, 26 feet; Tower,
44 feet: Relative gain, 7.3 db. For $2000: Power, 1 kw.: Boom, 40 feet; Tower, 75 feet;
Relative gain, 17.2 db.
Economically, c.w. telegraphy gives by far the most DX
per dollar. Not only is this true because more DX stations are available by c.w., but
also because of greater efficiency, expressed in db. as follows 17:
C.w. ............................................ 0 db.
D.s.b. a.m., order-wire
quality... +17 db. required
S.s.b. order-wire quality ........... + 14 db. required
S.s.b. DXers will nearly all aver that the table above should be corrected to
read "11 db." instead of "14 db" for s.s.b.
After reading this, it is fair to
ask: "What does a db. in DX e.r.p. really buy, after all, in terms of DX capability'?"
The answer is that, other things being equal, it buys a lot. Six db. buy, competitively,
a decisive advantage.
So far, system-design trade-offs have been discussed. The
matter of constructional alternatives is also important in station economics. The remainder
of this month's text is on miscellaneous antenna-construction comments. Antenna mounts
are frequently the major item of home built equipment.
First, re antenna towers. There are fine products on the market. These firms also sell
the numerous necessary and desirable accessories: brackets, clamps, clips, anchor, winches,
guys, and even gin poles. Only a small percentage of DXers use these products, because
the majority (a) can't afford them, and/or (b) home and neighborhood consideration won't
permit them. Speaking in generalized terms, short of all-out optimum performance, a practical
and almost universally applicable construction is to use telescoping pipe sizes, side-supported
to the house, with a hand-winch for running the antenna up and down. This is what is
done at W3AFM. Some particulars follow.
Fig. 6 - Compatible equipment complements
(with allowance for receiver advantage).
The cheapest and most universally available
mast structural element is water pipe. It comes in 21-foot lengths. It should be ordered
black, unthreaded. Local suppliers usually deliver. Prices run about $10.00 a length,
depending on weight. Figure 12¢ to 20¢ a pound, depending on discounts, location
etc. Sizes are confusing, because they are based on nominal i.d. of the standard weight.
"Extra strong" and "double extra strong" are of the same material, but smaller i.d. (same
o.d., to match fittings) for greater wall thickness, Some examples are given in the table
below, in which "XXH" means "double extra heavy":
Many Yagis are made to mount on 1 1/2-inch pipe.
Speaking in generalities, and depending on prevalent winds, antenna, etc., 16 feet of
unsupported height (i.e., 16 feet above guy attachment or last bracket) can be good design,
whereas 20 feet can be risky. It is wise, if using water pipe, to telescope section,
in such a way that the top 10 feet are single-wall, next 10 feet double-wall, next 1
foot triple-wall, etc.
Steel much better than water-pipe iron exists. Chrome-molybdenum
electroweld or seamless AISI 4142, heat-treated to 180 k.p.s.i. looks great - but costs
ten times as much per pound and seems almost impossible to get in less than mill lot.
A popular mast in the Northeast is Diamond "E" (1020 cold-drawn steel) 2 inches
o.d. by 0.25-inch wall X 20 feet long, selling for about $60.00. So far as known, one
of these has never folded.
Aluminum alloys have a modulus one-third that of steel. This can make them very willowy,
unless kept short, and thick-walled.
Fig. 7 - Compatible equipment complements
(DX e.r.p, optimization).
In the best installations, the mast or top
tubular-section of the antenna-mount, projects only a few feet above the main steel-lattice
tower. The rotator is then a few feet below the top of this tower. Sometimes, for reasons
previously mentioned, a lattice tower is not practical. In such cases, the pipe mast
is extended down to the ground, and the rotator mounted near the ground. Such an antenna
support is commonly clamped, loosely so the antenna can be turned, to the side of a house.
When this is done, it is important to spread the stress on the house structure. This
is done by angle-iron, channels, or wooden members, coupled typically by 1/2 inch threaded
bolts all the way through, for example, the attic walls. At W3AFM, a vertical 2 inch
X 6 inch X 12 foot plank is bolted to the side of the house, with 2 inch X 4 inch X 6
foot lateral stress-spreaders horizontally inside the attic wall. The strongest wood
commonly stocked is oak. Clear white oak, unfinished, and suitably stained, is used.
The vertical plank is attached by four 1/2 inch bolts, and projects 4 feet above the
peak of the roof. Three husky electrical clamps attach the mast to this plank. The second
21 foot pipe section up from the ground is slotted to fit over a 1/2 inch dowel in the
lowermost 21 foot pipe section; so the antenna may be lowered to a height reachable from
the roof by first raising it a few inches, then lowering the disengaged part to the ground.
Such a load requires work advantage. Boat winches, available from Sears or Ward's at
about $25.00, are well suited to this purpose. Half-inch polyethylene boat rope is a
It can be dangerous and expensive to economize on small hardware
fittings: eyebolts, U-bolts, clamps and the like. Items stocked at Sears, Ward's and
neighborhood urban hardware stores generally go only to 3/8 inch sizes, and are cheaply
made. For example, a 3/8 inch eyebolt, of the kind having a formed, unwelded eye, if
used at the top of the vertical plank mentioned above, could easily unwind and drop the
load on the ham below at the boat winch. Where does one get better hardware? Try industrial
suppliers, such as McMaster-Carr Supply Co., 2828 North Paulina Street, P.O. Box 4355,
Chicago, Illinois 60680.
An item not as widely known among amateurs as it should
be is the screw anchor. This is a long rod with eye at one end and an auger plate at
the other, by which it is screwed in to the ground. A common size is the Hubbard 7526
or Chance 6346, 66 inches long, 3/4 inch rod, 6 inch blade, which sells for about $5.00.
Fully screwed into good soil, these withstand 4500-pound pull. Many varieties are made:
swamp anchors with blades 15 inches in diameter; rock anchors, etc. A well-stocked supplier
is Graybar, with warehouses in most U. S. cities. Graybar also has excellent ground-rods
- not as cheap as you see in radio stores - but better. Typical sizes are Hubbard 9348,
5/8 inch in diameter X 8 feet, or Hubbard 9,450, 3/4 inch X 10 feet. They even make one
(No. 9697) 1 inch in diameter X 40 feet long. Graybar also stocks clips, clamps, thimbles,
arming bolts, eyebolts, shackles, etc.
If the terms used above, and other such
as: "gin pole," "tag line," and" come-along" are unfamiliar, some preliminary reading
or talking with persons having experience in rigging, is desirable prior to undertaking
a major antenna project. Alternatively, there are people who, for a fee, will take the
problems off your hands.
Rescue squads or fire departments which accept public
contributions, can be helpful in raising antennas to the top of, say 60, foot towers
or poles. A local amateur made a $25.00 contribution (tax deductible?) and found willing
and effective cooperation.
Raising a tower with a crane can be dangerous, though
it is common procedure commercially. A 130 foot, 24 inch tower was once being raised
with a 60-foot crane by wiring the tower base to its concrete foundation, and picking
up the tower by attaching the crane hook below the tower center. The base temporary wires
failed. The tower base whipped sidewise and killed a rigger instantly.
the most experienced ham riggers, W3MSK and W3GRF, prefer to assemble towers such as
the AB-105 vertically in place by carrying up pieces, bolting them on, climbing up to
the next level, etc., using a light gin pole and ground helpers to pull up materials
The reason why self-supported high towers are rare compared to guyed
towers, is that they cost several times as much. (The next installment will appear in
an early issue.)
17 "Median Signal Power
Required for Reception of Radio Transmissions in the Presence of Noise,"
Technical Report 5, U.S. Army Radio Propagation Agency . .June, 1961.
* Costs in parentheses are used for apportionment purposes in
construction of Fig. 6. Total costs in all cases are taken from the left ordinate.
Posted February 28, 2014
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