Station Design for DX - Part II
October 1966 QST Article
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
I), economics and construction
II), station configuration and receiver topics
III), and propagation quirks and operating tips
Station Design for DX - Part II
Part II - Economics of Station Design and Construction
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
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.
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,
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 17.2 db.
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
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
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":
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,
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 good value.
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.
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.
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.
Locally, 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 as required.
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,
* 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