January 1947 Radio News
[Table of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Something about the moniker "plumber's delight" for
homebuilt antennas always appealed to me. In the early days of radio it referred to antennas built with
threaded galvanized or soldered copper plumbing pipe or threaded black gas pipe. Today, it also includes
structures that incorporate sections of cemented PVC pipe (with wire elements strung along or within
them). A major issue with using threaded pipe is the potential for passive intermodulation products
(PIMs) being generated at the dissimilar metal junctions
(various degrees of oxidized metals). With
as spectrally 'dirty' as many transmitters as there were in the day, PIMs probably paled in comparison
to what was spewed as a matter of course. The presence of PIM products, unless severe, is not
usually noticeable in CW or phone operations, but with digital communications
(very common in modern Ham radio), the bit error rate (BER) can
be profoundly impacted by them.
Note: Thanks to Lynn L. for reminding me that the original use of "plumber's delight" applied
to antennas where the elements are physically (electrically) attached directly to the main mast (as
opposed to being insulated from it).
New Parasitic Beam Design
By R. G. Rowe, W2FMF
Design characteristics of a 4-element, close-spaced antenna array
featuring novel adjustable element length. Data applies to arrays for 28 mc. and up.
Table 1. Formulas for calculating the various element lengths (shown in Fig. 1) for
The following article describes a four-element, ten-meter, close-spaced array based upon the Plumber's
Delight, but having a distinctive feature in the way of adjusting element lengths not heretofore disclosed
to the best of the writer's knowledge.
While the design principles are not limited to any practical number of elements or element spacings,
from a mechanical standpoint they can be best applied to arrays for 28 megacycles and higher, due to
the relatively short element lengths required at these frequencies.
The novel mechanical design arises from the method of staggering quarter-wave sections of each element
along the central carrying tube, as shown in Figs. 1 and 3, thereby permitting simple adjustment of
element length from the center of each half-wave section and eliminating the need for telescopic sections.
The protrusion of the short length of the butt end of each quarter-wave section does not deleteriously
affect the gain or pattern of the array. The beam is made entirely from aluminum, with the exception
of one small piece of insulation in the "T" matching section, later to be described. The metal frame,
elements, matching section, and feed line are all electrically connected, permitting a single, permanent
ground connection to afford protection from lightning and static discharge.
Fig. 1 - Diagram shows how the 12 foot, 2 inch o.d. aluminum tube is drilled to support
the various antenna elements. Constructed unit is shown in Fig. 2.
The central carrying structure, or "frame," for the particular ten-meter beam illustrated is made
from a 12 foot length of 2 inch o.d. aluminum tube with a wall thickness of one sixteenth inch. An inspection
of Fig. 4 shows how the tube is drilled at the spacings indicated in Fig. 1. A "stagger" distance of
1.5 inches was used in this particular embodiment, with 0.5 inch diameter thin-wall elements. The stagger
distance should be minimized as much as possible, becoming progressively critical at higher and higher
frequencies. The frame holes should provide a snug fit for the butt ends of the elements, yet permit
them to slide when the clamping bolts are loosened. Small holes through the bottom of the frame, indicated
in Fig. 4, are drilled to permit the insertion of the so-called "J" bolts. These bolts may be formed
from eye bolts, "V" bolts or bent up from straight bolts. Tightening the wing nuts securely locks the
elements in place, whereas loosening them slightly permits simple adjustment of the length of each quarter-wave
The quarter-wave sections may be scribed or otherwise ruled off at their butt ends to facilitate
reading the over-all length. In the illustrated beam, short pieces of friction tape were wound around
each element near the butt end at a predetermined, measured length from the tip. Thus, by measuring
the short distance from the frame to the tape, it is possible to balance the length of each section
easily as well as to mentally calculate the over-all length rapidly, without using a long rule or tape
to measure the tip-to-tip length. All adjustments and measurements may be made at the center of each
half-wave section, greatly facilitating installation and tuning.
Fig. 2 - Photograph shows close-up view of "T" matching section.
Fig. 3 - 10 meter antenna constructed by author. Main support is made of 1/2" galvanized
Fig. 5 indicates a possible method for mounting the frame at its mechanical balance point on a short
length of grooved 2 x 4 with "U" bolts. A pipe flange is screwed to the underside of the 2 x 4 to take
a short 12 inch length of pipe, the i.d. of which just will slip over the o.d. of the supporting pipe,
providing a bearing for rotation. In the illustrated arrangement, a 1/2 inch galvanized water pipe is
used as the supporting pipe, for inasmuch as the array is close to the chimney bracket and the supporting
pipe is short, greater rigidity is not required. Many other ways to support and to rotate such arrays
have been fully described in the literature or will suggest themselves to the ham. In the illustrated
mounting the tube frame was grounded to the pipe flange by a short length of copper braid, inasmuch
as the supporting pipe and chimney bracket are permanently grounded to a vent pipe in the roof of the
house for lightning protection. A potential method for rotating the array is to bring the base end of
the supporting pipe through the roof. A section of small o.d. tube may be inserted inside the supporting
pipe, mechanically secured to the 2 x 4 and provided at its lower projecting extremity with a wheel
or lever for rotation.
For this type of beam the "T" match, delta match or folded dipole feed is most easily adaptable.
However, by drilling oversize the frame holes which carry the driven element, insulated bushings may
be inserted at this voltage node to insulate each quarter wave of the antenna section from the frame
so that other types of feed may be used.
The "T" match shown in Figs. 2 and 6 was selected for this particular application and the "T" section
is made up from two 33 inch lengths of 1/2 inch o.d. thin-wall aluminum tube, the same diameter as the
elements. A small block of 1/2 inch thick bakelite serves to mechanically connect and electrically insulate
the inner fed ends of the "T." The block is so dimensioned that the stagger distance of 1.5 inches is
maintained in the "T" section and is supported from the tube frame for rigidity. The shorting straps,
which are adjustable along the length of the elements and "T" section, are formed from one sixteenth
inch thick aluminum sheet, 1 inch wide. They are provided at each end with a hole for the passage of
a bolt and wing nut to tightly clamp the tubing. By loosening the two wing nuts on each shorting strap,
each strap may be slid in or out along the length of the element to minimize standing waves on the feed
line, after the beam has been tuned by anyone of several tuning procedures outlined in the various antenna
handbooks. 300 ohm twin-lead type feeders are used with the illustrated beam and connected as shown
in Fig. 6.
Fig. 4 - Construction details for mounting and fastening the antenna elements.
Before final installation of the illustrated "T" section made from aluminum tube and aluminum shorting
straps, a temporary "T" section using No.8 copper wire was used with excellent results. If a wire "T"
section is used the vertical spacing may remain 4 inches and the distance "T" determined by noting the
standing wave ratio.
A convenient, qualitative check for standing waves on the twin-lead type line may be made by running
a neon bulb along the line for a distance of some ten feet, on ten meters. If the bulb brilliancy remains
reasonably uniform, the line is reasonably flat. If the brilliancy varies, the distance "T" should be
readjusted. The total distance "T" for the separation of the shorting straps will be somewhere between
40 and 60 inches. While the illustrated beam has been adjusted for maximum forward gain and minimum
standing wave ratio at 29 megacycles, it has been used without readjustment from 28.1 to 29.4 megacycles.
At these frequency extremities the standing wave ratio becomes appreciable and coupling to the final
tank must be altered. However, with 600 watts input to a BC610E transmitter the 300 ohm twin-lead does
not break down.
The staggered element design is not limited to the particular element or frame sizes shown. In the
illustrated array the 1/2 inch thin-wall aluminum elements seemed rather light and flexible. Some lengths
of thick-wall aluminum pipe were found in a war surplus stock, the o.d. of which would drive fit the
i.d. of the 1/2 inch elements. Therefore, 2 1/2 foot lengths of the pipe were driven into the butt end
of each of the quarter-wave elements.
In working with arrays using a metal center structure of appreciable diameter, the writer has noticed
that the popular formulas for calculating the tip-to-tip element lengths seemed to give elements which
were too short according to maximum forward. gain measurements. It has been determined roughly that
by adding the width of the frame to the calculated lengths such an effect is obviated.
Fig. 5 - An ordinary two by four is used to mount the main frame assembly to the
Accordingly, it is to be noted in Fig. 1 that the dimensions for D 1, D 2, A and R, calculated from
Table 1, are measured from the outer wall of the 2 inch tube frame to the element tip. The formulas
give the dimensions for each quarter-wave section of each element, which is the measured distance from
opposing sides of the tube frame to the tip of each corresponding quarter-wave element, or the distance
which each quarter-wave element projects from the side of the frame. The total over-all cut length of
the various quarter-wave sections, to permit adjustment from 28 to 30 mega-cycles, is:
D1 & D2 = 8' 2"
A = 8' 6 1/4"
R = 9' 0"
It will be obvious from this that this beam requires for the elements, 4 lengths of tubing 8' 8"
long, 2 lengths 9' 0-1/4" long and 2 lengths 9' 6" long; for the frame, 1 length of larger o.d. tubing
12' long; and for the "T" section, 2 lengths of tubing the same diameter as the elements, 33" long.
The additional lengths are to take care of the portion through the center section.
Fig. 6 - Two views of "T" section show mechanical details of construction.
It will be appreciated that, by substituting a wooden supporting mast, this array may be oriented
to give vertical polarization. Further, with the array in the horizontal position; the feed line may
be carried parallel to the 2 inch tube frame either toward the front or the rear for any desired distance
and dropped down from that point. In this application it is to be recommended that the feeders clear
the tube frame by at least 3 to 4 inches.
In connection with feeding beam arrays some controversy seems to exist as regards the delta match
vs. the "T" or other matching systems. In considering the efficiency of a feeder system it may be stated
that the energy lost is equal to the energy delivered to the sending end of the line minus the energy
delivered at the load end of the line. At the lower frequencies, the energy loss consists mainly of
the 12R losses in the line, which manifest themselves as an unusable form of radiant energy,
namely heat. As the operating frequency is increased, however, another type of loss must be added to
the 12R loss.
Posted September 19, 2016