May 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Maybe I suffer
from cranial rectumitis at the moment, but I'm having a hard time with a statement made about coaxial feedline impedance,
to wit, "102-ohm line (52-ohm lines in series)." I must be missing something because I don't understand how placing two
52-ohm transmission cables in series results in twice the impedance. Aside from that, author John Avery presents an interesting
article on multi-impedance dipole antennas. Empirical data is presented on how the feedpoint impedance of a dipole varies
with distance above the ground. His results are very close to theoretical values which assumes non-sagging elements, perfectly
linear alignment, a perfectly conductive ground, etc. He then extended his investigation into 2-wire (4x impedance) and
3-wire (9x impedance) folded dipole antennas.
Closer Matching at Various Antenna Heights
By John D. Avery,* W1IYI
While looking at a chart in the ARRL Handbook that shows the impedance of a half-wave antenna at various heights above
ground, I began to wonder if a wire at my QTH would behave "like the book says." My location is at a lake shore, where water
and wet ground are always present, so any measurements could be made over a period of time without running the possibility
of a significant change in the electrical ground. ("Electrical" ground and the "surface" ground do not coincide - the electrical
ground is usually some feet below the surface, depending upon the characteristics of the soil and the radio frequency being
Fig. 1 - The solid line is a theoretical curve showing the variation in impedance for a single-wire half-wave
antenna at various heights above ground. The values for 2- and 3-wire folded dipoles can be expected to vary in the same
way. The small circles are experimentally-determined values for a single-wire antenna.
Fig. 2 - With three wires strung up in the air, one has the choice of connecting them as (A) plain dipole,
(B) 2·wire folded dipole, and (C) 3-wire folded dipole.
Fig. 3 - Constructional details of the multi-impedance antenna. Pine spreaders are used, boiled in paraffin
to make them water-resistant. The antenna wires (and jumpers) are connected to 1/4 inch brass screws in the end spacers
and in the Lucite terminal block. The terminal block carries a coax fitting - an open-wire line can be connected directly
to the brass screws.
The measurements were made with a single wire a half wavelength long, split in the center so that a 52- or 72-ohm coaxial
line could be connected. Two 60-foot towers were used to support the antenna, and a standing-wave bridge was available for
use in the coaxial line. The procedure that was followed was quite elementary - with a given coaxial line connected to the
antenna, the antenna was raised a few feet at a time until the minimum s.w.r. was indicated.
Starting with 52-ohm coax and the antenna, one foot off the ground, the s.w.r. was rather high, but as the antenna was
raised the s.w.r. dropped and was quite close to unity at around 35 or 40 feet. Substituting 72-ohm coax for the lower-impedance
line, the s.w.r. was higher. As the antenna was raised (now fed with 72-ohm line) the s.w.r. was dropping as the maximum
height of 60 feet was reached.
It is safe to say that practically everyone ignores the effect of "height above ground" on the impedance of an antenna.
This doesn't make any practical difference in many cases, but it can where you use a "flat" line and no means for adjusting
the match. This article tells how W1IYI didn't ignore the height factor, and how it led to some interesting results and
a slightly different concept in antenna design.
Having run out of height at my place, I managed to prevail upon a ham in Rhode Island who had higher supports to test
with a 102-ohm line (52-ohm lines in series), and he found the minimum s.w.r. to fall at around 90 feet.
The diagram in Fig. 1 shows part of the Handbook graph that started this whole thing, with the three experimentally-determined
points shown as small circles. Since they don't fall too far off the curve, they seem to prove that "the book is right."
This got me thinking about what might be happening to folded dipoles at various heights above ground. Since a two-wire
folded dipole shows a four-times step-up in impedance (and a three-wire dipole a nine-times step-up) I added these values
to the chart, on the left-hand side. A little study of this chart shows that, for low antenna heights (low in wavelengths)
such as one runs into on 75 meters, the first choice of antenna and feed line might not always be the best. For example,
a two-wire dipole only 25 feet off the ground should match better with 72-ohm coaxial line than with 300-ohm ribbon. A 3-wire
dipole 35 feet above the ground offers a better match for 300-ohm line than does the more conventional 2-wire folded dipole.
These statements are based on "electrical" ground, of course, a somewhat variable plane in most cases. Nine times out of
ten it can only be found by experiment.
The Multi-Impedance Dipole
In an attempt to make better use of this (to me) newfound knowledge, a simple spreader system was devised that would
permit changing quickly between two- and three-wire folded dipoles and a single-wire dipole. As shown in Fig. 2, the basic
three wires can be used in these three ways. The spreaders were made from soft pine turned down to size and then boiled
in hot paraffin. Fig. 3 shows some of the construction details. A center connecting block of 1/4 inch Lucite was built to
take terminals and a coaxial connector, for quick changing of the various feed lines. It is apparent from Fig. 2 that changing
from one antenna to another only requires changing a few jumpers, and perhaps disconnecting one feed line and connecting
With a system like this, it is not too difficult a task to find the best combination of line and antenna for the particular
height you have available. You will probably want to use the maximum available height for the antenna, so it isn't suggested
that you run the antenna up and down for a perfect match, although you may find the experiment interesting, as I did. For
any length of antenna there is one frequency at which the s.w.r. is a minimum, and the s.w.r. will increase slowly as the
frequency is changed in either direction. However, I noticed that the 2- and 3-wire folded dipoles, and the 3-wire dipole
of Fig. 2A, seemed to be "broad" in this respect and not at all critical.
Although it has been pointed out many times before, it is worth repeating here that you need suitable coupling at the
transmitter for each type of line. If you use an antenna coupler, small 2- or 3-turn links are adequate for coupling between
coupler and transmitter, but larger links may be required if no coupler is used. I use plug-in links and a variable condenser
in series with one side of the line. The largest plug-in link is 12 turns.
Right now I am using the antenna of Fig. 2A, fed with 52-ohm coaxial line. Local and DX results on 75 have been very
This is all that is required for a 20-meter antenna in which several different values of impedance can be obtained. An
antenna for a lower-frequency band would require only more wire and more intermediate spacers.
Posted April 28, 2016