BOOM MATCHING as a ROTARY DIPOLE on 40 / 30 Meters - N4KG
The loaded boom dipole trick works very well indeed! A 24 ft boom end
loaded by 20M (full size or trapped) elements is self resonant very near
40M ( resonance is not needed for efficient radiation). I have matched
my TH6 boom first for 40M and now use it on 30M where it has accounted
for >300 countries. Shorter booms will also work with slightly less
bandwidth.
The radiation resistance of a dipole depends on it's height above ground
so using someone else's exact matching system may not give identical
results unless the antennas are at the same height also.
A better approach is to describe the TECHNIQUE and make your own
matching system, tuned in place on the tower.
First, a matching ARM needs to be attached to the boom and brought
back to the mast. I like to use 3/4 inch CATV hardline as a semi-rigid
piece of tubing that can be easily bent and formed. (Besides, it's
free!)
I use a sloping feed, from near the 15M director back to the mast about
12 to 15 inches above the boom, and insulated from the mast by a piece
of slotted PVC over the aluminum tubing. I use 1/8 inch nylon rope to
lash the tubing/PVC/mast together. Smash the end of the tubing and
use a hose clamp to attach the arm to the boom. The arm may also be
suspended below the boom if the antenna has enough clearance
above the top of the tower. Either a sloping or parallel arm will work.
I like using an OMEGA match (see ARRL Antenna Book) because it can
easily match lower impedances up to 50 Ohms AND tune out the inductive
reactance of the arm. With this system, it is NOT necessary to find the
50
Ohm tap point (which can be a real pain!). If the beam is already on the
tower, place the arm attachment point out as far as you can safely reach.
The OMEGA match uses two capacitors, one from the arm to "ground"
(the center of the boom or mast, assuming a good boom to mast connection)
and another from the arm to the center of your feedline. The braid of
the
feedline is connected to the "ground"/mast/boom-center.
To determine the necessary capacitor values, I mounted two 300 pf
variable capacitors on a piece of plexiglas, with a plexiglas front, and
pointer knobs. A piece of 3" by 5" card (paper) is mounted behind the
knobs and lines drawn with calibration marks. (It helps to have a
capacitance or impedance meter available for calibration).
To match the arm, I mounted two receiving-type 300 pF variable
capacitors
on a piece of plexiglas and connected the stators together. A short wire
runs from this junction to the end of the arm where a hose clamp can be
used to make the connection. The rotor of one capacitor goes to the
"ground" connection at the mast or center of the boom. The rotor of the
other capacitor goes to the center of your feedline. The braid of your
feedline goes to the "ground" at the center of the boom or mast. Keep
these leads as short as possible.
The BEST way to tune the capacitors is with a battery powered
Antenna Analyzer. Your transmitter (at reduced power) with a helper,
two meter radio, and SWR meter (at the antenna) will also work.
It is a good idea to wear heavy rubber gloves and have good
communication (KEY, DONT KEY, etc.) with your helper.
Once the capacitor values are known, I make weather proof capacitors
from solid dielectric coaxial cables such as RG-8, RG-213, or even RG-59.
(Solid dielectric cables have much higher breakdown voltage ratings than
foam). Fifty ohm cables are approximately 30 pF per foot while 75 ohm
cables are around 21 pF per foot. To prevent arcing at the far end of
the
cable, I trim off 1/2 inch of braid and tape the end. The cable can be
coiled. I like to tape the coiled cables to the matching arm (isolated
from
the boom and mast). Be sure to attach the BRAID of the cable capacitors
to the matching arm. (This prevents arcing from the braid through the
outer jacket which has only a 600 V rating). The center conductor of the
shunt capacitor (cable) is then connected to the mast or boom center.
The
center conductor of the series capacitor (cable) is connected to the
center conductor of your feedline. Again, the braid of the feedline is
connected to the center of the boom (or to the mast). It helps to draw
a picture!
There are several advantages to matching the boom of a Yagi as a
rotary dipole compared to using an inverted Vee suspended below
the Yagi. One obvious advantage is that the antenna can be rotated to
maximize radiation in the desired direction. A less obvious fact is that
a flat horizontal dipole can have up to 3 dB more gain than an inverted
vee because radiation off the ends is minimized and interactions with
other antennas is minimized.
This technique of matching a Yagi boom as a rotary dipole has been
successfully copied by several others with good success. My own
systems continue to perform well after 20 years. Using various
rotary dipoles at 80 ft, I have worked over 300 countries on 40 meters.
Optimum heights range from 80 to 90 feet, where the radiation resistance
of a 40M dipole goes through a minimum, thus maximizing the current
(and GAIN) of the dipole. Because of this impedance / current / gain
relationship to height, further increases in height actually yield LESS
gain until approaching a height close to 1 wavelength (140 ft.) !
If you have followed me this far, have an adventurous spirit, and are
looking for a good 30 meter antenna in addition to 40M, I see no
reason that one could not put a second matching arm to the opposite
side of the mast and tuning that arm to match on 30M. If separate
feedlines are used, be sure to NEVER connect both feedlines to
separate radios at the same time!
Try it, you'll like it !
GL, Tom N4KG ( Originally posted October 1997 )
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