From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
if sending attachments: k1vr@gis.net
On Wed, 19 May 1999 07:03:46 -0400 "Bob Thacker" <k3gt@pgh.net> writes:
> Want to load the boom on a TH11 for 30, any suggestions?
To: K3GT
From: K1VR
Here's what I was able to produce from my files.
From: n4kg@JUNO.COM (T A RUSSELL)
To: TOWERTALK@contesting.com
To: <towertalk@contesting.com>
Date: 23 Oct 1997
Subject: 40M Boom Dipoles
............................................................
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 290
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, DON'T 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 !
Tom N4KG
==============================================
From: Doug - K1ZO <thecomshop@monad.net>
To: <towertalk@contesting.com>
Date: Tue, 06 Oct 1998
Randy, N1KWF and Doug, K1ZO spent a couple hours on the
tower feeding the TH6 boom at 93' for operation on 40
meters...and it worked! Randy later got the TL5 with one
call using 100 watts.
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