Hi Roger,
ONE of the truss wires can be fed but I see a few problems.
# 1 - You must insulate the wire at the mast
# 2 - The wire is fairly small in diameter compared to the boom
which results in a higher Q match (reduced bandwidth)
#3 - You are relying on a pressure contact between the thimble
and the supporting eye ( or whatever the attachment is)
For my TH6, I used non-conductive trusses (heavy duty
plastic clothesline...small philystran is another option)
and a sloping 3/4 inch aluminum tube (CATV hardline
with the end smashed and hose clamped at the 15M Dir)
Another option is to use either a sloping or parallel matching tube
suspended UNDER the boom. This will require that the
boom be 18 to 24 inches above the top of the tower.
My *guess* is that the truss wires can be left intact
above the boom.
K4RO has pictures of his sloping large copper wire feed and
matching unit on his web site www.k4ro.net/tcg/pix/40boom.html
73 / GL, Tom N4KG
--------- Forwarded message ----------
From: "Roger Borowski" <K9RB@bellsouth.net>
To: <n4kg@juno.com>
To: <towertalk@contesting.com>
Date: Wed, 15 Aug 2001
Subject: Re: [TowerTalk] BOOM MATCHING for 40 / 30 Meters (repeat post)
Tom, Do you think one could put insulators on the boom truss support
cables near
the mast and try feeding the cables as sloped "arms" by mounting a box
containing at first, a couple of variable caps, later to be replaced with
coaxial homemade caps of the appropriate value, on the mast up near the
attachment point of the boom truss support cables. I would attempt to
make the
box with a couple of feedthrough insulators as well as a couple of SO-239
connectors for the feedlines and match one side for 30M and the other for
40M. I
imagine I could also use fixed transmitting mica caps rather than the
coaxial
caps OR wind the coaxial caps in a loop and have them all contained
inside the
box?? Any suggestions on this / these proposed approaches from your
experience/s? Thanking you in advance....73, -=Roger Borowski-K9RB=-
BTW, I've got a TH11DX at 72 ft. and with their method of feeding the 6
element
center log section, the boom is quite "busy" with additional tubes and
insulators for the feed system, something the old TH6DXX and TH7's didn't
have...
----- Original Message -----
From: <n4kg@juno.com>
To: <TOWERTALK@contesting.com>
Sent: Wednesday, August 15, 2001 3:09 PM
Subject: [TowerTalk] BOOM MATCHING for 40 / 30 Meters (repeat post)
>
> 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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________________________________________________________________
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Juno offers FREE or PREMIUM Internet access for less!
Join Juno today! For your FREE software, visit:
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List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
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supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
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Problems: owner-towertalk@contesting.com
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