As I recall, when you use the inverted V configuration in either two half
waves in phase or a double extended zepp, you do not end up realizing the
gain at productive angles that you would expect from a flat top. A few years
ago I modled the effect of a flat top versus inverted v on a double-extended
zepp for 20m and what I found was that the gain in the V configuration was
not where I expected it (at the desired takeoff angle). Since I had been
working with a flat top, I was looking at around 20 deg. The predicted gain
was exactly as expected. I did nothing more than remodle with a "drooping
dipole" and at 20 deg...the gain was gone. I then did a 3D model and sure
enough, the right amount of gain over dipole was there, but at a MUCH
higher takeoff angle.
So I would advise caution in assigning increased performance to longer
center fed wires over a dipole, IF you change the configuration from a flat
top to an inverted vee. The gain will be there, but not necessarily at the
desired takeoff angle.
I then proceeded to model a 130' center fed wire, flat top versus inverted
vee. No matter where I looked the flat top produced more gain at a lower
takeoff angle on any frequency that was at least 2X the fundamental. I think
the problem is that the "two half waves in phase" aren't "in phase" any
longer once you move from flat-top to inverted vee.
73,
...hasan, N0AN
----- Original Message -----
From: <n4kg@juno.com>
To: <TOWERTALK@CONTESTING.COM>; <hurl8957@uidaho.edu>
Sent: Thursday, March 28, 2002 8:08 AM
Subject: Re: [Towertalk] I need to be louder on 40 and 80.
> The SIMPLE solution for 80M is an inverted vee under the TH7.
> If you feed with ladderline, you can also use this on 40M.
> You may want to supplement this with lower dipoles (50 ft)
> for close-in coverage if you can find a place to mount them.
>
> Note that on 40M, an 80M (horizontal) dipole acts as two
> half waves in phase which provides ~1.8 dB gain over a
> dipole but narrows the beamwidth to ~50 degrees with
> DEEP NULLS off the ends.
>
> The HF2V plays well on 40M where it is full size, but SUCKS
> on 80M where it is BASE LOADED, the LEAST EFFICIENT
> method of loading short antennas. I like the low cost ($160)
> MFJ 1792 80 / 40 vertical which is full size on 40M and has
> an isolating coil that acts as more efficient TOP LOADING
> on 80M in conjunction with a top hat. Bandwidth is restricted
> as for any loaded antenna and the maximum power rating is
> reduced to 1000W on 80M.
>
> For 40M I *highly recommend* matching the Boom of your TH7
> as a rotary dipole. KILLER ANTENNA. See below.
>
> Tom N4KG
>
> 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, 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 (10 / 23 / 97)
>
> On Wed, 27 Mar 2002 "Chris Hurlbut" <hurl8957@uidaho.edu> writes:
> > Hi everyone,
> > Maybe some of you can help me out. I'm looking to improve the 40
> > and 80
> > meter signal here at the University of Idaho, W7UQ. Right now the
> > antenna
> > is a ladder line fed inverted vee wire of unknown origin, up about
> > 50 feet
> > on the side of a wood chip silo. I've been thinking up ideas, but
> > I'm not
> > really sure what would work out best. Maybe some of you out there
> > have
> > experience with one or more of these and could help me out. Here is
> > what we
> > have here at W7UQ:
> >
> > On top of the powerplant on campus is our tower with a TH7 on top.
> > I am
> > guessing the tower starts at 50 feet, and is 50 feet tall. This
> > puts the
> > TH7 at about 100 feet. There isn't much room to stretch out wires
> > or
> > anything on top of the tower that would go out.... but something
> > might be
> > worked out with the powerplant people. Just below the TH7 is a
> > random wire
> > that seems to be worthless, but it is very large. I've never used
> > this
> > antenna effectively on any band. My goal is to be loud for DOMESTIC
> > contests... such as NAQP, SS, and the sprints. Very rarely would I
> > ever do
> > a DX Contest seriously from here. There is limited space, but a
> > wire
> > pointed east/west is doable (ie dipole, etc).
> >
> > I have had a few suggestions already about what might work, but I'm
> > not sure
> > if some of them would be worth the trouble. On the other hand, they
> > may
> > turn out to be great. Here is what I have thought up, or people
> > have
> > suggested:
> >
> > 1) Loading the tower for 80 meters, and hanging radials off the side
> > of the
> > silo. Sounds like a great idea to me, just not sure if the
> > powerplant folks
> > would go for it. Would it be THAT much better than a dipole? What
> > about
> > effectiveness in a domestic contest setting? Would it work at all
> > with 50
> > feet of tower and a tribander? Hanging radials?
> >
> > 2) Loading the guy wires as a sloper array if they're long enough,
> > shorting
> > at the breaks in them. Not sure how this would work, but it sounds
> > cool.
> >
> > 3) Dipole on each of 40 and 80, up as high as I could get them.
> > (Seems like
> > the best to me?)
> >
> > 4) Other wire type inverted vee array such as a double extended
> > zepp. Maybe
> > it has more gain than a dipole?
> >
> > 5) HF2V butternut vertical. We have one of these in the shack +
> > coax ready
> > to go up. Aren't they supposed to be ground mounted with a ton of
> > radials?
> > Not sure we could get it on the ground, or up in the air with enough
> > wire
> > for it to work.
> >
> > 6) 2 element 40m yagi fixed to states (I WISH! heh) But maybe a
> > wire yagi
> > with close spacing? It would have to be pretty close spacing... no
> > more
> > than about 12 feet I think, if that much.
> >
> > I really like the idea of the tower turned into a vertical, but I'm
> > not sure
> > if hanging radials would even work? I'm no antenna expert. Dipoles
> > seem to
> > work OK, at least better than some non-resonant wire.
> >
> > Any help would be appreciated, and it might even get you an extra
> > mult in
> > NAQP :)
> > Thanks,
> > Chris KL9A
> > kl9a@qsl.net
> > http://www.qsl.net/kl9a
> >
> >
> >
> > _______________________________________________
> > Towertalk mailing list
> > Towertalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
>
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