As Tim noted I built the K3LR version of the 3 element parasitic array
this past fall. I used my original and existing shunt fed tower as the
driven element. It is switchable in four directions and I have an
extensive radial system (120 radials)under each element.
My initial assessment indicates a forward gain of around 5 dB and a
minimum F/B of 25 dB. It performs very well from out here in fly over
country.
I am in the process of completing a paper detailing my construction and
experience with the array and will also be presenting this at the Dayton
Antenna Forum on Friday of Dayton.
I have spent the past 10 years improving my 160 meter RX systems here at
W5ZN and had reached a point of needing to improve the TX system beyond a
single shunt fed tower. I struggled with whether to build a stand alone 4
square for 160 meters however the appeal of the parasitic array is that I
could use the existing shunt fed tower to support the T elements without
having to erect an entirely new mechanical structure for a 4 square while
achieving basically the same result. I have been pleased with the
performance for the short few months it has been in operation.
Obviously it doesn't provide the broad bandwidth a 4 square does. I have
about 40 KHz between 1.5:1 points however in reality a 4 square has
limited bandwidth since beyond the resonant points you dump a lot of power
into the dump load!
73 Joel W5ZN
I have been using a 3 element parasitic vertical beam on 160 for at least
20
years. K9CT, NR5M, AA1K*, VE3EJ and most recently W5ZN have the same
array.
It has instant switching in 4 directions.
With 4 parasitic cut directors around the center driven element tower - it
is pretty easy to get over 5 dB of forward gain (over 40 KHz wide) and
over
30 dB of front to back (over a narrow bandwidth). Easy driven element
match
with an L network at the base. Each parasitic has 3 modes. Director,
Reflector or float. Going from directional to Omni is easy as well.
*AA1K has an additional director toward Europe. So he has 4 elements!
As was pointed out - the magic in any vertical antenna is all about the
radials. Each parasitic wire (4) and the center driven element tower has
120
radials that are 130 feet long (unless they cross the junction bus). I
have
67,000 feet of radials under my 160 array.
I use a 120 ft 24 inch face solid leg tower as the driven element. The
tower
sections are welded together to decrease any joint loss. The parasitic T
wires are #12 Copperweld.
The last three editions of the Low Band DXing book describe this antenna
in
the Yagi chapter.
73
Tim K3LR
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of W7RH
Sent: Tuesday, January 30, 2018 11:39 PM
To: Topband
Subject: Re: Topband: Adding a parasitic reflector to a vertical
I've played with parasitic elements in antenna arrays for almost three
decades and the current antenna system I have used parasitic elements
both director and reflectors.
With very careful tuning performance that of a all driven array can be
achieved. Tree is correct they due tend to be somewhat limited in in
bandwidth with relation to F/B ratio. Gain remains fairly constant.
The tuning procedure that Tree suggested is absolutely correct. You
detune all unused elements and adjust the center frequency of the
parasitic for best F/B one element at a time. Parasitic elements I might
add are no different than driven and must have extensive ground system
to be effective. No exceptions. You know you have right by F/B ratio.
You can go one step further and measure the actual antenna currents
which I have done. In my system the parasitic elements achieve 80-85% of
the theoretical current at the base.
de Bob W7RH
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