Topband: Adding a parasitic reflector to a vertical

[Joel Harrison]

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 at 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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