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Yagis and Quads

Tim Duffy, K3LR, needs no
introduction to readers of this
book. The way Tim runs his
Dayton Antenna Forum and his
own super contest station tells a
lot about the man. He is
thorough, well organized,
punctual, and a super host on
top of it all! No wonder there’s a
long line of operators who want
to operate from K3LR in the big
Tim is Senior Vice President
and Chief Technical Officer of
Fig 13-1—Tim Duffy,
Dobson Communications
K3LR, a well-known
Corporation (the 9th largest
Cellular Telephone company in contester and
superstation builder
the USA). He has been em­
from Western
ployed in the broadcast and
wireless engineering discipline
for over 28 years. Tim is a
graduate of The Pennsylvania State University and has
been a licensed amateur radio operator for over 32 years.
He currently maintains his large 9-tower, 12-operating
position multi-multi station in Western Pennsylvania and
experiments with large high-gain contest antennas. Tim
took the time to review this chapter on Yagis and Quads,
for which I am very thankful.

On the higher HF bands, almost all dedicated DXers use
some type of rotatable directional antenna. Directional
antennas produce gain to be better heard. They also show
directivity, which is a help when listening. Yagi and cubical­
quad antennas are certainly the most popular antennas on
those bands.
On the low bands, rotatable directive antennas are
large. Forty-meter Yagis and quads—even full-size—exist
in greater numbers these days. On 80 meters there are only
a few full-size Yagis and quads, while reduced-size Yagis
and quads are a little more common. They seem to come and
go, and are rather difficult to keep in the air. On 160 meters,
rotatable Yagis still belong to dreamland.
I have had the chance to operate a 3-element full-size

quad, as well as a 3-element full-size Yagi, on 80 meters, and
I must admit that it is only when you have played with such
monsters that you appreciate what you are missing without
them. The same is even more true on 40 meters, where full­
size Yagis and quads appear in ever-growing numbers. Until
the day I had my own full-size 40-meter Yagi, I always
considered 40 as my worst band. Now that I have the full-size
Yagi, I think it has become my “best” band.
Much of the work presented in this chapter is the result
of a number of major antenna projects that were realized with
the help of R. Vermet, ON6WU, who has been a most
assiduous supporter and advocate in all my antenna work.

In Chapter 11 on vertical arrays I discuss arrays of
antennas, where each antenna element is fed with a dedicated
feed line. During the analysis of these arrays I noticed that
elements sometimes exhibit a negative impedance, which
means that these elements do not draw power from the feed
line but actually deliver power into the feed system.
In such a case mutual coupling has already supplied
enough (or too much) current into the element. Negative
feed-point impedances are typical with close-spaced arrays,
where the coupling is heavy.
Parasitic arrays are arrays where (most often) only one
element is fed, and where the other elements obtain their feed
current only by mutual coupling with the various elements of
the array. To obtain the desired radiation pattern and gain,
feed-current magnitudes and phases need to be carefully
adjusted. This is done by changing the relative positions of
the elements and by changing the lengths of the elements.
The exact length of the driven element will not influence the
pattern nor the gain of the array; it will only influence the
feed-point impedance.
Unlike with driven arrays, you cannot obtain just any
specific feed-current magnitude and angle. In driven arrays
you “force” the antenna currents, which means you add (or
subtract) feed current to the element current already obtained
through mutual coupling. You could, for example, make a
driven array with three elements in-line where all elements
have an identical feed current. You cannot make a parasitic
array where the three elements have the same current phase
Yagis and Quads

Chapter 13.pmd


2/17/2005, 2:49 PM