[TowerTalk] Cushcraft X7 (long note, delete if uninterested)

[L. B. Cebik]

Mike,

I cannot speak to any particular antenna, but I can speak to gain in
general.

A 3-element Yagi (monoband) will run between about 7 and 8 dBi free space
gain, with good front-to-back.  It is possible to squeeze about 1/2 dB
more gain than the peak, if one let's the F-B go to pot.  This is for boom
lengths on 20 meters of from 16 to 24 feet, the longer the boom (with
element placement changes) the higher the gain.

By very careful selection of height, for example about 5/8 wl, 1.12 wl,
etc., one can get from almost any horizontal antenna (except W8JK and
similarly phased antennas) gain peaks.  These get smaller relative to gain
nulls as the height is raised beyond about 1.5 wl up.  I did some work for
Communications Quarterly some years ago showing the relation of height to
gain, F-B, feed Z, and TO angle for a wide variety of antennas.  Now at a
peak height,  an antenna might show up to 6 dB gain over real ground
realtive to its freee space gain, although at minima heights, it might
show only 5--for the sort of standard average of 5.5 we think of when
comparing models of dipoles in free space vs. at height over ground.
Hence a dipole might show on average above 1/2 wl up a gain of around
7.6-7.7 dBi, with peaks above 8 dBi and minima around 7 dBi.

By choosing a peak, one can apply the 6 dB figure to the monoband 8 dBi
free space gain and get up to 14 dBi over ground at some specific height
for some specific target design frequency.  Using high quality ground
helps a smidgeon in deriving such a figure.

More common would be a basic free space figure in the 7s (dBi) plus about
5+ dBi for height for a total above 12 and a half or so dBi plus for
common types of 3-element Yagis using 16-20' boom lengths (counting only
the elements in a multi-bander that apply to a frequency).

However, let me make it a little more complex:  under certain conditions,
the presence of elements for other bands can slightly increase gain on a
given band relative to what the antenna for that band would give in
isolation.  I ran into that phenomenon when I worked out the staggered
independent interlaced beams.  The most I obtained from models that did
not break the limitations of NEC-4 was about a half dB extra.  With some
very close spaced elements on NEC-4, one can get false indications of
extra gain that is not there (as verified by putting the same model on
either old or new MININEC in free space).

Notice that all gain figures I am citing are in dBi.  Except for slight
differences in TO angle, at any height where the Yagi gain is higher than
average for peaks and nulls, the dipole gain is also higher, so the net
difference remains the same (approximatley).  Yagis do show a lower TO
angles at lower heights (up to 1+ wl) than dipoles by just a bit.

So, it is possible to develop a model that shows 13.5 dBi gain at some
height with 3 elements or perhaps 3 elements for that band with other
elements in play.  However, I would not normally expect such a figure to
play all across any band.  Nor would I expect the antenna to outperform a
dipole by more than its basic design allows, given constraints imposed by
boom length, etc.

As modeling becomes more standard among many manufacturers, you can expect
to see more advertising in terms of dBi.  First, any such figure should be
listed as dBi and not just dB.  Second, unless free space is specified,
the figure should be accompanied by the conditions under which it was
obtained, including antenna height, type of ground (and ground calculation
system used by the program), gain across a relevant bandwidth (like 20
meters), and front-to-back (or -rear, treated either as worst case rear or
as averaged rear).  Third, the program from which such figures are
obtained should be named.  Fourth, such figures in dBi should not be
backed out from range testing, which yields a comparative figure in dB to
some standard antenna, unless the complete calculation set is shown.

Anything less is <cum grano salis>  (with a grain of salt).  I suspect
that the best we can do is develop a good sense of what good antennas show
in dBi, and then some practice in what that means relative to a real
dipole of similar construction at the same height so we can instantly get
the difference.  The old rules of thumb apply no farther than the thumb
these days, but we can get our whole hands on the gain picture.  A good
place to start is the collection of Yagi designs in the K6STI program YA
that accompanies the ARRL Antenna Book.  We can play with those monoband
figures for antennas of different length and element numbers, comparing
them to dipoles in free space to get a sense of the differences we might
see in operation.

Some manufacturers do tell us how they get their numbers--or will tell us
in detail if we ask.  That is a big help.  But it usually means going
beyond a small ad that simply announces a number.

I hope this is useful to some just getting involved in the numbers game.

-73-

LB, W4RNL


L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html




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