Practical_Antenna_Handbook_0071639586

C h a p t e r 1 3 : C u b i c a l Q u a d s a n d D e l t a L o o p s 317
important choices to make early on because, as a general rule, in neither Yagis nor
quads do maximum forward gain, F/B, F/S, and bandwidth occur at the same dimensions
or frequency.
With the availability of good modeling software, we can get at some of the comparisons,
but the ultimate decision as to which type of antenna is “better” will depend
on how these various parameters are prioritized. To expose some of the tradeoffs, the
author recently modeled (for 15 m) a quad loop against a dipole and then a two-element
quad against a two-element Yagi using EZNEC 5+. All were fed and oriented for horizontal
polarization. Subject to the assumptions in the author’s model for each antenna,
the results for a single element indicate:
• In free space a single-element square loop exhibits about 1.5 dB better broadside
gain than a single bent dipole whose center has the same orientation as the fed
leg of the loop. But since a bent dipole has lost about 0.5 dB of broadside gain
compared to a standard l/2 dipole that is horizontal across its entire length, the
loop is only about 1 dB better than a conventional l/2 dipole in free space.
• Compared to a conventional horizontally polarized l/2 dipole at the same
height above “average” ground as the top (horizontal) leg of a loop fed at the
center of either the top or bottom leg, the loop exhibits lower broadside lobe
gain until a height above ground of about 0.67l. At that point the loop becomes
better—and remains that way through at least a height of 2l. The differences
throughout that range are on the order of 0.5 to 1 dB. See Fig. 13.2.
• Throughout the entire range of heights from l/4 to 2l over average ground, the
dipole has a slightly lower elevation angle (angle of peak broadside radiation).
This is understandable, since approximately half of the RF energy feeding the
loop is radiated by the lower bent dipole, thus contributing some higher-angle
radiation to the loop’s overall pattern. In fact, careful examination of Fig. 13.2
reveals that the elevation angle of the quad loop’s peak radiation is the same as
that of the dipole when the midpoint of the quad (halfway between the top and
bottom horizontal wire segments) is at the same height as the dipole—i.e., when
the top wire in the quad is l/8 higher than the dipole. Another way of saying
this is that the quad is a two-element array and its net ground reflection factor
is based on the average height of the radiators in the array—i.e., the vertical center
of the array.
At first blush, a two-element quad does enjoy about 1.2 dB forward gain advantage
over a two-element Yagi at the same design frequency (in this case, 21.2 MHz) when
both are modeled with #12 wire for their elements. It’s possible to tweak the Yagi dimensions
to reduce that advantage to somewhere between 0.5 and 1.0 dB at the expense
of lower F/B and F/S ratios for the Yagi. However, in addition to element spacing and
lengths, element diameter is also a factor. In particular, a “practical” rotatable Yagi will
have elements of aluminum tubing instead of #12 wire supported by insulated spreader
arms. This improves the bandwidth of the Yagi for a given boom length, all other factors
being the same. Thus, after a few hours at the computer with EZNEC 5+:
• A two-element Yagi modeled with either #12 copper wire or 0.75-in aluminum
tubing and 0.16l spacing (roughly 7½ ft) between reflector and driven element
exhibited 6.5 dBi (decibels relative to isotropic) forward gain and more than 10 dB