The Maco has a 16 foot boom and an advertised gain of 12.5 db, while the JO
GUNN has an 8 foot boom and an advertised gain of 14.5 db. I know there is a
lot of smoke and mirrors involved with advertised gains and so my question
is...is it possible for the JO GUNN to perform as well as the Maco antenna?
I would be sceptical. The fact that both manufacturers are quoting
gain in dB, without saying what it's in reference to (dBi for
isotropic, dBd for dipole) is a bit of a warning sign. Assume it's
dBi - the numbers come out bigger, and that's attractive in ads.
I'd be sceptical of the numbers offered for both of these antennas,
but I'd be quite a bit more sceptical concerning the JO GUNN.
It's probably useful to compare the design, and the numbers, with
what's known and published about various reference Yagi designs and
the results that can be achieved. I looked at some of the
documentation that Cebik (SK) published on his excellent web site.
The Maco is advertised as a "6-element" antenna, but the drawing
doesn't show a full set of six elements on either vertical or
horizontal polarization. I can't tell whether they omitted showing
some elements to leave the drawing less cluttered, or whether it's
supposed to be a "three elements vertical plus three elements
horizontal, equals six elements" design. It looks to me as if it's
essentially a Quagi antenna - a quad element in back (reflector?) plus
one or more set of linear elements in a Yagi-style arrangement.
At 11.5 feet, it's about a third of a wavelength long... and for any
antenna of this general type, the boom length is likely to be the
parameter which dominates the maximum gain figure you can achieve.
Cebik's figures indicate that well-designed quarter-wavelength-boom
Yagis typically deliver somewher around 8 dBi of gain, and one-half-
wavelength-boom Yagis are up somewhere around 10 dBi. Front-to-back
figures in the 25 - 35 dB range seem to be achievable. There's
usually a tradeoff in the optimization process... for any given boom
length, optimizing for maximum gain will usually cost you F/B ratio,
and vice versa.
I'd guess that it's *possible* that the Maco actually achieves 12.5
dBi of gain (at least on paper) at some frequency with its
one-third-lambda boom length, but that's more than I'd expect.
This assumes that the Maco has separate feeds for horizontal and
vertical polarization, and that it feeds only one at a time (e.g. via
a switch) and that there are no losses involved. If it's actually
feeding power to both polarizations at once (either linear or
circular), subtract 3 dB from the effective gain to a
linearly-polarized vertical or horizontal antenna at the other end.
The JO GUNN... well, I'm not sure just what sort of antenna it thinks
it is, other than "tries to be snazzy looking". It seems to be a
three-element short-beam antenna (at 8 feet it's less than 1/4
wavelength long), and yet it's claiming a 14.5 dB gain. This is
around 6 dB more gain than Cebik indicates is delivered by Yagis in
this boom-length class. The F/B ratio, "side rejection" and "back
rejection" are all extremely high (40 dB)... such high numbers suggest
that these are the best-looking numbers, taken from the deepest
rejection lobes of a mathematical model of the antenna in free space.
The GUNN is of much stronger construction and a smaller windload so I'd
rather put it up, but the short boom makes me wonder if the gain and
rejection numbers are suspect.
Well, before I trusted these numbers at all, I'd want to get a NEC2 or
similar model of the antenna from the manufacturer, and run the model
myself.
My guess is that the Maco folks *might* give you a model deck if you
ask nicely, and that the JO GUNN folks with either refuse or say "What
are you talking about?" or claim that their antenna incorporates
principles that cannot be accurately modeled.
--
Dave Platt AE6EO
Friends of Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!