I regret that many of the notes, either to me or to the list, relative to
the Cushcraft published gain number, have taken an unfortunate turn. They
look like attempts to either support or detract from various
manufacturers, rather than appearing to be serious attempts to understand
spec sheet claims. I have no connection to Cushcraft, but there are
electrically correct and incorrect ways of criticizing the spec figures
published.
1. A blank dismissal of the 13.5 dB gain number (and its allies in the
list) is not correct. Although stated in dB, the number is possible when
interpreted as gain over ground in dBi. A 3-element Yagi may achieve this
number if well designed. Although very difficult as an engineering
exercise, incorporating this gain into a multiband antenna is not
impossible.
2. A dipole at 1 wl up over ground has a gain of between 7.5 and 8 dBi.
This is the frame of reference for determining the added gain of the Yagi
specifications. Let's call the difference between the Yagi and the dipole
about 6 dB. That is also feasible and sensible.
3. Translated into free space, the numbers would read about 2.1 for the
dipole and about 8.1 for the Yagi. I have presented models for Yagis that
can achieve these free space numbers and more. They are not part of the
Cushcraft design, but they do establish the numbers as high, but not
beyond belief.
4. Older 3-element Yagi designs largely aimed for gains in free space or
7 dBi and a little more. Perhaps we need to review a number of the
3-element designs in K6STI's YA to get a sense of what is possible now
that optimization of design is more readily feasible.
5. Some of question that we might relevantly pose to the spec sheet
involve the maintenance of that gain--or something close to it across the
amateur bands in question. Also, we might want to ask how the
front-to-back ratio varies across the band. Are the numbers sharp peaks
good for only a narrow bandwidth or is the curve smooth across the bands.
6. We can also raise questions about the methods used to derive the
figures. Are they straight modeling numbers, or are they translations of
range numbers into dBi? In either case, we can ask questions about the
methods used. For greatest accuracy, models need to be complete; yet
their are limitations with every NEC-based software package that may
defeat absolute modeling completeness. Likewise, range specifications are
needed in order to understand the basis for the numbers.
7. There are other anomalies in the specified numbers. The chief example
is that each gain figure was taken at a different physical height in order
to specify the height in terms of wavelengths up. This leaves hams, who
tend to mount multi-band beams at only one physical height per beam, with
questions about performance of the beam on bands where the height deviates
from the specified height.
8. Compared to existing multi-band Yagis, the numbers appear very
optimistic. I do not distribute numbers of commercial beams I have
modeled, because in all cases, there are remnant questions I have about
the adequacy of the models. However, in general, the best 3-element
multi-band models I have made show afree space gain of about 7.5-7.8 dBi
on 20. This is shy of the Cushcraft numbers. On higher bands, I have
sometimes gotten a bit over 8 dBi, which I attribute in part to pattern
reinforcement by elements on other bands as well as the configuration of
elements on the band in question.
9. Questions that some folks have raised have attempted to connect
impressions of Cushcraft's past performance with impressions of what is
electrically sensible. These are two separate questions types, and should
be separated. Belief in the claims--or disbelief--based on company
performance is a consumer question. What is electrically possible and
feasible is a wholly separate question. We all at this point simply lack
the data on the actual performance of the antennas to make any sort of
connection in order to answer the question: will the antennas live up to
their claimed performance?
10. Correlation of modeled data and range data is under active study in
several quarters. It may one day be possible to see performance
specifications in QST if the details of using well-developed models (or
EIA standard range tests) as the basis for such specifications can be
worked out.
I regret the length of this note, but it is the result of some acrid
responses to a very small note I distributed. The question was why one
might use the 9-element beam in place of the 7 element beam, and I
suggested the possibility (but not the actuality in the absence of
definite data) of going to a 9-element model for potential electrical
reasons. Some folks apparently thought I was supporting the manufacturer;
others thought I was against the manufacturer--or they wanted me to be for
or against the manufacturer. How anyone makes their buying decisions was
never elevant to my message.
I heartily approve the publication of specifications that are both
complete and well-documented with respect to the method of their
development. I have not seen such documentation (or completeness) in the
present case. Hence, what the antennas in question will or will not do is
mere speculation. The numbers are highly optimistic, but not impossible.
While we are waiting for testing or adequate modeling confirmation or
disconfirmation, we all might update ourselves on what is possible with
Yagis today and on how to correlate the various gain figures that emerge
from different contexts. Although winter is coming, I tend to prefer
light to heat when it comes to antennas.
-73-
LB, W4RNL
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