Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/bestant/best_ant.txt

Text File | 1996-06-30 | 22KB | 670 lines

     WHICH ANTENNA IS THE BEST

       R. P. Haviland, W4MB


I have always made a practice of log-
ging the information which comes
across in DX QSOs. Typically, this in-
dudes antenna type and height, and
rig type or power.
 Some years ago, a question of an-
tenna usage prompted me to do a
short analysis of the logs, to develop
antenna type and height data. Very
recently, a similar analysis by ZL1OI,
but of U.S. stations, was reported.
Also very recently, thoughts of a new
antenna system for W4MB prompted
an extensive review of log data. The
idea was to try to get an answer, or at
least an indication of answers, to typi-
cal questions, such as:

   Which antenna is the best?
   How important is height?
   How important is power?

This is a report of these studies.

Relative Antenna Usage

 The analysis started with a review
of usage, of the two studies previously
reported, plus the data from the latest
logs. The results are summarized in
Table 1.

 Results of the three analyses are
very similar, showing exactly the
same relative order of usage and sur-
prisingly small differences between
U.S. and DX stations. Very nearly
three-fourths of the stations use
beams, and one-fourth use simple an-
tennas. The Yagi dominates the beam
usage; almost three times as many as
its closest competitor, the quad. Verti-
cal antennas are somewhat more
popular than dipoles.

 The data at W4MB is primarily for
15-meter contacts, and is markedly
influenced by European practices,
since a majority of QSOs are with
these stations. Comments received
indicate that a major factor in the use
of verticals is space for installation.
This seems to be true for the suburbs,
where many of the verticals are at
ground level, and for the cities, where
antennas are at roof height. Other
comments indicate that the quad
would be more popular if space were
available, and that ready availability
of commercial triband Yagis is a factor
in their popularity.

 It will be noted that the difference
between any of the three usage fig-
ures and the average of the three is
small, at most just over 3 per cent.
This difference may not be significant.
For example, for the W4MB data, 3
per cent would amount to a ten- to fif-
teen-station change. However, there
is some reason to believe that some of
the indicated difference is real. The
last data are for a period of relatively
high solar activity, and reflects a
change in QSO area. For example,
the recent data include more USSR
stations, which are often users of
quads. As will be seen, the improved
conditions can be responsible for the
increased percentage of simple anten-
nas. No reason for the decrease in the
use of "other" antenna types has
appeared; these others include rhom-
bics, long wires, log periodics, and
SO on.

Signal Strength as a
Measure of Performance

 The only performance measure
available from the logs at W4MB is
the signal-strength report given to the
DX station. For the last five years, I
have taken some effort to make these
reports meaningful. The report is the
S-meter reading on my TS-820, with
enough attention being given to en-
sure that it was the maximum, inter-
ference-free signal. The meter calibra-
tion of the 820 was checked just after
purchase and found to be almost ex-
actly 5 dB per S-unit. The relative and
absolute values have been checked at
intervals since, most often by check-
ing the crystal calibrator signal, but
also by using a signal generator. Sig-
nal-strength-indication stability seems
to be excellent.
 In most radio circuits, signal
strength is a statistical variant follow-
ing a Rayleigh distribution.' How-
ever, this analysis is dealing with
many circuits and a combination of
factors, including transmitter power,
antenna design, and solar activity.
Thus, it seemed that the signal
strengths should follow the Gaussian
distribution fairly closely.t Also, it
seemed that the distribution data
could be used to check the results for
bias, since a systematic change would
change the distribution in some
respect .
 Accordingly, a section of the log
was examined for signal-strength
probability. The results are shown in
Fig. 1, the curve indicated by the Xs.
On this type of graph paper, a Gaus-
sian, or "normal," random distribu-
tion plots as a straight line. Since the
curve is not straight, the distribution
departs from Gaussian. However, it
should be noted that there are upper
limits, due to legal restrictions on radi-
ated power, and practical limits on
antenna height. These have the effect
of bending the upper part of the curve
downward, as observed. Neglecting

'Rayleigh distribution -- A mathematical state-
ment of a natural distribution of random
variables.

Gaussian distribution -- A distribution of ran-
dom variables comparable to that found in
nature, characterized by a symmetrical and
continuous distribution decreasing gradually
to zero on either side of the most probable
value.

this effect, the distribution below
about S7 is very nearly Gaussian.
 Also shown are points for two other
types of data, for contacts where
height was given, and where power
was given. (In QSOs, often only
power or type of transmitter is given).
These samples fall very close to the
upper curve. Overall, the data indi-
cates that the height and power data
are associated with about a one S-unit
stronger signal, as compared with an
average contact. The difference be-
comes smaller as signal strength in-
creases.
 Inspection of the raw data showed
a tendency for the QSOs where power
and height were given to be longer
than QSOs where they were not.
They tended to be more "rag chews"
than "hello-goodbye" contacts. Part-
ly, these occurred under conditions of
better signal or less QRM. Also, the
longer period gave a greater chance
of observing a short-term increase in
signal strength. Thus, the observed
difference in the three sets of samples
seems to be an observable fact, rather
than some fault of the method of
analysis.

Performance of
Simple Antennas

 The performance of simple anten-
nas -- dipoles and verticals -- was
considered first. For each value of sig-
nal strength, the percentage of sta-
tions using these antennas was deter-
mined. The results are shown in Fig.
2. The performance character is strik-
ing, and the trend is definite. Simple
antennas do get signals through, but
the signals tend to be on the low side;
markedly lower than the signals from
beams.
 The measured data can be approxi-
mated reasonably well by a simple re-
lation: for each unit increase of signal
strength, there will be about a five per
cent reduction in the number of sig-
nals from dipoles, and about another
five per cent reduction in the signals
from verticals; there will be a corre-
sponding increase in the number of
signals coming from beams.
 While these antennas tend to pro-
duce weaker signals, the performance
is not really that bad. Verticals, for ex
ample, account for about 18 per cent
of all antennas. Comparing this value
with the curve in Fig. 2 shows that the
average expected signal from a verti-
cal is nearly S6. For a dipole the
usage is 13 per cent, which intersects
the dipole curve at (18 + 13) or 31
per cent, again nearly S6. These are
perfectly respectable signals, some 25
dB above normal noise levels; not
broadcast quality, but excellent com-
munications quality. To the limits of
analysis accuracy, verticals and di-
poles, as used, give identical results.

Performance of
Beam Antennas

 For this analysis, the amount of
data restricted beam consideration to
only two types, Yagis and quads. The
percentage of these at each signal
level is shown in Fig. 3. This seems to
indicate a striking, indeed startling, re-
suit: as a family, Yagis seem to out-
perform quads. Over the range of S4
to S9 signals, those from quads are
nearly constant in percentage, while
over the same range, those from
Yagis increase in percentage, by
about 5 per cent use for each unit of
signal strength.

 Obviously, this is an important find-
ing. If it is really true, it could settle the
long arguments of quad os Yagi. Ac-
cordingly, some additional tests were
made. The first of these was to re- ex-
amine the source data for antenna
size. Since the usual quad has two
elements, any antenna with more
than two elements was considered a
"large quad." Also, since a two-ele-
ment quad is usually considered to be
equal to a three-element Yagi, de-
signs with four or more elements were
considered "large Yagis." While some
of the antenna notes recorded the
specific design, TA33, Th-6, and so
on, there were not enough recorded
as these or as mono-banders to per-
mit detailed evaluation.
 The results of this size analysis is
shown in Fig. 4. Here, the percent-
age of the smaller Yagis is plotted
first. The trend to increasing percent-
age with increasing signal still ap-
pears, but the larger Yagis seem to be
increasing at a faster rate. This sug-
gests that the apparent better per-
formance of Yagis as compared with
quads is partly due to larger antennas.
 A different way of looking at the
data is shown in Fig. 5. This is de
veloped from the number of antennas
of a given type producing a given sig-
nal, plotted as a percentage of the
total number of that type. It seems
clear that there are two main group-
ings, one for the simple antennas --
verticals and dipoles, and the other
for the beams -- the quads and
Yagis .
 On the average, the beam group
produces about 11/2 S-units, about
71/2 dB greater signal level than do
the simple antennas. Interestingly,
this is very nearly the theoretical gain
of three-element Yagis and two-ele-
ment quads. Also interestingly, the
beam group seems to produce a more
consistent signal: at low signal levels
the beams are about two S-units bet-
ter than the simple antennas.
 Data for the big antennas, the four
or more element Yagis and three or
more element quads, are also shown.
The number of such antennas is
rather small, so the points are rather
scattered. It seems to indicate about
one-half S-unit improvement in signal
as compared with the smaller beams.
Again, this is interesting in that it rep-
resents about the expected gain in-
crease.
 Plotted in this way, the type/signal-
strength data indicates that there is no
difference between quads and Yagis,
or between dipoles and verticals. In-
stead, the data suggests strongly that
the significant factor is the amount of
gain, rather than the type of antenna
which produces the gain, at least for
the types surveyed. However, before
considering this relative comparison,
factors common to all antenna use
should be looked at.

Effect of Antenna Height

 The early W4MB study included
evaluation of antenna height. This
was done again for this second study.
Results are tabulated in Table 2, and
plotted in distribution form in Fig. 6.
A small difference, up to about 10
per cent deviation from the average,
is found between the two data peri-
ods. This may be real, or it may be
due to "sampling noise." There are
some differences between the two
periods. The second covers a period
of improved propagation, which
would tend to increase the percentage
of marginal signal contacts, as ob-
served. The second period also in-
cludes a larger fraction of 10-meter
contacts, where height is known to be
less important.

 More important than the matter of
usage is the effect of antenna height.
To study this, signal reports were tab-
ulated for height blocks of 0-15 feet,
15-30 feet, and up to 120-240 feet.
The reason for this grouping choice
was the sometimes-used approxima-
tion that doubling the height will im-
prove the signal by one-half to one S-
unit. The results of this analysis are
shown in Fig. 7.

 While five height-groups are plot-
ted, only three curves are shown. The
data for the 0-15 height, shown by
Xs, follows a Gaussian distribution al-
most exactly. Data for the groups.
15-30 feet and 30-60 feet follow the
distribution fairly closely, but with a
different slope, and with a small in-
crease in average signal strength. The
change in slope indicates that the sig-
nal for these groups is more consistent
than that for the low group. The dope
change is also apparent for the
60-120 foot group, which shows a
further increase in average signal
strength .

 The 120-240 foot group seems to
fall along a line which is parallel to,
but above, the 0-15 foot group. A
check of the source data did indicate
that this high antenna group included
many simple antennas mounted on
apartment roofs. An attempt was
made to evaluate the effect of anten-
na type, but there were not enough
such contacts to be meaningful.

 Overall, it appears that high anten-
nas behave according to reputation.
There is a definite increase in average
signal strength, in the range of 1-2 S-
units. There is also good indication of
greater signal consistency; a high an-
tenna will make contacts, where a low
one may not.

 The first W4MB study did not eval-
uate power, but an evaluation was
done for this study. These data are
tabulated in Table 3, together with
the ZL101 data for comparison. The
influence of the one-package trans-
ceiver, of about 180 watts input,
seems clear. Comparison of the two
sets indicates a greater percentage of
high-power stations in the U.S.A.,
certainly in line with our reputation.

 There seems to be increasing use of
powers in the range of 1 to 30 watts.
It is most pronounced on 10 meters,
partly due to new commercial trans-
ceivers of this class, partly due to CB
conversion, and partly due to
power/band restrictions of some
license classes in some countries.
However, QRP operation is not re-
stricted to 10 meters; growth seems tc
be occurring on all bands.

 The results of the signal strength/
power study are shown in Fig. 8.
With one exception, the trend is deal
signals, and more consistent signals.
The average increase is almost exactly
in accord with power, i.e., two S-
units for 10 dB increase.

 The exception is for the power class
10-30 watts, which appears to be bet-
ter than the 30-100 watt class. This
may be real -- several QRP operators
have indicated that they took special
care with the antenna installation.
Many more samples would be needed
to separate the effects of power and
antenna size.

 Some time ago, W4MB ran a two-
month check of power benefit by
operating at 180 W PEP rather than
the usual 1500-1600 watt level. The
number of contacts made in a month
did not change greatly, but a differ-
ence in operating practice was neces-
sary to do this. Whereas, with the
linear, a short CQ had a high proba
bility of one or more replies, the CQ
replies dropped way down when oper-
ating "barefoot," and it was necessary
to start replying to DX stations CQs to
keep the number of contacts up.
Power does make a difference.

 Re-evaluation of
 Beam Antenna Type

  Several methods of checking the
 possible difference between quads
 and Yagi beams were considered.
 The two finally adopted are covered
 here.

 If one type of beam is truly better, it
 should perform better under poor
 conditions. This would be reflected as
s an increase in the percentage of con-
tacts using that type of antenna.

 The measure of conditions adopted
was the A index, the second numeri-
cal value transmitted by WWV. It was
found that a reasonable sample size
could be obtained by selecting con-
tacts where A was ten or less for good
conditions, and A of fifteen or greater
for poor conditions. Contacts for in-
termediate values, or for days when
the A index was not recorded, were
ignored. For comparison, the per-
centage of antenna use for all values
of A was also tabulated. The results
are shown in Table 4.

 First, it is noted that the variations
between the three sets of data is
small. The largest is for dipoles,
changing by 4.6 per cent from the all-
A-value column to the poor column.
For small Yagis, the percentages
show a small increase from poor to
good conditions, but almost exactly a
compensating decrease for large
Yagis. For quads, there is a decrease
of 4.2 per cent between good and
poor conditions; the small increase in
large quads does not compensate, so
there is a net decrease in the number
of quad contacts under poor condi-
tions.

 A check of the logs was made to
see if this decrease were real. One
factor noted was the almost complete
absence of USSR contacts under poor
conditions. Since a high percentage
of these stations use quads, the ob-
served change could be real. How-
ever, it must be remembered that one
percentage point is only two contacts
for the poor column. Accordingly, the
sampling noise is large.

 Because of this, a second method
of checking was sought. The logs
were again reviewed. It was found
that a fair amount of QSO data gave
height, power, and type of antenna.
Since height and power are shown to
be important, this data was checked
for two groups; one included all an-
tenna types, but only those stations
running linears or over 400 watts
(assumed to be the lower limit of lin-
ears). The second group included on-
ly Yagis, for stations operating at the
same power level.

 The results of this analysis are
shown in Fig. 9. The curve shown is
copied from Fig. 1, the contacts giv-
ing height, the other remaining vari-
able. The o-points are for linear-
power level contacts in general, com-
pared with the x-points for contacts
using Yagis. As seen, there is essen-
tially no difference in signal perform-
ance, and no difference when com-
pared with contacts stating height.

 The number of contacts used in this
compilation is not great, sixty-five for
the all-antenna group, and thirty-six
for the Yagi group. As a result, there
is appreciable chance for error. How-
ever, the indication that there is no
real difference seems clear.

 Comparing the four tests of beam
type makes it appear that there is little
or no difference between Yagis and
quads. The important factor, as noted
before, seems to be antenna gain.

Summary and Conclusions

 While there is some possibility of
error due to sampling noise, this sta-
tistical analysis indicates the following:
Beam antennas produce better sig-
nals than simple antennas on the
average, by almost exactly the anten-
na gain. For the common beams this
amounts to about two S-units. There
is good indication that the beams pro-
duce a more consistent signal.

 Higher antennas produce better
signals, by approximately one-half S-
unit for doubled antenna height. An-
tennas above thirty feet appear to
produce somewhat more consistent
signals than lower ones.

 Increasing the power improves sig-
nal strength, almost exactly a 10 dB
increase of signal for each 10 dB in-
crease of power. Higher power sig-
nals also appear to be appreciably
more consistent than low-power
ones.

 Although some tests appear to indi-
cate that the Yagi beam produces
stronger signals than the quad beam,
other tests indicate absolutely no dif-
ference. It appears that the gain of the
particular antenna is much more im-
portant than the type, and it may be
the only significant factor.

 The most important single variable
in received signal strength is propaga-
tion variability, typically a range of 35
dB over the 5-95 per cent limits. (The
range of signals in this log period is
from SB to S9 + 40 dB, a total range
of 85 dB). The range is so great that a
combination of legal power limit, and
maximum practical antenna height
and gain cannot compensate for it.
However, the super station is appre
ciably more likely to get some signal
through under poor condition.

 There is one overall conclusion:
The single most important factor in
getting good signal reports is choice of
frequency band and time, as needed
to catch optimum conditions. Using
this variable, any station can be a
good DX station.

Notes on Extending
the Study

 The solution to reducing the sam-
pling noise mentioned several times is
to increase the sample size -- the
number of contacts studied. For ex-
ample, a tenfold increase in the num-
ber of samples will reduce the noise to
about one-third. This may not be
easy. The data segment of the W4MB
logs is limited by rig and power
changes at one end, and antenna
changes at the other. It seemed best
to avoid these added variables. Even
so, some 1800 contacts were ex-
amined -- not all QSOs give useful
data.

 Since an improvement in analysis
will require looking at some ten to
twenty thousand QSOs, an extended
study might make a good club project
-- especially if the club has a comput-
er available. The extra number of
QSOs would allow some additional
studies, such as variations in usage
between continents or even countries.
Just for example, it appears that UA
antennas are much more likely to be
quads, and DLs are much more likely
to use linear amplifiers.

 If this club approach is used, some
checks of the data will be needed. Dif-
ferent operators have different report-
ing practices, and a "station correc-
tion factor" may be required. Devel-
oping this from the data will take extra
work .

 It is probably best to avoid analysis
of contest and pile-up data. Contest
reports are too stylized (5 x 9), and
pile-up QSOs are apt to be influenced
by the well known "DX-report" factor.

 Such an extended study would be
interesting. Well done, it should pro-
vide a definitive answer as to whether
it is gain that is important, or whether
antenna type also enters the picture.
And perhaps it could look at the less
common antenna types for usage and
performance.

 Oh yes: W4MB's new antenna? It's
a quad, except on 10 meters, where it
is a Quagi. Why? It seemed a good
idea at the time. It still seems so.