[TowerTalk] Figure of Merit

[Eric Gustafson]

Hi Y'all,   (long boring antenna discussion - you should probably
	     stop reading now)

I finally got to read the second edition of the tribander
comparison report.  It is very interesting.  Anyone wishing to
discuss their (Ward and Steve's) results intellegently should
first get a copy and read it carefully.  However, it isn't my
intention to discuss the merits of the test protocol or the
accuracy of the results here.

I have been having some off reflector discussions with Larry
Moore, KM6IU, about possible improvements to the test methodology
(most of which I've already mentioned here).  During the course
of these discussions, Larry said something about forward
staggering multiple Yagis on a single boom that tickled my
internal biases towards the use of the LPDA as a multiband
antenna.

Anyhow, I had recently been doing some modeling of the Tennadyne
T-12 antenna (One of the many reasons that the LPDAs are nice is
that they are relatively accurately modelable.) and I realized
that this antenna has a boom length (30 feet) that falls within
the range covered by the "larger boom" (24 to 32 feet) group
reported on by Ward and Steve.

One of the nicer ideas in their report is the "figure of merit"
catagories and calculations for ranking multiband antennas.  I
have long felt that it would be useful to include at least one
copy of an LPDA in the testing.  So I thought it might be a bit
of fun to do the calculations including the modeled T-12 as
though it had been tested with the others.  I just wanted to see
where it would fall in the rankings _IF_ its measured performance
had been equal to its modeled performance.

Yes, I know that is a very big _IF_.  But possibly, someday, the
testing will include this class of antenna and we can see how
much "implementation loss" there might be in the LPDA designs.
And, yes, I know this is a completely meaningless exercise until
those measurements are done.  But as I said, it was a bit of fun
to see where the rankings might fall.

I did the comparison two ways.  The first was to see how well the
T-12 would rank when evaluated as though it was an ordinary
tribander compared to other three band antennas.  The results
(ranked on the same basis - Gs - as the antennas in the test were
ranked in the report) are shown in Table 1.  Note that the other
three antennas are ones that apparently were working OK.  In
fact, they are the top three performmers from the testing in the
large boom category.

Table 1 - Comparison as three band antenna

           | Gain - dBd |   |-- dBd/foot --|
Antenna    Gs       Gavg    G(s)/ft   Gftavg
-------	   ----     ----    -----     ------

C31XR	   15.9	    5.3	    0.513     0.171

X-9	   15.0	    5.0	    0.536     0.179

T-12	   14.1	    4.7	    0.469     0.156

Skyhawk	   13.3	    4.4	    0.554     0.185


The second comparison was done as though we were interested in
antennas to cover all five ham bands in the range from the bottom
end of 20 meters to the top end of 10 meters.  The results of
this comparison are shown in table 2 below.  Those of you who are
interested in 17 and 12 meters as well as in "the big three"
should find this comparison interesting.  Note that this
performance on the two additional bands is obtained "at no cost"
once the LPDA has been purchased as a tribander.  By "at no cost"
I mean no additional antenna(s) to buy, no additional feedline to
run, no wories about how the stacking interactions are going to
foul things up, etc, etc.  And, if a future WARC adds another
band (unlikely) or moves one (possible) in the range between 14
and 30 MHz, your antenna is already in place.  But wait!!!
There's more!!!  The antenna even points in the same direction
for all the bands!

Table 2 - Comparison as five band antenna

           | Gain - dBd |   |-- dBd/foot --|
Antenna    Gs       Gavg    G(s)/ft   Gftavg
-------	   ----     ----    -----     ------

TD-12	   23.7	    4.7	    0.791     0.158

C31XR	   15.9	    3.2	    0.513     0.103

X-9	   15.0	    3.0	    0.536     0.107

Skyhawk	   13.3	    2.7	    0.554     0.111


I suppose I should include a table of EZNEC modeling results for
the T-12 for completeness.  So here it is (table 3).  This is a
model based on the dimensions found in the T-12 assembly
instructions.  Conductor loss was assumed to be the values for
aluminum.  Balun losses (if any) were not considered since I
don't think this anttenna uses a balun at the feedpont.  If it
does (and it shouldn't really need to), then adjust the gain
values downwards by 0.05 to 0.1 dB.  0.1 dB would be a high
estimate and would not affect the ranking results.  To get the
free space dBi gain figures add 2.11 to the dBd gains in the
table.

Table 3 EZNEC V2 model of Tennadyne T-12

Band    Gain (dBd)      F/R (dB)
----    ----------      --------

20m     4.88            21

17m     4.86            21

15m     4.76            21

12m     4.82            20

10m     4.42            18


Now, to go from the sublime to the ridiculous, there are twenty
nine (29) 0.5 MHz wide "Bands" between 14 MHz and 28.7 MHz.  If
you _really_ want to see the figure of merit numbers go
astronomical, do the evaluation as a 29 band antenna (even using
the lowest gain in the range for all the bands)!  This might be
enlightening for a person interested in doing serious SWLing
between 14 and 30 MHz.  SWL is _much_ more pleasant if you have a
pointable, up in the cear, efficient, reasonable gain antenna
with relatively good F/R characteristics.

So, are we having fun yet?

73, Eric  N7CL

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