I have been following the recent discussions of Mosley and KT-34XA
performance on the list with great interest. I am always amazed at how
rapidly a topic can change in a thread; the post that started it was tower
loading of Mosley antenna, and rapidly moved to the electrical performance
of Mosley antennas and the KLM KT-34XA.
Despite posts from many to the contrary, the original comments and included
data "make sense" to me from a theoretical point. I offer the following
comments, some of which may clarify the situation and some of which may
muddy it further;
1. The measured rapid drop off of gain with increasing frequency of both
antennas is consistent with the predicted (modeled) performance of Yagi
antennas. As the frequency is increased, the gain of a Yagi usually slowly
rises to a maximum value and then drops rapidly. This is evident in several
published curves of gain vs. frequency and is particularly pronounced with
longer Yagis at VHF, but it is a real effect with even a 2 element Yagi at
HF.
This effect is often compounded into a problem by the antenna assemblers
desire "not to screw up" when measuring element lengths. The assembler
usually measures and cuts elements a bit long on the theory that it is
easier to cut a bit off than add some on later. This lowers the gain peak
and drop off into the ham band and causes low gain in the upper end of the
phone band.
With respect to gain; an antenna cut for the phone band will usually do
better in the CW band than an antenna cut for the CW band will do in the
phone band.
The moral of point 1 is to measure exactly or err on the short side when
measuring element lengths.
2. It is difficult, if not impossible, to optimize the gain, front to back
ratio and SWR bandwidth simultaneously for a single band Yagi. This effort
is compounded by two issues; the usual design having maximum gain and
maximum front to back ratios at differing frequencies and the maximum gain
figures usually being accompanied by narrow SWR band widths. Modeling
clearly shows these effects. The situation becomes even more difficult, and
probably impossible, when the same optimization is tried simultaneously on
3 or 5 bands.
3. The efficiency of multiband trapped tribanders usually drops as more
elements are added. The radiation resistance of a 2 element Yagi spaced
0.15 wavelength with modest gain (4dBD), front to back ratio (10Db) is
about 45 Ohms. If we try to get the additional 2 dB or so gain
theoretically available from a 3 element beam, then the radiation
resistance will drop to less that 20 Ohms, and the trap losses will become
more significant and reduce efficiency.
Telrex originally approached this problem more than 30 years ago by using
traps in the driven element only and using separate reflector elements for
each band. KLM approached the problem by using lower loss "linear traps" in
the KT34 as well as using a two element feed to raise the radiation
resistance. More recently Force 12 has approached the problem by using
closely coupled resonators and separate reflector elements. The result is a
two element tribander that out performs many 3 element tribanders. Of
course two element multiband quads have no such limitation and the lack of
traps with their associated losses in these antennas has accounted for much
of their excellent reputation, particularly with respect to the ubiquitous
3 element tribander.
4. In Yagis, an unusually large bandwidth is usually a good indicator of
excessive losses somewhere in the system, which are usually accompanied by
lower gain than expected. While this is usually true of antennas in
general, there are exceptions, such as the log periodic. The log periodic
is usually thought of as a poor trade off with respect to gain achieved for
aluminum in the air, but the poor measured gain values for the PRO77 show
that investing a similar amount of aluminum in a triband Yagi is no
guarantee of better performance.
5. Location is probably more important than antenna performance. A poor
antenna at a good site can outperform a good antenna at a poor site. Of
course, this is no justification for having a poorly performing antenna, an
a good antenna at a good site is a tough combination to beat.
6. These results are consistent with measurements made nearly 20 years ago
by Wayne Overbeck, N6NB. Wayne measured a number of mono band Yagis,
tribanders and quads using a two element tribander at the same height.
Wayne's data showed the rapid drop in gain at the high frequencies. In some
cases it was quite dramatic. The mono band Yagis were much better than the
trapped tribanders, and the Yagis were better than the multielement
(greater than 3) quads. One of Wayne's conclusions was that the three
element tribanders were not really all that much better as a class than the
two element tribander he used as a reference, A TH2 if I recall correctly.
I realize that there was some controversy in Wayne's measurements at the
time, as there is with Larry's now, but both provide hard data to talk
about instead of anecdotes.
This has grown in length past what I intended, but I hope it is of some use
to readers. If you have further questions, the posts at L. B.'s page on two
element Yagis are a good place to start. Actually L. B.'s pages are a good
place to start for nearly any antenna question;
http://funnelweb.utcc.utk.edu/~cebik/radio.html
Lord Rayleigh's comments about only knowing something after you have
measured it are probably appropriate here. Keep up the good work Larry. I
know that these are difficult measurements to make. - Duffey KK6MC/5
James R Duffey KK6MC/5 DM65
30 Casa Loma Road
Cedar Crest, NM 87008
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