I'm making my final decisions about what antennas to populate my newly
erected tower with (MANY thanks to all who have helped with my questions
and concerns over the course of this long project -- the actual raising
went really well. Why it took me so long (over two years) in retrospect is
a mystery, other than a lot of non-radio related personal crisis'. But by
waiting until the weather here in Libby has turned bad I've ensured that
the tower will stand forever and the antenna have all gained at least 3+
dB! But that's already been covered).
Brief description:
90' of Rohn 45G, guyed at 29', 59' and 90'
I plan to stack tribanders just above the top (92') and each set of guys
(30' and 60' roughly). The top antenna will be a tribander + 2 el 40M
A new wrinkle to consider was prompted by a discussion with Natan Huffman
of Force 12:
As well as the 2el 40M at 92', why not also make the lowest antenna a
tribander + 2el 40M and switch them -- both/top only/bottom only.
I hadn't considered 40M at 30-32' feet, it had just seemed too low. And
after running some quick modeling of 40M at 92' and 60' stacked, the
additional gain and diversity didn't seem worth the extra trouble and
expense. But maybe I am missing something.
Here are the figures for both situations (the actual dBi numbers may or may
not be realistic, what is important is the relative change in dBi and angle
of max radiation):
40M at 92' and 60':
Antenna Gain Angle
92' 12.5 dBi 20 degrees
60' 11.4 dBi 27 degrees
both 12.5 dBi 22 degrees
49M at 92' and 30':
92' 12.5 dBi 20 degrees
30' 8.7 dBi 37 degrees (very broad lobe)
both 11.8 dBi 23 degrees
In case one, there is only 7 degrees difference between one or the other
antenna and effectively, no increase in gain with the stack. In case two,
there is a wide range of angles covered with either the high or the low
antenna, but not much difference between the upper antenna and the stack,
and actually a decrease in gain with the stack.
I am located in extreme northwest Montana (equally far from all DX!) and
have mountains to my west that preclude really low angle stuff, according
to Terrain Analyzer. Will a set up with the lower 40M antenna at 30' be a
useful addition for closer in and high angle stuff (is this the "domestic
contest" antenna for 40M or just a short-range rag-chew antenna).
As always, I value the years of experience and expertise of the Towertalkians.
Steve Zettel KJ7CH
Libby, MT USA
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