My 86 or 89 foot crank-up tower project seems to be progressing through my
County review boards. So now I am getting really excited for my first
tower.
I am now in ham nirvana because I have to start making up my mind on which
antenna(s) to put on the 15’ mast. Here is my first dilemma. Should I
put
up a tribander (10, 15 and 20M) or 3 monobanders. My intuition said there
was no way a tribander with the same 20M boom length could beat 3
monobanders having a 20M antenna with the same boom length.
So, just as an exercise, I looked at Force 12 antennas. (I am not
endorsing Force 12 antennas, although they do seem to have a good
reputation. I am also aware that the performance specs of antenna
manufacturers need to be taken with a grain of salt –this is not meant to
be a criticism of Force 12.)
I started with the Force 12 model C-31XR tribander. It has 14 elements on
a boom length of 31’, a wind load of 10.7 sqft and published figures of:
20M gain: 14.0dBi, a F/B of 20dB,
15M gain: 14.3dBi, a F/B of 22dB, and
10M gain: 15.6dBi, a F/B of 21dB. Published price: $2,970
Then, I looked for monobanders from Force 12 that had a boom length for
the
20M monobander close to the C-31XR and a gain in the vicinity of the of
the
tribander for each of the 3 bands. Here is what I found:
EF-420 (20M with 4 elements) has a boom length of 30’, a gain of 14.5Dbi,
a F/B of 22dB, a wind load of 5.5 sqft, and a cost of $1,300.
EF-415 (15M with 4 elements) has a boom length of 20’, a gain of 14.6Dbi,
a
F/B of 22dB, a wind load of 2.8 sqft, and a cost of $869.
EF-510 (10M with 5 elements) has a boom length of 18’, a gain of 15.1dBi,
a
F/B of 23, a wind load of 2.5 sqft, and a cost of $799.
The total wind load of the 3 monobanders is 10.8 sqft and a total cost of
$2,968.
So, at first glance, the price and wind load of the tribander is
practically the same as the sum of the 3 monobanders. There is no more
than an additional 0.5dB of gain (and 0.5 dB loss of gain on 10M) with
monobanders; a gain of 2dB F/B (and 2dB more F/B on 10M) for monobanders.
However, I am aware that there will be a degradation of the performance of
at least the middle monobander if I put 3 of them, in a Christmas tree
fashion, on a 15’ mast. I haven’t been able to find a good discussion of
the amount of degradation—even the 22nd Edition of the Antenna Book has
only a few words to say about this. Please direct me to a discussion of
this topic if you know one. I may have to model these antennas on EZNEC
to
see the impact of 3 antennas confined to a 15 foot mast.
On the other hand, there has to be a compromise when 3 antennas are
mounted
on a single boom.
I would really appreciate your help in finding all of the specs I should
be
looking at in making this decision. Any other comments that would push me
one way or the other would also be appreciated.
Please help me with my first tower!
73, Wayne KK6BT
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