Wayne,
Another consideration in addition to the degradation from 20/15/10 monobanders
stacked on a 15 foot mast
is whether you want antennas for the WARC bands on the mast later too.
I have an 18 foot mast coming out the top of my tower and I have a Force 12
WARC 2/2 which has 2 elements on 17 and two elements on 12,
mounted on top, then 8 feet below is a Force 12 C3S. Thanks to Tom Schiller /
Force 12, I learned that the 8 foot spacing
was the sweet spot in spacing. It exhibits zero negative interaction on any of
the bands and actually adds .5 DB of gain on 15 meters with these
two antennas at 8 foot spacing.
Take a look at the Force 12 C19XR it is a killer. 3 elements on 20, 3
elements on 15 and five elements on 10.
Compare it with non-optimum spaced monobanders and I think you will be pleased.
Anyway my two cents, good luck !
Bob
K6UJ
On Apr 18, 2013, at 2:06 PM, Wayne Willenberg wrote:
> 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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