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Re: [TowerTalk] "Magic" length from tower to first insulator

To: jimlux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] "Magic" length from tower to first insulator
From: Larry Loen <lwloen@gmail.com>
Date: Wed, 16 Mar 2016 09:01:51 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Thanks, Jim.  A little too much "magic" in your formula, though.  I'm
trying to understand this as well as "cook-book" it.

So, I did the "obvious" thing and constructed my own table based on
multiples of 1/2 wavelength.  Took the classic 468 and divided it by the
upper and lower bounds of each frequency band.  Then I "deducted" 10 per
cent from the shorter length (higher frequency) and "added" 10 per cent to
the longer length (higher frequency).  So, for each frequency, multiple of
1/2 wavelength, I calculate an upper and lower bound (distinances
in-between are also on the "bad idea" list).

That produced this little table:

                                 0.5 wlength                1.0
wlength                1.5 wlength
 3.50         4.00               147.09       105.30       294.17
210.60       441.26       315.90
 7.00         7.30                73.54        57.70       147.09
115.40       220.63       173.10
10.10        10.15                50.97        41.50       101.94
83.00       152.91       124.49
14.00        14.30                36.77        29.45        73.54
58.91       110.31        88.36
18.06        18.16                28.50        23.19        57.01
46.39        85.51        69.58
21.00        21.45                24.51        19.64        49.03
39.27        73.54        58.91
24.89        24.99                20.68        16.85        41.37
33.71        62.05        50.56
28.00        30.00                18.39        14.04        36.77
28.08        55.16        42.12
50.00        54.00                10.30         7.80        20.59
15.60        30.89        23.40

First off, it tells me that if you try and account for 6 meters, you're
going to end up with a very short initial cable length.  Nobody has told me
yet whether there is a minimally practicable minimum size for that first
length of guy.  I guess I will have to hope that at 6m, at least, that any
antenna will be high enough up the mast as to not matter (which should be
true according to our plans).

This still gives me acceptable results, or should, if I pick around 10 feet
as my "consensus" length.   It might disturb the pattern on the upper end
of 15 meters, but it should be well "out of synch" with everyone else.  At
least that's what my all-too-quick study of this suggests.

I don't know if there is a big structural difference between, say, 10 and
20 feet, and perhaps even 3 feet wouldn't matter (except for the sheer
logistics of getting the cable tied off), but 10 feet looks like the
overall safest best.  So, absent further discussion from those far more
learned than I, that's what I am going to pick.  I know what the initial
configuration is, but I'm trying to account for every use.  Unless this
becomes a 15 meter monoband tower someday (very doubtful) and the upper end
of 15 becomes important (actually, kind of expendable given my interests if
_something_ has to go), then this looks good to me.

Comments?  Dissents?  What did I overlook?

On Wed, Mar 16, 2016 at 5:25 AM, jimlux <jimlux@earthlink.net> wrote:

> On 3/16/16 12:31 AM, Larry Loen wrote:
>
>> A friend of mine told me that someone, somewhere had calculated a "magic"
>> length for guy cable.  If you went down 10 or 11 or 13 feet (or whatever
>> it
>> is) off of your tower and installed your first insulator there, your guys
>> would be sufficiently 'non-resonant' so that they wouldn't interfere with
>> the pattern of the antenna(s) above.
>>
>> What is this magic length and where is it documented?
>>
>
> There's a table in the ARRL Antenna Book of "good" guy segment lengths..
>
> I do it by setting up an excel spreadsheet that calculates the number of
> wavelengths for my frequencies of interest for a given length, and then
> just making sure they're not close to a multiple of 0.5
>
> if cell C3 is the frequency (in MHz) and cell A5 is the length (in ft)
> C4 = 3.28*299.7/C3                      wavelength in feet
>
> C5 = $A5/C$4 - floor($A5/C$4)   frac part of guy length in wavelengths
>
> Then conditional format C5 to be red if it's in the range 0.45-0.55  (or
> whatever tolerance you want)
>
> I put a sample sheet out on google docs
>
>
> https://docs.google.com/spreadsheets/d/1rXeWjPMej_TlzN0h8OfzEFAU2mwKGiar_QaxK94tozA/pubhtml
>
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