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

Wed Mar 16 13:49:45 EDT 2016

On 3/16/16 9:01 AM, Larry Loen wrote:
> 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.

That's my 3.28 * 299.7/f -> feet/meter * meters/sec / freq
to compute the free space wavelength in feet. (a  half wavelength would 
be 491/f)
( I used that rather than the "resonant dipole" formula of 468, which is 
about 5% lower, figuring that later, I'd be comparing with a 10% 
tolerance anyway)

  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 works.. I figured I'd put band centers, or upper and lower in the 
list and look.

I just take the guy length and divide it by the wavelength: so a 1/2 
wavelength winds up as 0.5, a 3/2 wavelength is 1.5, etc.

if you take x-floor(x) that just removes everything to the left of the 
decimal point, so you get numbers between 0-1.
I then compare against 0.45 to 0.55 (which is 10%)

>
> 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 at earthlink.net
> <mailto:jimlux at 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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