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@earthlink.net
<mailto: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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