On Fri, 20 Jun 1997 01:25:53 +0000 "Sean McCarthy" <wx8l@akorn.net>
writes:
>Hello all,
>My lot (105' x 310') will accommodate up to 80' of tower guyed at 80%
>"by the book". What compensation would I have to make if I were to
>sneak it up to 90 or 100' (72 and 65%) ? Would the extra 10 or 20
>feet be worth it?
>The tower will be 25G with a TH7 at the apex and 40-2CD 12' above.
Here's a little comparison of 80 vs 100' on 40 thru 10.
It shows that you gain about 1.9 dB at the lowest angles
by adding the height. But, you lose a lot at the higher
angles. If being loud in the USA is important to you, even
80' is too high for your 10 meter antenna. It's time to
start thinking about stacking something lower, at about 40
or 50 feet. Angles up to 20 degrees are useful on 10 thru 20,
and sometimes even higher angles work. One interesting
possibility for you would be to use 100' of Rohn 45 or
preferably 55, and engineer it such that it
required guying at only two levels. If you guyed it at 95'
and 47', you would have room to rotate another TH7 on a
sidearm or ringrotator at the 48' level, even if the
anchors were only 65' away at ground level. This approach
would also give you another 2-3 dB at the low angles, plus
a lot of flexibility.
The following data is from a little spreadsheet program that
I use. The "ERR" entries are where the program falls apart
because of some divide-by-almost-zero errors, and in those
cases the total gain is less than minus 20 dB.
-----------------------------------------------------
height in feet: 80 height in feet: 100
frequency, Mhz: 7.15 frequency, Mhz: 7.15
wavelengths: 0.581 wavelengths: 0.727
ant gain dBi: 6 ant gain dBi: 6
tot gain tot gain
reflection/ reflection/
angle antenna angle antenna
- - - -
1 -11.92 1 -9.98
2 -5.92 2 -3.99
3 -2.43 3 -0.52
4 0.03 4 1.91
5 1.91 5 3.77
6 3.42 6 5.24
7 4.68 7 6.45
8 5.75 8 7.47
9 6.66 9 8.33
10 7.45 10 9.05
11 8.15 11 9.67
12 8.76 12 10.20
13 9.29 13 10.65
14 9.76 14 11.02
15 10.18 15 11.33
16 10.54 16 11.57
17 10.85 17 11.76
18 11.12 18 11.89
19 11.35 19 11.97
20 11.54 20 12.00
height in feet: 80 height in feet: 100
frequency, Mhz: 14.2 frequency, Mhz: 14.2
wavelengths: 1.154 wavelengths: 1.443
ant gain dBi: 9 ant gain dBi: 9
tot gain tot gain
reflection/ reflection/
angle antenna angle antenna
- - - -
1 -2.97 1 -1.05
2 2.97 2 4.86
3 6.38 3 8.20
4 8.71 4 10.43
5 10.43 5 12.03
6 11.75 6 13.19
7 12.77 7 14.02
8 13.55 8 14.58
9 14.15 9 14.90
10 14.57 10 15.00
11 14.85 11 14.89
12 14.98 12 14.56
13 14.98 13 14.01
14 14.85 14 13.19
15 14.59 15 12.07
16 14.18 16 10.55
17 13.61 17 8.46
18 12.88 18 5.45
19 11.94 19 0.50
20 10.76 20 -12.87
height in feet: 80 height in feet: 100
frequency, Mhz: 21.2 frequency, Mhz: 21.2
wavelengths: 1.724 wavelengths: 2.154
ant gain dBi: 9 ant gain dBi: 9
tot gain tot gain
reflection/ reflection/
angle antenna angle antenna
- - - -
1 0.48 1 2.39
2 6.34 2 8.16
3 9.60 3 11.27
4 11.72 4 13.17
5 13.17 5 14.32
6 14.14 6 14.89
7 14.72 7 14.97
8 14.98 8 14.57
9 14.93 9 13.63
10 14.58 10 12.04
11 13.89 11 9.49
12 12.81 12 5.14
13 11.24 13 -5.33
14 8.95 14 ERR
15 5.43 15 ERR
16 -1.14 16 ERR
17 ERR 17 ERR
18 ERR 18 ERR
19 ERR 19 ERR
20 ERR 20 ERR
height in feet: 80 height in feet: 100
frequency, Mhz: 28.2 frequency, Mhz: 28.2
wavelengths: 2.293 wavelengths: 2.866
ant gain dBi: 10 ant gain dBi: 10
tot gain tot gain
reflection/ reflection/
angle antenna angle antenna
- - - -
1 3.92 1 5.80
2 9.66 2 11.39
3 12.71 3 14.16
4 14.53 4 15.56
5 15.56 5 16.00
6 15.98 6 15.57
7 15.85 7 14.19
8 15.15 8 11.47
9 13.80 9 6.08
10 11.52 10 -20.62
11 7.66 11 ERR
12 -0.71 12 ERR
13 ERR 13 ERR
14 ERR 14 ERR
15 ERR 15 ERR
16 ERR 16 ERR
17 ERR 17 ERR
18 ERR 18 ERR
19 ERR 19 ERR
20 ERR 20 ERR
Dave Hachadorian, K6LL
k6ll@juno.com
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