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G8MNY > TECH 24.09.21 08:23l 68 Lines 3351 Bytes #999 (0) @ WW
BID : 47610_GB7CIP
Read: GUEST
Subj: Your own EMF Spreadsheet
Path: IW8PGT<I3XTY<GB7COW<GB7YEW<AL0Y<KD8FMR<PY2BIL<TI0RC<CX2SA<GB7CIP
Sent: 210924/0717Z @:GB7CIP.#32.GBR.EURO #:47610 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Sep 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is my attempt at an EMF spreadsheet. It is straight forward with no fiddle
factors added. It is for "far field" square law & not the cube law near fields,
as they are very condition dependant. So expect differences at distances under
a wavelength!
Aerial gain _.--~~\ Hi Gain |Large Sphere
/-/-/-/ <_ ) Front Lobe |(distance)
| ~~-__/ |Needed
| | |
Tx Feeder loss | | Low Gain
Power ===================== / Height Angle
-' Small Sphere
Done in logical bits, so not too complex to follow, with simple Maths used,
e.g. straight fractions, dB convertions "Exp (dB/10x2.303) x Watts", to...
Calculate the RF at the aerial, & with aerial gain dBs the Isotropic ERP, then
calculate the area of a sphere needed to get the required field strength limit,
then the radius of that sphere for the minium distance (Area = 4 Pi R2),
rewritten as radus R = "square root of (area/(4 x Pi))". Then calculate aerial
off lobe field strenth distance if relevant.
Type this example into your spread sheet. N.B. Column "B" is for inputs & the
formulae (quoated in square brackets [=...] in the Notes column "D") to make
the result outputs. Note the number of brackets () needed in spreadsheets cells
in order to do the maths in the right order!
-----------A--------|--B-|---C---|------------------D--------------------
1 E. M. F. Far Field Distance Calculator
2
3 Notes
4 Tx Power > 200 Watts
5 FM/Process SSB > 100 % FM=100%, SSB=10%, Processed SSB 25%
6 Use Duty Cycle > 50 % Max overs compared to 8 mins
7 Average Power is = 100 Watts Mean power [=B4*(B5/100)*(B6/100)]
8 Feeder Length > 10 m
9 Feeder loss /100M > 2 dB/100m Feeder used loss figure
10 Coax Loss is = 0.2 dB Resultant dB loss [=B8/100 *B9]
11 Aerial Power is = 95.6 Watts [=EXP(-B10/10*2.303) *B7]
12 Aerial Gain > 8.7 dBi dBi figures are 2.15 above dBd (dipole)
13 Isotropic ERP is = 708 Watts Illuminating [=EXP(+B12/10*2.303) *B7]
14 Max Field Strength > 2 Watt/m Investigation level for that frequency
15 Sphere Area need = 354 m2 Area for max illumination [=B13/B14]
16 Min Aerial Distance= 5.31 m R=SQRT A/4pi [=SQRT(B15/(4*3.142))]
17
18 With Beam Offset Angle
19 Off Beam Loss > 10 dB From Aerial elevation or polar diagram
20 Off Beam Power = 70.8 Watts Result [=EXP(-B19/10*2.303) *B13]
21 Sphere Area need = 35.4 m2 Result [=B20/B14]
22 Min Aerial Distance= 1.68 m with offset loss [=SQRT(B21/(4*3.142))]
Where ">" means input "B" cell follows, "=" Calculation "B" cell follows,
"[= ...]" means formulae used, put it into the "B" calculation column.
From here you can see the step by step process for each Tx signal for your site
calculations & hopefully they agree with the Ofcom estimate. And will help you
write your EMF complience document.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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