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G8MNY > TECH 19.04.04 00:32l 156 Lines 6400 Bytes #999 (0) @ WW
BID : 53430_GB7CIP
Read: DL7NDF GUEST OE7FMI
Subj: RF Signal Calculations
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0GOS<DB0EEO<DB0RES<ON0AR<GB7CIP
Sent: 040418/2224Z @:GB7CIP.#32.GBR.EU #:53430 [Caterham] $:53430_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Published BATC's CQTV 177, packet version Dec 03)
With several radio path loss programmes now available to amateurs, it becomes
possible to predict the P grade (or S meter for phone) for any contact.
Data is needed for these 5 parts of the problem.
TX IERP TX power.
Coax & connector losses.
Aerial gain dBi.
PATH LOSS NGRs for distance, )
Aerial height, ) used with a terrain database
Frequency ) for loss calculations.
RX SIGNAL Aerial gain dBi.
Coax & connector losses.
ALL THE ABOVE
RX SENSITIVITY Sky Nose + Noise figure (preamp & Rx).
Bandwidth.
'R' or 'P' GRADE S/N Conversion.
Most people can only make a guess at these, or use makers figures where
believed!
TX ISOTROPIC EFFECTIVE RADIATED POWER.
This is a straight forward calculation best done in dBW.
TX IERP = TX dBW - LOSS + GAIN
PATH LOSS.
This has 2 parts:- the Free space loss and other losses. The free space path
loss can be calculated directly from the distance and frequency, where D is in
km and F is in MHz.
FREE SPACE LOSS dB = 20LogD + 32.4 +20LogF
However, on VHF & UHF it is heavily modified by obstructions like hills,
buildings, trees etc. The use of Path loss programme with a UK heights database
can take into account hill diffraction loss, reflection loss etc, but this will
still be a best path guide. Under lift conditions losses can get close to the
free space loss. If you don't have a programme you will have to guess at these
additional losses.
PATH LOSS = FREE SPACE + DIFFRACTION + REFLECTION + OTHERS.
RX SIGNAL.
The level is now just the Rx losses added to the other figures & converted to
dBm. (Decibel ref to 1mW)
SIGNAL (dBm) = TX IERP +30, -PATH LOSS + AERIAL GAIN - COAX LOSSES.
RX SENSITIVITY.
This should include the sky noise term, but I have ignored this as @ VHF & UHF
it is usually only a dB or so above the thermal noise, so it does not really
alter a prediction by much.
The Rx sensitivity calculation is slightly more complex with bandwidth (in Hz)
& overall noise figure (dB) needed to get the noise floor with the -174dBm/Hz
from Boltzmann's constant (at room temp). The bandwidth figure assumes an ideal
square I.F. response shape, so for example a Sat Rx with normal I.F. filtering
will be 30MHz wide, a dedicated FM ATV RX good on sound & colour Rx may be
13MHz, & a narrow DX RX for B/W Dxing only may be 6MHz wide! On 70cms AM TV
will be 5MHz unless a narrow I.F. is used. Wide FM is 200KHz I.F., NBFM 25KHz
system uses 20KHz I.F., 12.5KHz system should be using a 7KHz IF, SSB uses
2.4KHz I.F., & CW whatever I.F. you have.
Noise figures of preamps & Rx are additive, but as long as the Rx noise is
drowned by the preamp noise by 10dB or so, then take just the preamp noise
figure as the system noise. Noise figures of just the preamp device itself can
be very misleading as input socket, input tuned circuit & radiated/PCB losses
can often add 1-2dB to this.
NOISE FLOOR (dBm) = NF -174 + 10Log Bandwidth (in Hz)
RX SIGNAL/NOISE RATIO.
This is just the difference between the noise floor & the Rx signal. This is
what is in the IF before the FM discriminator or AM detector.
S/N (dB) = SIGNAL - NOISE FLOOR
'P' GRADE CALCULATION.
This is an arbitrary conversion, assuming for ATV P5 is 35dB S/N (eg good VHS
tape) going down to P0 at no signal. With FM there is a capture affect & also
the visible noise is quite different, but with narrow FM ATV (not a Sat Rx) the
effect is not as far from linear as you might expect.
I have found these formula gives quite an acceptable conversion.
P Grade = (S/N +3) x 0.13
'S' METER CALCULATION.
This is also a simple conversion, assuming 3dB/S point, that gives a good
27dB S/N @ S9 on VHF & above, but with an added 1dB to get started makes 28dB
S/N, which is a very good contact.
S Meter = (S/N -1) x 0.3
Note for HF 6dB/S point is normally used as these Rx are quite deaf & the band
noise is usually 10-30dB above thermal noise, & gives a greater meaningful
range for the meter.
-------------------------------------------------------------------------------
Fig.1 Example of a Path loss Programme
Path loss : 134
--------------
Free Space Loss: 129.3 dB
Diffraction Loss: 0.0 dB
Reflection Loss: 0.0 dB
Siting Loss: 0.0 dB
Round Hill Excess: 4.4 dB
At Frequency: 1248.0 MHz
Over a Distance of: 55.80 km
TQ335643 to SU850919
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ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
TQ335643 Quit (Q) or run again (return) SU850919
G8MNY Croydon GB3HV High Wycombe
-----------------------------------------------------------------------------
Fig 2 Example of my basic programme calculator.. (I you want a copy I can send)
CALCULATED SIGNAL FROM G8MNY TO GB3HV
INPUTTED PARAMETERS:-
FREQUENCY 1248 MHz POWER 20 W = 13 dBW
TX COAX -2 dB TX AERIAL 17 dBi
RX COAX -1 dB RX AERIAL 9 dBi
PATH LOSS -134 dB NOISE FIGURE 1 dB
BANDWIDTH 13 MHz
CALCULATED RESULTS:-
TX IERP = 632.4 Watts = 28 dBW
RX SIGNAL = -68 dBm = 89 uV (in 50 ohms)
RX NOISE FLOOR = -101.9 dBm
RF SIGNAL to NOISE = 33.9 dB
'S' METER = S 9
'P' GRADE = P 4.7
Why don't U send an interesting bul?
/QSL
73 de John G8MNY @ GB7CIP
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