G8MNY > TECH 27.11.21 09:47z 176 Lines 7369 Bytes #29 (0) @ WW
BID : 54207_GB7CIP
Subj: RF Signal Calculations
Sent: 211127/0941Z @:GB7CIP.#32.GBR.EURO #:54207 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Published BATC's CQTV 177, Updated Nov 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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 not via
Data is needed for these 5 parts of the problem.
Tx Aerial IERP Sky Noise -> Rx Aerial ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
ÚÄÄÄÄÄ¿ Gain \³/ Ä> : : : : : : : : : : : Ä> \³/ Gain ³ Rx Noise Fig ³ S/N
³ Tx ÃÄÄÄÄÄÄÄÙ Path Losses Rx ÀÄÄÄÄÄÄÄÄÄ´ Rx Bandwidth Ã>Output
³Power³ Feeder (Free space) Field Feeder ³ S Meter Mode ³
ÀÄÄÄÄÄÙ Loss ( + Others ) Strength Loss ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
1/ TX IERP TX power.
Coax & connector losses.
Aerial gain dBi.
2/ PATH LOSS NGRs for distance, )
Aerial height, ) used with a terrain database
Frequency ) for loss calculations.
3/ RX SIGNAL Aerial gain dBi.
Coax & connector losses.
With 1/ & 2/
4/ RX SENSITIVITY Sky Noise + Noise figure (preamp & Rx).
5/ 'R' or 'P' GRADE S/N Conversion for that mode.
Most people can only make a guess at these, or use makers figures where
TX ISOTROPIC EFFECTIVE RADIATED POWER.
This is a straight forward calculation best done in dBW.
TX IERP = TX dBW - ALL LOSS + AERIAL GAIN
This has 2 parts:- the Free space loss & other losses. The free space path loss
can be calculated directly from the distance & frequency, where D is in km & 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 only 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. I use a programme called Tempath.
PATH LOSS = FREE SPACE + DIFFRACTION + REFLECTION + OTHERS.
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.
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 500Hz, & FT8 will be 6.25Hz.
Noise figures of preamps & Rx are additive, the preamp adds some noise to the
signal but making the signal much bigger, but as long as the Rx noise is
drowned by the amplified band noise + preamp noise by 10dB or so (not much more
as you loose headroom performance), then take just the preamp noise figure as
the system noise as this will then be quite accurate without calculating it.
Noise figures of just the preamp active device itself can be very misleading as
input socket, input tuned circuit & radiated/PCB losses can often add as much
as 1-3 dB 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, AM or product detector.
S/N (dB) = SIGNAL - NOISE FLOOR
'P' GRADE CALCULATION
This is an arbitrary conversion, assuming for ATV P5 is 35dB S/N (e.g. 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 this 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 fairly
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 Rx thermal noise, & gives a greater meaningful
range for the meter.
Fig.1 Example of a Path loss Programme (Tempath + Graphics to Print)
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
153- _ . , - '_³ ³
³\ _ . , - ' / \ ³
³ ³ _ . , - ' /\-/ ³ _³
³ \| ' __/\ /\_ ³ \/ ³
³ ³ __/\___---/ ³ ³ \/ ³
³ -\_ /\ ³\ ____----/ V ³
4³_ ----__/ \_/ ³___-- ³
TQ335643 Quit (Q) or run again (return) SU850919
G8MNY Croydon GB3HV High Wycombe
Fig 2 Example of my basic programme calculator.. (you want a copy I can send)
CALCULATED SIGNAL FROM G8MNY TO GB3HV
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
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 (in 13MHz)
'P' GRADE = P 4.7
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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