Here are our requirements for signal strength at the receiver front end, and what the current systems provide.
AM ATV (if we were to use it)
|We want||-53 dBm|
|We need at a minumum||-63|
|We want||-63 dBm|
|We need at a minumum||-73|
|We want||-73 dBm|
|We need at a minumum||-94|
- P5 signal from the ATV circuit
Using FM ATV, P5 occurs at > 50 uV (-73 dBm) Using AM ATV, P5 occurs at > 150 uV (-63 dBm) At 5 uV (-93 dBm), AM and FM are at the same level of "snowyness." It is best to exceed the needed signal requirement by 10 dB to allow sufficient fade margin.
For FM ATV, we need -63 dBm to give P5 picture with 10 dB of fade margin.
For AM ATV would need -53 dBm for P5 and 10 dB fade margin.
What signal levels are needed for WiFi?
- A SINAD that allows 11 Mbps on the WiFi circuit
This SINAD can be obtained with -83 dBm at the WiFi card. A SINAD allowing the lower limit of 1Mbps on the WiFi circuit is acheivable with -94 dBm signal at the WiFi card. It is best to exceed the needed signal requirement by 10 dB to allow sufficient fade margin.
For WiFi we need -73 dBm at the receiver to give 11 Mbps and the desired fade margin.
Antenna and amplifier gains
The path loss due to free space can be found using an equation from Andrew ® Catalog #33; p8;
Lfs (dB) = 96.9 + 20log10 D + 20log10 f
D is in miles; f is in GHz;
Also the path loss can be calculated using an equation found in the ARRL Antenna Book; P17-6;
Lfs = 20 Log10 4(pi)d / lambda(free space)
where d and lambda are in the same units
The freespace loss equation from the Andrew catalog will be used for this calculation. The worst case path is the distance of our spacecraft at apogee, which is 10 miles or 52,800 feet.
At the ATV frequency, 1.277 GHz. Lfs = 96.6 + 20log10(10) + 20log10(1.277) = 118.72 dB (loss)
At 2.422 GHz Lfs = 96.6 20log10(10) + 20 log10(2.422) = 124.28 dB (loss)
Add up all the gains and losses from rocket to the ground...
The FM ATV exciter puts out 50 mW (+17 dBm) this needs to be attenuated by 7dB for it to match the required +10 dBm signal input of the M67715 power amplifier. This is done with a Pi attenuator added to the input of the power amplifier board. The R values are 130/47/130 ohms.
The P number system is a metric for picture quality. P5 is best "closed circuit quality", and P1 is barely descernable. See ARRL Handbook. Tom O'Hara W6ORG provided the signal strength information needed for various P levels.
The units of dBm at the transmitter is the only place where reference to milliwatts is applied. All other dB values are unitless ratios, thereby the final result is in dBm.
dBd = +2.15 dBi
dBm = 1mW @ 50ohm @ 1GHz (absolute reference level)
Reciever preamp KU2000LSF from Kuhne Electronic, GMBH. Typical gains of their Ga As components are 35 dB or better, with noise factors around 1.0 or less.
Receiver preamp for 1.2 GHz, part number DEM 23 LNA from DEMI had 15 dB gain with 0.7 NF.
Install a hybrid "brick" linear power amplifier such as Mitshubishi M68719 New link -> http://www.g8ajn.tv/images/M68719.pdf between the transmitter and antenna on the spacecraft. This puts out +42 dBm at 1dB compression (16W), consuming about 5 amps at 13.8 VDC supply (69 W). Thats an efficiency of 23%! Mitshubishi M67715 http://www.qsl.net/n9zia/atv/67715.pdf puts out 3 watts, using 1.5 amps of 12 VDC
If a receiver with 6kHz bandwidth performs well at -87 dBm, a receiver of 6MHz bandwidth (1000 times more spectrum to fill with energy) will need (10 log10 (1000) = 30 dB) -87 + 30 = -57dBm signal to perform similarly.
Other preamp products for signals below 1GHz can be found at Advanced Receiver Research.
Information on the 2.4GHz amplifiers:
Noise Factor, Noise Figure, Signal strength, SNR, SINAD, channel capacity?
WiFi bandwidth related to signal strength?
Helix gain and half power beamwidth? Helix bandwidth?
SINAD From Wikipedia: http://en.wikipedia.org/wiki/SINAD From Aeroflex, a manufacturer of SINAD analyzers: http://www.aeroflex.com/products/signalsources/signalgens/articles/sinad.pdf
BER From Wikipedia: http://en.wikipedia.org/wiki/Bit_error_ratio From HF Electronics article: http://www.highfrequencyelectronics.com/Archives/Jan03/HFE0103_Tutorial.pdf
Antenna Engineering Handbook
Tom O'Hara W6ORG; Signal strength needed for ATV