Power Output Calculation

[torps04]

guys pano po b macompute ang power output ng isang FM transmitter, ung mga sinasabing 1 watt or etc.
tapos pano po ito marerelate sa distance na kaya ngang icover?
thanks po

[MX45]

o yung mga engineer design dyan.. pasok...

[akhen]

may mga local and international standards po yan sa power output ng mga transmitters...

AFAIR, sa mga bts(cell sites) 25kW ang standard... sa gm di ko na mtandaan...^_^

[labgruppen]

i think yung distance magdedepende rin sa sensitivity ng receiver. so kung may data ka sa typical sensitivity ng FM receiver, at alam mo yung tx power, then makukuha mo yung "system gain" from which you can compute the maximum FSL allowed. Having max FSL allowed, you can derive the distance. as for the tx output, usually measured using power meters. or computed from the plate current using the manufacturers formula or table.

[rdpzycho]

line of sight ba ang question o 'yung coverage area?

kung 'yung coverage area kasi meron pang computations 'yan, ask niyo sa broadcast instructor/professor niyo. depende 'yun sa gain at radiation pattern ng antenna, HAAT (height above average terrain) ng antenna, at 'yung mga nakapaligid na environment. makukuha mo diyan 'yung ERP (Effective Radiated Power).

alam ko merong standard ang FM at TV na pwede mong ma-receive 'yung signal. kaya kapag ginawa 'yung plot nito, merong different zones, kung saan nakalagay ang field strength ng signal sa zone na 'yun.

pero basic range niyan e 'yung makukuha mo sa FSL.

isa pang note, 'yung omnidirectional antenna na ginagamit sa broadcast e hindi 'yung typical na monopole, madalas phased array 'yun dahil wala namang sasagap ng signal sa taas ng height ng tore. then sa typical monopole, maliit naman ang signal sa ilalim ng antenna, shadow area.

[torps04]

thanks po sa mga nagreply,
ganito po kasi scenario.
naggawa po ako ng FM tx (1watt) for my thesis, and for educational purpose lng po xa, im using a whip antenna w/c is 1ft in height.
nang nagmeasure ako using cellphone fm rx, nakaabot ng 90meters.
eh ang problema ko ay pano ko mapprove na 1watt ung nagawa kong FM Tx. i need the formula on how to compute that, or pano ko nlng mamemeasure ung power output nya using only a tester, wala ako power meter eh

[labgruppen]

check this out.. http://www.io.com/~n5fc/rfprobe2.htm or this http://www.qsl.net/n9zia/wireless/pics/rfprobe.gif ..  your antenna i think is too short, quarter wave at 100MHz is about 75cm.

[rdpzycho]

kung wala kayong power meter, pwede mong i-rectify 'yan tapos i-filter, then feed sa opamp. doon mo na i-measure 'yung average voltage output at ang approximate power output mo ay V^2 / R.

load-an mo muna ng 50ohms or kung ano man 'yung load na matched sa transmitter mo.

BTW, use fast diodes like 1N4148, or Germanium type diodes. kung 1W 'yan, dapat mga 7V ang makukuha mo.

[torps04]

@rdpzsycho
meron na po atang rectifier diodes sa schematic ko, dun ko b mismo sa rectifier diode imemeasure or sa op amp? meron n din atang op-amp na transistor ung ckt ko eh. tapos merong RG-58 na coax na nagfeed sa whip antenna ko.

@labguppen
pang laboratory lng po ang purpose ng ginawa kong TX, kaya po 1ft lng or 1.5ft ang antenna height

[5Zero]

Hi Torps04,
Actually, FM transmitters' output power can easily be measured by a power meter within its frequency range and terminated with a resistive load of 50 ohms. Meter commonly used are Bird Thruline by broadcast engineers. Antenna and other peripherals has nothing to do with the transmitter output if your purpose is just to determine the RF power out. Knowing the Field strength after propagation from antenna is not easy to determine TX out, since transmission line loses, antenna gain on its front lobe, distance etc., are needed. 
In the absence to VHF power meter, and since you're the one who did assembled the transmitter, you can take a look on the power transistor or tube characteristic curve and have the voltage supply level you used and operating frequency can have an approximate power it can generate. Sometimes, operating efficiency of the power output stage is required to know its factor for the true power.  Broadcast transmitters has a standard load impedance of 50 ohms.
And since it's your thesis, I'm willing to help you if you so desired.