When is more than a few watts power needed?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by N0PKG, Oct 27, 2015.

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  1. N0PKG

    N0PKG Ham Member QRZ Page

    OK, so I had this thought while looking for a mobile dual-band radio: Why or when would you ever need more than a few watts of output power?
    For example, the FM satellites have less than 1 watt, with omnidirectional antennas, and we can pick up their signal over several hundred km with cheap HTs, sometimes with even just a rubber duck antenna. So in what cases would more than a few watts of power be needed for communications on earth? Simplex is usually no more than 10-20 km, repeaters about 70-100 at max. What are the factors that affect how much power we need to get through?
     
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Satellite to ground paths are direct paths, so the only loss is that from the inverse-square loss of plane waves spreading out through space.

    On a terrestrial path, say from your car to a repeater, there are many additional losses that must be overcome. Obstructions (buildings, trees, etc.) which may be very numerous from the car to the repeater site, curvature of the earth which causes loss as the signal must refract or scatter to overcome that at distances greater than a few miles, and multi-path interference which can drastically reduce the signal strength at either end.

    If you live where there are tall mountain repeaters then the path may be mostly a direct one with lower losses, and greater distances can be achieved with less power.
     
    VK2KRR and KE7W like this.
  3. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    I always need a KILLERWATT on HF. I can't work across the street without 1000 W!!! But on VHF/UHF, I can usually key up the repeaters with a HT.
    The difference is that on HF, usally more than just me is calling the DX and I have the WEAKEST SIGNAL out of the West Coast, even with the KILLERWATT!!!! I eventually do make it through the pileups if I work at it hard enough, but i am always 60db down from the rest of the callers and get a 3x3 report at best (unless it is a contest, then I am 5x9)
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    50 watts in a automobile is nice to have.

    Blasting 50 watts at a satellite is a waste.

    Weather maters, Weather you can make a fully quieted signal. :D
     
    K9ASE likes this.
  5. WA8ZYT

    WA8ZYT Ham Member QRZ Page

    Terrain, efficiency of your antenna system, noise floor at the receiving stations location, among other things.
     
  6. W4SEX

    W4SEX Ham Member QRZ Page

    I'd like to have a dollar for every time I've heard someone tell people, " you need to turn up your power as we can barely hear you". Then they come back strong with the comment that they had their radio on the low power level setting.

    Hams for some reason, love very poor audio quality and lots of noise in their radio. They accept the .25 cent speaker in their radio, and live with it producing the poorest sound quality known to man. Always straining to hear the other person through lots of noise and static, caused by someone with very low transmitting power. A simple law of physics. More power, then better the signal is going to cut through.

    I can't hit the local simplex net with my HT with 5 watts, but hit it great with 65 watts in my mobile unit.
     
  7. W4SEX

    W4SEX Ham Member QRZ Page

    I've been wanting to ask that question, why is satellite work different than all other forms of transmitting a signal? I know the second post referred to the signal coming from a satellite down to Earth, but getting say a 1 mw signal versus a 50 watt signal up to a sat, be like normal Earth signals where you are fighting all types of stuff?


    Thanks in advance.
     
  8. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Because it's always line-of-sight.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Mo' power helps a lot.

    Since "most" mobile and portable operations are actually via repeaters and not simplex...repeaters commonly output ~100W. If you want them to hear you about as well as you hear them (so the repeater "fades out" to you as you are also "fading out" to it), you need to use similar output power.

    Operating simplex, for each 3 dB increase in transmitter power both station use, point-to-point coverage can improve by a few miles. Going from 1W to 64W is is an 18 dB power boost; this can increase direct range by 20-30 miles, easily.

    In the "old days" of VHF-FM work (1950s-1960s) almost all mobile stations ran 30 to 100W output power, as they were using converted taxicab/police commercial 2-way gear that used tubes and produced that kind of output. It's a reasonable balance between having enough power and not overloading a mobile electrical system.:p
     
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    It sucks when you run out. :eek:
     

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