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I want to buy a sub-gigahertz radio module which has a TX power of 16 dBm. With the inverse square law, I can see that it has a power of -90 dBm at 50 km. I don't know if this is the correct way to determine power at a certain range or not, but either way, this is not enough.

Can I use something like an op-amp to just increase the output power and get higher range?

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    \$\begingroup\$ That strongly depends on the laws and regulations in the place where you will operate the transmitter. Also, 1 kHz and 100 MHz are both sub-gigahertz but might perform differently, can you specify the operating frequency within an octave at least? \$\endgroup\$
    – The Photon
    Dec 7 '21 at 18:16
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    \$\begingroup\$ You'll need a radio-frequency amplifier. You can buy one for the appropriate frequency (but be careful to get an amp that can tolerate 16dBm input power). Also, keep in mind that transmitting at high-ish powers without a license is illegal. \$\endgroup\$ Dec 7 '21 at 18:16
  • \$\begingroup\$ I don't know if this is the correct way to determine power at a certain range or not No it is not. If your receiver needs -85 dBm then you cannot reach 50 km but my receiver is more sensitive at -95 dBm and can be used at 50 km away. Using lower datarates and/or different modulation techniques can also change the required signal level so there is no clear yes/no answer if 50 km can be reached. \$\endgroup\$ Dec 7 '21 at 18:32
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    \$\begingroup\$ Does your radio module receive as well as transmit? If so, adding a power amplifier is not a simple matter - while receiving, the power amplifier must be bypassed so that the receiver sees the antenna. \$\endgroup\$
    – glen_geek
    Dec 7 '21 at 18:34
  • \$\begingroup\$ If you don't have line-of-sight then you won't make it 50km even with 1,000 times the power. \$\endgroup\$
    – hobbs
    Dec 7 '21 at 18:37
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The Friis transmission path-loss equation in decibels is this: -

Path loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is in MHz and d is in kilometres. So, If I assume 100 MHz (sub 1 GHz) and a distance of 50 km, I get an attenuation of 32.45 dB + 40 dB + 33.98 dB = 106.4 dB.

And, this agrees with you expecting a receiver level of -90 dBm from a carrier transmit power of +16 dBm.

I don't know if this is the correct way to determine power at a certain range or not

Well, it's a good start. You can make directional antennas to increase the receiver level by many decibels. You can also be prudent with the bandwidth of the information you wish to send to get a few more decibels too.

but either way, this is not enough (receive power).

Well, that depends on the bandwidth of the information you wish to send (and you haven't told us that). The formula for a decent receive level (ignoring fading) is this: -

Power (dBm) needed by a receiver is -154 dBm + \$10log_{10}\$(data rate)

So, if your data rate is (say) 1 kHz, the receiver power needed is -154 dBm + 30 dBm = -124 dBm, then with an extra 20 dB added in to account for fading, you'd be looking at -124 dBm.

But, of course, your bandwidth might be much bigger than what I've assumed.

Can I use something like an op-amp to just increase the output power and get higher range?

No, you'd never do this for two reasons: -

  • You might exceed the regulatory specifications on power emitted (ideal op-amp)
  • Ideal op-amps might do the job but, real op-amps wont, ever.

Extra information here and here.

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  • \$\begingroup\$ The frequency is 900 MHz. As a follow up question, does a directional antenna only on the receiver increase the power level? \$\endgroup\$ Dec 7 '21 at 20:08
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    \$\begingroup\$ It does but it’s not as good as at both ends. \$\endgroup\$
    – Andy aka
    Dec 7 '21 at 21:17
  • \$\begingroup\$ So if I did want to amp the power from an retail transmitter, how would I go about doing that? For something like a CC1101, for example \$\endgroup\$ Dec 7 '21 at 21:48
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    \$\begingroup\$ @SagarPatil that is a question beyond the scope of probably this site without some effort shown by yourself at investigating some possible products. Maybe see what Mini-circuits can offer as a start and figure out if there's a product that can be used. See if there are any app notes on the CC1101 that give circuits for higher power like the CC1190 \$\endgroup\$
    – Andy aka
    Dec 8 '21 at 7:58
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Can I use something like an op-amp to just increase the output power and get higher range?

No, that has no sub-GHz BW nor power gain capability.

  • 1st you find out why your range is inadequate.

    • Tx/ Rx antennae gain, not line of site with tree leafs or buildings blocking and reflecting signals, cable loss, Receiver sensitivity, adjacent or co-channel interference.
  • 2nd you determine what is the local rules for radio interference and safety from human near-field exposure.

You do this before you consider boosting the Tx.

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