I built an RF power amplifier with about 10 to 13 dB gain operating at about 100 MHz. It amplifies signal from about 50 to 100mW to about 1W. Inspite of the amplification, I don't see a real improvement in the range of coverage. I just observe a range of about 20 to 40m. There are a lot of buildings and walls here. Is it because of the walls that the range is not improving? I am just transmitting Frequency modulated analog audio.

  • 3
    $\begingroup$ How have you measured the actual RF output of the transmitter? What happens to the received signal that makes you say when you've run out of range? $\endgroup$
    – mike65535
    May 31, 2018 at 14:09

1 Answer 1


Transmit range involves a number of factors.


At 100 MHz your signal range will be largely limited by line of site (LOS) although you may experience some fringing and reflections. This means that no matter how much you increase the transmit power, you cannot predictably increase your range beyond LOS. Increasing the altitude of the transmit or receive antenna to achieve LOS is the only way to correct this condition.

Obstructions often attenuate RF signals. At 100 MHz manmade wood structures are largely RF transparent. Concrete and other materials have a higher loss factor.

Antenna System

If your antenna system is not a good impedance match for your amplifier's output impedance or your antenna system is quite lossy, your transmitted power will be substantially less than you might expect.

If the polarization of the transmit and receive antennas are not matched, you can experience substantial losses. Losses in the order of 20 dB or more have been reported. Either match the polarization or make one of the antennas circularly polarized as a compromise.

Design Error

If you have not verified the increased output power with a reasonably calibrated watt meter, you may not have achieved the 10 dB gain you are anticipating

Minimum Discernable Signal (MDS)

Every receiver has an MDS power level that must be met before a received signal can be detected. If your system does not deliver sufficient power to the receiver's antenna to meet or exceed this threshhold, your 10 dB increase is not sufficient. You could improve upon this by further increasing the transmitted power or increasing the efficiency or directivity of your antenna system.

  • $\begingroup$ RE: "Either match the polarities or make one of the antennas circularly polarized as a compromise...." Just to note that the radiation being transmitted & received is an a-c waveform, which has no single polarity. Its polarity changes from positive to negative and vice-versa at 180 degree intervals of the r-f waveform,, Polarization is the correct term, and (as used) it applies to the physical orientation of the radiated e-field vectors with respect to the surface of the earth. $\endgroup$ May 31, 2018 at 22:00
  • $\begingroup$ @RichardFry Thanks for the correction. That is what I meant but didn't type. $\endgroup$
    – Glenn W9IQ
    May 31, 2018 at 22:01
  • $\begingroup$ I do observe a significant increase in range where LOS is available. Hence I guess 1 W of power at 100 MHz is not enough to cross the concrete walls in the way. I guess I have to increase the height of the transmitter then. $\endgroup$
    – user11096
    Jun 4, 2018 at 9:07

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