I was wondering what calculations would need to be performed to find out if a receiver will blow out in the given conditions.

Portable AM receiver with ferrite rod antenna 200 ft away from MW antenna producing 1kW RMS into 1/4 wave GP antenna.

I know there is no clear cut answer due to different receiver tolerances, but I need to know if it is just way off and nearly impossible to happen or if it is very likely.

  • $\begingroup$ Plenty of car radios have driven by 100+ kW AM broadcast towers just off the side of the HWY. They mostly still work. $\endgroup$
    – hotpaw2
    Sep 8, 2019 at 1:35

1 Answer 1


A major edit to my original answer to capture the comments below.

The Friis equation, that I originally suggested, does not apply in this situation since the receiving antenna is only around one wavelength away from the transmitting antenna, meaning that it is still in the near field. At these low frequencies, also the conductive ground plays a major role. Accurate results can be obtained from simulations.

You can test the withstanding of your receiver by gradually moving closer and observing what happens. As the ferrite core antennas have somewhat high losses in the ferrite, the first sign might be that the antenna starts to heat up. On the other hand, if you are listening to the TX antenna, I think the first effect will be that the sound starts to clip when the output voltage from your pre-amplifier reaches the DC bias voltage.

My gut feeling is unchanged, I don't think the receiver will break down. Please let us know if I'm wrong.

  • 3
    $\begingroup$ 200 feet is too close from a MW antenna to be in the far field, so the Friis transmission equation does not apply. $\endgroup$ Sep 8, 2019 at 2:44
  • 1
    $\begingroup$ The Friis equation is valid only for free-space, far-field propagation paths, not for paths at or near reflecting/obstructing surfaces such as the earth. With more detail provided by the OP, the approximate field intensity for given transmit and receive conditions could be calculated by other means, such as provided by NEC, or Longley-Rice software. $\endgroup$ Sep 8, 2019 at 10:10
  • $\begingroup$ It is possible (but not common) for an antenna not resonant on the broadcast frequency to pick up enough signal to cause the other end of the coax to arc, so I'd never say that it's impossible to damage the receiver. If this was an external antenna, I'd say measure it before connecting the radio. But with an internal loopstick antenna, this would be impractical. Next best bet would be to approach slowly and make sure the radio isn't heating up. :) $\endgroup$
    – user10489
    Sep 8, 2019 at 14:03
  • $\begingroup$ Thanks for the comments! I made major editing to get rid of the wrongness of my original answer. $\endgroup$
    – OH2FXN
    Sep 8, 2019 at 17:02

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