I am looking at antennas for a rather unusual application: mounted under a utility pit lid (manhole cover).

The goal is to receive commercial MW/LW stations. The antenna has to be attached to the underside of the lid with a magnet. To make it easy to install we want it to be as omnidirectional as possible, because the installer can't hang around aligning it.

What type of antenna is best suited to this? We are willing to consider things like having two switched/combined ferrite rods at right angles, if that will work.

Should have mentioned, the received signal can be very poor and still be fine for our needs. Even if it's 95% noise that's fine, as long as there is some small amount of signal. We have made successful tests with consumer MW/LW receivers with ferrite antennas. The main issue was alignment.

  • $\begingroup$ you ask for "The goal is to receive commercial MW/LW stations" and in a comment you say "In fact we have tested a consumer AM radio in a few pits under a lid and it works fine" ... I am wondering: what is the actual problem you are trying to solve ? Please update your question with what you are trying to do, keeping in mind the answer(s) given already. $\endgroup$ Nov 28 '16 at 9:20
  • $\begingroup$ Unfortunately it's commercially sensitive so I can't elaborate too much. The bottom line is we need to record the audio but even extremely low quality / massive amounts of noise is fine. $\endgroup$
    – user1211
    Nov 28 '16 at 15:13
  • $\begingroup$ So if you've tested a consumer AM radio and that works fine, why not just go with that? It almost surely has a ferrite stick antenna, which is generally an excellent choice for performance considering typical cost and size limitations. $\endgroup$ Nov 28 '16 at 18:55
  • $\begingroup$ If it is "commercially sensitive" then it sure is not "Amateur Radio" related. I agree with Phil that if AM is working fine on a consumer radio, why not use that. Anything further I would suggest that you hire a consultant in RF/radio/electronics to scope and design your commercial application. $\endgroup$ Nov 29 '16 at 8:35
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    $\begingroup$ I'm voting to close this question as off-topic because this question is about a commercial application, which is has a "sensitive nature" where the OP cannot post all details. $\endgroup$ Nov 29 '16 at 8:37

A lid where a magnet attaches is made out of iron.

Iron is a good conductor.

You're trying to build an antenna in a place which is especially well-shielded against radiation. That will be very very difficult.

Omnidirectional from under a manhole cover means "in the earth". If that isn't dry, then there will be little signal there.

Yes, radio waves can penetrate wet soil – as an estimate, with a very sensitive receiver and a very powerful transmitter, you could do up to 10m ground penetrating radar.

But this will not be possible for any receiver that is supposed to receive an AM signal and is remotely related to ham technology (making it on-topic here). For that kind of sensitivity, you'd need extremely low noise figure, and that will be extremely expensive.

All in all, I don't know what kind of radio broadcast receiver for mole people or explosive trigger device you want to build, but I trust physics enough to believe this won't work at your level of insight.

The fact that you're asking about commercial MW inside populated areas (i.e. where there are manhole covers) might indicate you're Britain-based. Now, that'll also mean the broadcasters won't be pushing incredibly much power, making this a bit harder.

To make things more complicated, omnidirectional won't work easily. The reason is simple – the MW/LW wavelengths are far bigger than a manhole, and thus, you can only use high-Q antennas (ferrite core antennas, for example), and those tend to have a very clear preferred direction. Having arrays of these probably won't work either – they'd interact (being all within $\ll\frac12 \lambda$).

Frankly, think about it: As you know, the typical propagation mode for commercial LF and MF is surface wave – the earth's conductive surface forms kind of a waveguide together with ionospehre – and that means that wokring MW reception is based on the fact that at the point of surface, there's little power transport. You could try to make an antenna pick up the displacement currents – but that's going to be freaky business, especially anywhere there's electrical infrastructure (which happens to be the places with manholes).

  • $\begingroup$ In fact we have tested a consumer AM radio in a few pits under a lid and it works fine. We could have been lucky I suppose, but as far as I am aware we aren't near any transmitters. We are UK based, as you guessed. The received signal does not need to be great, even listenable, in fact we can handle <5% SNR quite easily. $\endgroup$
    – user1211
    Nov 28 '16 at 9:15
  • $\begingroup$ I don't think the shielding is quite as dire as you say, a low NF at HF is easily realized for less than $5 in parts, and the Q of the antenna does not make it much more directional than a dipole. Compare the radiation patterns of an infinitesimal Hertzian dipole to a half-wave dipole: they are nearly the same. $\endgroup$ Nov 28 '16 at 12:24

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