Why radio receivers won’t tune 800-900 MHz

guest · Dec 15, 2020

Why radio receivers won’t tune 800-900 MHz
November 28, 2020
https://computer.rip/2020-11-28 the verboten band.html

[...] people who come to radio from all kinds of different perspectives run into one odd problem: the curious refusal of any receiver to tune to certain frequencies in the 800-900MHz range.

A lot of people have a general knowledge that this has to do with some kind of legal prohibition on reception of cellular phones. That's roughly correct, but to fully explain the matter requires going into some depth on two different topics: FCC regulation of radio devices, and the development of cellular phones.
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Generally speaking, most electronic products manufactured or imported into the United States are subject to regulation by the Federal Communications Commission. Specifically, they generally require an "Equipment Authorization" from the FCC prior to being marketed. For purposes of this regulatory scheme, electronic devices can be broadly divided into two categories: intentional radiators and unintentional radiators.

In general, the whole thing is sort of bizarre. Broadly speaking, it is legal to listen in on any radio communications in the US, but cellular phones have repeatedly gotten a special carve-out.
Repeatedly? That's right.
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Because listening to the modern digital cellular modes wouldn't be particularly interesting or useful anyway, and this rule doesn't really deter anyone with the motivation and ability to decode those modes anyway, there are two lasting impacts of this rather particular rule:

1) SDRs and other receivers made today must implement this particular and peculiar restriction in order to receive US equipment authorization, which is probably part of the reason that a lot of SDRs... don't.

2) To comply with the specifics of the regulation about rejection, many receivers use a notch filter around 850MHz in their frontend. This means that reception throughout the 800-900MHz range is particularly poor, a real irritation as various public agencies and private agencies (especially railroads) use land-mobile radios elsewhere in the 800-900Mhz range.
Basically, more than a decade after any of this made sense, we're all still hassling with it.
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