I'm developing a type of ground penetrating radar that does not use pulses. It's extremely narrow bandwidth. As narrow as a good crystal can get it. The frequency I'm shooting for initially is somewhere between 150MHz to 900MHz, but that's not set in stone.

So I'm wondering what the maximum legal power is for such a device in the United States, California. I don't have any HAM license. I've seen GPR units for sale that produce hundreds of watts, but they produce pulses. My device will not produce pulses and will not modulate the frequency. The signal will initially slowly rise to peak, and from there it will maintain a constant power for about 100 seconds at a fixed frequency, followed by a slow decay when it's done.

I've tried googling this. The only thing I find is regarding HAM radios (up to 1500 watt limit). Also there are pulsed GPR units for sale. Perhaps there are less restraints for non-modulating fixed frequency signals. My GPR unit will be slowly rotating. The device will be used far out in rural areas. Perhaps 10 to 200 miles away from any city. I'm guessing that even if the unit exceeds legal power limits there's almost zero chance of being caught since there'll be nobody out there except some hunters and campers and perhaps forest rangers. If caught, then I'm wondering if the authorities will take away my GPR. The GPR unit wouldn't be heard on radios since its at an extremely narrow BW. So the Rangers should be okay. They'll probably be more curious then anything else. At say 300MHz the GPR yagi antenna will be around 7 elements.

Any help is greatly appreciated!

  • 1
    $\begingroup$ Interesting, but it's not really an amateur radio question. Note that even an unmodulated signal can cause trouble; you don't want to accidentally block a police, aviation or satellite distress signal. Why not obtain a ham license and then use it to transmit legally, in the appropriate part of the amateur band, to perform your experiments? $\endgroup$
    – tomnexus
    Jul 24, 2016 at 17:57
  • $\begingroup$ 150-900 MHz is a pretty enormous range, and it includes a large number of radiocommunication services: commercial broadcasting, cell phones and amateur allocations, just to name a few. Can you be more specific about the frequency range you want to operate in? $\endgroup$ Jul 24, 2016 at 18:29
  • $\begingroup$ I'm not sure what will be the best frequency but 2 meters is a good first choice. Does it take a long time to get a license? I heard they no longer offer novice and advanced. $\endgroup$ Jul 24, 2016 at 19:09
  • $\begingroup$ So even if the signal is not modulated it would interfere? $\endgroup$ Jul 24, 2016 at 19:11
  • $\begingroup$ @Paul: While it's true that the old levels named "Novice" and "Advanced" no longer exist, they have been replaced with levels named "Technician" and "Amateur Extra". $\endgroup$ Jul 24, 2016 at 20:41

4 Answers 4


An amateur "ham" radio license in the US will not allow the kind of transmission you are attempting to do. Therefore, amateur radio is not your solution path.

The FCC regulations part 97 that covers the Amateur Radio Service dictates precisely the types of transmissions that can be made. In particular, the purpose is maintain two-way communications between two stations. At least one station must be a licensed amateur radio station and if the other station is not a licensed amateur radio station then it must fit into a very small category such as special MARS (Military Affiliated Radio Service) station, or possibly a licensed but non-amateur emergency services station where the amateur station is participating in emergency services communications.

Your desired transmission is called one-way in FCC Part 97 regulations and the only valid one-way communications allowed are each explicitly dictated by the regulations. Operating a ground penetrating RADAR unit is not one of them. These legal one-way transmissions are allowed for a variety of reasons such as: brief test signals, transmissions designed to raise a responding licensed station, and a few other reasons.

The full detailed list of the legal and and restricted (illegal) transmissions made by an licensed amateur station are covered in paragraphs: 97.111 "Authorized Transmissions"; 97.113 "Prohibited Transmissions".

Other specialized one-way transmissions include beacon stations outlined by paragraph 97.203; and a space station 97.207; a space telecommand station as described in 97.211; telecommand of model craft 97.217; and telemetry as described in 97.219.

Other than these sections I list here, there is other language such as the opening paragraphs, of FCC 97 that do not allow the type of transmissions you describe for a ground penetrating RADAR.

However, having said all this about amateur radio regulations, it is possible to obtain a special license for short-term usage of some band of frequencies for scientific experiment usage. This would be by request to the FCC but I expect the format and content to be specific to the precise needs of the experiment, duration, and so forth. Definitely not something for some general usage application.

  • $\begingroup$ Thank you. Very detailed to my device. I still don't see how people can legally operate GPR units in the U.S. I've never heard of GPR owners requiring a permit to operate the unit. Your answer gave me the idea to ask a company that sells GPR devices. $\endgroup$ Jul 25, 2016 at 1:11
  • $\begingroup$ @Paul In this article you will find some comments regarding the FCC licensing of ground penetrating radar. I just briefly read it and certainly not the whole document but it might give you hints on other places to look. I have googled "FCC licensing of ground penetrating radar" and there may be better search keys to use. See: transition.fcc.gov/Bureaus/Engineering_Technology/News_Releases/… $\endgroup$
    – K7PEH
    Jul 25, 2016 at 2:00
  • $\begingroup$ @Paul And, this article seems to be more appropriate for your questions. researchgate.net/publication/… $\endgroup$
    – K7PEH
    Jul 25, 2016 at 2:04
  • $\begingroup$ Thank you. Looks like a 2002 date stamp. There might be an updated version. I was just reading a GPR website that says there's no frequency restrictions, and does not mention any license requirements. FCC rules are extensive and difficult to learn. Maybe there're lawyers specializing in this lol. $\endgroup$ Jul 25, 2016 at 4:00
  • 1
    $\begingroup$ @K7PEH That's a valid argument (at least semantically; I can't argue in terms of the legislation). My basis is the wording in US § 97.3(a)(4) and the ITU regulations both about technical investigations. That of course assumes that this is done for technical investigations and not something else. Compare life.itu.int/radioclub/rr/frr.htm, article 1, section III, § 1.56. That said, I'm not positive that such an argument would hold legal water, so best might be to get a special experimental permit; I'm simply reluctant to categorically rule it out the way you did in the answer. $\endgroup$
    – user
    Jul 26, 2016 at 14:17

So I'm wondering what the maximum legal power is for such a device in the United States, California. I don't have any HAM license.

If you do not have an Amateur license then you are limited to “Part 15” operations and a very low output power — 1 watt or less depending on frequency and antenna gain ("citation needed" as they say; I found only unclearly-written sources).

The GPR unit wouldn't be heard on radios since its at an extremely narrow BW.

In general, your question is supposing that your device will interfere less if its frequency is stable and unmodulated. This is false.

While it does mean you are less likely to interfere with any given other user (since you are less likely to be within the channel they're using), it also means that the severity of the interference is worse.

For example, anyone using FM two-way radios (“walkie-talkies”) for communications on the same frequency you're using would be unable to communicate if their signal power is not higher than yours — they would hear only silence. Other modes will display different effects, but there will always be interference.

I don't know for sure whether it is legal to operate a radar in amateur bands at all (since it is not a transmission for communications).

If it is, then using an amateur band is actually a better choice than any other portion of the radio spectrum. This is because amateurs are prepared to change frequencies if needed to avoid other users, whereas most other radio services use fixed frequencies. And for this purpose, your narrow bandwidth is advantageous.

However, you will need to modulate your signal, because except in the case of radio-control transmissions (model airplanes and cars), you are required to identify yourself within the transmission. This identification can simply consist of CW / Morse code, i.e. periodically interrupting the signal in a specific pattern.

  • $\begingroup$ That sounds like one watt is legal, but that makes me wonder why certain commercial GPR units that greatly exceed one watt are legal. Btw it'll be a directional yagi antenna where the signal will be directed into the ground with some reflection. Maybe I'll direct the reflected signal so it bounces in Russia for the decades of their Woodpecker signal lol. $\endgroup$ Jul 24, 2016 at 20:56

If you transmit without a license in a band allocated to a licensed service and/or with unlicensed equipment, you're breaking the law and possibly messing up someone else's communications. Don't do that. Even if you're unlikely to get caught, there's a reason the FCC is in charge of frequency allocations: the EM spectrum is a public resource, and no one gets to use it to the exclusion of others.

If you want to experiment with new (non-commercial) uses for radio technology, get a ham license – that's what it's for! The entry-level Technician license is easy to study for with free online tools or inexpensive books, and you can find a club near you that offers license exams. I'm a newbie myself and have loved the freedom to learn about radio technology by experimentation and from talking with more experienced hams.

  • 1
    $\begingroup$ "If you transmit without a license in a band allocated to a licensed service and/or with unlicensed equipment, you're breaking the law" Not completely correct. You don't need an amateur radio license to transmit on ~2.4 GHz with your microwave oven (or WiFi access point), even though there's an amateur allocation covering the frequency range in use. ISM isn't the only example; there are several amateur allocations where amateur radio is a secondary user. It would be more correct to say that transmitting without a license in a manner requiring a license is illegal. $\endgroup$
    – user
    Jul 27, 2016 at 16:53
  • $\begingroup$ Good point - I hadn't taken secondary allocations into account. $\endgroup$ Jul 27, 2016 at 16:54

Let me word this differently:

If you do NOT have a license, you may NOT transmit and "radar" signal. (full stop, no exceptions)

Even when you do this in "license free" frequencies/bands. These bands and frequencies, while excempt from license requirements, are still allocated, and only for use for certain services and type-approved-equipment. Which does not cover what you are asking.

When you have a license, then you are bound by the terms of the license. And for your question; this will be a very specific license.

IANAL: I do not believe that a Amateur (HAM) Radio license includes permission for transmissions as you describe.

In regards to other very valid answers given. Most answers (and comments) refer to "allocations" in the frequency spectrum.

Lets be very clear: EVERY FREQUENCY IS ALLOCATED in some form or other; there are NO "free bands" !

.... exception below 9KHz, which is in the VLF spectrum, good luck with your radar signal there. Even at that; you are still bound to rules and regulations from local/federal authorities !

so, the answer is: sorry, but no.

  • $\begingroup$ I've never seen any FCC language state that. A few years ago I know numerous engineers who were developing PI metal detectors said the FCC allows up to 5 watts. That's in agreement with present walkie-talkies, which don't require a license to operate. BTW these engineers were field testing new metal detectors that did not have a UL sticker. $\endgroup$ Jul 26, 2016 at 15:45
  • $\begingroup$ Also while doing some research a few years ago I learned there are no laws requiring electronic devices to get UL tested; i.e. those stickers on electronic devices that say UL listed. But if you want to sell the device to large companies such as Home Depot then it will need to be UL listed and each unit will need to display the UL sticker. Sorry I don't have references off hand right now. That was research I did a few years ago. $\endgroup$ Jul 26, 2016 at 15:45
  • $\begingroup$ UL (formerly Underwriters Laboratories) is a private company that certifies equipment as being safe to operate so that it won't burn your house down. That has nothing to do with FCC type acceptance. $\endgroup$
    – rclocher3
    Jul 26, 2016 at 16:05

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