# How to tune an FM transmitter to a specific frequency?

I am planning to organise a simple fox hunt, a family event. This includes building a transmitter. I am currently considering the following schematics:

Both target the 88-110 MHZ band, but that range of frequencies requires licensing. I live in Ireland and according to the local rules the following bands seem to be available:

• 26.957 – 27.283 MHz (no further requirements)
• 40.660 – 40.700 MHz (no further requirements)
• 49.82 – 49.98 MHz, 10 mW ERP, no other requirements
• 169.4 - 169.475 MHz, 500 mW ERP, Duty cycle < 10% Max 50 kHz channel spacing
• 433.050 – 434.790 MHz, 10 mW ERP, Duty Cycle ≤ 10 %

This is how I understand the document https://www.comreg.ie/media/dlm_uploads/2015/12/ComReg0271R9.pdf

Now, from the list above 169 MHz range seems to be the closest to 110 MHz. My plan is to first build the transmitter within the 88-110 MHz range, verify using common radio receiver that it works and then tune up to 169 MHz.

I assume that if I play with the coil inductor (add or remove turns) and/or capacitor the shift from 110 to 169 MHZ will be possible. I found a surprisingly affordable frequency counter on Amazon and hope it will help me to validate the frequency (so that I do not skip to 200 MHz). https://www.amazon.co.uk/gp/product/B01B3ZCP3U/

Would that work? Is this how tuning is normally done?

EDIT: The desired communication distance is up to 100-200 meters from the fox box in a park (within line of sight).

• If you have a few Euros to spare, you can get a comercial FM band transmitter. The ones with 50 nW EIRP are license-free in Europe and this might be enough for a foxhunt. They're cheaper than making on on your own and have a battery included. – AndrejaKo Mar 7 '18 at 9:30
• @AndrejaKo well, on the other hand, 50 nW + Free Space Path Loss with a path coefficient that incorporates Irish weather (i.e. spherical accumulations of water in the air in a steady downwards movement) means you'd need pretty low-noise receivers pretty soon, unless that fox hunt is very locally limited. What about simply getting a SRD handset? – Marcus Müller Mar 7 '18 at 9:35

I will assume you will get the legal issues sorted so I will address the circuit itself.

If you look at the second circuit you referenced, you will find L1 and VC1 in parallel. These form a tuned circuit that sets the output frequency of the transmitter (ignore the polarity symbol on VC1, it is a remnant of the circuit editor on this site):

simulate this circuit – Schematic created using CircuitLab

The formula for the resonant frequency of this circuit is:

$$Frequency=\frac{1}{2\pi\sqrt{LC}} \tag 1$$

where Frequency is in Hertz, L is the inductance in Henries and C is the capacitance in Farads.

In your case you may find it helpful to solve equation 1 for LC:

$$LC=\left(\frac{1}{Frequency*2\pi}\right)^2 \tag 2$$

where resonance is achieved by any combination of inductance and capacitance where their product equals the right side of equation 2.

So with L1 = 0.1 $\mu$H, VC1 would need to be set to about 26 pF to hit the middle of the FM band (98 MHz).

If you wish to adjust the output frequency to ~170 MHz, VC1 would need to be set to approximately 9 pF according to equation 1. But the recommended capacitor for VC1 probably will not go that low so select a different variable capacitor that can be adjusted to this value; or place a 10 pF capacitor in series with VC1; or use a 10 pF fixed capacitor instead of VC1 and simply stretch or compress the windings of L1 a bit to adjust the frequency. Your frequency counter will make this a straight forward process.

As the capacitor value gets quite small, the circuit may not behave as expected due to stray capacitance. You can overcome this by reducing the inductance value. For example, 4 turns of wire wound on a 0.25 inch form and spaced out to make a coil that is about 0.5 inches long will form an inductance of ~0.04 $\mu$H. A 22 pF capacitor in parallel with this will get you into the 170 MHz range.

The circuit will be sensitive to the presence of your hand or any metal tools. You can make the inductor adjustment with a small wooden stick to avoid this problem. A stiff piece of plastic (e.g from a plastic milk jug) can serve as a poor man's adjustment tool for the variable capacitor.

• Also: 9pF is really little, even compared to capacitances of PCB traces. You won't be able to reliably make this work on a breadboard thingie! – Marcus Müller Mar 7 '18 at 15:33
• @MarcusMüller I edited my answer to address your concern. – Glenn W9IQ Mar 7 '18 at 18:37

not quite sure what to say about "how is it normally done":

"how it can be done" depends on the transmitter architecture you're using, and any non-my-first-diy-fm-transmitter will be a bit more complex in the way they generate the RF oscillation :)

However, the principle for many of these will be the same: you change an energy-storing element (either a capacitor or an inductor (coil)); you measure what that does to your operating frequency, and with that info calibrate the frequency scale on your device.

Regarding legality:

Can't really comment qualifiedly, have measured none of these circuits, but "1 General Information" demands

All radio and telecommunications terminal equipment must comply with the essential requirements and other relevant provisions of the R&TTE Directive before being placed on the market or put into service in Ireland.

That requires suppression of out-of-band spurs as well as sufficient stability to be sure that your transmitter doesn't drift away from the frequencies it's allowed on. The latter might be a real problem: Your circuit is pretty temperature-dependent and will generally tend to drift (which is why real-world transmitters typically a bit more complex). However, nothing says it's impossible to make, by carefully trimming, and pampering your circuit, to make it stable enough.

Whether your location application or your transmit power is legal in your band: I honestly have no idea, and would have to read that document in full. You also have zero means to measure emitted power, raising completely different questions regarding legality than frequency!

Regarding band selection:

I think table 1 is pretty clear here, the band between 169 and 173 MHz can only be used with very low duty cycles, your transmitter must be off for 99% of time. I doubt this is optimal for fox hunting, unless you have extensively clever receiver design (as in, a computer-aided TDMA system. Not happening.). So, 169 MHz is right out.

Your best bet is the 173.2375 – 173.275 MHz band, where no duty cycle restrictions apply. However, that band is only 37.5 kHz wide. This means your transmitter needs to be pretty frequency-exact. I predict that this will be a major obstacle when employing such simple, uncompensated circuits – any temperature, supply voltage, antenna condition change (rain, potentially wind, close large objects moving) will influence the operation frequency.

So, if you really want to build a transmitter yourself instead of eg. buying a cheap SRD handset that you can program to that frequency you should probably stay away from instructables.com, and the other websites you're using, and would need to aim for more advanced circuits.

Personally, I'm a computer person (in fact, I'm a Software Defined Radio person, where we control such parameters in software). I'd frankly directly go for something that a manufacturer has calibrated and with which I can talk to digitally and simply tell it what frequency to synthesize. Now, that would require a relatively large engineering effort for you to build – cannot recommend. Annoyingly, most cheaply available programmable oscillators end at 150, 165 or 170 MHz. Just below what you need...

• Thanks! I will look into buying a ready-made transmitter indeed. Regarding power and harmonics: would the proposed designs be that strong to cause problems? I thought the signal would die off at, let's say, 300 meters and therefore nobody will be offended if the transmission is already within a shared-use band. – Alexander Churanov Mar 7 '18 at 12:00
• About R&TTE, it's now in final stages of being replaced with RED. RED has some exceptions, if a small number of devices will be produced, as far as testing is concerned. – AndrejaKo Mar 7 '18 at 12:28

Let me chime in, as I am in Ireland, and have at least some knowledge of the regulations. However I can see at other answers have already addressed this, so I am not going to repeat this.

Let me give you a solution for your family-day-fox-hunt

• 100 mW EIRP TX
• legal
• cheap
• unlimited FREE RX devices for your family memembers

If it is just for the family fun day (and lets forget about the fun building electronic circuits) you can do the following:

• get a cheap/old Wifi router
• most of them run on 9V or 12V, so you can run it on a battery of some sort
• some you can turn down the power output if you want to make it challenging
• get mobile phones and install a "Wifi tracking or finder app"
• configure the Wifi router with a recognizable SSID
• hide the Wifi router where you have your family day.
• try to find it with phones....

Lots of fun, everyone has a phone, so you have not only solved the TX problem, you have given everyone a method of RX, for all ages !

Building transmitters if fun, and this is probably something where you could say: "we have all done it". The frequency which is most versatile to experiment with is 87-108 MHz, which is the FM broadcast band. You will need to experiment with filtering and the like. And with a max ERP of 50 nW there is not much of a range.

You do have to start somewhere when you want to do this, but it will be a while before you can build a transmitter with larger power output, which can be used for your intended family day. (as other answers have already covered this)

[EDIT after reading some more documents about the legality]

I was surprised about the frequency ranges mentioned, hence my edit here.

It seems to me that most of the frequencies you are listing have a very specific license exemption, e.g. cordless phones, baby monitors, radar-to-wall, and other applications.

I would strongly advice to stay away from those "seems to be good to use" frequencies. These frequencies are NOT for "Private Mobile Radio" (PMR for short). The license exempt allocations for PMR are:

• 434.04 – 434.79 MHz (10 mW)
• 446.00 - 446.10 MHz (500 mW) channelized
• 446.10 - 446.20 MHz (500 mW) channelized digital only

However for those license exempt frequencies there are actually type-approval requirements in the appropriate legistlation.

The relevant documents can be found on Electronic Irish Statute Book:

• S.I. 93 of 1998
• S.I. 405 of 2005
• S.I. 160 of 2006

Clearly stipulating that the equipment used for those need to be "type approved", and will bring a extensive list of requirements for such approval.

Furthermore, Document 06/47R stipulates the following:

All radio and telecommunications terminal equipment must comply with the essential requirements and other relevant provisions of the R&TTE Directive before being placed on the market or put into service in Ireland.

Which was already mentioned in another answer, this will bring requirements in regards to spurious emissions and harmonics.

Under Irish legislation (The Wireless Telegraphy Acts 1926 - 1988), all apparatus for Wireless Telegraphy requires a licence unless that apparatus has been specifically exempted from licensing under Irish legislation by means of an Exemption Order.

Which comes down to "you may not own a tranmitter, unless licensed, or unless license exempt" And we already know that the license exempt equipment need to be type approved.

So your options are getting narrower once you start reading the detail of the legislation.

The PMR devices (properly type approved and license exempt) are widely available in many stores around Ireland. They are called PMR446 devices, and sold as "walkie-talkie". These will have a 500 mW output and can be loads of fun. Some of them have microphone inputs where you can feed it with a signal of some sort.

Conclusion

For a "family day of fun in the park" and therefore in a public place, I would higly recommend you stick to type-approved equipment, or the Wifi I mentioned before.

Personal Opinion

Don't let this stop you from experimenting, there are some good answers here which can get you going in building simple transmitting circuits. Just don't expect to build anything with large power output and be sure you are able to measure and adjust/filter spurious emissions and harmonics... this will be a learning journey.

I believe that many experimenters do so in the 87-108 MHz FM broadcast band, and keep the power output under 50 nW. This I mention above as well. Technically speaking this license exemption for Short Range Devices, also need type approval, and also a CE mark certification. But I could not find any other frequency allocation for such experimentation.

I am not even sure if there is an allocation you could "officially" use for experimentation.

Did I built transmitters in that frequency range?

Yes I did, learned a lot, had some fun. But never with large power, and never used them for "public" functions/family-days-in-the-park.

• Thank you so much for suggesting the idea! A report on how it went: purchased 4 routers (€2 each) and battery boxes to produce 12 volts (€3-7 each). Built 2 Faraday Cage boxes out of food packaging cardboard and cooking foil, left 1 side open. Used the WiFi Sonar app, direction was detected by the sound (higher pitch — more power) after rotating the box few times. The game was fun! Downsides: routers ran out of power within few minutes during test. Purchased most expensive Energiser batteries (36 AA = €48), lasted for few more minutes (10-20 minutes total play time). – Alexander Churanov Apr 23 '18 at 15:01
• I am glad you had a good day with some fun radio games ! – Edwin van Mierlo Apr 24 '18 at 10:55

As buying everything pre-made would take away the fun of building something yourself, I'd propose to look into kits which you can at least assemble yourself. The transmitter is the one on which legal restrictions apply, so that one might be a good candidate for getting it pre-made, or in kit form. If you want help, get in touch with a local ham radio club. Your mileage may vary, but quite certainly there's someone very eager to help you out if you want to make frequency and/or power measurements. Just sent them an email and ask, you might be surprised. Helping others (or elmering) is supposed to be highly valued among hams (as is building things yourself). Or maybe someone has an ardf transmitter that you can lend, and build your own receiver for.