OH8GAD

70 MHz (4 metre) FM transceiver

70 MHz, a.k.a 4 metres, sounds like a good band to play with. It is close to HF, so that construction techniques can be on the simple side, but also high enough so that you don't need huge aerials - a half-wave dipole will only be about 2 metres across.

Because my work space, skills and tools available are pretty limited, I am going to use cheap kits as much as possible. Buying kits from the likes of Banggood is a lot easier and cheaper than getting all the components seperately. You also get a PCB thrown in, so no mucking about etching your own either. The only drawback is the delivery time, anywhere from about two weeks a coupe of months. The plan is to start with a small kit, modify it, add to it and so on. Grow it, evole it and learn about what you're doing. So, here goes...

The plan:

The overall plan for the 70 MHz transceiver is:

  1. Build basic 70 MHz transmitter
  2. Add varactor diode tuning
  3. Add digital control of tuning*
  4. Build a receiver
  5. Add narrow filter
  6. Build a basic, proper aerial
  7. Boost power

* This needs some sort of connection to and control from a PC. I have decided to use Python

circuit diagram for 70 MHz FM wireless microphone

I bought three FM "70 MHz" wireless microphone kits. I bought three, so that I would build the first "as is", to give a base point to work from, which I'll use as a reference signal source, set to 70.450 MHz, the FM calling channel. Then, the second one, I will build with mods and add-ons straight away. the third kit will, hopefully, become the receiver. In the end, it will become a proper 70 MHz transceiver.

I built the first FM "70 MHz" wireless microphone kit. The information provided states that the default frequency is about 70 MHz. I used the GY560 Frequency Counter to check this, and... It's actually just above 80 MHz. I have actually build a couple of these in the past and in all cases, they are just over 80 MHz. The 4 metre radio amateur band is from 70.0 to 70.5 MHz. So, we're way off to start with.

70 MHz FM wireless microphone Inital frequency

The instructions suggest that you can change the frequency by stretching the coil, but when you make a coil longer, you lower the inductance, so the resonant frequency increases.

I did a little test by pushing my thumb nail between the turns of the coil to stretch it a little bit. You can see that the resonant frequency increases:

Stretched coil Frequency shift

If you are making this as a "wireless microphone" and want to receive it on a regular Band II (88 to 108 MHz) FM radio, then you should increase the length of the coil until you can recieve the signal on your radio.

In this case, however, we need to lower the frequency. So, we need to either increase the inductance or the capacitance. To increase the inductance, we would need to add more turns. This isn't practicaly possible for this kit - unless you can find a suitable bit of copper wire. The other way is to increase the capacitance value.

To make things easier, I found an online inductance/capacitance/frequency converter (See link below). I put in the values I knew:
Capacitance = 30 pF
Resonant Frequency = 81.5989 MHz

That gave me the value of the inductor as 126.809094104 nH.
I then put that value in the L box and the frequency I want, 70.450 MHz, into the ƒ box, which then gave me the value of capacitor I need:
40.246476833 pF

The nearest value of fixed capacitor to 40.246... pF is 39 pF. Which would give a frequency of about 71.5 MHz, which is too high. I could also put several capacitors in parallel, to make the desired value. For example, 30 + 8,2 + 1 + 1 pF, would eaqual 40.2 pF, which would result in a frequency of 70.49 MHz. Which is about 10 kHz, almost one channel - the 70 MHz band uses 12.5 kHz channel spacing in that part of the band. Also, the inductor is still unreliable, as it can move and change the frequency.

I think the best bet is to put some glue on the coil, to make it more rigid and then fit a variable capacitor, something like 10 - 50 pF. One other thing to note, is that because the board is tiny, any additional components will need to be attached to the board with wires. At these frequencies, even short wires act as inductors, so a variable capacitor would help esure these extra inductors can be compensated for.

I ordered a 4 to 27 pF trimmer capacitor from my friendly local neighbourhood electronics shop, but while it was in-transit, I found in my collection, a 2 to 20 pF trimmer.

I soldered the trimmer capacitor across C4 (30 pF) on the underside of the PCB - it was a surface-mount device, so I was able to solder the trimmer's tabs directly to the same pads that C4 was soldered to.

The result was on the new frequency was just over 71 MHz. I then played around, turing the capacitor and found the circuit now tuned from just over 71 MHz to just under 80 MHz. The goal is 70.450 MHz."

Trimmer capacitor on underside of PCB Frequency shift with extra capacitor

One other change I have made so far, was to remove the small ON/OFF push switch. It is quite fiddly to use and also, touching the baord can cause small shifts in frequency. I replaced the original switch with normal toggle switch, attached to the board with a couple of wires. I can now turn it on and off without touching the PCB.

I swapped the 2-20 pF trimmer for a 4 to 27 pF one. The initial frequency was just over 66 MHz. It was quite difficult to get the frequency close to the desired 70.450 MHz, as the slightest touch to the trimmer would send it several hundred kHz in one direction or the other. I finally got it fairly close, at 70.4526 MHz.

4 to 27 pF trimmer Frequency in band

Because both the coil and the trimmer capacitor are mechanical components and very small, they are affected by the slightest movement. Even using a metal screwdriver to adjust the trimmer meant I couldn't see the true frequency until I removed the blade. The result is that the frequency can drift around.

Frequency drift

To be continued ...

70 MHz / 4 metres

One thing to note, though. The 70 MHz band is not available in all countries. 70 MHz was freed up when countries closed down their VHF (Band I (VHF low) 54–88 MHz) TV networks. Finland was one of them.

Finnish allocation is now from 70.0 MHz to 70.5 MHz.

Frequency Use Notes
70.000 - 70.100 MHz Propagation Beacons This area of the band is allocated to beacon stations with a maximum transmission bandwidth of 1kHz.
70.100 - 70.250 MHz Narrowband Modes (CW/SSB/MGM) Narrowband modes with a maximum bandwidth of 2.7 kHz: CW,SSB and MGM
Most CW and SSB activity will be conducted around 70.200MHz.
70.250 - 70.294 MHz All Modes The 70MHz band is unique insofar that it has an AM.calling frequency on 70.260MHz.
70.294 - 70.500 MHz All Modes (Channelised Operations – 12.5 kHz spacing) These are narrowband FM (NBFM) channels with 12.5 kHz spacing and in this sub-band area you’ll find FM telephony, packet radio, RTTY and internet gateways.