A Low-cost Professional UHF Room Transmitter
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For a change, I thought I’d talk about my favourite bit of surveillance kit, namely UHF bugs. Ultra high frequency (UHF) describes a range of electromagnetic waves whose frequency is between 300 MHz and 3 GHz. Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz.
UHF Bugs typically transmit on frequencies between 300MHz and 900MHz. You’ll notice that the main wireless spy camera frequencies (1.2GHz and 2.4GHz) are in the UHF band too. Commercial radio stations transmit on VHF between 88MHz and 108MHz.
Typically VHF bugs are much cheaper than their UHF counterparts, because VHF bugs do not use any electronic components to stabilise the transmitting frequency. VHF bugs tend to drift in transmitting frequency due to environmental fluctuations such as temperature and background electro-magnetic radiation. UHF bugs make the use of a crystal oscillator to stabilise the transmitting frequency, adding circuit complexity, extra cost, but do offer improved audio clarity.
The majority of ’spy shops’ sell products at grossly inflated prices. What’s even more frustrating, is that people pay those prices too! It’s rare to find a UHF bug for less than £150, but I used to sell them in the Spy Warehouse for just £85 each. Sadly, these devices are no longer manufactured, so I cannot sell them any more.
There are two variations of the UHF bug based on the same circuit design. One used a CR2032 lithium coin cell battery, which gives the bug an overall size of just 27mm x 16mm x 15mm. The battery lasts about 20 hours, and it transmits audio in perfect clarity on 434.075MHz.
The bigger brother to the CR2032 version is the AAA battery version of the UHF Bug. The circuit is identical, but the transmitter uses 2xAAA batteries to give 80 hours transmission time (rather than 20), with a similar range of 800 metres and only slightly larger to accommodate the larger batteries.
What I really like about these bugs, is that both are small enough to be concealed in a large number of objects. If you wanted to do a long term surveillance operation, you could use a mains adapter to avoid using batteries. You could modify the CR2032 version, connect a smoothing 3V power supply, and you’re off! I say the CR2032 version so that you can keep the design small, but its not that important really.
Sadly, these devices are no longer made now. I’ll update the blog with information about any new devices that I discover.
