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RF (Radio Frequency) Circuits

500 mW VHF Video Transmitter

500 mW VHF Video Transmitter

Description

Q1 (BC548C) is a modulated oscillator operating in the Commercial VHF Television Band and Q2  (BC548C or 2N4427) is an VHF RF Amplifier.

Tuned circuits consist of C4, L1, C8, L3, and the two 15 pF trimmer capacitors across the collectors and emitters of both transistors. Other NPN transistors like BC54, 2N3642, 43, etc should also work.

The circuit is designed for simplicity, so "No Audio" has been included as this would involve adding at least an RF coupling transformer. The main use for this circuit is for "Security Monitoring".

L1 and L3 can be molded chokes, however, you could use 3-4 turns of insulated copper wire about ¼” diameter. Just spread them until you see a signal.

[VR] is the video balance. It’s adjustment will depend on the type of video input you provide.

If you use a 2N4427 or similar transistor for Q2 (output), you must use a heat sink. The 2N4427 is capable of delivering 1 Watt of RF at VHF frequencies so be sure that either a 50-75 Ohm dummy load or antenna is connected at all times before applying power.
 
Any composite output device will work. Cameras, game consoles, etc.  Tune the transmitter to an unused channel.

Note: You should use an old Analog Tune TV with an indoor antenna.

Designer & Author: Special thanks to Laszlo Kirschner.

Simple No-Tune Wi-Fi Sniffer

Simple No-Tune Wi-Fi Sniffer

Description

Here is a circuit of a very basic No-Tune Wi-Fi Sniffer. It’s about as simple as it gets, using only two transistors. It could detect my notebook at 10 feet with a tiny vertical. No doubt, a Well Tuned Dipole would increase it’s range.

Q1+Q2 are RF amplifier stages tuned to receive 2.4 – 2.5 GHz. L1+L2 are molded chokes. The critical components are L1, C3, L2 and C6.

LED1 is the signal indicator. As you approach a signal source, the LED [1.3v], will become brighter. It’s most useful in dim to dark areas, this is due to lack of current, however, there are now more efficient LED’s available, so it may be useful during full daylight. Alternatively, you could use an old VU, or MA DC meter instead of a diode.

The needle will gradually rise as you approach the signal source. 50K potentiometer should be placed across the meter and adjusted near a Wi-Fi for Full Scale.  The sniffer should be built into a metal box to minimize hand capacitance and stray RF effects.

For the imaginative, a Low Current DC Buzzer could be used.

Be aware that most strong RF signals can produce a reading. Harmonics from mobiles, portables, CB's, microwave ovens, and even mobile phones, or their towers are among the most common causes. Either way, it will indicate weather it’s worth while or not to open your notebook and check out a signal.

Designer & Author: Special thanks to Laszlo Kirschner.

Simple Cell Phone Jammer

Simple Cell Phone Jammer

Description

A “Cell Jammer” is just way of saying “Dirty Transmitter” which happens to transmit within the Cellular Phone Bands. Reality is, the dirtier the better.

The 555 timer [8 pin] IC simply makes a noise. It’s coupled via C4 [electrolytic] to modulate the MRF transistor oscillator. With C1 set at roughly 1/3rd, you will be close to 900 MHz. By sweeping the C1 trimmer capacitor, you can swing the output frequency from 800 MHz to 2 GHz with the transistor and values shown.

You could replace the 555 chip with an electret microphone and listen to yourself talk on a scanner, so the unit could easily couple as a UHF Bug.

Instead of a single Tapped Coil, I’ve used two molded inductors for ease of construction.Values for C1,C2,L1,L2 are critical for the frequency range.

You might want to build the unit into a metal box, add an on/off switch in the batteries + line, and maybe even add a LED. Connect an old 800 MHz cell phone antenna to C5.

Would you believe the whole thing can be built on top of the 555 IC itself when using surface mount components, and the lot will fit onto a nine volt battery clip. Output is reasonably good, although the current drain is a bit high, so a new 9 Volt battery will only run about an hour, [if you are lucky].

The “Cell Kill Distance” is around 10 – 15 feet, ample for most purposes.

Designer & Author: Special thanks to Laszlo Kirschner.

2.4 GHz RF Pinhole Camera Amplifiers

2.4 GHz RF Pinhole Camera Amplifiers

Description

Here are two simple Junk Box but extremely useful circuits. They are designed to amplify those flimsy 2.4 GHz Pinhole cameras with such annoyingly low power and short range.

The two transistor version [A] is merely a cascaded version of [B] for greater output. The one transistor version in [B] will easily penetrate two walls of an average household, while the cascaded version [A], will penetrate two floors. Needless to say, a pinhole RF camera’s Antenna connection connects to the Input of either amp. Either amplifier’s ground [Negative] connects to the camera’s Negative, and off course, Positive to Positive. The amps should use the camera’s DC voltage if possible, especially if running the camera off a DC plug pack. All component leads and connections should be short as possible. [point to point connections]. Antenna connects to the RF output.

Note: All inductors are molded chokes. They are critical. Other Higher Gain transistors could be used, but values would have to be altered appropriately.
For those intending to build either project, I strongly recommend you begin with the one transistor version for two reasons. It’s simpler and cheaper, and, when using surface mount components, will easily solder directly on the back of the camera’s circuit board.

When built with care and success, results will be excellent, and will not only encourage you to build the higher power version, but you will have acquired the skill to do so successfully. Most 9 Volt RF Cameras will work quite safely on 12Volts.
 
Caution: You will be broadcasting Video and probably Audio. The two transistor version is capable of transmitting around ½ Kilometer with the camera’s original antenna, and a great deal further with an extended antenna. It makes Public Listening and Viewing!. As far as I’m aware, the power outputs far exceed the Legal Limits set for such devices in most countries, so, if you live in a built up area, please keep this in mind.

Designer & Author: Special thanks to Laszlo Kirschner.

Wide Band RF Amplifier with Input Range 10MHz - 500MHz

Wide Band RF Amplifier Schematic

Description

The circuit is a wide band RF amplifier with an input range of 10MHz to 500MHz. It can be used as a pre-amplifier due to its low noise characteristics.

Amplifier Specifications:
Supply voltage : +12V
Current : 10mA
Input / output impedance : 50 Ohm
Frequency range : 10MHz – 500MHz
Noise figure : 3.5dB typical
IP2 : 110dBuV
IP3 : 105dBuV

The desing is a classical RF amplifier concept. The feedback resistors R3 and R2 determine the gain. By changing these values we can get a higher gain at low frequencies. Coil L2 is used to increase the gain at high frequencies. On the other hand, as gain increases stability starts to decrease.

RF Signal Attenuator up to 30dB for 1MHz - 500MHz Input

PIN Diode Attenuator Schematic

Description

This is a PIN diode based attenuator circuit operates with 1MHz - 500MHz input. PIN diodes are one of the most common parts used in RF applications generally as switches in transmitter and receiver circuits.  They are usually placed at antenna or amplifier parts. In some applications PIN diode functions not only as switch but also only as potentiometer.

Resistance of PIN diode is high when no current passing through it. When current starts flowing, resistance decreases and diode conducts. In this circuit we use PIN diode as a potentiometer. You can use this circuit in a wide variety of applications. Some examples;

  • Level control in signal generators
  • Regulation of antenna signal at receivers
  • Power control in RF amplifier drivers
  • AGC control at receivers...

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