These are the wideband active antenna circuits for the SW/MW/FM radio receiver. We can try to test it with many brands of radios.
Why should we use this circuit?
In normal, we short-wave radio receiver. We need to use a long antenna.
But with the helping of this circuit, we can use the only 18-inch antenna. We can receive signals from many band stations around the world more clearly.
I am going to show you 2 circuits ideas. There are differences, in the simplicity and the details of the circuit. Of course, a simple circuit, the quality will suffice for normal use.
But the second circuit has more equipment. So, it has higher efficiency.
Which type will you choose? Start with a simple circuit first, right?
Simple Active Antenna amplifier circuit
This circuit is the easiest with only one FET. And other equipment just a little more.
In experiments using this circuit with many a short wave radio. They have similar results with the shortwave radio antenna size 20-30 feet.
When used with a conventional MW radio receiver. Previously, some stations were poorly received. It becomes a clearer signal.
And, when using the FM tuner will make for a better signal than the antenna the attached receiver.
What is more? Keeping Read…
How it works
See in the circuit. Is it easy? Yes, it is an untuned circuit. And, the output high impedance is able to work well with a cheap radio.
Which designer wants to use with an untuned antenna like in the car radio.
But we use the active antenna with radio amateur receivers. That has low impedance input. It will not work well.
This circuit is small. But there are details that must be considered as follows.
Important to select the L1. That is the RFC – Radio Frequency Choke (RFC) to the appropriate band to use.
In-circuit we coil L1 is 470uH. Because want to amplify the so signal at lower frequencies.
For those who want to apply to the short-wave band. We may use the L1 value as 20 uH.
- This circuit can be packed into a plastic box. If a metal box would be better to reduce noise that may occur better. To make a circuit can work well in the circuit wiring. And connect the antenna cable into the jack and the output should be as short as possible.
- Use the shorter coaxial cable of 50 ohms or 75 ohms from the antenna to this circuit and the radio receivers, too. It will not lose too many signals. The cable is used should be the low capacity like use in the car and should not be used without a shield cover.
- Use DC power supply between 6VDC to 15VDC. In-circuit use a 9V battery.
When some problems occur
In the case of some problems such as:
- There is a strident sound. When listening to some stations. It may cause oscillated with the active antenna and radio receivers. maybe possible that it oscillates with the active antenna and radio receivers. The solution is to reduce the value of L1.
- The power of the 9V battery runs out fast than normal. We should check or change Q1 new.
- Quiescent current is normally of Q1 (MPF102) about 5mA. But maybe increased to up to 20mA. Which needs more power of the battery.
- On using an external power supply circuit. Put capacitor 0.047uF across the rail of power supply to filter any noise. And should be placed close to the Q1.
Parts you will need
- Q1: MPF102, JFET VHF amplifier, N-Channel Transistor Buy Here
- L1: RFC 470uH or 20 uH see text
- R1: 1M, 0.25W resistor, tolerance: 5%
- C2,C3: 470pF 50V Ceramic capacitor
- C1: 0.047uF 50V Ceramic capacitor
- B1: 9V battery
If the efficiency of the first circuit Not enough for you. Let’s look at the second circuit.
High Gain Wideband Active Antenna amplifier
In addition, this signal amplifier from this antenna, also useful as impedance matching.
Between the antenna (high impedance to the operating frequency Because the length is too large) to match the input impedance of the radio receiver. Which is mainly designed for 50 ohms impedance antenna.
Learn: High impedance preamplifier
How it works
This active antenna circuit can be designed to be used with any frequency band. But normally we use with low band frequency – VLF (100kHz or more) Up to 30 MHz.
Because of the antennas of these frequencies are very long. Until most people are unable to find enough space to install the antenna.
See the diagram below, high gain active antenna circuit. This circuit has a gain of around 14-20 dB, covering short-band frequencies to certain amateur frequency bands in the range of 1-30 MHz.
Low frequencies will have a higher gain than a high-frequency band, such as 1-18 MHz will have a high gain of about 20 dB and the gain will be reduced to around 14dB when using a 30MHz frequency.
In general, the length of the antennas used with shortwave radios is usually 1/4 of the wavelength. Therefore the antenna has a very high reactive impedance. We should use the input of this circuit as a FET better than the normal transistors.
We set it as the source follower circuit. It results in a circuit with a very high input impedance. It is a bridge that makes it very compatible. In connection with the short antenna characteristics That needs to be used with a wide frequency range in the VLF.
We can use many numbers of Q1 as MPF102, 2N3819, or 2N4416. These FETs can amplify high-frequency signals well.
Transistor Q2 is arranged as an emitter follower. In order to use Q2 as a load with high impedance to Q1. While it drives with low impedance to Q3. Which this Q3 is a signal amplifier in the common emitter. For amplifying the voltage gain of the incoming signal.
One of the important parameters of Q3 is the voltage drop across R8. The higher the voltage across the R8, the greater the gain. (Especially at lower frequencies greater than at high frequencies).
Transistor-Q4 is an intermediary for modifying the high-impedance output of Q3 to have a sufficient lower impedance, to drive the signal to the radio receiver. Which has an impedance input 50 ohms. The antenna used with this circuit We can use any type, such as solid wires or Antenna of general radio.
How to build
See the copper PCB layout and component layout below.
Choosing and setting circuit
If you want to use it with the frequency range 1-30 MHz. Should find the LC tuning circuit in the middle frequency range instead of R1 in the circuit to prevent other frequency bands Easy to disturb.
But if you want to use with very low frequency (VLF), try increasing the values of C1 and C3. It will make the circuit respond better to lower frequencies.
Same as the first circuit. In this circuit, we can use a wide power supply of 6V to 15V.
The voltage at various points makes the circuit work correctly Which has a discrepancy of 20 percent.
Parts you will need
- Q1: MPF102 or 2N3918 JFET VHF amplifier, N-Channel Transistor
- Q2, Q3, Q4: 2N3904, 0.4A 40V NPN transistors
- B1: 9V battery
0.25W Resistors, tolerance: 5%
- R1: 1M
- R2, R10: 22 ohms
- R3, R11: 2.2K
- R4: 22K
- R5: 10K
- R6, R9: 1K
- R7: 3.3K
- R8: 470 ohms
- C1,C3: 470pF 50V Ceramic capacitor
- C2, C5, C6: 0.01uF 50V Ceramic capacitor
- C4: 0.001uF 50V Ceramic capacitor
- C7, C9: 0.1uF 50V Ceramic Capacitor
- C8: 22uF 25V Electrolytic capacitor
Here are a few related Circuits you may find helpful, too:
- Wide band high frequency amplifier
- Signal amplifier circuit diagram with set input-output ratio
- The twin-T complementary amplifier circuit with filter selector
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