VHF FM Antenna Booster: New horizons in 3-metre band DXing 1

Published:2011/8/23 2:17:00 Author:Amy From:SeekIC

N. S. Hori Sankar VU3NSH

Together with a good directional antenna this high performance two-stage antenna amplifier for the VHF FM broadcast band will enable you to capture far removed (DX) stations. Alternatively, it will drastically improve reception of FM signals you’ve come to accept as marginal and noisy in your area.

FM broadcast transmitters are normally meant to have a useful range of 30-100 km. Fine, but if you are in a fringe area, all you may get is a very weak signal, possibly even blotted out by a power­ful station around the corner.

For various reasons, an increasing number of people are not satisfied with the quality of radio signals that can be received via cable systems. Unfortu­nately, cross-modulation, and other nasty effects created in the head end station are in stark contrast with the superb quality of high-end FM tuners that can be bought commercially. Not surprisingly, owners of such tuners will often prefer to have their own antenna on the roof. Add to that a high-performance KM antenna booster like the one described here and you can start DXing as well as enjoying high-quality stereo reception. The author employs the present amplifier in combination with a Sangean ATS-803 World band receiver, a Philips RR-571 tuner and a single-element cubical quad direc­tional antenna at a height of just 15 feet. Using this setup he is able to receive FM stations as far off as 500 km.

Circuit description

In the design we’re about to describe you’ll find the following important parameters coupled: low noise figure (approx. 1 dB); high gain (up to -10 dB) and low susceptibility to intermediation products. Yet die amplifier is inex­pensive and easy to build.

The circuit diagram in Figure 1 reveals the secret: dual-gate MOSFETs in posi­tions T1 and T2. The first of these, T1, is configured mainly far low noise and antenna matching and the second, T2, for high gain. Unusually, the antenna signal is applied to T1 via its source (S) terminal, which is convenient because unlike one of the gates (G 1 and G2) it already represents a fairly low imped­ance. None the less, for impedance matching with the 5011 coax cable a tap needs to be used on tuned filter L1/C2.



For VHF and UHF amplifiers, IDG -MOS­FETs represent a good alternative to cheap but noisy bipolar transistors and very expensive and difficult to get Ga-As FETs.

The gain of the second DG-MOSFET in the circuit is adjustable using preset PI which varies the bias voltage to T2’s G2 terminal — this is the classic way of controlling the gain of a DG-MOSFET and it still works very well. Such a control is included in the design to allow you to get the exact amount of gain required for your particular appli­cation. For example, if you live near a powerful VHF FM or TV broadcast transmitter then you’ll find that a lot of gain produces cross-modulation and other unwanted effects like coupled oscillation and ’birdies’ within the FM band.

For stability the supply voltage is decoupled at several locations by elec­trolytic and ceramic capacitors for the low and high frequencies respectively. To enable it to be mounted as close as possible to the antenna, the amplifier is powered via the download coax cable, i.e., over coax connectors con­nected to K2 and K3. In the supply, the RF signal is taken off the coax core by capacitor C19.

The supply voltage is adjustable to some extent with pot P2, which will also allow a degree of gain control. The current through ’power on’ LED D2 should be about 10 mA. If a low-cur-rent LED is used, then R8 has to be increased accordingly. With electrical safety in mind we strongly suggest the use of a mains adapter with 12 VDC output. Depending on the gain set and the DG-MOSFETs used the circuit will consume about 50 mA so a I00-mA or slightly more powerful adapter will be fine in most cases.

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