VHF FM Antenna Booster: New horizons in 3-metre band DXing 2

Published:2011/8/23 2:28:00 Author:Amy From:SeekIC

N. S. Hori Sankar VU3NSH

Construction

The amplifier is built on the single-sided printed circuit board shown in Figure 2. The inductors in the design are all very simple to make, see the parts list for construction details. LI, 1.2 and L3, are wound on a 4.5-mm dia. drill or pencil. LI then needs to be stretched to a length of about 10 mm. You’ll find that a relatively large resis­tor is needed to wind choke L4 on — we used a 0.5-watt carbon-film resis­tor from the junkbox (metal film resis­tors seem to have taken over com­pletely). The value 1 Mil is uncritical, what we’re after is ’a lot’ of carbon tor the core so 820 k or 1.2 M will do just as well.



The DG-MOSFETs are not only sensi­tive to static discharges but also easy to fit the wrong way around. The non-SMD versions require 5-rum holes t be drilled In the PCB. Make absolutely sure you know each MOSFET’s final orientation on the board before solder­ing it in place — check, think hard and refer to the component drawing shown in Figure 1. You may find that the legs ate a bit too long for the PCB but don’t use your cutters until the very last moment because without the stud marker (source terminal) you will be lost for device orientation.

Several VHF/UHF DG-MOSFETs may be used in this circuit — see Table 1. With some dexterity it is also possible to use SMD devices. In general, you should aim to use a BF9xx with a low noise figure, although that may imply a slightly reduced overall gain. Remember, however, that the amplifier’s gain is secondary to the noise figure — in practice, any gain between 25 dB and 40 dB will be just fine.



The power supply parts rue not accommodated on a PCB but may be connected up using flying lead construction in a small case.

The finished PC should be cleaned with isopropyl alcohol to remove solder residu.

The amplifier must be housed in a metal case with proper coax connectors used for Kl and K2. The author used F-type sockets as customary with satellite TV rigs. They are cheap and easy to obtain. However, BNC sockets may be used equally well. The connections between the sockets and the amplifier input and output should be kept as short as possible using thin coax cable like RG174/U.

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