ACTIVE ANTENNA FOR PORTABLE SW RADIOS

One of the problems encountered in getting good performance from a SW portable is the the necessity of using a severely limited antenna. This is generally a collapsible whip antenna around one meter in length. The antenna, together with the chassis of the radio, acts as an unsymmetrical dipole which is much too short to present a reasonable impedance over the LF, MF, and lower end of the HF spectrum. For the LF and MF ranges (100KHz-5 MHz), a ferrite loop antenna is often used but it is generally small and while satisfactory for AM broadcast listening, is generally ineffective for weak signal or DX listening. The SW receiver may very well have sensitivity under a microvolt and a good noise figure, but the severely mismatched whip antennas and the very small ferrite loop antennas limit performance compared to that which can be obtained with a larger antenna. A 5 to 10 meter length of wire can often be attached to the radio but this is very often inconvienient. And also, the lack of an effective ground adds a very high impedance in series with the antenna and ground equivalent circuit, and the extra length is found not to be very helpful. An example of this situation is found in an AOR AR1000XLT pocket scanner we have that covers 500 KHz to 1300 MHz. With the supplied whip antenna (25 cm long) the scanner performance is less than useless below 5 MHz. While sensitivity measures less than 1 microvolt at 1.5 MHz, AM BC performance is very poor and unusable. For instance, the scanner only receives three 50 kW AM broadcast band transmitters from a New York City location, and reception is way too noisy for comfortable listening. A 25 cm whip feeding a low impedance input (50 ohms) is all but useless in this case. This is not the fault of the scanner, as it is being asked to cope with a hopeless antenna situation. A longer whip does not help as the scanner has very little chassis area to serve as an effective ground plane.
This same situation was encountered when trying out a new Sony ICF-SW100S pocket portable. This receiver covers 150 KHz to 30 MHz as well as 76-108 MHz FM. While the performance of this receiver is excellent, and respectable even with the built in ferrite loop, it was rather poor for weaker AM and longwave signals. Since one of our main interests is AM and LW DX listening, the use of an active antenna was considered. The receiver comes with an active antenna setup. However, this was found to lack performance on the lower frequencies and appeared susceptible to cross modulation from strong SW broadcast stations. This setup seemed mainly to be intended for SW use and did well there, but was of little help for AM or long wave. By the way, the receiver measured better than 1.0 microvolt for 10 dB S+N/N ratio over most of its range, falling off slightly at 28 MHz and above. So, therefore, little could be expected from an ordinary external preamp as the receiver itself seemed excellent, with few if any spurs, excellent selectivity good SSB/CW reception, and remarkable dynamic range performance, considering its 3 volt power supply. The active antenna jack was noted to supply DC voltage to the active antenna unit. We decided to use this feature to see if we could power our active antenna circuit directly. Lab tests showed 2.8 V DC (with somewhat used batteries installed) and a 10 ma drain using a 270 ohm resistor produced little voltage drop, and no observable change in receiver performance. However, the active antenna must operate with a supply as low as 2 volts and draw preferably under 10 ma so as not to have much effect on battery life. The existing active antenna preamp requires 30 to 40 ma at 12 volts. This is much too high a power consumption for use with a portable radio operating from a battery supply.
To solve this problem, the active antenna circuit was tried with the following modifications:
All other circuit components were left unchanged. Performance was measured at 2 V and found to be satisfactory, using a 47 ohm load resistor. Current drain was 7 ma at 2.8 V. Some signal handling capability is of course sacrificed with these changes, but this proved not serious in actual tests, as the receiver proved to be the limiting factor. The circuit board was was assembled and then built into a small plastic box (Hammond 1590 series), which in addition served as a a base for the 95 mm collapsible whip antenna used. Any style whip antenna may be used, and can be salvaged from a junked radio or TV set or purchased from Radio Shack. 3 meters of small coaxial cable type RG 174/U was used to connect the circuit via a 1/8 inch single circuit plug to the receiver's active antenna connector (an ordinary 1/8 inch single circuit jack). The DC block unit is not needed in this application, as the radio directly supplies the necessary interfaces to the active antenna
Performance was excellent, with AM broadcast stations 200 miles away being received in daylight with good listenable volume. The extremely high input impedance of the preamp circuit provides matching to the high capacitive whip antenna impedance In addition, the longwave band 150-530 KHz was found to be filled with signals previously barely audible. No AM broadcast signal breakthrough was noted, with clean reception. SW performance proved to be better than with the supplied original equipment active whip antenna, and with a smaller overall package as well.
The same scheme was tried with the aforementioned AOR scanner and a tremendous performance improvement was also noted. Above 30 MHz this performance fell off, but this active antenna is not intended to work at VHF or above, the original whip antenna being adequate. In this case DC was supplied to the active antenna through a DC block consisting of a 330 ohm resistor and 0.01 mfd capacitor, and a 6 volt supply.
While this scheme was used with specific radios, it should be easy to adapt it to most SW portables. The performance improvement below 5 MHz is literally spectacular. The original ferrite antenna of the Sony ICF-SW100S also acts in conjunction with the whip antenna and some directivity will be noted. With suitable phasing and level controls, it should be possible to use the setup for producing a pattern favoring a specified direction. Some portable radios used by owners of small pleasure boats have this capability, for emergency DF use on the AM and LF bands. With this technique the active antenna is used as a sense antenna.
Disconnecting the active antenna returns the radio to its original configuration and no radio modification is necessary in most cases as power for the preamp can be supplied via a separate battery or a DC block circuit

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