An Active Whip Receiving Antenna
September 17, 2010 4 Comments
In my quest to find small antennas that work well I decided to try to build an active whip, a small vertical antenna connected to a preamplifier. The role of the preamplifier in this case is to transform the high impedance of the 50Ω of the receiver and the coax cable.
There are several designs for such amplifiers on the web. The one I built is Chris Trask‘s wideband complementary pull-push amplifier whose schematics are shown on the right. One JFET (the bottom one) functions as a current sink, another to transform the high input impedance to a lower one, and a pair of NPN and PNP transistors to drive the 50Ω load.
There are simpler designs for such amplifiers. Charles Wenzel’s design uses only one JFET and one NPN transistors, and is also wideband. Todd Gale presents several designs, both wideband and tuned narrowband (here and here). All of Todd’s designs use two transistors in a cascode configuration.
Chris’s web site actually presents not one, but two designs. In addition to the one I used, he also presents a design that uses two wideband transformers to eliminate the need for an PNP transistors. I decided to use the simpler design with the PNP transistors, since Chris wrote that the amplifier works reasonably even with the common 2N2907 transistor (but he also writes that it works better with more expensive RF transistors).
I thought that the two extra transistors in Chris Trask’s pull-push design were worth the effort and built that one. I built it into an aluminum box salvaged from some small surplus VHF amplifier, and used a 50cm telescopic antenna as the receiving element. A longer antenna would work better, but that’s what I had. If I find a longer one, I’ll replace it. Power is supplied through the coax, using a bias T near the receiver and using an RF choke to separate the DC from the RF in the amplifier.
The amplifier does not seem to overload or generate intermodulation, even though it is very wide band. This correlates with Chris’s analysis and measurements. In my usual location, the antenna worked reasonably. I was able to receive numerous shortwave broadcast stations, as well as radio amateurs on 14MHz. On the lower bands, receiving amateurs was more difficult. I didn’t hear anything on the 10MHz PSK31 frequency. On 7MHz I was able to pick up PSK31 transmissions, but very few. Even on 14MHz, signal strengths were weaker than with a 1.25m tuned loop, but that’s not surprising.
I left the receiver and computer working overnight, sending PSK31 reception reports to pskreporter.info. The map shows most of what I received during one weekday evening and night. The data collection was done with a Softrock Ensemble hardware, sdr-radio.com software, and DM780‘s PSK31 decoder and collector. DM780 decodes only some fraction of the signals, not all of them (partially because I use a trial version of VAC to route audio), so these are not the only stations I received. But the map shows that the antenna performs reasonably. You can also see on the map the one 7Mhz station received. The receiver was tuned to 14MHz most of the time, so don’t read too much into this lone reception, but as I wrote, signals on 7MHz were very weak.
The final transistors get quite hot. The dissipate about 300mW each (50mA across 6V). They can dissipate up to 500mW at 25C without a heat sink, so the 300mW is not way too much, but it’s close. Chris wrote to me not to worry about this unless the amplifier is used in the sun in the tropics. Tel-Aviv in the summer probably qualifies as the tropics for this purpose, so I should probably put a heat sink on the transistors.
I can still receive signals when the bias T, and hence the amplifier, is not powered. Signals are weaker, but they are still there. Chris wrote to me that this is caused by the signals leaking through the unpowered transistors.
In summary, this is a reasonable antenna for receiving strong broadcast stations and even for amateur signals on the higher bands. The amplifier does not overload easily. It does not require tuning, so you can quickly jump from one frequency to another, which is nice. With a short receiving element, signals are weak, especially on the lower bands.