Another Active Antenna

Ron, G4GXO recently drew the attention of the QRP community to an engineering report released by the US government describing a Voltage Probe Antenna (VPA) Design 5 kHz to 500 MHz. The design dates back to April 1973 and was produced by GTE.



The objective of the overall development of the Voltage Probe Antenna (VPA) was to produce a miniature, mast mounted, omni-directional antenna that would operate over the band from 5 kHz to 500 MHz for communications reception. In addition, it was desired to optimize the antenna at 150 MHz and 400 MHz.



The report of the report can be found at:



http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA030178



The report itself is at: http://handle.dtic.mil/100.2/ADA030178



As Ron reports, "The images but they are all "washed out" by the copying process. However, there are some illustrations of the test range which suggest a short pole (about a couple of feet high) onto which the antenna is mounted. At the base of the pole is a ground pane to simulate salt water - presumably this aerial was for covert use when the submarine was at snorkel/periscope depth. The aerial element appears to be small, probably around 30cm high by 15cm diameter. There is also mention of the "probe" being a bi-cone, a wideband VHF/UHF antenna - note that one of the design objectives was to provide LF to UHF coverage optimized at 150MHz and 400MHz; VHF aviation/marine and UHF aviation bands?"







The circuit is quite interesting since at first glance it looks like a conventional FET input active antenna similar to the PA0RDT design that we are all familiar with, with the modification that an MMIC is used in place of a transistor in the second stage. However on a second look (an reading the text) I see that there is an interesting subtly to quote from the report:



"The amplifier consists of two active stages; the first stage consists of a low-

noise field-effect transistor used for active impedance matching, and the second stage consists of a low-noise microwave integrated circuit that provides actual voltage gain.



"The field-effect transistor is operated in a cource-follower(sic) configuration with an additional gate~to-source capacitor C2 added to alter the input impedance characteristics and to improve high-frequency performance. At low frequencies where the gate and source of the transistor have an in-phase voltage of nearly equal magnitude, capacitor C2 essentially disappears from the circuit as very little voltage appears across it. This provides a very high impedance at lcw frequencies that approaches 1.5 megohms shunted by 4 pF.



"At high frequencies where the voltage on the source of the transistor begins

dropping and the phase shift becomes important, signal voltage begins to develop

across capacitor C2. This provides a signal path around the first-stage transistor Q1. As the frequency goes progressively higher, the impedance of capacitor C2 drops quite low until the input impedance of the amplifier approaches that of the second-stage amplifier A1. At high frequencies (above 100 MHz), the first-stage amplifier Q1 adds very little to the performance of the circuit.



"Diodes CR1 and CR4 provide burnout protection for the input of the amplifiers. Resistor R3 is provided to suppress transients on the output cables so the voltage limitations of the output capacitor are not exceeded."



The amplifier performance specifications, measured in a 50-ohm test system,

are given in the report are shown below:



Frequency Range 5 kHz to 500 MHz

Gain 11 dB +/- 1dB

Noise Figure 8dB at 150 MHZ and 400 MHZ

Output Power

(1 dB Compression) 6 dBm

Intercept Point

(Third order) 17 dBm

DC Power 45 mA_at +12 Vdc

15 mA at -12 Vdc

Physical size 1x1x0.5 inches



It would be interesting to redesign the antenna using more modern components for use as a wideband scanning antenna.



Stewart/G3YSX