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Active antennas.

An active antenna is simply an antenna that consumes power, other than from the signal environment, in order to function. This briefing treats the classic case of an electrically short monopole antenna element integrated with a high impedance amplifier and intended for receiving only.

We consider the use of active antennas whenever we require electrically short (antenna length |much less than~ wavelength) elements and they must work over a broad range of frequencies without tuning. They have traditionally been used in the VLF-HF frequency ranges but are increasingly being employed up through UHF. Applications include communications, EW, navigation and scientific instrumentation, especially for man-portable and vehicular systems. What are their disadvantages?

First, being electronic, they consume additional power, yield noise and are less reliable than mere passive elements. Second, they are so broadband that the receiving system designer must be on guard against overload through adequate preselection, AGC, etc. Third, they tend to be sensitive to "burnout" due to overvoltages such as ESD, precipitation static, co-located transmitters, lightning, etc. And finally, active antennas can be a significant source of intermodulation distortion.

How Do They Work?

The classic active antenna is essentially a voltage probe. An electrically short antenna element (whip or dipole half) looks slightly capacitive (e.g. 10-25 pF/m length) and a source follower JFET circuit acts as an impedance transformer serving to take the signal from the high impedance antenna side to the low impedance (typ. 50 Ohms) transmission line input side of the receiver. For frequencies where the amplitude |Z.sub.in~ |is greater than~ |Z.sub.ant~ then |V.sub.out~ is approximately |V.sub.in~. And as long as the circuit noise is close to, or less than, the ambient electric field noise received, the S/|N.sub.out~ cannot be significantly improved upon by using a longer antenna element. What is important here is the S/N ratio and not the absolute signal strength delivered.

In practice, an active antenna's schematic will often include variations on the amplifier and additional elements such as filtering, surge protection and switching. An active dipole antenna may be constructed by having each element of a passive dipole feed a separate high impedance amplifier and summing the resultants in a balun. Information on other active antenna variations (active loops, active transmit antennas, adaptive arrays, etc.) are also available in the literature. Some useful terminology and formulas follow:

GAIN: |G.sub.a~ = 10 log (|P.sub.o~/|P.sub.i~), where

|G.sub.a~ = Gain of an active antenna |dB~ |P.sub.o~ = Power output to the receiver from active antenna |W~ |P.sub.i~ = Power available from the passive antenna element |W~, calculated as the product of the passive antenna element effective area |m2~ and the signal strength density |W/m2~.

Note that this definition of an active antenna's gain is a transducer power gain and does not include directivity. Do not confuse this term with antenna factors or conversion factors which merely calibrate output power or voltage with respect to the incident electric field strength.

NOISE FIGURE: NF = 10 log (|P.sub.n~/kTB), where

NF = noise figure of an active antenna |dB~

k = Boltzmann's constant = 1.38 X |10.sup.-23~ |J/K~

|P.sub.n~ = noise power |W~

T = temperature |K~

B = bandwidth |Hz~

|P.sub.n~ is measured at the output with the active antenna's input terminated in the passive antenna element's equivalent impedance and the output terminated in the transmission line impedance (e.g. 50 Ohms). When computing system S/N, the contribution of active antenna noise must be added to the ambient electric field noise and the receiver noise.

EQUIVALENT NOISE FIELD: |E.sub.n~ = |{(|P.sub.n~/B)Z}.sup.-1/2~/|h.sub.eff~, where

|E.sub.n~ = Equiv. E noise field strength |V/m/|Hz.sup.-1/2~~

|h.sub.eff~ = passive element effective height |m~

Z = Input impedance of RCVR |Ohms~

|P.sub.n~/B = noise power density |W/Hz~

External radio noise data is often described in terms of an effective F or noise factor. Conversely, it is useful to be able to express external radio noise, or active antenna noise, in terms of an equivalent electric noise field strength. REFERENCES

The following selections should make a good start. In particular, do not neglect foreign sources nor the amateur/hobbyist electronics periodicals.

1. Burhans, R.W. "Low-Frequency Receiving Techniques, Building and Using VLF Antennas," Radio-Electronics reprint, 1984, Gernsback Publications Inc.

2. Gosling, William, Ed., Radio Receivers. Peter Peregrinus Ltd., IEE, London, 1986, pp.33-34.

3. Goubau, G. and F. Schwering, Eds, Proc. ECOM-ARO Workshop on Electrically Small Antennas (May 6-7, 1976), US Army Electronics Command, Fort Monmouth, NJ, Oct 1976.

4. Rohde, Ulrich L. and T.T.N. Bucher, Communications Receivers, Principles and Design, NY, McGraw-Hill, 1988, pp. 183-199.
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Title Annotation:EW Reference & Source Guide: Technical Manual; electronic warfare
Author:Smith, Kevin G.
Publication:Journal of Electronic Defense
Date:Jan 1, 1994
Words:802
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