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1 An Active Short Dipole Antenna with Remote Tuning by Chris Trask / N7ZWY Sonoran Radio Research P.O. Box Tempe, AZ Senior Member IEEE 18 May 2009 Revised 7 March 2011 Trask, Active Short Dipole Antenna 1 7 March 2011
2 Introduction Active antennas are of interest to a wide range of users, from shortwave listeners (SWLs) and radio amateurs to designers of aircraft radios. SWLs and radio amateurs living in confined areas such as apartments or in communities having antenna restrictions find small antennas and especially active antennas to be a practical solution. Physically small antennas that have a directional radiation pattern can be very useful when nulling out interference is desired. Small loop antennas have this characteristic, and small shielded loop antennas provide a good degree of immunity from low frequency electric field noise, which is highly desireable in urban environments. Both forms of loop antennas require some form of tuning in order to accomodate the high inductive reactance of the terminal impedance. Designs that employ parallel tuning require an amplifier to couple the resultant high antenna impedance to the feed line, while those that employ series tuning can be passively matched to the feedline and have exceptional noise figure (NF) and intermodulation distortion (IMD) performance. A half-wavelength dipole has a terminal impedance of roughly 72 ohms. As the antenna becomes shorter with respect to wavelength, the impedance increases and is best represented as a large resistor in series with a small capacitor. This is in direct contrast with the terminal impedance of a loop antenna, which is often represented as a small resistance in series with a large inductor. Also, as the dipole becomes smaller with respect to wavelength the efficiency and gain decreases, however the directional characteristics are retained, much the same as with a small loop. The high terminal impedance of a short dipole antenna requires that an amplifier with a similarly high input impedance be employed in order to conduct the antenna signal voltage to a suitable feedline, which is typically 50-ohm or 75-ohm coaxial cable. The NF and IMD performance of such an amplifier constitutes the Achilles Heel of active dipole antennas, and a considerable amount of attention to detail is required in order to arrive at a suitable design the provides a high degree of dynamic range, The purpose of this paper is to present an alternative small directional antenna that is capable of both wideband and tuned narrowband perfromance and which can be made with readily available and inexpensive parts and materials. Short Active Dipole Antennas Dipole antennas share radiation pattern characteristics that are similar to those of loop antennas. Typically, dipoles are designed such that their length is a half wavelength of the desired operating frequency although other lengths, although other lengths such as that for the Zepp (short for Zeppelin) are found as they have a narrower major radiation pattern. Fig. 1 - The Datong AD270/370 Active Dipole Antenna Amplifier Trask, Active Short Dipole Antenna 2 7 March 2011
3 meaning low NF and high IMD performance, while at the same time keeping cost and power consumption to a minimum. The Datong AD270/370 Active Dipole Antenna One of the more popular commercially available active dipole antennas was the Datong AD270 and AD370, the former intended for indoor useage and the latter for outdoor. The amplifier consisted of a balanced JFET input stage using transformer-couple drain-to-source feedback, followed by a pair of bipolar transistor amplifiers using transformercoupled collector-to-emitter feedback and then a balanced-to-unbalanced (BalUn) transformer. DC power for the antenna was conducted through the coaxial cable feedline. The overall design of the amplifier shows that a good amount of attention was given to both NF and IMD performance, however it does have a somewhat critical flaw. The JFET input stage is shown in abbreviated form in Fig. 1. The two antenna elements are connected to the JFET gates through a series of coupling capacitors together with some static voltage suppression circuitry, which, together with other details is not shown here for reasons of clarity. The transformer provides a means of drain-to-source negative feedback, a configuration that was made popular by Burhans (1, 2) as well as others (3, 4) Fig. 2 - Lankford Active Dipole Amplifier Trask, Active Short Dipole Antenna 3 7 March 2011
4 and which was used in the design of the North Country active short monopole antenna (5) which used a $US20 transistor and consumed a large amount of DC power, both aspects of which detracted from the desirable high dynamic range performance of the design. Those who are experienced in the design of active antennas will undoubtedly recognize that there is no common signal path between the two antenna elements, nor is there any provision for providing immunity from strong common-mode signal voltages that can occur when in close proximity to high-power broadcast stations as well as high intensity noise sources such as lightning. This is an aspect of active dipole design that is often overlooked (6, 7, 8). One could argue that the balanced nature of the amplifier provides for some commonmode immunity on the output side, however the much preferred approach to the problem is to provide such immunity at all instances, and it is essential that it be applied at the earliest opportunity. This aspect of the design will be discussed at length later. The Lankford Active Dipole Antenna An additional active dipole design worth mentioning is that of Dallas Lankford (8), the full schematic of which is shown in Fig. 2. Just as with the Datong amplifier, the Lankford design lacks a common signal path between the two elements. More seriously, the design lacks any provision for immunity from common-mode signals, the output coupling transformer T1 notwithstanding. The design is essentially a pair of monople amplifiers in parallel and undoubtedly has very poor common-mode performance. In addition to the lack of common-mode signal immunity, the Lankford design requires the use of a pair of obsolete transistors (2N5160) which cost $US10 each. Further, the use of table lamp zip cord for transmission line is highly questionable if for no other reason than the very high dielectric losses in the insulation material. It also requires that a separate conductor be provided for supplying DC power, which is an unnecessary installation burden. An Improved Balanced Active Dipole Antenna Amplifier The Lankford circuit has little if anything to offer as a basis for an efficient and effective design, however the balanced feedback topology of the Datong circuit shown in Fig. 1 is a very good place to begin. To start with, we add a BalBal transformer between the antenna input terminals and the JFET gates, as shown in the functional schematic of Fig. 3. This provides the best possible immunity from strong common-mode signals and also allows for the elimination of the 220K gate biasing resistors, which in turn provides a very high load impedance for the short dipole antenna. For wideband operation, the added input transformer can be a simple 1:2CT (aka 1:1:1) transfomer made with four or more turns of a trifilar twist of enameled wire through the holes of a suitable binocular core, such as a Fair-Rite This turns ratio provides a noiseand distortion-free voltage gain of 2. Fig. 3 - Basic Balanced Active Dipole Amplifier Trask, Active Short Dipole Antenna 4 7 March 2011
5 The JFET biasing resistors R3 and R4 are chosen to provide about 5-10mA of bias current for each device. These may be changed as desired if a different bias condiciton is desired Rather than have two separate gain stages as with the Datong amplifier, the JFET and bipolar devices are combined into a single non-inverting stage having a very high transconductance. The output impedance char- acteristics of the amplifier remain essentially the same as before, however the higher open loop gain provides significantly better IMD characteristics for the JFET input stage. The amplifier gain is now controlled by the turns ratio of transformer T2. This approach eliminates the bipolar stage feedback transformers T2 and T3 of Fig. 1, which further simplifies the design. Transformer T3 is a familiar 1:1 BalUn which establishes the balance of the output side C1, C2, C3, C4, C6, C8-0.1uF C5-22uF 16V electrolytic C7-0.33uF 50V electrolytic D1, D2 - MVAM109 or MVAM115 (see text) D3-1N5236B Q1, Q2 - J309 or J310 (preferred) Q3, Q4-2N2222 or MPS6521 (preferred) Parts List R2-10K R3, R4-100 ohms (see text) R5, R6-470 ohms R7-12 ohms T1-1:2CT Transformer (see text) T2, - 2:2:12 Transformer (see text) T3-1:2 Balanced Autotransformer (see text) T4-1:1 Balun Transformer (see text) R1-1.0M U1-78L05 Fig. 4 - Active Short Dipole Antenna Amplifier with Remote Tuning Trask, Active Short Dipole Antenna 5 7 March 2011
6 of the amplifier as well as providing a convenient means of conducting the DC power from the coaxial cable feedline. An Active Dipole Antenna with Remote Tuning The basic active dipole antenna amplifier of Fig. 3 is now further modified to include additional gain and remote tuning, as shown in Fig. 4. Here, a pair of varactor diodes D1 and D2 provide a means of adjusting the parallel resonant frequency of a tank circuit. This feature requires that the input transformer T1 now be made so that it has an inductance value that will provide tuning over a desired range. For coverage of the more popular shortwave broadcast band (SWBC) frequencies, the inductance of the secondary side of T1 is approximately 3.5uH, and for this value T1 is constructed using 15 turns of #34 trifilar enameled wire on a Micrometals T50-6 toroid core. For different tuning ranges, the value of T1 can be adjusted as needed. For wide tuning ranges hyperabrupt varactor diodes should be used and may be MVAM109, MVAM115, NTE618 (available from Mouser), or any suitable equivalent. Remote tuning is applied as a voltage across resistor R2, which is derived from the amplifier power and tuning voltage that varies from 8V to 22V from a control unit, which will be described later. The zener diode D3 provides a voltage drop of 7.5V so that the supply voltage to the amplifier is always greater than 8V while the tuning control voltage varies from 0.5V to 14.5V. With the parts described for T1, D1, and D2 this provides a tuning range from 5.5MHz to almost 15MHz. For wideband usage, these components may be set aside and T1 constructed as a wideband transformer, as was discussed earlier Transformer T3 is added to provide additional open-loop voltage gain of the amplifier as well as a convenient means of power supply decoupling. Although not obvious in the schematic, T3 is actually a Guanella 4:1 BalBal impedance transformer (9, 10, 11). Fig. 5 - J309 Characteristic Curves (horizontal 1V/div, vertical 2mA/div, 0.1V/step) Fig. 6 - J310 Characteristic Curves (horizontal 1V/div, vertical 5mA/div, 0.2V/step) Trask, Active Short Dipole Antenna 6 7 March 2011
7 Fig. 7 - MPS6521 Characteristic Curves (horizontal 1V/div, vertical 2mA/div, 5µA/step) The 5V regulator U1 provides a stable bias voltage for the amplifier stages while the supply voltage is varied from 8V to 22V for the remote tuning. The small value for the electrolytic capacitor C8 (0.33uF) is prescribed in the manufacturer s datasheet for applications where the device is located at an appreciable distace from the power source. Transistor Selection There is little that needs to be said about the use of a J309 or J310 (preferred) for Q1 and Q2 in the input stage. Either of these devices will provide the high impedance needed to realize a usable signal voltage from the short dipole antenna and to provide a high degree of tuning selectivity from the tuning network. The J310 is known to provide a slight edge in terms of IMD performance over the J309 when used as a source follower, though other devices such as the 2N3819 and 2N4416 will deliver better IMD performance in common-source applications. Fig. 5 and Fig. 6 show the transfer characteristics of the J309 and J310, respectively. Fig. 8-2N2222 Characteristic Curves (horizontal 1V/div, vertical 2mA/div, 10µA/step) The MPS6521 was chosen for the NPN bipolar devices Q3 and Q4 as it has acceptable characteristics (primarily capacitances and f T ) for HF applications together with a fairly high gain (h fe ). The 2N2222 may also be used for Q3 and Q4, though the IMD products may be a bit higher due to the lower h fe. Both devices have very good linearity characteristics, especially in the saturation region, as shown in Fig. 7 and Fig. 8, and are readily available from a number or sources. Transformer Construction The transformers used in this design are key elements in the overall performance of the amplifier as they provide linear voltage gain (T1 and T2), feedback coupling (T3), and output signal combining (T4). They need to be constructed in such a manner as to minimize parasitics such as leakage inductance and intrawinding capacitance which affect the high cutoff frequency, but at the same time maximize the coupling coefficient, which affects the low cutoff frequency and the overall amplifier gain. Commercially available transformers such as Trask, Active Short Dipole Antenna 7 7 March 2011
8 With twisted wires, coupling to the core is minimized, which results in lower losses, lower intrawinding capacitance, and lower IMD products that result from magnetic field nonlinearitites in the core material. To that effect, all three transformers are constructed using some form of twisted wire where it is convenient. The construction of the input transformer T1has been discussed earlier for both wideband and tuned applications. Fig. 9 - Transformer Construction Details for T2 (top), T3 (centre), and T4 (bottom) those available from Mini-Circuits are convenient, however their overall performance is insufficient for high performance applications such as this. Despite some unfortunate remarks made in the Technical Topics column of Radio Communications (RADCOM) some years ago (12), it is entirely possible for hobbyists of average ability to construct wideband transformers that will easily have performance equal to and even surpassing that of most commercial offerings, provided that simple guidelines concerning the design and construction as well as the selection of materials are adhered to (13, 14, 15, 16). One of the leading causes of poor transformer performance is the construction of the windings. Many designs, including most commercial products, make use of monofilar (meaning single-wire) windings, which results in less than ideal coupling coefficients, regardless of how they are arranged. Twisted wires, either bifilar (two wires) or trifilar (three wires) offer the best possible coupling, and one only has to learn how to design transformers using combinations of wires with 1:1 or 1:1:1 ratios. Transformer T2 requires a turns ratio that is not convenient for having all wires twisted together. However, the split primary winding is best realized with a twisted bifilar pair to ensure good balance, while the secondary winding is monofilar. Since the DC bias current passing through the two halves of the primary winding is essentially balanced, there is very little if any bias flux in the transformer core, which relieves most of the concerns of distortion due to flux compression in the core material. Therefore, T2 is constructed first with two turns of #34 bifilar twisted wire with the free ends exiting from one end of a Fair-Rite binocular core, followed by eight to twelve turns (depending on the amount of signal gain desired) of #34 wire exiting from the opposite end, as is shown in Fig. 9. Transformer T3 is also constructed on a Fair-Rite binocular core, this time winding four turns of #34 biifilar wire through the two holes and along the outside, as shown in Fig. 9. Finally, a third such binocular core is used for constructing T4, this time winding four turns of #34 trifilar wire between the two holes, which is also shown in Fig. 9. For lower frequencies, a core made from Fair-Rite 73 material should be used, and for higher frequencies cores made with either Fair- Rite 61 material or a Micrometals powdered iron material such as Carbonyl E (mix 2) or Carbonyl GQ4 (mix 8) should be considered. And Trask, Active Short Dipole Antenna 8 7 March 2011
9 Control Unit The active dipole antenna described here was designed to make use of the control unit that was described in my September/October 2003 QEX article on active loop antennas (17, 18) as a matter of convenience since this control unit has been serving me well for various active antenna designs, and I have come to rely on it for trouble-free operation. Fig Amplifier Protype for lower frequencies such as LF and VLF where more turns will be required, a smaller size of wire will be required. Prototype Construction and Testing A prototype amplifier was constructed on a 1.1 x2.0 piece of Ivan board (0.80 squares, 0.10 apart on 1/16 G-10 epoxy fiberglass, similar to FR-4), as shown in the photograph of Fig. 10. In the photo, transformer T1 is the toroid core on the far left, followed by transformers T2, T3, voltage regulator U1, and then transformer T4 going from left to right. The SMA connector was later removed when the assembly was installed in a 1 1/8 x 2 1/8 x 3 1/4 aluminum enclosure together with an SO-239 connector for attaching the coaxial feedline. Fig. 11 shows the completed amplifier housing assembly attached to a pair of 1 meter long antenna elements made from 1/2 copper pipe. A pair of PVC electrical conduit nipples with a filler sleeve made from 1/2 PVC pipe are used to attach the antenna elements to the aluminum enclosure. Fig. 12 then shows a waterproof housing made from PVC plumbing components that the assembly of Fig. 11 is mounted in. Referring to the schematic of Fig. 13, the unit begins with a DC power supply consisting of a 24V control transformer T1, a full-wave rectifier consisting of diodes D1 through D4, and the electrolytic capacitor C1, which provides about 30VDC. The LM317 regulator U1 provides a fixed 26V for the system, and LED D5 is simply a pilot light. Transistors Q1 and Q2 and their related components form a current limiting circuit with LED D6 providing an indicator that the unit is in current limiting. Resistor R7 determines the maximum current that the unit will provide, with the value shown (15 ohms) limiting the current to approximately 50mA. Potentiometer R10 is the antenna tuning control, and given the tuning sensitivity of the antenna it should be of the 10-turn variety. Meter M1 is the tuning indicator, and potentiometer R8 is a full-scale adjustment which is set by adjusting R10 to it s maximum setting and then adjusting R8 for a full-scale reading on M1. Transistor Q3 together with transformers T2 and T3 comprise an augmented lossless feedback amplifier (19, 20), and the amplifier transistor Q3 also acts as the pass device for controlling the supply and tuning voltage to the antenna ampifier. The construction of the amplfier transformers T2 and T3 is not necessarily obvious. Transformer T3 is identical in construction to trans- Trask, Active Short Dipole Antenna 9 7 March 2011
10 Fig Active Dipole Antenna Assembly Fig Active Dipole Antenna Housing Trask, Active Short Dipole Antenna 10 7 March 2011
11 Fig Active Antenna Power and Control Unit Schematic (from refeence 13) Trask, Active Short Dipole Antenna 11 7 March 2011
12 Fig Active Antenna Power and Control Unit Prototype Front View (top) and Rear View (bottom) Trask, Active Short Dipole Antenna 12 7 March 2011
13 Fig Control Unit Transformer Construction Details for T2 (top), and T3 (bottom) former T1 of the antenna amplifier, the details of which are shown in Fig. 8. The construction of transformer T2 is a bit more demanding, and it is necessary to have to violate the no monofilar windings rule For this transformer, you begin with six turns of #34 wire, with the ends exiting from one end of the binocular core (the left end, as shown in Fig. 8). Then, you wind six turns of #34 trifilar wire, with the ends exiting from the opposite end of the binocular core (the right end of Fig.15). The wires of the trifilar winding are then interconnected so as to form the 1:2 output winding of the transformer, and the various ends are connected to the other circuitry as shown in the labeling in Fig. 15. The completed protype of the control unit is shown in the photographs of Fig. 14. The antenna amplifier and tuning can be Fig Biasing Tee Schematic (see terxt) Fig. 17 -Bias Tee Assembly easily powered and controlled by way of a simple bias tee, such the one depicted in the schematic of Fig. 16. Here, the transformer T1 is made with 6 turns #26 AWG bifilar wire on a Fair-Rite or similar balun core. As shwon in Fig. 17, this bias tee fits comfortably inside a 1 1/8 x 2 1/8 x 3 1/4 aluminum enclosure and is very convenient for other applications such as powering mast-mounted preamplifiers. Synopsis The active dipole antenna described here was designed to make things as simple as possible and yet retain a high degree of dynamic range. The addition of a transformer to the input provides noise-free voltage gain that improves the NF, while at the same time providing much-needed initial immunity from strong common-môde signals. The combining of two gain stages into a single negative feedback amplifier with a high degree of open-loop gain serves to improve the IMD performace of the Datong amplifier, from which this design is derived. Both tuned and wideband (untuned) versions are available, which gives the user a good degree of flexibility. The performance compares well with that of active loop antennas, though passively tuned loops perform better. Trask, Active Short Dipole Antenna 13 7 March 2011
14 References 1. Burhans, R.W., Active Antenna Preamplifiers, Ham Radio, May 1986, pp Burhans, R.W., VLF Active Antennas, Radio-Electronics, February 1983, pp Gentges, F., The AMRAD Active LF Antenna, QST, September 2001, pp Rohde, U.L., Wideband Amplifier Summary, Ham Radio, November 1979, pp Sheets, W. and R.F. Graf, A 100kHz-30MHz Active Antenna, Popular Electronics, August 1997, pp , Salvati, M.J., High-Gain Broadband Active Antenna, Electronic Design, 6 January 1997, p Salvati, M.J., Frequency-Selective Gain Increases Dynamic Range of an Active Antenna, Electronic Design, 14 December 1998, pp Lankford, D., Complementary Push-Pull Output Active Whip and Dipole Antennas, 14 Jan 2007, revised 22 July 2007, (online resource). 9. Guanella, G., New Method of Impedance Matching in Radio-Frequency Circuits, The Brown Boveri Review, September 1944, pp Guanella, G., High-Frequency Matching Transformer, US Patent 2,470,307, 17 May Guanella, G., High Frequency Balancing Units, US Patent 3,025,480, 13 March Hawker, P., New Low-IMD Mixer, Technical Topics, Radio Communications, October 2000, pp Trask, C., Designing Wide-band Transformers for HF and VHF Power Amplifiers, QEX, May/April 2005, pp Trask, C., 'Wideband Transformers: An Intuitive Approach to Models, Characterization and Design,' Applied Microwave & Wireless, Vol. 13, No. 11, November 2001, pp Walker, John L.B., Daniel P. Meyer, Frederick H. Raab, and Chris Trask, Classic Works in RF Engineering: Combiners, Couplers, Transformers, and Magnetic Amplifiers, Artech House, Snelling, E.C.Soft Ferrites, Properties and Applications, 2nd ed., Butterworth, Trask, C., 'Active Loop Aerials for HF Reception, Part 1: Practical Loop Aerial Design,' QEX, July/August 2003, pp Trask, C., 'Active Loop Aerials for HF Reception, Part 2: High Dynamic Range Aerial Amplifier Design,' QEX, Sep/Oct 2003, pp Trask, C., 'Distortion Improvement of Lossless Feedback Amplifiers Using Augmentation,' Proceedings of the 1999 IEEE Midwest Symposium on Circuits and Systems, Las Cruces, New Mexico, August Trask, C. Lossless Feedback Amplifiers with Linearity Augmentation, US Patent 6,172,563, 9 January Trask, Active Short Dipole Antenna 14 7 March 2011
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I am selling a Datong AD 370 Outside Active Antenna in excellent
condition. It is in dipole configuration, but no longer than 9 ft end
to end. With this configuration, it is less affected by local
electrical noise. The interface unit has a selectable 12 dB preamp
built in. 12 volt supply included. No antenna tuner needed.
Receives 200 khz to well over 30 Mhz.
Price: $80 plus shipping CONUS. Selling as I have enough room to
erect my 90 ft SGC T2FD.
Datong Active Antenna Model Ad3701
Thanks,
Victor Trucco KD6PTO
Passive Antenna
Drake R-7
Lowe 225 Europa
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