Remote Termination of Beverage and Ewe Antennas
This is a variation on the approach taken by Steve Byan in his remote Beverage termination article (reference 1). The VTL3A27 Vactrol mentioned in his article is no longer manufactured. EG&G Vactec turned production of similar devices over to PerkinElmer. Their VTL5C4 part (reference 2) is a usable substitute. I have already used this successfully for control of the terminating resistance on Pennant and Kaz antennas (references 3, 4, 6). Silonex part number NSL-32SR2 may also be used.
Figure 1 (click here to view) shows the overall set-up if using a twisted-pair Beverage or Ewe antenna. Any two wire conductor (such as lamp cord, speaker wire, TV twinlead, or coaxial) can be used. Twisting the leads is not necessary in this application. Note that three boxes are identified: the (in-shack) Controller, the Feedpoint Box, and the Termination Box. These are all very simple circuit-wise.
Figure 2 (click here to view) illustrates the fabrication of the impedance transformer which will be used in the Feedpoint Box. The drawing shows a binocular type core such as Fair-Rite 2873000202 or Amidon BN-73-202. For simplicity, a 2-turn primary and 2-turn secondary is shown, but for this application we'll be using a 9-turn primary (450 ohm) and 3-turn secondary (50 ohm) for a 3:1 turns, or 9:1 impedance, ratio. That's because a Beverage or Ewe presents an impedance of approximately 300 to 600 ohms.
Figure 3 (click here to view) shows the Controller Box. You could put a 0-50 mA panel meter in series with the arm of the potentiometer (as Steve Byan did in his circuit). Don't use a cheap potentiometer. Go for something rated at a minimum of 1 watt. I like the 10-turn wirewound types such as made by Vishay-Spectrol (Mouser stock number 594-53411102). Use 2.2 mH chokes that can handle at least 50 mA DC. Mouser and other vendors have suitable types. You could probably go down to 680 uH or up to 6.8 mH in RF choke inductance value with little appreciable difference in performance. If you want to put an amplifier in the box, look into W7IUV's design (reference 5). You'll need a 3-pole 2-throw switch if you want to be able to switch the amplifier in or out.
Figure 4 (click here to view) shows the Feedpoint Box. This can be located a substantial distance, maybe up to 100 m, from the operating position if fed with good-quality coaxial cable. A ground rod or mesh at the feedpoint (and not connected to other grounds) can sometimes be used to reduce local electrical noise pick-up. The switch on this box allows you to try a so-called noise-reducing ground or to use the coaxial shield (common) ground. Whichever gives the best signal-to-noise (without messing up directivity) "wins". Use of the low-noise ground at the feedpoint was not part of the original Byan design. It has proved useful on Newfoundland DXpeditions and elsewhere. See the previous notes (with Figure 2 description) about transformer construction.
Figure 5 (click here to view) is the Vactrol-based Termination Box. The test point allows output resistance to be measured by means of an ohmmeter clipped from this terminal to the Ground System jack. The Vactrol range is about 54 ohms to greater than 20K ohms. Because two 39 ohm resistors (78 ohms) are used in series with the Vactrol termination (for smoother adjustment in the critical 400 to 1400 ohm range), the measured output resistance will be in the range of 132 ohms to greater than 20K. A "sanity check" of the three boxes can be done on the test bench by the following procedure: Two clip leads would take the place of the twisted-pair antenna going from the Feedpoint Box to the Termination Box. With the Controller potentiometer set for an arm voltage of 0 volts, maximum resistance is noted at the Termination Box. With the Controller potentiometer set for an arm voltage of 9 volts, minimum resistance is noted at the Termination Box. Because of the voltage-dropping resistors in the Feedpoint Box, a maximum voltage of about 5 to 6 VDC should be measured across the two antenna input jacks of the Terminator Box. If the antenna set-up is going to be permanent rather than "DXpedition temporary", some sort of static protection could be added (as Steve Byan did in his design).
(Note: Over time Web URL's may change. If this occurs, it may still be possible to retrieve the articles by going to known DXer Web sites or to search engines for links. Hard copies are likely to be available from the National Radio Club and International Radio Club of America reprints services.)
1. Remote-Controlled Termination Beverage Antenna, Steve Byan, 1996
2. Vactrol Data Sheet (includes VTL5C4)
3. Testing Two 'Kaz' Squashed Delta Antennas, John Bryant, 2001
4. Pennant and Kaz Antenna Tests, Mark Connelly, 2001
5. W7IUV Amplifier
6. Pennant Antenna with Remote Termination Control, Mark Connelly, 2000
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