From: Remote Termination of Beverage and Ewe Antennas
Mark Connelly, WA1ION - 31 JAN 2002; revised 14 JUL 2005 =
This is a variation on the approach taken by Steve Byan in his remote = Beverage termination article (reference 1). The VTL3A27 Vactrol = mentioned=20 in his article is no longer manufactured. EG&G Vactec turned = production of=20 similar devices over to PerkinElmer. Their VTL5C4 part (reference = 2) is a=20 usable substitute. I have already used this successfully for control of = the=20 terminating resistance on Pennant and Kaz antennas (references 3, 4, = 6).=20 Silonex part number NSL-32SR2 may also be used.
Figure 1 = (click here to=20 view) shows the overall set-up if using a twisted-pair Beverage or = Ewe=20 antenna. Any two wire conductor (such as lamp cord, speaker wire, TV = twinlead,=20 or coaxial) can be used. Twisting the leads is not necessary in this=20 application. Note that three boxes are identified: the (in-shack) = Controller,=20 the Feedpoint Box, and the Termination Box. These are all very simple=20 circuit-wise.
Figure= 2=20 (click here to view) illustrates the fabrication of the impedance=20 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=20 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=20 ohm) for a 3:1 turns, or 9:1 impedance, ratio. That's because a Beverage = or Ewe=20 presents an impedance of approximately 300 to 600 ohms.
Figure 3 (click = here to=20 view) shows the Controller Box. You could put a 0-50 mA panel meter = in=20 series with the arm of the potentiometer (as Steve Byan did in his = circuit).=20 Don't use a cheap potentiometer. Go for something rated at a minimum of = 1 watt.=20 I like the 10-turn wirewound types such as made by Vishay-Spectrol = (Mouser stock=20 number 594-53411102). Use 2.2 mH chokes that can handle at least 50 mA = DC.=20 Mouser and other vendors have suitable types. You could probably go down = to 680=20 uH or up to 6.8 mH in RF choke inductance value with little appreciable=20 difference in performance. If you want to put an amplifier in the box, = look into=20 W7IUV's design (reference 5). You'll need a 3-pole 2-throw switch = if you=20 want to be able to switch the amplifier in or out.
Figure 4 (click = here to=20 view) shows the Feedpoint Box. This can be located a substantial = distance,=20 maybe up to 100 m, from the operating position if fed with good-quality = coaxial=20 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=20 switch on this box allows you to try a so-called noise-reducing ground = or to use=20 the coaxial shield (common) ground. Whichever gives the best = signal-to-noise=20 (without messing up directivity) "wins". Use of the low-noise ground at = the=20 feedpoint was not part of the original Byan design. It has proved useful = on=20 Newfoundland DXpeditions and elsewhere. See the previous notes (with = Figure 2=20 description) about transformer construction.
Figure 5 (click = here to=20 view) is the Vactrol-based Termination Box. The test point allows = output=20 resistance to be measured by means of an ohmmeter clipped from this = terminal to=20 the Ground System jack. The Vactrol range is about 54 ohms to greater = than 20K=20 ohms. Because two 39 ohm resistors (78 ohms) are used in series with the = Vactrol=20 termination (for smoother adjustment in the critical 400 to 1400 ohm = range), the=20 measured output resistance will be in the range of 132 ohms to greater = than 20K.=20 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=20 antenna going from the Feedpoint Box to the Termination Box. With the = Controller=20 potentiometer set for an arm voltage of 0 volts, maximum resistance is = noted at=20 the Termination Box. With the Controller potentiometer set for an arm = voltage of=20 9 volts, minimum resistance is noted at the Termination Box. Because of = the=20 voltage-dropping resistors in the Feedpoint Box, a maximum voltage of = about 5 to=20 6 VDC should be measured across the two antenna input jacks of the = Terminator=20 Box. If the antenna set-up is going to be permanent rather than = "DXpedition=20 temporary", some sort of static protection could be added (as Steve Byan = did in=20 his design).
References:
(Note: Over time Web URL's may change. If this occurs, it may still = be=20 possible to retrieve the articles by going to known DXer Web sites or to = search=20 engines for links. Hard copies are likely to be available from the = National=20 Radio Club and International Radio Club of America reprints = services.)
1. Remote-Controlled Termination Beverage Antenna, Steve Byan, =
1996
"http://www.ultranet.com/~oakridge/Beverage_info/RCT_Bevera=
ge/RCT_Beverage.html"
2. Vactrol Data Sheet (includes VTL5C4)
"http://www.qsl.net=
/wa1ion/vactrol/vactrol.pdf"
3. Testing Two 'Kaz' Squashed Delta Antennas, John Bryant, =
2001
"http://h=
ome.comcast.net/~markwa1ion/exaol2/kaztests.pdf"
4. Pennant and Kaz Antenna Tests, Mark Connelly, 2001
"htt=
p://home.comcast.net/~markwa1ion/exaol2/pennant_v_kaz.htm"
5. W7IUV Amplifier
"http://www.qsl.net/w=
a1ion/amp/w7iuv_amp.htm"
6. Pennant Antenna with Remote Termination Control, Mark Connelly,=20
2000
"http://ho=
me.comcast.net/~markwa1ion/exaol2/pennant.pdf"=20
and
"http://ho=
me.comcast.net/~markwa1ion/exaol2/pennant.htm"
Click here for: RF Circuit = Building=20 Blocks Page