Voltage Feeding SCV Loops

L. B. Cebik, W4RNL (SK)

We have known since the first days of the development of the antenna types that we could feed half squares and bobtail curtains at a high voltage point. The dangling 1/4 wl vertical wires exposed the high-voltage, high-impedance point and placed it within relatively easy reach.

The circuit for effectively feeding a voltage node of the open-ended SCVs is in Fig. 1. The top--or a tap near the top--goes to one of the half square verticals or to the middle vertical of the bobtail curtain. This remote circuit can be set and weather sealed. If the tips of the vertical are a significant distance above ground, then the center wire of the bobtail may be extended toward the ground. The matching tank circuit is then positioned so that its lower end forms a contact with a good RF ground (which is not the same as a radial system). The current magnitude at the top of the tank circuit will be higher than at the tip of an end vertical, but it will reach close to the zero point in line with those end-vertical tips. Maximum current will still be at the tops of the verticals, where they join the horizontal wire. The maximum center-vertical current magnitude, of course, will be twice the maximum magnitude of the end verticals on a Bobtail current. On a half square, the two top vertical currents will be about equal.

Many newcomers to antennas are skeptical about the equality among the feeding systems for the open-ended SCVs, so let's pause to create a small demonstration. Let's feed the center vertical of a bobtail curtain at a series of positions: the top (at the junction with the horizontal phasing lines), at the center of the vertical, at the base of the vertical, and finally at ground level (by extending the center vertical to ground). In terms of some essential data, we obtain Table 1.

Table 1. Performance of a bobtail curtain using various feedpoint positions.
Note: all dimensions are the same, except for the length of the vertical for ground-level feeding.
Feed Method Gain dBi TO Angle Feedpoint Impedance
Vertical top 4.87 18 deg 43 + j5 Ohms
Vertical center 4.98 18 73 + j8
Vertical Bottom 4.99 18 4000 - j2400
Ground level 5.07 18 500 - j5000

As we lower the feedpoint position, the impedance climbs until we reach the normal tip of the center vertical. Feeding at this point represent the placement of a parallel resonant tank circuit, such as shown in Fig. 1, or its equivalent in the form of a network, at the vertical tip. We may presume that a ground line runs from the tank to a ground rod. However, the tank circuit represents a very high impedance in series with this lead. Therefore, no significant antenna current appears on this line.

The table shows only slight variations in gain and none in the TO angle. The current distribution of the relative current magnitude does not change with a change in feedpoint position. Fig. 2 provides a demonstration of this fact.

The only variation in dimensions for the bobtail occurs with the final case in which we have brought the center vertical to ground level. At this level, we install the parallel resonant tank circuit for matching to a feedline. (I shall assume that anyone using this type of system has taken every useful safety measure to prevent human or animal contact with the high-voltage on the wire.) In the table, the impedance is indicative of the values we might obtain, but it will vary in practice according to the components that for the tank circuit. The model also includes a ground rod for the base of the tank. Note that the matching point is not precisely a true high-voltage point, but occurs below it. Hence, the current rises somewhat at the feedpoint, producing a lower impedance. However, with a tapped tank, we may easily find the correct matching positions for the antenna and for the lower-impedance feedline to the shack. Within the boundaries of the original length of the vertical, we find a completely normal current distribution curve.

The loop versions of the SCV--the triangles and rectangles--present a different problem, as revealed in Fig. 2. They have no exposed ends. Moreover, the maximum current point of effective SCV operation with a maximum of vertically polarized radiation is way off to the side and elevated.

The shame of it all is that the loops can be used as general purpose wire antennas for most of the ham bands above their resonant SCV frequencies. However, they operate better in this role if fed at the center of the bottom. You will need balanced feeders and an ATU, just as you would for a doublet.

Just for initial comparative purposes, Table 2provides some numbers for a right-angle delta, resonant for SCV use at 7.15 MHz, with its base at 35' and its apex at 65' up over average soil, as sketched in Fig. 4. The first numbers list the gain, take-off angle, and source impedance if we attempt to feed the antenna on all bands at the SCV side feedpoint, about 12% up the triangle leg. The second set give the same modeled data if we feed the antenna at the center of its horizontal base leg.

Table 2.  Modeled performance of a right-angle delta using side- and bottom-feed
Right-angle delta:  side fed
Frequency      Gain dBi       T-O angle      Source Z (R+/-jX)
 7.15 MHz      1.97           16 degrees       49 + j   0
10.1           4.60           57             5700 + j4500
14.15          6.92           37              115 + j 135
21.2           6.42           22              270 + j 295
24.95          7.96           18              790 - j 105
28.5           8.10           15              435 + j 510

Right-angle delta:  bottom-center fed
Frequency      Gain dBi       T-O angle      Source Z (R+/-jX)
 7.15 MHz      5.83           42 degrees      255 + j  75
10.1           7.57           29             3510 + j2100
14.15          7.47           35              175 + j 120
21.2           7.90           14              235 + j 350
24.95          8.91           68              665 - j1150
28.5           8.49           15              615 + j 415

Where SCV-type operation is not involved, on every band except 12 meters, the bottom fed loop shows either higher gain or a lower take-off angle than the side fed version. Hence, for general use, the bottom-center position is the better feedpoint. Of course, it is also more convenient than the side position.

It would be nice if we could get SCV operation at the same point. We can.

For SCV operation, we cannot simply place the feedline in series with the wire, as we would for general operation. Instead, we must separate the wires a bit at the very center of the bottom. This is the equivalent to the first step in converting the antenna into a half square. This position is a high-voltage point where the current reverses polarity. Separating the wires at this point does not materially affect the gain or take-off angle of the antenna. In other words, it does not affect the ratio of vertically polarized to horizontally polarized radiation.

However, as shown in Fig. 5, we shall not change the shape into a half square. We shall retain the delta shape. The spacing of the break in the wire is not critical--2" to 6" appears to make no significant difference. For the particular right angle delta model we are using, opening the bottom did not change the gain, take-off, angle, or source impedance. More precisely, the source impedance with side feed changed by only a fraction of an Ohm. Moving the side feedpoint to the bottom changed the impedance, but not the gain or take-off angle.

We shall leave one end of the wire at the bottom-center break unattached to anything. The other end, we shall attach to the very same parallel tuned circuit we might used to voltage feed a bobtail curtain. The antenna source impedance will be complex and high, with the resistive component in the 3500 to 4000 Ohm range and the reactance between 8000 and 10000 Ohms. A good high-Q coil with well-spaced turns to prevent arcing and a good variable capacitor, also with wide spacing between plates, will handle the job easily. The only task is patiently finding the right tap points for both the antenna and the feedline so that a coax line sees 50 Ohms.

However, since we are making this move to use the antenna with parallel feedline, the situation is not so critical. The use of the parallel tuned circuit is still recommended, since the impedance of the antenna end is still very high even with 600-Ohm parallel line. However, we need only find a tap that approximates the line impedance and let the tuner in the shack do most of the work. In fact, with this system, once we find a good setting, we can replace the variable capacitor with a door knob high-voltage capacitor and protect the expensive variable from the weather. We should also run a wire from the base of the tank to ground--a good RF ground.

We shall still need a weather proof case for the capacitor and the coil. So we might as well add either a knife switch for manual operation or a relay for remote operation. The switching job is this: when we wish to use the antenna as an SCV, one side of the base-leg break goes to the tank circuit and the other goes to nothing. The feed line goes across the tap(s) on the coil. When we wish to use the antenna for general operation, the tank is disconnected and each side of the base-leg break goes to each side of the parallel feed line. The switching, suggested in Fig. 6, may require 3 sets of contacts.

This system should work equally well on rectangles as well as triangles. If you are inclined to try this system, I recommend that you start with a manually switched system to see if it will suit your needs before you invest in a remote switching system.

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