Antenna Components

Assuming that low-loss 50Ω coax will be used as a feed line, the antenna feed will need to perform three tasks:

  1. Transform the 50Ω feed line impedance to 200Ω,
  2. Convert the unbalanced currents of the coax to balanced currents, as needed by this symmetrically fed antenna. Failing to do so, would severely upset the radiation patterns.
  3. Prevent both conducted and induced RF currents from flowing back on the outer sheath of the coaxial line towards the shack. Doing so, will eliminate the most common source of radio frequency interference (RFI) occurring with off-center fed dipole antennas.

Current Balun

Tasks 1 & 2 can be performed by a 4¸1 impedance-transforming balun, preferably of the current-type. Voltage type baluns also work, but will be less effective in forcing the right currents on the antenna when nearby metallic objects couple with the antenna. However, do avoid autotransformers or ununs having a common rail between input and output, but without a center tap to earth. More information on selecting the right current balun can be found here.

A balun for an OCFD needs to be sturdy because it will also need to compensate for the compounded effects of forward power, reflected power and sheath current. The same holds for the sheath current choke. In terms of the power handling capacity as stated on commercial baluns, beware that 1kW SSB is less than 1kW CW, is less than 1kW RTTY, due to the increasing duty cycle of each respective mode. The use of a balun built within a tuner is not recommended because all too often, such baluns happen to be too flimsy.

Furthermore, digital modes and FM have duty cycles of up to 100%. This meaning that if you want to operate full legal power in a digital mode, you really would like to have a balun rated at 5kW CW/SSB. For such applications, I highly recommend the 4115ocf balun devotedly hand-assembled by Robert J. Rumsey, KZ5R, the sole owner of Balun Designs. I particularly like the model with the «N-connector» option.

Balun Designs
Balun Designs 4115ocf balun

Coax Sheath Current Choke

The third task is often overlooked in many practical realisations of Windom antennas, resulting in their unfair reputation of being provokers of RFI. Although a current balun will effectively prevent conducted RF currents from flowing along the outer sheath of the coax, it may not always be sufficient to prevent the antenna itself from inducing sheath currents on the coax line. Off-center-fed antennas are more prone to this effect because the coax line is positioned asymmetrically with respect to the antenna.

Induced sheath currents can easily be prevented from flowing back to the shack. One should place a sheath current choke right behind the balun and optionally also between the PA and the grounded metal bulkhead or entry plate through which the coaxial line enters your shack. (As a matter of fact, a grounded metal entry plate forms also an excellent lightning protection since it may act as a diversion of last resort for any high currents on the outer sheath of the coax.)

What happens, you may visualize as follows: Sheath current chokes will be most effective at locations where the sheath current would like to see a relatively low impedance in order to flow. However, at the before-mentioned positions, what it encounters will be the relatively high sheath impedance of the choke. In reality, a sheath current choke with its magnetic material constitutes a new boundary condition due to which the sheath current will not be able to exist.

Capacitor

f (MHz)VCser (V)
3.647464
7.09962
10.1251310
14.174209
21.224403
28.594652

This table lists, for an input power of 2500W, the voltage over the center-loading capacitor at the different operating frequencies. The highest voltage is present when the antenna is operated at 10.125MHz.

Door-Knob Capacitor

door-knob
capacitor

If the power were only 150W, the maximum voltage would still measure 321V. At 5W; 59V.

A doorknob capacitor with a standard value of 220pF and a voltage rating of at least 2kV can safely be employed at the 2.5kW power level.

More information about different types of high-voltage capacitors can be found here.

Inductor

An inductor of 4.95ÁH can take on many sizes and shapes. However, the loss and hence quality factor of an inductor will to a certain extent be dependent on its form factor and conductor size. At the end of the day, it all depends where you want to compromise.

Since I was not satisfied with the accuracy and formulas on which most inductance calculators are based, I designed my own inductance calculator, especially for this project. However, feel free to employ it also for your own projects.

Here is a both practicable and satisfactory design suggestion for the center-loading inductor:

According to my inductance calculator, an air coil of length ℓ = 150mm with a mean diameter D = 150mm and N = 6.75 turns of diameter 1/4" (d = 6.35mm) copper brake pipe will yield an inductance of 4.98ÁH at 10.125MHz; close enough for this application. With these dimensions the unloaded quality factor will be a staggering 2220!

The self-resonant frequency of this coil is 31.633MHz, so well away from the highest used odd harmonic, 10.125MHz in our case.

As for the coil's construction technique, I prefer AD5X's plumbing recipe.

Insulators

Hams tend to forget tend to forget about the high RF potentials that are present at the ends of any dipole antenna. Cheap, poor, dirty or broken insulators may form through the humid hang ropes an additional path to earth. The resistance of this path may be of the order of mega-ohms. Nonetheless, due to the equally high end-potentials these hidden losses will be quite considerable!

Rope, Knots, Pulley & Weight


pulley

pulley


Horizontal wire antennas are often hung on both ends from ropes. Due to gravitation flexible wires will hang in the shape of a catenary or hyperbolic cosine cosh(x). Considerable tensile forces are needed to straighten up the wire a bit. On a stormy day, the wind force may put extra strain on the wire until it eventually snaps. A pulley with weight system at one end of the antenna will prevent this from happening. As mentioned before, a pulley system will also greatly simplify the trimming of your antenna for resonance as well as its general maintenance.

Click here for: