Instruction manual

The resistance to the flow of RF current is closely tied to the linear inch of material parallel to and surrounding the
wire, and it goes up by the square of the number of turns. It is actually a complex impedance, with X decreasing
and R increasing with increasing frequency. At some frequency X=R (loss tangent equals effective permeability)
and the Q becomes unity [Q=1]). It is this property which makes their use in RFI suppression so ideal.
As emphasized above, the effectiveness is directly dependent on the number of turns through the ferrite material,
and the particular ferrite material in question. Unfortunately, most factory supplied ferrites are not specified as to
mix, but mix 31 is a good place to start.
So how many turns are needed? Assuming mix 31 ferrite material, a correctly installed antenna with an adequate
image plane under it, and low power (100 watts or so), 6 turns may prove adequate. However, 8 turns, and
preferable 10 turns, are required in most cases.
The major stumbling block to this is, antenna manufacturers tend to make their control leads rather short! This
requires adding length to the wires. Typically, size 20 is adequate for most mobile installations unless the runs are
long (i.e.: motor home or RV).
Many beads aren't large enough to take 8 or more turns, so what do you do? DX Engineering, Palomar-Engineers,
Amidon, and others carry mix 31 split beads. The .5 ID units are ideal in most case. However, if your wire size is
larger than size 20, then use the .75 ID units.
In extreme cases, it may be necessary to use a ferrite material with a higher ui. An FT114-61 toroid core will hold
15 turns of #20 Teflon
®
wire, and will provide a magnitude more bypassing than the aforementioned mix 31 cores.
Regardless of the number of turns your antenna manufacturer recommends, they don’t have any control over how
and where you mount your antenna. Thus, the higher above RF ground your antenna operates, the more RF flows
over the control wires, and even the outside of the coax (parallel line currents). This points out the need for proper
mounting. The rule of thumb is, it's the mass under the antenna, not along side, that counts!
The beads should be placed at the very base of the antenna. Think of it this way; any length of wire from the
antenna to the position of the bead, is part of the antenna, and has RF impressed on it!
In some cases (high power operation, poor mounting position or method, etc.), additional bypassing is required.
Shielding the control wires from the antenna to their entry into the vehicle, and bypassing them to ground with
parallel .1 and .01 ceramic caps adds a magnitude of RF suppression.
Since the reed switch (if equipped) isn’t use with the TC/SC, here’s a tip. Connect the leads from the reed switch
directly to the mast of the antenna. In this case, you won’t need any bypassing on these leads. Remember, they’re
already at the same RF potential as the antenna.
Antenna Matching
The TC/SC is not an antenna tuner; it is an antenna controller! Depending on the
antenna, its mounting location, and its mounting methodology, the input impedance
will vary between 18 and 50 ohms (80 through 10 meters). This represents an SWR
between 2.7:1 and 1:1. Therefore, some form of antenna impedance matching is
required in most cases. However, the matching device cannot be an external antenna
tuner.
A simple inductor, installed across the coax connections, will provide the needed
impedance transformation. Here’s how it works. A small amount of capacitance is
borrowed from the antenna (the antenna is tuned slightly above true resonance).
When combined with the inductance of the coil, they form a highpass LC network.
As the frequency increases, the effective impedance transformation decreases. This
provides a low SWR over the operating range of the antenna (typically less than
1.6:1 from 80 through 10 meters). The coil also DC grounds the antenna lessening
static build up.
8