8 October 2017
AIf everything was as described (i.e., only passive reactive components), then indeed no power would be dissipated in the filters. I’ve sketched a simple RF band pass filter example in Figure
4. This is a single-pole filter ( 3 dB per octave roll-off).
However, the filters I purchased are five-pole. You can see
there are no resistors on that schematic. The only problem
is that it’s not telling the whole truth.
Both the capacitors and inductors in that simple filter
contain stealth resistances; some of which only come into
play at radio frequency. We’ll start with the capacitors.
As you might know, there are many different ways
to construct capacitors. To make them smaller, often a
material with a higher dielectric constant (higher electric
permittivity) than air is used to get more and stable
capacitance in a limited space. This happens because
molecules in the material become polarized by the
electric field applied. In the case of RF fields, the electric
field changes cause charged particles making up those
molecules to shift around with the fields.
There is a kind of friction associated
with the charge movements that effectively
causes the material to heat. Remember that
temperature is a measure of average kinetic
energy in a material. This, of course, means
we’re losing some RF energy to this heat
production. This is similar to a resistor.
Inductors are subject to even more
losses. Since they are typically constructed
from potentially long lengths of wire, there
is a conventional resistance associated with
them from the wire itself. In addition to
this, there is a similar effect to the dielectric
losses above. We often use materials like ferrite — a kind of
magnetic ceramic — to increase the inductance (increasing
the magnetic permeability) of coils of wire.
This results from the magnetic field moving magnetic
dipoles in the material. With radio frequency fields, the
dipoles are rapidly moving, and some energy is again
transferred to the material as kinetic energy, resulting in
Even if we assume that the losses aren’t very big (let’s
say 1% or 0.09 dB), they are still significant. At 4,500W,
that’s 45W to be dissipated as heat. Some of that 45W
will be concentrated in critical areas where changes in
temperature could significantly change the electrical
characteristics. So, I think I’d want a fan on there. Note that
the manufacturer rates the filters at 2,500W without forced
More information on these filters can be found at
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; FIGURE 4. Typical single-pole RF band pass filter topology.