Jim Brown wrote:
> In general, we want to use resistance, NOT inductance in chokes
> designed for EMI suppression, because we want to dissipate the RF,
> and because we don't want the X of the choke (either capacitive or
> inductive) to resonate with the length of the cable and increase the
> current rather than reduce it.
Although it's a good idea to dissipate the incident RF power,
a choke can be very effective if designed with parallel
resonance at RF frequency. Series impedance will be very high
(many tens of kOhms, and even > 100 kOhm), and RF current
will be blocked.
> I've used both the MFJ-259B and the AEA CIA-HF to measure these
> chokes. Neither device could be considered a serious bridge,
I agree. Such instruments are quite good around 50 + j * 0 Ohms,
and useful up to a few hundred Ohms. But they are useless for choke
measurements.
Higher impedances can be measured (or at least estimated)
by measuring insertion loss. Let's take a 50 Ohm generator
(5 W TX with some attenuation will do) and a calibrated
50 Ohm detector (even diode detector and DVM can be used).
Let's measure voltages V1 and V2 as follows:
V1 is measured with direct connection from generator to detector;
V2 is measured with generator terminated in 50 Ohms,
and connected to the detector via choke.
Choke impedance can now be estimated as:
Z ~ (V1 / V2) * 50 Ohms - 75 Ohms.
73,
Sinisa YT1NT, VE3EA
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