Topband: 160m noise

[Sinisa Hristov]

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