Topband: 160m noise

[Jim Brown]

Of course -- IF the interfering signal is only at a single frequency, and if you have time to 
tune each and every choke to each and every line. Rarely do the stars line up that way. 
My ham station operates from 160 meters to 450 MHz. I want more bandwidth from 
suppression, not less. If all I was trying to do was keep a single broadcast station out of 
a single length of line and I needed a lot more suppression than a good resistive choke 
could provide, I might consider your suggested method. Otherwise, I think your method 
is more an academic exercise than a practical one. 

Jim

On Wed, 10 Nov 2004 19:52:13 -0500, Sinisa Hristov wrote:

>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