For below 160m (472 KHz & 136 KHz), you need Mix-75 toroids or similar.
Also, if you saw "a few dB" improvement in noise with the choke at the
antenna - every dB counts, so put a second choke out there!
73
Lloyd - N9LB
-----Original Message-----
From: RFI [mailto:rfi-bounces+lloydberg=tds.net@contesting.com] On Behalf Of
Tony
Sent: Tuesday, April 06, 2021 9:15 PM
To: Rfi List <rfi@contesting.com>
Subject: Re: [RFI] Common Mode Choke Materials
All:
Thanks for the info on LF/MF common mode chokes. Jim's (K9YC) cookbook is
very informative. I did some preliminary testing on 160 and 80 meters with a
couple of 2.4 inch mix31 cores I had on hand using 16 turns of RG-178.
I was surprised to see that the choke was much more effective when placed at
the SDR Play receiver than it was at the antenna (active loop). The noise
floor dropped 15db with this configuration and only a few db with the choke
placed at the antenna.
My guess is that the bulk of the noise is coming from the PC connected to
the SDR since the only things on in the shack were the PC and monitor. I
placed another choke on the USB cable that connects the two devices, but
that had a minimal effect.
I'm seeing -120 dbm noise floor on 160 and -130 dbm on 80 with the simple
mix31 choke and the active loop so I'm happy about that. It doesn't do much
for bands below 160, but I guess that's to be expected.
Tony -K2MO
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