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[TowerTalk] ferrite saturation and IMD

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Subject: [TowerTalk] ferrite saturation and IMD
From: (Tom Rauch)
Date: Mon, 13 Apr 1998 09:40:37 +0000
Among some suggestions and questions I sent Tony in an e-mail, I 
promised Tony I'd do a two tone test on some of the small 73 mix 
cores he used.

I measured RF current through a string of four beads  (one turn) 
shunting a 50 ohm system, and watched IMD as power was increased. 
There was absolutely no change in IMD until I applied almost ten 
watts across the 50 ohm load. That's over 22 volts RMS!

The RF current flowing through the bead-path had to exceed 100 mA 
RMS  before any measurable increase in  IMD appeared. (Noise and IMD 
floor is over 85 dB below one tone of a two tone test in the set up I 

By the way, the RF current level that produced an increase in IMD 
also caused the beads to become very warm!

I would conclude from this data it is very unlikely the IMD is from 
core saturation. Most receiver front ends would almost certainly be a 
problem (or actually suffer component damage) long before the 
transformer generated IMD.

It's a sure bet the problem is at the receiver 
front end, a preamplifier, or other that contain semi-conductors. 

Perhaps the IMD is external to the antenna, and the antenna is just 
picking the IMD up from an external mixer that is re-radiating the 
IMD products.

73, Tom W8JI

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