[600MRG] Is this snake oil?

[Tom W8JI]

It is sad how little our community knows and understand the way common 
mode is created, and how that common mode enters a system signal path. 
It leave us open for all sort of odd devices and theories.

With a coaxial line, common mode can only enter the signal path at a 
flaw or break in the shield. It works this way in everything from loop 
antennas to our feed lines. A "shielded  magnetic loop" does not receive 
signals from the wire inside the shield, for example.  The wire inside 
the loop shield is just a transformer winding that couples to the shield 
inside layer, and that layer couples to the shield outside at the shield 
gap where the necessary electric field appears to drive the outside of 
the shield. The outside layer of the shield is the only actual antenna 
if the system is built properly. If it is improperly built, then the 
feed line shield and everything connected to the feed line shield it is 
involved.

This is even why a small e-probe antenna gets more sensitive when we 
mount it higher, it has more feed line shield to pick up signal.

In ALL of these cases , the point of common mode ingress into the 
receiver signal path is always at a discontinuity or break in the 
shield.  In a shielded loop it is at the shield gap. In a e-probe it is 
at the probe element, where the shield center is continued out past the 
shield by the "probe".  This causes people to miss what is really going 
on, and they make up a well intentioned but seriously flawed theory.

If the shield has good integrity to the receiver input any ingress at 
the receiver would be negligible compared to ingress at the antenna or 
an upstream gap or break. The meaningful leak-in point, unless there is 
a flaw in the cable shielding, would be at the antenna.

Another thing often missed is the antenna (which by definition needs 
common mode on it to function as an antenna) receives signals directly 
from noise sources through local induction fields or long distance 
electromagnetic radius fields. Even if the feed line shield has unwanted 
common mode noise, the noise level on the shield has to exceed the 
desired antenna's pickup of noise.

One of the very worse antennas for this is an E-field probe, because the 
lack of a stable infinite ground means the shield is as much the antenna 
as the probe is!! There is no way around this without an RF ground plane 
(electrical mass) at the probe! This is why elevated probes become more 
sensitive with more height...because the shield is the bulk of the antenna.

All of this is important to get our heads around to build better and 
cleaner systems. To me, while this isolator has a possible function (I 
designed one for DX Engineering 20 years ago so I obviously thought they 
have a place) it would not be when it is placed at the receiver. It 
would be when placed somewhere outside where the shield path should be 
broken before the reaching the ingress point, which is usually at the 
antenna. I sure wouldn't put one at my receiver or at my receiver 
multiplexer panels.  A single proper bead would be way better in my 
shack, if I need anything.

73 Tom


On 3/25/2023 12:26 AM, D.J.J. Ring, Jr. wrote:
> https://www.bonito.net/hamradio/en/galvanic-antenna-isolator-gi1000/
>
> The Galvanic Antenna Isolator GI1000 suppresses noise reaching the 
> ground connections on the receiver. The GI1000 covers a wide-band 
> range of 50 kHz – 1 GHz (typ. 1,5dB insertion loss). This unit can be 
> used in many receiving applications, It will also work with 
> frequencies up to 1,25 GHz but above 1000 MHz insertion loss may 
> increase by up to 3db. The unit also has integrated double 
> over-voltage protection and input and output are blocked for DC voltage.
>
> The galvanic isolator GI1000 separates the path of the direct current 
> between the outer shield of the coax cable and the shielding of the 
> antenna feeding line in order to suppress interference caused by 
> potential differences. This is achieved by using a small toroidal 
> transformer. The inner conductor of the coax cable is insulated with 
> capacitors providing coarse and fine voltage surge protection.
>
> Close connection please
> The GI1000 should be connected to the receiver as closely as possible 
> to avoid other interference. We offer short very adapter cables with 
> different connectors.
>
> Galvanischer Antennen Isolator GI1000
>
> GI1000 & CCMC30 a great team
> Even a cascaded use of the GI1000 directly at the receiver input and 
> the CCMC30 following it directly after the coaxial power inserter can 
> in some cases lead to an increased efficiency. Especially in the lower 
> frequency range, surprising improvements can occur:
>
> CCMC30 mit G1000 kaskadiert
>
> Galvanic Antenna Isolator GI1000 Specifications:
> Excellent protection against sheath waves and voltage surges.Double 
> voltage surge protection. On the antenna side Gas Discharge tube with 
> an ignition voltage of 90v. On the receiver side you will find an 
> ESD-diode (30KV; max.pulse power 350W (8/20μs) Input and output has 
> been blocked for direct currents of max. 50V.
>
> Tested by Clint Gouveia
> The well known DXer Clint Gouveia (Oxford Shortwave Log) is using the 
> GI300 (previous model) on nearly all his DX-Expeditions. He wrote a 
> Test including Videos. Please ready his tests here: read the test
>
> 73
>
> DR
> N1EA
>
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