Topband: QST Jun 06 RX Loop

Tom Rauch w8ji at
Sun May 14 22:12:46 EDT 2006

> One question that some readers of this forum might ask: 
> If that is the
> case, how does an inductively coupled network, where two 
> resonant inductors
> are separated by a Faraday shield, transfer energy?

We can modify the direct capacitive coupling (change 
impedance), but we cannot stop the time-varying electric 
field without stoping the time-varying magnetic field. Take 
either to zero and the other goes to zero. Nothing goes 
through a solid shield. It can't because inside the shield 
wall the electric field goes to zero.

> Recall the old B&W HDVL series plug-in coils with 
> swinging-link coupling.
> feedline.  But if "essentially nothing goes through that 
> shield," how is the
> fundamental-frequency energy from the tank coil 
> transferred to the pickup
> link?

The same way it does in a loop.The outer shield becomes the 
actual radiator. The inner conductor loop couples to the 
inside of the shield, the inside of the shield has a voltage 
(potential)  difference across the gap, and that potential 
causes current to flow on the shield outside. The current on 
the shield outside, which has a voltage difference along its 
length and current flowing,  is what actually couples to the 
tank coil.

It's no different than a larger loop.

> My explanation is that in the case of the link-coupled 
> tank circuit, we are
> talking about what is essentially a transformer, where the 
> primary and
> secondary coils are magnetically coupled, and the shield 
> prevents capacity
> coupling.

It also prevents the magnetic field from passing directly 

What you really have is a three layer transformer. The inner 
conductor's fields set up a longitudinal voltage that is the 
same as the inner conductors voltage. This voltage appears 
across the gap. The voltage difference across the gap causes 
current to flow on the inside and the outside of the shield. 
The current inside the shield flows the opposite direction 
of the center conductor current and equals the center 
conductor's current. The current on the outside flows the 
same direction as the current in the center conductor, but 
opposite the current on the inside of the shield. (This can 
actually be measured on a bench with a few boxes made from 
something like double sided blank PC boards.)

What happens if we short the gap is we force the voltage 
across the gap to zero. Now we have zero time-varying 
voltage outside the cable shield, and we also have zero time 
varying current (assuming a zero ohm short).

Any time we take the time-varying voltage to zero we do the 
same for current, and vice versa.

In a "shielded link" the shield does exactly the same thing 
as in a larger loop antenna. The outside of the shield 
becomes the actual link.

We can measure the loss in Q and increase in heating caused 
by current having to flow over the additional length on the 
inside length plus the outside length of the shield.

Of course what you say about the antenna is absolutely true. 
The electric and magnetic fields in a "radio wave" are 
always inseparable. The field impedance very close to the 
radiator can be changed as can balance of the system, but 
the link behaves just like the loop. It all follows the same 

Even the shields around our transmitters or a screen room 
behaves the same way. The screen room doesn't let a 
time-varying magnetic field in unless we crack the door open 
and allow a voltage across the gap, and then we have both 
time-varying voltage and current. The outside of the screen 
room becomes the "antenna".

73 Tom 

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