[600MRG] 630m background noise

[Tom W8JI]

Hi Dave,

There are a few myths passed around that are useful correcting.

We can't build a shielded loop that is really shielded, so don't fret 
about your loop not being "shielded". It is physically impossible to 
"shield" a time-varying electric field without also taking the 
time-varying magnetic field to zero.

What really happens is the current on the center conductor(s) induce a 
current on the inner wall of the "shield". That current causes a 
longitudinal voltage at the shield gap. The voltage at that gap excites 
a current ion the other shield wall, and that becomes the antenna. This 
is why a well-designed shielded loop always has the shield gap 
diametrically opposed and equal distances from the bottom, and that is 
where the mounting is attached. If this is not done the shield 
unbalances the antenna and introduces common mode that is very difficult 
to get rid of.

The shield if properly implemented simply balances the antenna and 
becomes the actual antenna! The inside stuff is just transformer coupled 
to that outer wall. This is Lenz's Law. It is the same reason we cannot 
have common mode current on the center conductor of a coaxial line, and 
all common mode on a frequency above the skin depth of the wall just 
moves to the outer shield wall via shield gaps.

If you properly balance a single turn loop the radiation and induction 
fields are identical to a shielded loop.

If I were going to build a LF loop I would either link couple with the 
capacitor up top and the link at the bottom and use multi-turns to 
reduce capacitance size, or do a *PROPER* shielded loop with shield gap 
up top and dead symmetrical each side to ground.

The second key point to remember is loops are magnetic field dominant 
due to the strong magnetic induction field near the loop, but they also 
have an electric field. As a matter of fact at 1/8th wave from the loop 
the combined magnetic and electric fields are all even. It has no 
specific favored field impedance. Beyond 1/8th wave the small loop 
becomes electric field dominant, or a high field impedance! Between a 
1/2 and a full wave away the field impedance from all small antennas is 
equal, we couldn't tell if it came from an "e-field probe" or "magnetic 
loop", the field impedances are all equal.

  Electric field probes, unless ground mounted, are extraordinarily 
difficult to remove common mode. The common mode is why they sometimes 
gain sensitivity when installed higher. When installed higher, we are 
just making a longer antenna.  If I used a probe and didn't have a 
suitable ground plane of significantly lower impedance than the common 
mode Z of the feeder, I would ground mount it.

I think we could do a lot better with receiving antennas than we are 
currently doing.

One really good basic design was a link coupled system I saw posted on 
here. I can't understand the need for balanced line in the link, but it 
appears to be one of the more sensible approaches.

I have a selfish interest in getting the best possible receive antennas. 
The better solutions the more people there will be to work.

I'd like to set up an SDR here on a good omni antenna someday. If I only 
understood computers.

73 Tom


On 1/30/2023 12:43 PM, Dave Riley via 600MRG wrote:

> The current loop antenna here for 630m is 5 turns of #14 wire, 
> resonated and not shielded in a 5' dia.
>
> It out performs previous antennas here so far.
>
> With a 50 ohm load connected to the receiver input I set the audio out 
> ( no agc ) to 0 db
>
> Replacing the load with the loop antenna I get + 7 db of noise with no 
> signals
>
> Is this about average for 475 khz?? Does anyone know what to expect at 
> these frequencies??
>
> Dave/W1FRV
>
>
>
>
>
>
>
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