[600MRG] 630m background noise

[Dave Riley]

Thanks Tom, I'm on a good learning curve now..

I tried the shielded vs. no shield and really couldn't see any difference..

Previous to this loop was a 'passive e-probe' in the form of a 20' 
vertical wire tied to the top  end of a 100 turn winding w/ #75 ring 
with the other end going to it's own ground directly beneath the 
vertical.. With about 15 turns wound on the low side and using twin lead 
it was fairly low noise and very broad band, thence another decoupling 
#75 ring xfmr at the receiver.. Dallas Lankford style..

Thanks for your response.. I will be processing for a while..

73s de Dave / W1FRV


On 1/30/2023 1:59 PM, Tom W8JI wrote:
> 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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