[600MRG] 630m background noise

[Ed Cole]

Interesting.  I intend to build a rotatable 6-foot diam shielded loop 
using some discarded 1/2 inch Heliax (LDF4-50A).  Transmitting antenna 
40-foot high tee vertical using 80 and 40m  dipole wires as top loading 
and my old Inv-L loading coil at the base.  Will try to load on 160m, as 
well.

I have a ckt board to make a 600m preamp for the loop.  Probably next 
summer.  I have an old TV rotator for this, if it still works.

73, ED - KL7UW

On 1/30/2023 9:59 AM, 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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