[RFI] Shielded vs unshielded loops - difference

[Tom Thompson]

Several of you have shown interest in the loop  antenna.  I have updated 
my web page to show more details.  If you have it cached, you may have 
to refresh it.  The URL is: 
http://tomthompson.com/radio/ReceivingLoop/loop.html

Let me know if some of it is unclear.

Tom   W0IVJ

On 3/28/2014 1:53 PM, Tom Thompson wrote:
> Hi Ken,
>
> One thing to keep in mind when you choose a loop is the antenna 
> factor.  I believe your RFI is mainly on 80m.  A small (10")  loop 
> that is not tuned will have a terrible antenna factor on 80 m. You may 
> not even hear the noise from your house.  Also, your receiver probably 
> does not have a very good attenuator in the front end. When you get 
> close to the noise, you can detune the loop and get some attenuation.  
> You need to keep the signal right at the noise level in the null for 
> the AGC NOT to work if the S-meter is not very good.  When the AGC is 
> NOT working, you can listen to the strength of the signal through the 
> speaker.  Again, remember the null is looking through the loop with 
> the feed at the bottom.  Also, I always put a choke balun right at the 
> loop so I don't get false readings from picking up signals on the coax 
> when the signal gets strong.
>
> When you start your hunt, find the direction of the noise by nulling 
> the loop.  You now have an ambiguity in direction.  Peak the noise by 
> turning the loop edgewise and walk in one direction. Keep rotating the 
> loop back and forth looking for the peak signal to get stronger.  If 
> the peak is getting weaker, walk in the other direction.  When you get 
> very close to the RFI, use the null feature of the loop and attenuate 
> the signal by switching in an attenuator or detuning the loop.
>
> I have been able to trace a grow light almost to the room in the house 
> by not getting on the residential property.  Once I find the house and 
> if the RFI is on a timer, I will position myself in front of the house 
> and wait for the noise to appear to make sure I have the right house.
>
> If you get this far, email me and I'll tell you how I approach the 
> grower.  That is sometimes tricky and could be dangerous so I do not 
> want to generalize here.
>
> Tom   W0IVJ
>
> On 3/28/2014 1:17 PM, Kenneth G. Gordon wrote:
>> "The unshielded loop will...respond to electrostatically induced noise."
>>
>> While:
>>
>> "The shielded loop , on the other hand,  is almost insensitive to
>> electrostatically induced noise."
>>
>> In other words, the shielded loop is NOT a good choice to DF the 
>> types of
>> noise we are most often bothered by, while the un-shielded loop is
>> specifically sensitive to such noise.
>>
>> So, my use of my NM-20B, with its shielded loop, was counter-productive.
>>
>> Kenneth G. Gordon W7EKB
>>
>> "Courage is being scared to death but saddling up anyway."--- John   
>> Wayne
>>
>> _______________________________________________
>> RFI mailing list
>> RFI at contesting.com
>> http://lists.contesting.com/mailman/listinfo/rfi
>>
>
> _______________________________________________
> RFI mailing list
> RFI at contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>