[RFI] Shielded vs unshielded loops - difference

[Tom Thompson]

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
>
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