[RFI] Loop antennas for handheld HF EMI sniffing

Cortland Richmond ka5s at earthlink.net
Tue Jan 25 14:25:06 EST 2005

Here's something I wrote to a poster on another list who wanted an antenna
to chase down HF EMI with his handheld Icom receiver. 


While NO passive, small antenna is very sensitive on HF, for EMI sniffing I
like shielded loops on handheld receivers. There are different ways to make
one. One I like to use is easily made from Radio Shack coax, and two coax
adapters.  You'll need a 12 inch RG-58 jumper with PL-258 connectors, a
UHF-series T connector, and a UHF to BNC adapter to fit on the radio.

On the jumper, cut off the crimped end of one connector's center pin so it
is no longer holding the center conductor. With pliers, pull and wiggle the
center pin until it and the plastic insulator on which it is mounted comes
out of the connector shell. This will leave the splayed out braid where it
was, and the bare center conductor sticking out. Bend the center conductor
to one side and force the insulator -- with the center conductor off to the
side, NOT in the pin -- all the way back in so it forces the center
conductor into contact with the braid and the connector shell. Trim as

Connect the modified jumper to the female ends of the T connector so the
jumper forms a loop with the T connector pointing downwards and. away from
it.  Now you can put a finger, pencil or other object in the loop to find
the balance point it hangs from.  Remove about a quarter to half inch (not
critical) of outer insulation and shield braid from the coax at this point.
Be sure the shield is completely severed (not even one strand of copper
making contact) and that the center conductor is not cut. It won't hear
anything if there's contact between the two halves of the shield here.

Screw the shielded loop you made onto the UHF-BNC adapter and connect to
your R-10.

Tune in an AM broadcast station and note the reception nulls when rotated.
With the radio held outstretched,  looking right through the loop, nulls
will occur when you are either looking right at the incoming RF (and
station) or away. In the very near field the loop can be used "transformer
style" parallel to a conductor to tell which of several wires  are "hot."

This loop is not as simple as it can be, but has the advantage of being
made easily from available parts and is pretty much self supporting.

It's possible to make a simpler loop that requires only one (BNC, or maybe
SMA) connector. In this case, you strip off the shield and insulation from
the center connector at one end, making sure the center conductor sticks
out an inch or more. Strip a circle of insulation from around the coax near
the connector (don't cut the shield) , and wrap the center conductor around
the shield here. Solder or tape securely. Screw onto receiver. This loop is
best made with semi-rigid coax; as soldering RG58 weakens it and it tends
to fall over. 

The simplest loop is merely a BNC-to-Banana terminal adapter, with a loop
of wire running between them. This lacks a certain (shall I say?) snob
appeal -- grin!  It is unshielded, so the null can be upset by reception as
a wire rather than a loop. This often doesn't matter. You can wrap a foil
shield etc. around it as long as the shield is unterminated at one end or
open. A tin can slotted with tech connector press fit through a hole in one
end would work -- and hide the kludge.

Good hunting!
Spelling/typing errors in the original have been corrected. 


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