Topband: RX Loops
w8ji at contesting.com
Tue May 18 09:59:43 EDT 2004
> If you live in the country, with no power lines, no
utilities, and no cable,
> that is certainly true. But it is definitely NOT if you
live in the city. My lot is
> 150' x 40'. On 160 meters, there are a dozen home in the
near field or
> quas-near field. Also within the near field are a power
line (10 ft from my
> property line and on my property), an electric powered
metro train line
> (60 ft from my property line), telephone lines (on my
property), and two
> cable TV systems (running below the power lines).
There is a tendency on this reflector or reflectors in
general to take things from context to prove others wrong,
more than to give good reliable information.
I agree (as I'm sure everyone does) all those nearfield
wires are coupling to the antenna, and the nearfield
response of antennas certainly does have different field
impedances. But the field impedance near an antenna only
affects noise or signals generated INSIDE the nearfield area
The fact is (and it IS a fact, whether we like it or not) a
small closed loop is only magnetic field response enhanced
(low field impedance) within 1/8th wl of the antenna! There
is absolutely NO guarantee that the low field impedance will
reduce noise response for noise generated within that small
area. The noise source field impedance is a matter of pure
random luck. As a matter of fact if the unwanted source is
1/4 wl away and has a HIGH field impedance, it couples more
effectively to the small loop than to a "voltage probe"!
We ALWAYS have to allow a time-varying electric field in or
the response goes to zero. We can add one, two, or ten
shields over a properly balanced loop and the field
impedance of the loop won't change. The shield does affect
loop balance (and adds some conductor losses); it can make
the balance worse or better. Like a shielded drop lead from
a Beverage, the outermost shield simply becomes the actual
The point many people miss is noise from UNintentional
transmitters like sparks, arcs, and BPL is absolutely no
different than noise from intentional transmitters in the
way it propagates or in the composition of the arriving
signal. Noise isn't "blue", signals aren't "yellow", and an
antenna can't sort them by using some sort of "field"
filter. When it all hits that mess of wires near your
antenna, the signal and noise all are affected the same way.
Anything happening to distant signals from 9V1GO is also
happening to noise from a thunderstorm a few miles away in
the same direction.
Think back at the cardioid pattern from a loopstick and a
small whip sense antenna that appears many places, like in
Handbooks or actual DF systems. One antenna is a "magnetic"
loop, the other is an "E-field probe". When that system is
calibrated from a broadcast station, it also works on an
arcing insulator. If the BC station is using a large
vertical, a short vertical, or a loop the calibration
remains accurate. This is also why a BC FSM can use a loop
to accurately measure farfield electric field intensity.
The sole difference we can rely on is the directivity of the
antenna, and is only totally reliable in a generally clear
area. In a congested area, you have to experiment. We might
find ANY random antenna better than another, and to my
knowledge no one on this reflector ever said anything
differently. What works is simply a matter of how the cards
fall. The examples of 40 meter dipoles working for some
people are examples of how luck plays a role.
There is enough confusion on this issue without distorting
the true overall view or mechanisms at work for obscure
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