[600MRG] Loop on the ground

[Bart Lee]

Here at K6VK, the best VLF antenna for a WiNRADiO G33 SDR is a a pair of
verticals at 40+ feet (One Alpha S9+43 and a Hustler 6BTV, as simple
e-probes), into an impedance reducer (an emitter follower circuit from W6BM
 -- maybe a 9:1 transformer would also work). The difference between this
set up and the 6BTV alone is 30 dBm on VLF at 25 KHz.

Many experimental loops, big and small, dot the property. The are all full
of 60 cycle harmonics, in whatever configuration. We have 21 KV lines on
both the East and West sides of the property. The loops are fairly
directional and the verticals omnidirectional. General EMI is fierce, but
the verticals seems least affected, and they are orthogonal to the lines
themselves. I'm told that the power lines are emitting mostly magnetic and
not electric interference, which is why my shielded coax loops still get
the harmonics. At HF, the verticals are noisier, I'm told because most HF
noise is vertically polarized.

Four AM ("O'Dark:30") is fairly quiet. I see sometimes strong WSPR signals
on both 2200 meters and 630 meters. QTH is east of San Francisco.

If you are anywhere near power lines, a loop is problematic. If you are far
enough away, a *tuned* big vertical loop in the direction of interest would
likely outperform a vertical array.  It will null broadside. But maybe get
some verticals up and see what happens.  I'm told nested schedule 80 PVC
pipe will stand up by itself (some guying is still good) and can maybe get
you up sixty feet, with a thick wire down the pipe. Its output will be very
high impedance, of course, but that can be managed.

Have fun!!

73 de Bart, K6VK
-- --
Bart Lee, K6VK, CHRS, AWA, ARRL

Texts only to: 415 902 7168

www.bartlee.com

{Bart(dot)Lee(dot)K6VK(at)gmail(dot)com}
<http://www.lawforhams.com/>



On Sun, Feb 9, 2020 at 3:04 PM Rudy Severns <n6lf at epud.net> wrote:

> Dave,
>
>
>
> The answers to RX antenna questions will depend strongly on the particular
> situation at a given QTH.  I spent the last year frantically trying many
> different RX antennas but ended up learning about reality.
>
>
>
> First, how close, in wavelengths, is the RX antenna to the noise
> source(s).  Anything less than 1/4 wave means you’re in the inductive
> near-field.  A quarterwave on 630m is about 500’ and 3600’ on 2200m.  When
> you model RX antennas within that distance you have to look at the
> near-field pattern which is often very different from the far-field pattern.
>
>
>
> Second, power lines are a combination of horizontal wires with occasional
> (or frequent) service transformers and drop lines which have a vertical
> wire from each transformer to a ground stake.  This means you have both
> vertical and horizontal RFI currents.
>
>
>
> The magnetic field from the horizontal lines will couple very nicely into
> a horizontal loop, in the air or on the ground.  They also couple nicely
> with vertical loops parallel to the lines.  The vertical ground wires
> couple nicely into vertical loops perpendicular to the lines.   In my case
> where the lines are perpendicular no matter what direction I point a
> vertical loop I get really great reception of the line RFI.  I have a loop
> mounted on a rotor which shows a very sharp null for sky-wave signals but
> no help at all for the line RFI other than to move the bars on the
> waterfall display around.
>
>
>
> I have the worst of all: power lines, all well within the near-field, in
> the form of a T.  My greatest distance from a line is 400’.  On my south
> boundary I have a polyphase industrial service running E-W forever.  On my
> west boundary I have a branch line, also polyphase, running N-S.  In most
> urban areas (I’m out in farmland BTW) this  would be a typical geometry.
> You are likely to be surrounded with lines.  Erecting a vertical loop is
> the same as adding a secondary winding to a transformer, i.e. the power
> lines are the primary.  Forget radiation, this is just plain old inductive
> coupling. Horizontal loops, BOG’s, Beverages, etc, same problem.  Laying
> down a 700’ BOG greatly improved my reception of the power line RFI.
>
>
>
> In my situation at least, the noise is mostly horizontal from the lines
> and to a much lesser extent vertical from the vertical grounding wires.
> The near-field usually falls roughly as the inverse of the cube of
> distance.  My best antenna is a simple vertical located in the far N-E
> corner of my property.  As discouraging as my situation is all is not lost,
> I still decode VK4YB WSPR on 2200m regularly.
>
>
>
> 73, Rudy N6LF
>
>
>
>
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