Understanding the nature and pattern of a single antenna pattern is key to
it's use. One bright possiblility, since the loop is so small to begin
with, is to overcome pattern with multiple loop antennas and phasing
(that's multiple antennas in separate locations). If you are in a
situation where a bad noise is off somewhere on the side of an important
direction, put the two loops on a line toward the wanted RX, e.g. NE/SW in
the eastern US. Orient them both to kill the local noise. Then phase them
to null the SW direction. If the spacing is serendipitous to the property,
some modeling will be needed to determine proper phasing to get the general
SW null. But you will get at least as good as phasing a pair of RX
verticals, plus the severe notch on the local noise. You WILL need a
separate amp on BOTH loops (just don't bother trying to get away with one,
you'll get buried in common mode noise). You may wind up with a superior
solution in a needed direction.
Three of them in a tight triangle after the fashion of one of the 3 element
RX array is not ridiculous either. It mostly depends where you need the
notches.
Mentally, just consider one of the loops as omnidirectional "with
advantages", and as much fodder for array making as anything else. We're
always looking for a silver bullet on a single mounting post. Just ain't
gonna happen on 160. Vary the height of a mag loop listening to the noise.
No rules here, because of miscellaneous conductors (including buried)
everywhere. You may find a sweet spot. Again, no rules for this, just
check it out.
73, Guy.
On Sun, Apr 15, 2012 at 10:00 AM, W2PM <w2pm@aol.com> wrote:
> Don. No doubt mag loop is far less sensitive to interaction than the flag
> or pennants where metal objects within 50 ft will certainly distort the
> pattern. Mostly the foward lobe but modelling and observation shows the fwd
> lobe is broad anyway.
>
> I have heard a notable 160 dx ER say the height of a mag loop can
> frantically affect its ability to reduce local noise however. He claimed
> having the bottom of the ring 10 ft up is optimal but also suspected that's
> purely a condition of the local noise character. I can tell you strong
> power line crud and be very spotty too eminating more strongly from certain
> spots on the span than others. Since I use a MFJ nuller as well found sense
> antenna location critical to success. Have also used a magnetic loop as
> the noise sense antenna quite well broadside the loop to the noise.
> Provides a far more table noise source and thus makes nuller more effective
> and easier to tune.
>
> Btw I use a SW (VK) facing half flag hidden in a tree Which happens to
> have its rear null direct to nasty power line noise 500 ft away and it's
> great. I only use 6db or ago of preamp on it tho. Beyond that it could
> pickup too much local noise in my setting.
>
> 73
>
> Sent from my iPhone
>
> On Apr 11, 2012, at 5:41 PM, Don Kirk <wd8dsb@aol.com> wrote:
>
> > W2PM said,
> >
> > "A comparison of the Wellbrook product which is a magnetic loop is not a
> good one to a AY loop or other low angle directive receive antenna. Mag
> loops are not "directive" in the same way they only have a very sharp null
> to two opposite directions and omni directional vertically. Nice for
> nulling strong local point source noise but that's it. It will enable qsos
> not possible otherwise and some DX I suppose but Wellbrook is not a good
> choice for any serious DX work. No mag antenna is."
> > ----------------------
> >
> > And so far my testing fully supports the above comments by W2PM, and
> calculated RDF values along with takeoff angle really appears to help
> predict the S/N performance of one antenna system versus another when
> dealing with normal atmospheric noise that's coming in from all directions
> or at least say multiple directions (with a higher RDF value and lower
> takeoff angle generally being desired for DX which is not a strong point of
> Mag loops compared with AY loops, Flags/Pennants, etc.).
> >
> > For local point source generated noise such as a Plasma TV, the Mag loop
> really helps if the offending signal is 90 degrees (or 270 deg) to the
> desired signal. Whereas the Pennant/Flag really helps when the offending
> signal is 180 degrees to the desired signal.
> >
> > I have a gut feeling through preliminary testing I've done that a
> shielded mag loop has less interaction (less distortion of its pattern) due
> to nearby objects when compared with a small rotatable flag, possibly
> making a well balanced mag loop usable where the Pennant/Flag might not be
> (very small lot size, indoor operation, etc.). Detailed pattern
> measurements need to be done in order to quantify this casual observation.
> >
> > I should also mention that I only obtain an approximate 2 db improvement
> (on average) in S/N performance using my half size pennants compared with
> my 68 foot tall base loaded TX vertical (but in reality I believe this is
> close to what I should expect based on calculated RDF values). I also
> notice that the S/N difference is much greater between my pennants and TX
> vertical for stations close in versus DX which is likely due to differences
> in elevation patterns between my pennants and my TX vertical. Contrary to
> most folks opinion, I set my preamp gain on my pennants to provide a noise
> floor identical to that of my TX vertical for S/N comparison measurements
> (I find it impossible to quantify S/N performance between two antenna
> systems any other way under real life operating conditions, and I
> continuously switch rapidly between the two different antenna systems in
> order to evaluate the average S/N improvement).
> >
> > I am by no means an expert on this topic, but I wanted to share my
> comments/experiences since I've been doing a lot of testing of shielded mag
> loops and small pennants and flags during the past 3 years.
> >
> > 73's
> > Don (wd8dsb)
> >
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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