How large are these loops, and how high up?Quote:
Originally Posted by N8YX
The DRRR is definitely not what is considered a loop.
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How large are these loops, and how high up?Quote:
Originally Posted by N8YX
The DRRR is definitely not what is considered a loop.
These look like small magnetic loops to me. The Germans used something like this as far back as WWII on the backs of their vehicles for NVIS work. I've wanted to try putting one on top of the tonneau cover for my pickup.
Magnetic loops like this have extremely strong magnetic fields in close proximity to them. I'd be very, very careful about where it was mounted on the roof. I also don't understand the orientation they have chosen for the loop - I would think that a horizontal loop would work better. I used a small mag loop in the house here, and horizontal worked better for me.
How big is the roof? It might be possible to fit a full-sized 80 meter loop on it. It does not need to be square by any means - you can pull the ends into the middle, to make it much smaller, too. The March 1994 QST has an article by K1KLO, Andy Pfeiffer on what is known as the 'Maltese Quad' because it's shaped like a Maltese Cross. You can apply the same principles to a single loop. You can feed it directly with coax and it will operate on all harmonics of the design frequency. With open wire feeders, it will work at slightly below the design frequency and at all higher frequencies. 1004/F should get you in the ballpark for total wire length. On 75 meters, 3.8 MHz,
that would be 264 feet, or 66 feet on a side with a square. It can be almost any shape you desire, though.
.
The DRRR is vertically polarized.
The "loop" current (very close to the ground plane)
acts to cancel any horizontal polarized radiation,
it serves mainly as linear loading.
The radiation patterns of large (one wavelength)
versus small loops are entirely different.
(Check out the difference in any good antenna book (or website).
For NVIS, a horizontal loop (vertical axis) less
than a quarter wave above ground is the norm.
A small magnetic loop on a vehicle needs to have
a horizontal axis (for best NVIS coverage).
In response to the OP, and cutting through the noise here...
Don't get all loopy, DDRR is not the same as a "standard" loop, it just looks similar. It is vertically polarized, it's a type of monopole. It ain't no "Squalo" either. ;)
Normally, a vertical antenna is not going to perform as an NVIS antenna, although you can "tweak" the DDRR installation to get a stronger "skywave", it's not optimal for NVIS. On the other hand it is a damn good antenna design for HF in small spaces.
http://www.orionmicro.com/ant/ddrr/ddrr1.htm
http://www.w1npp.org/events/2010/201...D/711202~1.PDF
It is certainly true that vertically oriented "standard" loops have horizontal or vertical polarization, depending on the feed point placement.
Art Bell's "monster loop" is a "standard" 1500+ foot loop, with two wires in the loop. I presume this is similar to the idea used with a cage dipole to widen the bandwidth. Dunno.
Art Bell's set up covers acres vs feet. That is what he said on the air. I've seen pictures somewhere. Towers as supports, not masts.
Thank you very much for the suggestion of the "Maltese Cross." This may make it possible to fit a full wave 80 meter loop above the rooftop (granted, maybe only 10-12 feet above it). Some work I've seen by K6SOJ and W0IPL indicate that such a height above "ground" (the rooftop) may well be very effective for NVIS work.