I'm a newbie here. Definitely and antenna-student and not an antenna theory teacher. Has anyone ever seen, or better yet, used one like this?
http://www.hilomast.com/images/uploa...op_Antenna.pdf
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I'm a newbie here. Definitely and antenna-student and not an antenna theory teacher. Has anyone ever seen, or better yet, used one like this?
http://www.hilomast.com/images/uploa...op_Antenna.pdf
That's a special vehicles antenna. What or where, are you mounting?
A length of wire is your best friend.
I agree with the barkeep. That antenna is designed specifically for local mobile communications on HF which is pretty much opposite of what most of us hams want.
By "special vehicles" he means a radio truck at a military field command post. Previously radio communication was plagued by the skip zone between the direct wave and sky wave returning from the ionosphere where communications is skipped over, a dead zone. By using NVIS the skip zone is eliminated, picture a large umbrella over the truck, signal returning from the ionosphere over a wide area.
Thank you to all three for your responses. The probably site will be on a small part of a flat roof on top of a 5-story apartment building. The primary intent is for NVIS communication within Florida and possibly a bit of adjoining states in case of emergencies (we're overdue for an East Coast hurricane or two, to say nothing of the tornadoes that are already paying visits to this area.) While we can, in fact put out a pretty "long piece of wire", it would only be a few feet above the roof-top. We have to be very careful about "stealth" installation to keep the property owner happy.
That design looks similar to the old AEA 10-30 loop, which is a derivative of a DDRR (directional discontinuity ring radiator).
The ARRL Antenna Handbooks of the late 70s and early 80s featured several DDRR construction projects, all using so-called "plumber's delight" construction. Namely, copper and aluminum tubing.
If I was the one putting up such a construct, I would build it from 1-2" copper pipe and scale the outside dimensions so it'll tune from 3.5-10MHz. This will give you 80/75, 60, 40 and 30M - the latter band being just about the highest frequency where optimum NVIS propagation can be obtained. Use PVC tees, pipe and flanges to space it about two feet off the roof; underneath the antenna, a ground plane made from chicken wire or similar mesh is laid down. The antenna is tuned with a motor-driven variable capacitor whose size and plate spacing will be dependent on the loop characteristics and the amount of power you wish to run through it.
A homebrew version is going to be a lot cheaper than its commercial equivalent, and will be practically invisible from the street if built as I described.
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Looks can be deceiving. The NVIS loop and DRRR are both
constructed of large diameter conductors because of the
lower radiation resistance they share. But other than that,
they have little in common. The DRRR requires a ground plane,
and is vertically polarized, the exact opposite requisite for NVIS.
Although a vehicle top is usually made of metal, a loop antenna,
by its nature, does not not require a ground plane.
Depends on how it's being fed. A tuned loop design can drive the loop against a ground via series feed (resulting in vertical polarization) or via a balanced feed, resulting in horizontal. In the case of a balanced feed being worked very close (in terms of wavelength) to a ground plane, there will be a significant high-angle radiation lobe.Quote:
Originally Posted by W9JEF
While not a true DDRR, for purposes of discussion the latter method is very similar in concept.
An ideal implementation would use vacuum relays to change the feed method and a pair of motor-driven capacitors to resonate the loop in each mode.
To change from DRRR to a true loop would requireQuote:
Originally Posted by N8YX
more than relays and motor-driven capacitors.
The DRRR "loop" axis is vertical, but a real loop's axis
is horizontal, with no ground plane.
You might want to argue that point with Art Bell and a number of others who successfully employ horizontal loops at their stations.Quote:
Originally Posted by W9JEF
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.
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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.