Tiny NVIS and DX antenna

[KG4RQO]

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

[KG4CGC]

That's a special vehicles antenna. What or where, are you mounting?
A length of wire is your best friend.

[NQ6U]

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.

[kb2vxa]

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.

[KG4RQO]

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.

[N8YX]

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.

[W9JEF]

.



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.

[N8YX]

.
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.

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.

[W9JEF]

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.

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 require
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.

[N8YX]

To change from DRRR to a true loop would require
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.