NVIS Antenna designs
WARNING! WARNING! WARNING!
NVIS antennas operate at low levels and are within easy reach. Apart from a hazard of being cut or tripped by low wires - RF BITES! ALWAYS take care to keep people away from the wires.
The Military NVIS antenna... (US Type ANTENNA AS-2259/GR)
NVIS antennas are by their nature often simple 'long wire' arrangements.
The first example is a popular one based on the military multiband NVIS antenna.
40m dipole section 80m dipole section
Top cap arrangement common 40m & 80m links
The antenna consists of a single pole (a decorators roller extension handle or PVC pipe?) approx 12' - 15' (3.5m to 4.5m), fixed to the top is the top plate which has an SO-239 mounted in the centre. The top cap is fixed to the support pole by right-angled strap with 2 screws or bolts, leaving room to attach a PL-259 from the HF rig/ATU. Ideally the pole and the top cap should be well earthed. The two 'hot' ends of the antenna are attached to the centre tag of the SO-239 and the earth ends are attached to the plate.
Each 'leg' of the antenna is 45' (13.7m) in total with the nylon rope end sections after the hf isolators forming the supporting guy ropes for the mast and antenna going to a tent peg. It is important NOT to over tension these lines or the feed point may become damaged.
The knee level... (W2IK Kit)
at knee level and at approx 2m above ground
The "W2IK NVIS Antenna Kit" is shown in the picture above. This type of antenna, if designed and deployed properly, will not create "skip zones". This makes it ideal for 40 or 80 meter emergency work. The RF pattern will resemble a half grapefruit with reliable signal coverage 400 or so miles in every direction.
A simple static vehicle NVIS antenna...
This antenna uses a vehicles HF mount and runs a long wire from the 'hot' side. The vehicle acts as the counterpoise/earth. the wire is 33' (10m) long with an option to add a second 10m section and a jumper to make an 80m antenna.
The Buddipole Quick-fix for NVIS on the Lower Bands...
You can take a standard Buddipole and turn it into an excellent antenna for local communications on 40, 60, and 80 Meters.
Make up two identical wire assemblies as follows: Wind 31 feet of wire on each of the linewinders. Solder or crimp a 3/8” ring connector on the loose ends of each assembly. Attach the ring connector to the female opening at the end of each coil by using the 3/8 by 24 threaded bolts. The wires on each side of the coils will slope down from the T when the mast is raised to about 9 feet off the ground. Keep the wires off the ground by at least two feet by using non-conductive electric fenceposts (or similar stakes) for that purpose. Follow the chart below to determine the wire lengths. You can paint colored marks on the wire to show the lengths of that wire needed for the band you wish to use. The coils are not tapped for 80 Meters, but are tapped for 60 and 40 Meter operation. Note: Always count tap settings from the WIRE SIDE of the coil.
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