A Portable NVIS Antenna    On HF, one of the factors that determine how far your signal goes is the takeoff angle. Since HF signals refract off the ionosphere back to earth, the lower the angle, the longer the distance.   One of the aims of DXers is long distance contacts and they will often go to great lengths to have a low angle of radiation. But, what if you want reliable communications over short distances?  This article looks at an antenna designed for short distances. Figure 1 shows how radio waves travel. They will take off, and assuming the ionization of the ionosphere is high enough, the signals will be refracted back to earth.  If the takeoff angle is low like the blue signal, it will return a long distance away. If the signal goes nearly straight up, the return distance is relatively short. The half wave dipole is a common antenna on the lower HF frequencies.  The takeoff angle of a dipole is dependent on its height above ground. As the dipole is raised, the angle will be reduced.  If the dipole is near the ground, the majority of the signal will be more or less straight up. That is the principle of the Near Vertical Incidence Skywave Antenna (NVIS).
 The impedance of a half wave dipole in free space is about 70 ohms. As it is lowered, the impedance drops.  By the time it gets to 7-8’ high, the impedance is down to about 12 ohms.   This will not provide a good match to 50 ohm coax.  One way around this is to use a folded dipole. The impedance of a folded dipole is about 200 ohms.  The impedance of the regular dipole dropped by a factor of 4 when it was lowered to 7’.  Starting at 200 ohms, and reducing it by a factor of 4 gives you 50 ohms, a perfect match! A folded dipole is the same length as a regular half wave dipole, but it is made of twin lead. The ends are shorted.  One of the conductors is cut in the middle and the cut ends connect to the coax braid and center conductor.  It is also a good idea to put some ferrite beads over the coax to prevent current from flowing on the outside of the coax and messing up the pattern. Since the dipole is so close to the ground, it will induce currents in the ground.  This will result in losses. Laying wires on the ground will help reduce these losses.
 Figure 1. Effect of take off angle on distance covered.
 Construction My interest in the NVIS antenna started at an ARES (Amateur Radio Emergency Service) meeting. The local Red Cross liaison was asking what we could do to provide communication outside the usual 2 meter VHF range and if anyone knew anything about the NVIS antenna.  No one did, so I thought I would investigate them.  After some reading I decided to build one and make it available to the local ARES group in case of an emergency.  Since my primary interest in ham radio is contesting, and since the annual Field Day exercise was only a few weeks away, I decided to give it a try on 40 Meters. The intended purposes of this antenna required that it be portable and quick to set up.  I decided to use fiberglass military mast for the support. Two sections will get the antenna up about 7’ which is perfect.  I used a PVC pipe Tee as the center section support. The center section of the dipole is taped to the TEE. Tie-wraps would be better for a permanent set up.  RG-58 is the coax since it is not anticipated to be used with more than 100 watts or so.
 Figure 2. Portable NVIS antenna construction