[Milsurplus] PRC-74 & NVIS

[Gene Smar]

Ben:

     What an NVIS antenna does is put a signal into a zone on the ground that is an annular region extending from about 200-500 miles or so from the transmitter.  That is, it is intended to fill in the <skip zone> that is normally created by using high dipoles, long wires, verticals, etc.  

     Of course, if both sides of the transmission were using NVIS antennas, and they were located from 200 to 500 miles or so away from each other, then both signals would be very loud on receive.  The TO angles of the antennas would be optimized for the distances between the stations.  However, if only one side had the NVIS antenna, then IT'S signals would be louder at the opposite end than would the other station's be at the NVIS end.  

     An NVIS antenna is NOT intended as an all-purpose transmitting wire; it is NOT for low TO angle DX paths.  What it IS intended for is to fill in the skip zone, for frequencies up to about 12 MHz only, that would otherwise exist around a transmitter with a non-NVIS antenna.  

     As an example, last year at the Gilbert, PA MRCA meet, I brought the NVIS crossed-dipole array (two inverted V wires on the same coax) for use during one of our HF field exercises.  We set up in a mountain valley about 20 miles from base camp.  There were at least two 2,000-foot tall ridges between us and the base.  When we called the base camp, we were heard 59+ while we were running 15 W AM from a milrad (not sure of the model.)  We were also heard loud and clear by a station in upstate New York, a distance of a couple hundred miles from our location in the Pocono Mountains of Pennsylvania.  The 80M wires for our NVIS antenna were at the top of my sectionalized poles - 19 feet AGL.  We continued to operate while we lowered the wires to within three feet of the ground.  We were still heard L/C at base camp and our sigs diminished somewhat in New York.  

     BOTTOM LINE:  An NVIS antenna is not a general purpose Ham antenna, but one that is intended to communicate via ionospheric paths on HF up to 12 MHz or thereabouts between points that would otherwise be missed by either ground wave or by first-hop e-layer paths.  


73 de
Gene Smar  AD3F
P.S.  The US Army found in its tests that NVIS signals are hard to DF as they seem, at the DF'ing end, to originate from overhead.


From: Military1944 at aol.com
Date: 2008/06/14 Sat AM 06:14:07 EDT
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] PRC-74 & NVIS

In a message dated 14/06/2008 01:05:04 GMT Daylight Time, WA5CAB at cs.com writes:    Theses antennas are essentially low-to-the-ground horizontal dipoles that   exhibit high take-off angles for low-band HF signals.  The high T.O.   angles of NVIS antennas permits signals of up about 12 MHz to be returned to   earth within 300-500 miles of the transmitter (between ground wave and first   e-layer skip range) with very little attenuation.
----------------------------- What happens if you are using a NVIS aerial but the other guy is running a long wire, high dipole, vertical etc etc.  Your signal might be going straight up and down hence he gets good copy of you but if his signal is low angle your not going to hear him too well.  Don't both ends of the path need to be using NVIS aerials?  Ben G4BXD   

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