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Mobile NVIS

 

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Email:Trish
[Last updated 08FEB2000]

My work with NVIS antennas first began when, as a Systems Maintenance Technician with Santa Clara County in the early
1980's, I met a dispatcher there by the name of
Ralph Libby.

 

At that time, Ralph was active in the U.S. Army Reserves and, at the rank of CW4, he was one
of the leaders in the "Communications for Armor" program out of the MOS Training Center at
Camp Parks. Through that connection, I was introduced to "Army Communicator" magazine.

CW4 Libby gave me a couple of old copies of Army Communicator, and I promptly wrote a
letter to the magazine's staff at Fort Gordon, asking to be placed on their mailing list. As a
member of the Public Safety Communications Community, it seemed that there would be
some spin-off benefits to Public Safety Comms via articles in their monthly publication.

Approximately 3 issues later, articles started appearing on the subject of NVIS that were
authored by Lt. Col David Fiedler. At this point I was hooked. I've always maintained that HF
radio is simple and reliable when used effectively. Any method to improve the reliability of
HF performance was of genuine interest here. Any form of wide-area communications other
than HF radio requires a sophisticated infrastructure such as multiple transmitter and
receiver sites on the ground, or satellite phones pointed to satellites that fly overhead out in
space. (Remember the famous Galaxy IV incident when your pager went dead?).

After moving on to be a Communications Technician with the City of San Jose in 1987, and
promoting to Senior Commtech some 18 months later, Lt. Col Fiedler's work came to mind
when I was asked to come up with an equipment recommendation for HF, VHF, and UHF
radio equipment that would be part of the new Emergency Operations Center. This EOC
was to be managed by the City's Office of Emergency Services.

Lt. Col Fiedler's experience paid off for us big-time, as NVIS techniques were used in the
aftermath of the Loma Prietta Earthquake in 1989. NVIS would now be part of the new
dispatch center's design. HF antenna wire and hardware used for an end-fed HF system on
the dispatch center were obtained from Mackay Communications in Berkeley, California.
Mackay's shop was responsible for radio and antenna maintenance on any of the larger
ships that arrive in ports of the San Francisco Bay Area. If anyone would stock quality strain
insulators and related hardware, it would be a shop that works on the larger container ships
operated by Matson® or tankers owned by Texaco®, or Chevron Oil®.

It is important to note that as early as the first part of World War II, NVIS antennas were
used by German Forces. This was due to the fact that early in the war, HF radio was of
major importance to widely-dispersed forces under a centralized command and
control structure. Even with the advent of Tactical FM radio later in the war, HF radio
was of significant importance with forward recon units.


Many people over the years have contacted me regarding info on Near Vertical Incidence
Skywave (NVIS), and the field work that I have done over the years since then. This webpage
construction started as the need grew for members of the NVIS discussion site to have
content in a common area. A call has gone out via the mailing list for URL's from subscribers
and references to the work of others in this arcane area of communications.

My original work is located at the website of the City of San Jose's Office of Emergency
Services. Rather than have you go there, this primary body of work is now available at:
My Field Work
. The articles go into the detail of my own field work which was done between
the fall of 1990 and the spring of 1999,

THOUGHTS RELATED TO MOBILE ANTENNAS:

I've recently been experimenting with a mobile antenna that is optimized for high-angle
skywave. This type of system is loosely patterned after the "notch", or "trans-line" HF
antennas that are used on Coast Guard, and other military  helicopters, and is similar to in
design to the one tested by the New Jersey Army National Guard on a long-wheelbase van.  

Here is my 1966 military Land-Rover 
with a home-built Notch antenna of 
my own design: photo2      
                                                                                                     

 

 

 

 

 

 

This system is used with a Motorola MICOM-2R HF radio and the companion autotuner that
is designed to operate with the radio. The hot lead from the antenna tuner runs through a
feed-thru which is machined from a 1-inch diameter piece of Delrin, inserted from the
outside of the vehicle, and held in place with a hex-nut, also made of Delrin, on the inside of
the vehicle. This feed-thru allows sufficient space between the antenna lead-in and the metal
body of the cab so that the high RF-voltage on the hot-lead will not arc to the body of the
vehicle.

photo4
Next the lead-in attaches to a standard AB15 antenna base assembly that is mounted via the antenna bracket found commonly on the side of the M151-series Jeeps. The bracket is called an "AB450/G".

The connection from the AB15 antenna base, to the actual antenna rail assembly is via a small piece of aluminum flat-stock. The dimensions are about 1/4 inch thick by about a half-inch in width.

The purpose of the interconnecting flat-stock is to allow as little movement of the entire antenna system assembly, with respect to the body of the vehicle. In systems such as these, any change in physical spacing will drastically
effect tuning of the antenna system. The connecting flat-stock prevents detuning as much as
possible.

The connecting link is drilled at each end for screws having wing-type heads on them. These
screws allow for fast removal of the connecting link. With the link removed, a standard
vertical whip antenna can then be installed in the AB15 base. The whip antenna then allows
a low-angle, conventional radiation pattern when operating, for example, on 10 or 11 meters.

A standard description of the Notch Antenna is that the far end of the antenna rail is
grounded to the surface of the vehicle, or in the case of a helicopter, to the tail-boom. This
connection couples the antenna to the body and allows the body of the vehicle to act as one
half of a loop antenna. The rail-radiator serves as the first half of the loop.

Anyone who has worked with short-length loop antennas has found that a capacitor is
commonly placed in series with the loop, usually at the side opposite that of the feed point.
The results of my particular adventure in Notch Antennas resulted in some "cranky" tuning
on the part of the Motorola F2265A Autocoupler when used on 75/80 meters.

Satisfactory operation was found by opening the connection between the far-end of the
antenna rail and separating the physical connection to the roof of the vehicle via the
placement of a machined piece of G10 fiberglass material and a suitable mechanical
clamp. The clamp provided the holding power to keep the rail in place, and still have the
rail insulated from the roof of the vehicle. 

This can be seen in this detail: 

 

 

 

 

 

 

 

 

73 From Patricia Gibbons

                                                                                                                        CDRACES

Link to the Nationwide RACES Organization Here

 

 

 

 

 

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