AS-2259/GR NVIS Antenna

and NVIS in General

5980-00-106-6130
© Brooke Clarke, N6GCE 2003 - 2009

Carry Bag	Installed	Top

NVIS Background

The AS-2259, NSN 5980-00-106-6130, is rated for 2 to 30 Mhz and up to 1,000 Watts.  The antenna is said to be compatible with radios that typically use a 15 foot whip antenna.  Also known as the Collins 637K-1 antenna or Telex Wireless Model 1990.

Military H.F. radios typically use a vertical whip antenna, like the AS-1320 on the PRC-47.  The whip works against ground by means of a number of radials.  The main lobe is at right angles to the centerline of the antenna or parallel to the ground.  This works well for distances up to about 70 miles (ground wave) [FM 24-19 says 10 miles for a man portable radio and 15 foot whip] or for very large distances corresponding to one or more hops off of the ionosphere (greater than 300 miles).  If the H.F. radio is located in a valley or jungle these whip antennas do not work very well and so the Near Vertical Incidence Sky wave antenna was developed.

An NVIS antenna has it's main lobe pointing straight up (or nearly so).  The idea is to bounce the signal off the ionosphere and have it reflect straight down.  This provides a strong signal out to 200 to 300 miles in all directions.  This is the largest coverage area you can get with a single receiver transmitter without a repeater.  Note that squad radios operating in the 30 to 88 MHz VHF low band are designed NOT to have long range to prevent the enemy from eavesdropping.  Classical H.F. radio using antennas on high towers is designed to provide long haul communications.  NVIS is ideal at providing solid communications that fill the gap between these two modes, out to a radius of 200 ot 300 miles.

The "D" layer gets charged by the sun on a daily basis.  So near noon it has the most charge.  The "D" layer has a resonance around 1.6 (? memory) MHz and that's where it has the highest loss.  The farther away from 1.6 Mhz the less D layer adsorption there is.  So NVIS is not a mode that will work for the AM broadcast band.  This is also why most military field portable HF radios have a frequency range like 2 - 12 MHz.  The lower end is limited by the D layer and the upper end is limited by the NVIS mode since it's very difficult to put a dipole up a 1/4 wave in the field.

When radials are used with a whip antenna the coupling to the Earth is lossy.  Antennas that are balanced like a dipole have higher efficiency because they do not suffer from ground loss.  NVIS antennas are about an average of 10 feet off the ground and do not suffer from ground losses.  Some NVIS antenna systems, like the Eyring, operate a foot or so off the ground. 

The AS-2259 was designed for light weight single man carry and is NOT optimized for NVIS performance.  The electrical performance has been traded for weight.  The NVIS mode can only be used at the low end of the H.F. frequency range around 2 to 9 MHz depending on the ionosphere.

The GRA-50 is a NVIS type dipole made for use with the GRC-19 vehicle mounted HF radio.

In Vietnam the enemy H.F. operators were using NVIS antennas, probably for the reasons stated above, and the loop antennas on the PRD-1 direction finder receivers would not give good bearings on signals coming almost straight down.  (It seems that someone would have made an Adcock antenna attachment for the PRD-1 to counter this problem?)

The term NVIS was coined either late or just post Vietnam era, but the mode was understood and used much earlier.

My house is located in a forest canyon with 80 foot high trees all around.  By using NVIS I can communicate with stations out many hundreds of miles, but it's difficult to get a "low angle" signal out either on HF or on VHF-UHF.

The Harris PRC-150 (replacement for the PRC-104)  lists the following antennas:

• OE-505 10-foot (3 m) whip (1.6 to 60 MHz)
• RF-1936P (AS-2259) NVIS (3.5 to 10 MHZ) - two versions, one the stock AS-2259 the other on a taller mast
  
• RF-1940-BNC dipole (3 to 60 MHz)
  

AS-2259 Description

The Antenna System 2259 consists 7 mast sections, each about 2 feet long that are 50 Ohm coax hard lines. There's a male end and a female end.  At the top there are 4 antenna elements that are used as guy lines to support the antenna.  The short antenna element is 2 * 24'10"  long and the long antenna element is 2 * 36'10" long.  A short and a long element are connected to the outer (ground) side of the mast/coax and the other short and long elements are connected to the mast center (hot) conductor.  The AS-2259 does not include the base adapter.  There are a number of different base adapters to connect the AS-2259 to a number of different radios or to use it as a stand alone antenna.

A half wave dipole that's 49'8" long (15.1 meters) has a self reasonant frequency of 9.9 MHz and a half wave dipole that's 73'8" (22.5 meteres) long has a self reasonant frequency of 6.7 MHz.  These get transformed by the 14 foot long 50 Ohm impedance mast.

Sewn on the inside of the canvas carry bag are the Instructions for Installation and a Parts Breakdown.  I've made these into a pdf document AS2259Ins.pdf about 290 kB..

Harris makes wersions of this antenna called the RF-1936 (same as AS-2259), RF-2236 and the double sized RF-2238

Base Adapters

MX-9313	NSN 5985-00-172-6518 base adapter is used with heavy H.F. radio sets such as the AN/TRC-75, AN/MRC-83, AN/MRC-87, AN/TRC-15 or similar radios that use a 1-8 male thread on the antenna base or coupler.  The AT-1011 is an antenna base has thread. The Telex model 1996 is also a 1-8 thread adapter.   Flying ground wire. (maybe the same part with a different model #?)
MX-10618/GRC-193A	Base adapter allows the AS-2259 to sit on top of the GRC-193 mobile HF antenna base and has a ground wire to connect to the vehicle ground.  Of course the AS-2259 can only be depolyed when the vehicle is parked. Bottom hole is about 0.635" I.D. and has male fingers to fit into a hole in the mating antenna base.  Note the threads for the MS-116 start some distance down the hole.  This is the AB-652 base that uses MS-116 type mast elements or any base that accepts the MS-116 type antenna elements.
	The MX-10618 is shown here installed on the MP-68 Base that the heart of the RC-292 VHF Low Band antenna.  The MP-68 takes the MS-116 Base element both as the lower vertical element and as the first of the 3 ground plane elements. Note the AB-15 Antenna Base is part of the MP-68 and is the actual base that the MX-10618 is designed to be used on.
NSN 5985-00-106-6130 PRC-47 Base Adapter	is a base adapter that screws into the antenna connection on a PRC-47.  The 3/8-16 threaded stud on the bottom screws into the antenna terminal on top of the PRC-47.  Note when the support legs are unfolded on the PRC-47 the front panel is facing the sky.  This supports the use of a whip connected directly to the front panel high voltage antenna terminal. It includes an antenna tuning chart attached by chain, for the PRC-47.  I tlooks similar to the one on the PRC-47 used with the AS-1320 whip, but the curves are for the AS-2259.
Home made adapters based on PRC-47 base adapter. The PRC-47 is probably the most common base and so is often used to make other types of bases.
	If 4 holes are drilled & tapped up into the bottom of this base adapter it can be mounted on a metal box that has a clearance hole for the 3/8-16 stud.  Then a coax connector can be installed on the metal box with a wire between the coax center and the 3/8-16 stud using a wire terminal and nut. These photos are from Mike prcradio at yahoo.com
	There's a 6-32 screw holding the coax center conductor terminal to the center conductor of the PRC-47 base adapter.  Not sure if that tapped screw hole is already part of the adapteror if it needs to be added.
	Edmund supplied photos of this base.
Telex 1994	is like the Telex 1995 except there is no transformer and it's rated for 1 kW.  Thumb screws for ground and hot wires.  This is one of the bases with the flat metal plate with a number of holes.  The mast part will fold flat aginst the metal base to make carryng in a pack easier.
Telex 1995	Rated 50 Watts.  Has a built in 4:1 transformer allowing easier matching to a PRC-104.  The base is a square metal plate with a hinge. For ground mounting.  BNC connector.  NSN 5820-01-065-4495 Another flat metal base plate unit.
Edmund supplied photos of this base. Top Label: BASE, ANTENNA P.N. 1995AA HY-GAIN Div - TELEX COMMUNICATIONS, INC. MFG CODE 37919 Bottom Label: MAXIMUM POWER INPUT 50 WATTS RF
Telex 1997	Adapter has male 3/16-16 thread to screw into the PRC-47 and 1-8 male threads to screw into the 1996 adapter.  Using this method two adapters are used to connect the AS-2259 to a PRC-47.
Telex 1998	base adapter mates to the AB652 Antenna Base.  Flying ground wire.
AB-1241/PRC-104	Although this looks like a patio umbrella base, it's only about 8 inches in diameter (some refer to it as the hubcap base).  There's an R.F. transformer inside so an Ohm meter shows a short between all the possible terminals. I tried using this base with the AS-2259 and a 50 foot long RG-58 coax to the AM-6874/PRC-104 RF Amp/Ant coupler and it seems to work at 18 MHz, but it's questionable at 4 MHz when the AM-6874 is set for a 50 Ohm antenna (i.e. no tuner).  The TM for the PRC-104 says to use a 2 foot cable between the Radio and this base.  That's consistent with the idea that the balun is not providing a match to 50 Ohms, but rather is bringing the impedance of the antenna into a range where the PRC-104 antenna coupler can work with.   Note impedance has real and imaginary parts and in this case the real part is probably what's mainly being transformed.  (At 2 MHz a diploe is much much longer than the wires on the AS-2259 and so it's real impedance is very low, too low to match directly.)

At 18 MHz if the coax + Antenna Tuner mode is selected the PRC-104 tries for about 12 seconds to tune up then sounds a warning tone.
All this testing is with the SA-2365 Antenna switch disconnected.

4 Aug 2004 - used a couple of prototype 55590BA Battery adapters to power the PRC-104 and about a 1 foot long BNC cable to connect to make the antenna connection.  In the BNC-Tuner mode the radio tuned up in just a few seconds on 3996 and 5371.5 kHz, so my base is OK.

Since the AS-2259 can be used over a wide range of frequencies there's no way any passive circuit in the AB-1241 can provide a 50 Ohm match, except by using a termination, and that would not be a good thing to do with a low power battery operated transmitter.  It's just a 4:1 auto transformer that brings the low real part of the antenna impedance up to some value that can be more efficiently matched and maybe adds some inductance to an antenna that looks like a capacitor.

Some experiments done by David Brock indicate that by clipping wires onto the longer AS-2259 elements to bring them into resonance (make a low dipole), the performance improves to nearly what you would get with a higher dipole.

References

The Most Secret War, Army Signals Intellegence in Vietnam, James L. Gilbert, ISBN 0-16-051284-0 GPO 

TM 11-5985-379-14&P - is available at ETM online.

Contains data and theoretical design considerations for an NVIS antenna.  If the ends of the antenna are supported at a higher elevation the antenna will be more efficient, i.e. ground loss will be minimized.  The AS-2259 was designed to work with the PRC-47 in Vietnam.

It's mentioned in the manual that when using a vehicle or shelter mount only enough mast sections should be used to get the highest point of the antenna up 16 feet off the ground.  Placing the antenna higher will start changing the propagation from NVIS into conventional horizontal polarizatiton and degrade the signal for distances out to 300 miles.

Related NVIS Antennas.

GRA-50 1.5 to 20 Mhz Reel dipole where the reels are at the far ends, not the center.  If there are trees or some way to get this antenna up at least 1/10 of a wavelength it will work well.

AT-984/G 150 foot wire

Hy-Gain HA-4000, 18TD, Collins TD-1, 637T-2 - a dipole made with two steel tape measures as the center part.  You pull out the tapes, marked in the metric system and you're good to go

Links

Antenna Products - makes a number of NVIS type antennas including the ST-230 sprial
Antenna Systems & Solutions - No images of antennas, seem to be a distributor
Broadband Propagation - South Australia offers HF antennas that look similar to the Bushcom units.
Bushcomm - BBA100 - eHam reviews - 3 wire T2FD type - in U.S.  Array Solutions makes the wire antennas here BBA-100 Antenna fitted with Ladder Line Feed Kit (LLF)
KV5R: Getting Started in NVIS Antenna and Propagation Experimentation -

NVIS - What It Is and How To Use It

  - By Patricia Gibbons - WA6UBE
Tactical-link - NVIS Antennas - W.W.II German recon vehicles By Patricia Gibbons - WA6UBE
Near Vertical Incident Scattering Antenna by Dr. Carl O. Jelinek - palns for a Ham Radio friendly version of the AS-2259
Field Manual 24-18 Appendix M - 2-to 4-MHz range at night and to the 4- to 8-MHz range during the day . . Operators are able to operate from protected, dug-in positions. Thus tactical commanders do not have to control the high ground for HF communications purposes. . . NVIS energy is received from above at very steep angles, which makes direction finding (DF) from nearby (but beyond ground-wave range) locations more difficult.
637K-2B Antenna Tuner made for the AS-2259
The Buddipole N-Vee - Combines the Buddipole with an adjustable dipole
High-frequency radio returns to transformation Army in brigade combat teams - photos of German W.W.II NVIS vehicle ant.
NVIS Propagation Maps - The top link " Inospheric Map - NVIS PROPAGATION North America" has the foF2 data for NVIS. 1 hr updates.  But if the Solar X-Ray flux gets too high the "D" layer will absorb the singals.
Near-Real-Time F2-Layer Critical Frequency Map - 5 minute updates, but whole world, harder to read than the N. Amer. map above
FM 24-19 Chapter 3 - Antennas
FM 24-18 - Appendix M Near-Vertical Incidence Sky-wave Propagation Concept
Automated HF Communications good for Nap Of the Earth flying -
Mobile NVIS: the New Jersey Army National Guard approach
Near-vertical-incidence skywave (NVIS) propagation: the Soviet approach
Skip the "skip zone": we created it and we can eliminate it
See "The Dave Fiedler collection" for NVIS articles.
Some NVIS: From the Backyard to Professional Installations, Some Notes on NVIS Cloud Burners,
Modeling the T2FD, Notes on the Terminated Wide-Band "Folded Dipole", Horizontal Heights and Sound Bites A Short Note Relevant to NVIS Antennas, by L. B. Cebik, W4RNL
NVIS Com - US Q-Mac outlet

Build Your Own AS-2259 Type NVIS Antenna - step by step with photos
Near-Vertical Incidence Sky-Wavve Propagation Concept -

NVIS Antennas and EZNEC - Modified AS-2259 - by David McGinnis K7UXO - 80 meter dipole at different heights
The modified AS-2259 has 4 legs with the following lengths:
57',  56' 4",  28.5'+ <40 meter trap>+18",   28.5'+ <40 meter trap>+18" (works on 40, 60 & 80 meters)
Typical Construction details for the T2FD antenna  -
David McGinnis Antenna Modeling various NVIS types including a sprial
a NVIS-ALE Log-Sprial ANtenna for 2.5 - 12+ MHz by L. B. Cebik, W4RNL
Antenna Farm - Two All Band NVIS Antennae by N2CKH/AAR2EY - ladder line fed dipole and long wire
W8JI - NVIS page with some Vietnam era test data -

breck k4che - AS-2259 Antenna Notes - AS-2259 Base Connector - AS-2259 Modeling and Suggested matching - keeping the wire lengths at the stock values
HF Systems - Hourly HAP progagation charts -
HF Link Yahoo Group - Hourly Worldwide NVIS propagation map -
Stealth Telecom - ST-940B - Mag Loop Roof Rack appearance antenna

Click to subscribe to NVIS

This is the 14011 hits since time this page has been accessed since since 25 Nov 2003.