AS-1729/VRC Antenna

1.0 General Information

1.1 Description

This is a VHF Low Band, 30 - 76 MHz Antenna composed of the following elements.

• MX-6707 Base Matching Unit
• AS-1720/VRC Lower 52" Antenna Element that is like a coax with inner and outer conductors
• AT-1095/VRC Upper 66" Antenna Element is a single conductor with a MS-11x style thread
The band switch can be controlled manually or by means of a 12 conductor cable.  There is a rotary solenoid that rotates the switch until it gets to the desired band position then stops.
The overall length from the bottom of the mounting flange to the tip of the safety cap is 123 inches.

Used with vehicle installations which have the radio sets AN/VRC-12 Series, AN/VRC-53, AN/VRC-64, AN/GRC-125 or AN/GRC-160. The matching unit MX-6707/VRC tunes the antenna elements automatically or by manual selection via a selector knob on the base.


The MX-6707 base tuning unit uses capactance and inductance to match the antenna, not lossy resistance as is used on the newer matching bases that are now needed for frequency hopping radios like the SINCGARS.  So the AS-1729 is more efficient than the newer antennas.


1.2 Photos

Bottom of MX-6707 - Showing:
• Clockwise only band switch 30-33, 42-47.5, 47.5-53, 37-42, 33-37, 53-56, 65-70.5, 70.5-76, 60-65, 56-60  Notice that the bands are not in order.
• 12-pin connector PCI  U-291/U The radio connector would be the 12-socket version.
• BNC(f) connector with a protective tube around it.
• Drain Screw - put in a straw, coffee stirrer, or somethng to break the vacuum once every few months to get the water out!
AT-1095 Upper antenna element
Tie Down Kit -
CX4722A/VRC (20 Ft. 0 in)  cable NSN 5995-00-985-8090  CAGE 55928 (3/93) has a 12-socket connector U-290/U  0CS66 that mates with the MX-6707 and a right angle 12-pin connector that looks like the one on the base of the MX-6707.]
Looking into the plug the pins are:
        B
      A   C
     D E F H
      J K L
       M N

The photo to the left showes the inside of the MX-6707 after the bottom cover has been removed.  In order to take off the bottom cover all the connector nuts, the knob,and the nut under the knob need to be rermoved BEFORE removing the scrows that hold the bottom cover to the flange.



SCD-189023  Scoop mounting base.  Common 2" muffler "U" bolts fit the existing holes so it can easily be mounted to a fence post or pipe.
SCD-446058 Corner bracket, Typically installed on the rear corner of vehicle.
Extender for SCD-446058 used to raise the antenna base up.  Important for higher power (more than 5 Watts?) so that someone will not ge as likely to get an RF burn. 

3.0 Operation Instructions

3.1 DC Resistance of Antenna

The DC resistance looking into the BNC connector is an open circuit for all bands except for the 70.5-76 MHz band where it is a short.  This means that this antenna can not be used with a direct connection to a  PRC-126, etc. radio that requires a DC path to ground in order to sense that a 50 Ohm antenna is connected.

It should work with a any of the radios in the PRC-68 family when my 68AA Antenna Adapter is used becasue the adapter supplies the DC return needed for the PRC-126 or PRC-128 (low band in this case).

It does work with the OF-185 in either the AMPL or BYPASS modes.

3.2 Mounting

For military vehicle mounting there are standard brackets that bolt on the vehicle.  There are also extension brackets that raise the antenna base both to keep bystanders from getting RF burns and also to increase the range.

There is also a mast adapter that allows the AS-1729 to be mounted on a GRA-4 or RC-292 mast.  This adapter has 4 holes that appear to be for ground radials, but they are not threaded for the AB-2 or MX-116 type elements.  What goes there? ans in AB-903 TM, 4 counter poise tubes (snap into the unthreaded holes) and then a wire is connected to the tips of the tubes froming a square shape.

The Antenna Adapter (NSN 5985-089-5020) is used with the AB-903 (NSN 5985-933-2197) 30 foot crank up mast.  Both made by FSCM 81868.  See TM 11-5985-263-15 for the AB-903 which includes the Antenna Adapter.  It's also called a "counter poise adapter unit" and it's stated that it's for only the MX-6707 matching unit base.

The adapter has two permanent pins and two "T" head bolts to allow attaching the AS-1729, or probably any other antenna base that uses the standard military 4 bolt pattern.

Some ground radials or a counterpoise system probably should be used when the AS-1729 is mast mounted.

Pole adapter for MX-6707 - this bracket weighs just over 1 pound, it must be made of Al.  The CX-4722 needs to be fed through the bottom of the bracket and mated to the MX-6707 before the MX-6707 is attached to the bracket.  The MX-6707 can only be installed in 1 of the 4 possible positions so that the cable will be in the notch on the bracket.

Needs Ground

When VSWR testing the MX-6707 the specification MIL-A-55288C calls for mounting the base in the center of a 10 foot square sheet of copper.  So for best performance the base needs a good ground.  I tried to test the AS-1729 with the base just sitting on the ground and got poor VSWR.

For now I am going to use a narrow bandwidth Hamstick 9106 on a Diamond K400-3/8C 2-axis adjustable mount for drive testing the OF-185.

3.3 Remote Antenna Band Selection

Using a BA-2 or 22.5 Volts DC with negative connected to pin C (Red)  

Band	Positive to	MHz	CX-4722 color code A= Black
A	A & D	30-33	Green
A	A & E	33-37	Orange
A	A & F	37-42	Blue
A	A & H	42-47.5	White/Black
A	A & J	47.5-53	Red/Black
B	D	53-56	Green
B	E	56-60	Orange
B	F	60-65	Blue
B	H	65-70.5	White/Black
B	J	70.5-76	Red/Black

Other Pins are:
 

Pin	Wire Color
B	White
L	Orange/Black
K	Green/Black
N	Balck/White
M	Blue/Black

Pin C is the negative grund pin
Pin A is bank select, set to +22.5 for 30 - 53 MHz, open for 53 - 76 MHz
D,E,F,H, & J are sub band selects
N is always +22.5 (or +25) when the MX-6707 is operational (note on some vehicle mounts this there is a jumper so that you dont drain the vehicle battery).
B, L, K, & M are not used

Reference Information

Manuals

054769.pdf  TM 11-5985-262-24P    ANTENNA, AS-1729/VRC  NSN 5985-00-985-9024
restricted      TM 11-5985-262-14 ANTENNA, AS-1729/VRC  NSN 5985-00-985-9024 - has the schematic diagram and testing methods.
TM 11-5985-262-15 Maintenance
Block Diagram of MX-6707 - Notice that the BNC coax center conductor goes to the center antenna terminal and  also goes through a network that is switched and then to the outer sleeve of the antenna.

Schematic Diagram - from TM 11-5985-262-14

Testing 7 June 2004

I attempted to plot the VSWR using the 4395A, but the VSWR looks poor in the 30 to 50 MHz range and is better from 50 to 76 MHz.  Not sure why.  Maybe need to have the antenna in a vertical position or may need a ground plane.  Tried a ground connection to the ground strap, but that didn't make too much difference.  When the AM-2060 switch changes the MX-6707 know spins to match the band select position.  Usually either a full turn minus one notch or one step.

The theory of operation for this antenna is not clear.  The lower section seems to be a coax extension for the upper section.

Opening the Bottom Case

1. Remove both the coax and band select cables
2. Remove MX-6707 from any mounting bracket
3. Remove the band select knob using a "-" screwdriver, not that there is a groove that aligns with the pointer side of the knob so you can put it back on correctly.  The pointer and the knob position should match the position set remotely.
4. Remove the nut that's under the knob.  This nut is threaded on a plastic shaft that's easy to break so be gentle.
   
5. Remove the nut securing the band select connector
6. Remove the nut securing the BNC(f) coax connector
7. Remove the screws holding the base to the rest of the MX-6707 using a "+" screwdriver.
8. The metal cover is located with a couple of pins, but it's a good idea to mark the cover and MX-6707 so you know how to put the case back on. 
   
9. Lightly tap the band select connector and BNC connector and pull the cover off.

Related

Web Links

Steve Haney - antennas and other mil radio stuff, recommended by Brooke
PS Magazine #559 pg 48 - Water's a Pain, Unless You Drain
Fair Radio Sales -
Tacticom - Antennas - Cables -
Component Products - Antenna -
Shakespear - Mil Antennas -4242-MK2 broadband, centre-fed, dipole antenna.  The antenna base is physically identical to that of the MX-6707 with the exception that it utilizes broadband circuitry that requires neither mechanical nor electrical tuning.  The frequency coverage is from 30 MHz to 88 MHz. The matching is accomplished with loss, but the performance is about the same as the As-1729?
Trival Antene - Kamnik - Sovenija - 0.4 to 3,000 MHz
Telex® Wireless - Model 4331, 4332 and 4334 VHF Antennas -
TM 11-2300-352-15-5: Installation of Radio Sets AN/VRC-46, AN/VRC-53, or AN/GRC-125 in Truck, Cargo, 3/4-Ton, M37 - Drilling Details for Antenna AS-1729/VRC

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