“The AS-1729/VRC is an omnidirectional, vertically polarized whip antenna assembly that provides transmission and reception of radio signals (very high frequency (VHF)) between 30 and 76 MHz.” TM 11-5985-262-15
BACKGROUND: This antenna was designed for installation on various vehicles as part of Radio Sets AN/VRC-12 and AN/VRC-43 through 49 (RT-524/RT-246 radios).
These as replacements for the 1950’s AN/GRC-3/8 systems with their AB-15/GR base and MS/AB series antenna masts.
The AS-1729 antenna is seen below on the M-151 “Mutt” drivers side, held bent over by the tie-down (range killer) rope kit. The AB-15/GR antenna/masts for the R-442 auxiliary receiver is also seen here tied down on the passenger side. The setup is either a VRC-12 or VRC-47.
The AS-1729 was also used with the vehicle-mounted AN/PRC-25 and PRC-77 systems including the AN/VRC-53, AN/VRC-64, AN/GRC-125 and AN/GRC-160 respectively.
This antenna system with associated radios was deployed beginning in the early-mid 1960’s and was widely used in Vietnam on jeeps, trucks, armored vehicles, shipboard, combat craft, PT Boats and fixed installations.
Below is a photo of an AS-1729 antenna mounted on Fast Patrol Torpedo Boat PTF-17 along with 2 HF antennas, the AS-390 UHF “spider” antenna and radar antenna. A second AS-1729 antenna was later mounted on top of the mast to increase range and to improve the radiation pattern.
A pair are seen below aboard a STAB: Strike Assault Boat in Vietnam.
These systems continued on into the 1990/1991 Gulf War as the SINGCARS frequency-hopping systems continued to replace it in Line combat units. They soldiered on well into the 1990’s and beyond in Reserve and NG units until replaced.
DESIGN: The antenna consists of two mast sections, the upper AS-1095 and the lower AS-1730 connected to the MX-6707 Matching Unit/Base assembly. The 2 fiberglass mast sections are center-fed in a 10′ high, vertical dipole configuration.
Note that this antenna is NOT a base fed, “quarter-wave” vertical that would ordinarily require a ground plane to control the base impedance and to reduce ground losses. It’s a different animal. As a center-fed vertical it ideally does not need a ground plane.
The center-fed design works by keeping the highest RF currents (that cause the actual radiating) further up the whip near the center and further away from vehicle/installation obstructions near the base. This, versus the max current/radiation that actually occurs in the spring (!) area of a base fed vehicle antenna closer to the lossy ground and buried next to a pattern-disturbing fenders and other metal parts. It’s a very good design.
The internal coaxial transmission line driving the center feed point is terminated by a choke balun residing inside the lower AS-1730 mast section, denoted as L1 in the schematic.
The MX-6707 provides switchable matching components over the entire frequency range as well as the base mount and spring assembly. The base unit matching circuits can be set to the radio’s operating frequency by remote control via a cable from the radio or manually selected by a rotary switch located at the base.
Ten bands are provided from 30-76 MHz. The below image is from a “parts set”; band switch segments as shown. Note that they are non-sequential. The manual band switch is to be rotated clockwise. Note the missing screw from the water drain hole below the BNC connector.
The performance specification states that the VSWR will be below* 3:1 within any selected frequency band; the nominal feed point impedance at the BNC connector is “50 Ω”. The power handling capacity is 70 watts.
(*In practice the VSWR can exceed 3:1 depending upon the actual installation in the field, Ref. 112. It will then be significantly worse when tied down on a vehicle as seen in the top photo.)
The Gain is not specified as its measurement is subject to many caveats in a multi-frequency system and deployed in random installations as these are. However subjective, it would approach that of any center fed vertical antenna of the same length minus small matching circuit losses. (Gain in dBd = decibels as referenced to a dipole)
A 10′ long center-fed half wave dipole has a natural resonant frequency of around 46.8 MHz. Operating it above or below that frequency requires some reactive elements to transform the resulting impedance closer to 50+j0 Ω. How was that implemented?
The MX-6707 base unit contains a 12 position band switch for selecting the appropriate reactance elements to achieve the best match / lowest SWR within each selected band segment. These include inductors and a piston type capacitor, all adjustable per the schematic. There are 2 null positions. See below:
The Depot Level tuning and adjustment procedure requires an Admittance Bridge to measure the network conductance/subceptance under test. Also required is an elaborate holding fixture which includes* a 10×10 foot square metal ground plane to control the systems’ local environment while being adjusted. Lacking that, I just used some trial-and-error adjustments on the bench and then tested in-place with a simple SWR meter. It works. Improvise, adapt, overcome.
* I assume the Depot procedure made an attempt to control the test antenna’s local environment to minimize that variable during factory alignment or repair.
IN HAM RADIO SERVICE: I have used these antennas mounted on the old ’71 Bronco using the SC-D-189023 “sugar scoop” vehicle mount. It is connected to the VRC-7 (RT-70) VHF FM radio system on the 6 meters Ham band. These are very effective and rugged mobile antennas for low-band VHF applications, as designed.
I am currently using this antenna in a fixed location mounted on a building as seen below. This installation uses the Lightweight Mount, NSN 5985-01-215-9404. When adjusted and manually band-switched accordingly, the antenna provides good performance on both the 6 meter and 10 meter US Ham Radio bands.
To use the antenna for 6 meters Ham Radio operation (in my case centered on 51.0 MHz) just set the Band switch knob to the 47.5 to 53 MHz band and it will work fine. To further reduce the in-band SWR at 51.0 MHz I adjusted L5 in the base unit for minimal SWR at that frequency.
For 10 meters operation (out of band) centered around 28.5 MHz I set the band switch to the 30-33 MHz position and adjusted C8 for minimal SWR.
As installed above, the SWR on 6 meters is 1.2:1 and on 10 meters it is 1.6:1, fed by a short length of RG-213/U coax to the radios. Both quite acceptable with good “DX” performance.
(The antenna will probably work adequately further out of band on the 27 MHz CB band if necessary although I have not tried to retune this one down that far. When set to the 30-33 MHz position and further adjusted to 28.5 MHz as above, the SWR inside the 11 meter band is below 2.4:1. That’s certainly usable if needed in a pinch, especially if the coax is pretty short as in a vehicle. This question comes up occasionally.)
The antenna is nominally designed for mounting on a vehicle which includes a “grounded” surface in the near-field by default. (Note that in Navy service aboard fiberglass PBR and STAB boats as well as plywood deck structures on PTF’s there is no significant metal near the base.)
To simulate “a vehicle” in my base station installation I ran a short ground braid strap to the adjacent 8 foot aluminum gutter thinking this would “help”. This is obviously NOT a “ground plane”.
It turned out this “ground” configuration made the SWR significantly worse in this installation on a wood-frame building with installed gutter. Although aluminum rain gutters make surprisingly good antennas, this indicates that the antenna is sensitive to metal objects in proximity in the near-field.
I removed the ground strap, all is well. YMMV. (Lightning protection or striking HV power wires in vehicle operation is another matter; lightning around here is very rare but something to consider.)
AVAILABILITY: As the SINGCARS radios with their AS-3900 broad band (resistively-loaded) antennas continue to proliferate, many tens of thousands of these narrow band antennas have become excess to the US DOD. They show up on the usual online places and at ham radio and military vehicle swap meets. However “asking prices” seem to be crazy expensive as military vehicle guys seek them for “that look”, often just as a proper accessory for that era vehicle or maybe for a flag pole on the fender. Why not…..
BE AWARE: Make sure you perform your preventative maintenance checks. Pay attention to your Specialist – She knows What’s Up! US Army Preventative Maintenance Monthly “PS” Magazine, June 1999:
The MX-6707 had some water intrusion issues over the years as noted in the cartoon above. The leaks were primarily caused by cracks in the Lexan housing caused by over-torquing the vehicle mounting ring bolts or the spring bolts. Different gasket seals were also fielded to reduce capillary intrusion by water. Life aboard ship in a corrosive sea water environment was especially tough. The drain hole in the bottom was no accident; if you ARE letting water out, you already have a problem.
PERFORMANCE: With this base antenna I can work 6 meter repeaters up in the Sierra Nevada mountains up to 75 miles away (nearly LOS) with a 5 watt FM signal. It also worked great on the truck at remote campsites working my buddies with the PRC-25 and PRC-6 sets on 51.00 Simplex.
It also works great with my VRC-10/RT-68 VHF FM transceiver on 6 meters simplex at home.
As we experience good propagation conditions in 2023 on 10 meters, I can easily work cross country or into South America, Asia and Europe on CW or SSB with the PRC-174 man pack set running 20 watts. The antenna works great on both Ham bands.
If you find one in decent shape they make excellent, rugged antennas for mobile or fixed base ham radio ops.
For further reading and a Deep Dive into the performance of these antennas in ship board and in more general applications (including “out of band” performance at an antenna test range) see Reference 112: ANTENNA AND RF DISTRIBUTION SYSTEMS APPROACHES TO MEET AN/VCC-2 COMMUNICATIONS REQUIREMENTS ON AMPHIBIOUS SHIPS. Technical Document 196