Once known as Saigon, and now Ho Chi Minh City, the city at the heart of the southern part of Vietnam is now a bustling metropolis. Among multiple medium wave AM stations there, the Voice of Ho Chi Minh City (VOHCM) has been a local voice for almost 50 years. Now that voice is about to get louder!

Modern HoChiMinh City, home of VOHCM
Modern HoChiMinh City, home of VOHCM

Voice of Ho Chi Minh City, officially Voice of Ho Chi Minh City’s People is the official broadcasting station of Ho Chi Minh City. The station is located on Nguyen Dinh Chieu Street, District 1, Ho Chi Minh City. The station broadcasts separate services on AM 610 kHz and FM 99.9 MHz, both of which can be heard throughout southern Vietnam. The VOHCM website is at http://www.voh.com.vn/news/default.aspx. Hear a VOHCM-AM audio clip at http://www.asiawaves.net/audio/vietnam/.

VOHCM 610 kHz in HoChiMinh, Vietnam
VOHCM 610 kHz in Ho Chi Minh City, Vietnam

Historically operating on 610 kHz with 100,000 watts; the station is now increasing power to 200,000 watts. At the same time, it is adding a new Nautel transmitter and has engaged LBA Technology to supply a complete new DRM – ready antenna system: tower, ground system, and ATU.

Projected daytime coverage map for VOHCM, 610 kHz, 200 KW
Projected daytime coverage map for VOHCM, 610 kHz, 200 KW

LBA engineering evaluation of the old antenna site determined that a power increase was not feasible with the existing tower and ATU. Not only was the existing equipment showing its age, but the higher voltages at 200 KW required a complete upgrade of tower insulators, lighting transformers, and other features. The previous facility was from an analog era, and didn’t consider bandwidth and phasing requirements for DRM transmission. Furthermore, RF exposure standards dictated a safety exclusion area be incorporated in the design. From the site photo, it can be seen that public exposure to even the 100,000 watt RF field was problematical.

VOH 610 kHz 100 KW antenna site before upgrade
VOH 610 kHz 100 KW antenna site before upgrade

As a result, the engineers at LBA Technology designed a completely new antenna system to replace the existing one.

The Tower and Ground System

The heart of the installation is a base insulated 400 foot, quarter wave, tower mast custom designed and fabricated for LBA by Magnum Towers to be compliant with Vietnamese standards and environment. LBA provided a high voltage stay guy wire insulation system using the LBA PH – series of corona ring protected, shedded insulators. ICAO standard obstruction lighting was isolated through an Austin dual winding toroidal transformer at the tower base.

Typical dual winding Austin lighting transformer
Typical dual winding Austin lighting transformer
LBA Model PH - series high voltage guy wire insulator
LBA Model PH - series high voltage guy wire insulator

To ensure excellent coverage, a new buried ground system was provided. This consisted of 120 radials of #10 AWG copper wire with a ground screen and copper strap bonding system. This system was specially designed to accommodate the high RF currents encountered at 200,000 watts and to provide a good lightning ground for the tower and ATU facility.

The antenna tuning system (ATU-200)

LBA Technology has designed a phase adjustable “L network” ATU circuit rated at 200 KW with 125% modulation for the Voice of Ho Chi Minh City medium wave antenna tuning system.  The ATU-200 design has been value engineered to fit into a new ATU building utilizing prefabricated circuit modules for the 610 kHz system.

This system is designed to operate with an input impedance of 50Ω ±j0Ω unbalanced from the transmitter and is manufactured to match the design tower impedance of 38Ω +j24Ω from the 400’ mast. This design is DRM capable with a VSWR of less than 1.05:1 at operating frequency and less than 1.4:1 ± 7.5 kHz.

The components of the ATU-200 and associated service equipment were fabricated to be assembled as modular units complete with all interconnecting piping, wiring and cables. Other components included a 6 1/8” coaxial input, high voltage RF output window, high voltage 120A toroidal current meters for monitoring the antenna and input currents, remote readout units for the transmitter building, carbon sphere lightning gaps and a static drain. The modules were pre-assembled and initially tuned at LBA Technology and shipped ready to install.  The photo shows all components and modules set up in the LBA plant in Greenville, NC and ready for engineering tests.

Voice of Ho Chi Minh City medium wave antenna tuning system
Voice of Ho Chi Minh City medium wave antenna tuning system

System construction is now underway. When complete, the result will be a vastly increased coverage area, better metropolitan reception, and modern digital DRM broadcast capabilities for Ho Chi Minh City and much of Southeast Asia.

LBA Technology has provided a full range of AM medium wave antenna and transmission equipment, engineering, and field support worldwide since 1963. Check out our complete offerings. Contact Jerry Brown or Javier Castillo to discuss your requirements.

About The Author

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The group is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection and EMC equipment for broadcast, industrial and government users worldwide; the professional consultancy Lawrence Behr Associates and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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