WD2XSH/7

Last updated - 26 October 2011

WD2XSH/7 is one of the participating stations in the ARRL 600 Meter Experimental Project. WD2XSH is operating under a Part-5 experimental service license granted by the Federal Communications Commission. The project is designed to obtain data for a later attempt to obtain a new amateur radio band allocation in the United States of America.

The WD2XSH/7 transmitter is located in Natchitoches, Louisiana, USA, at Grid Square EM31JS or North Latitude 31 46.629 and West Longitude 93 11.029 in WGS84 format.

The maximum radiated power allowed for the WD2XSH stations is 20 watts. Because of antenna and ground system losses, the WD2XSH stations require a substantial transmitter power output (TPO) to achieve the 20-watt limit. On the case of WD2XSH/7, a TPO of 200 watts is required. (The previous 36 foot tall antenna required almost 800 watts to achieve the 20-watt level.)

The antenna system consists of a 72-foot high, base insulated, series fed vertical antenna of 6-inch diameter. The antenna is constructed from several sections of 6-inch diameter aluminum irrigation pipe. The antenna mast is guyed at three levels and in four directions with non-conducting Dacron guy lines. The guy line anchor points are elevated six feet above ground, and they are located at a distance of 40 feet out from the base of the antenna mast.

The topmost set of guy lines is constructed in two parts. The first 21-foot section extending out and downwards from the top of the mast is made from a three-strand length of steel and hard drawn copper wire, which is commonly used for high-tension power lines. These conductive lines act as a top load umbrella for the antenna, and serve to make the antenna electrically taller, thereby increasing its efficiency. The remainder of these top-load guy lines is made from 5/16" black Dacron line.

Anti-corona rings are installed at the junction of all twelve of the guy lines and the antenna mast. Anti-corona rings are also used at the outer ends of the top loading wires to prevent the possibility of high voltage corona discharge from melting or setting fire to the Dacron lines.

The ground system consists of 480 feet of #6 bare solid copper wire laid out on the surface of the ground (the grass covered it quickly) and 19 - 8 foot long by 5/8 inch diameter copper-clad ground rods driven to full depth. The ground rods are connected to the copper wire network and spaced apart in such a way so that the field of influence of the ground rods covers the entire area for a radius of 50 feet outward from the base of the antenna.

The antenna system was modeled using EZNEC+ V5.0. A complete model was of the antenna and ground system was constructed, and numerical calculations were then run with all the conductor losses set to zero. The result of this computation gave the radiation resistance of the antenna. Working backwards from the allowable radiated power under the WD2XSH license, it was then possible to determine the required antenna current through the calculated radiation resistance of the antenna to produce the allowable power level.

After the antenna system was constructed, it was found that to produce the calculated RF current into the antenna, it was necessary to apply about 200 watts to the antenna system. This equates to roughly 10% antenna efficiency. A video of the raising of the antenna may be seen on Youtube at: http://www.youtube.com/watch?v=rmdqvQk5aV8

The antenna matching system consists of a main loading coil and a smaller variometer for fine-tuning. A smaller tapped inductor is used to match the transmission line from the transmitter to the antenna system. The main loading coil consists of 150 feet of 1/4 inch diameter copper tube space wound into a coil 14 inches in diameter by 23 inches long. The coil is supported by fiberglass rods and HDPE plastic to ensure low loss. The coil is mounted outside of the main tuning cabinet and it is protected from the weather by an enclosure made from HDPE plastic panels. The variometer is also wound with 1/4 inch copper tube. The variometer allows for about +/- 30-uHy inductance variation, to allow for compensation of normal environmental changes in tuning. Major frequency changes between the WD2XSH/7 assigned frequencies of 495.035 KHz and 508.900 KHz is accomplished by means of a bolt-on tap on the low end of the main loading coil.

The transmission line is a buried 1-inch diameter 75-Ohm impedance coaxial cable approximately 150 feet long. The transmission line enters the radio room through a grounded metal bulkhead, which has Alpha-Delta surge suppressors installed on all of the transmission lines.

For QSO operation, the RF chain starts with a Flex-5000A, or a Yaesu FT-747GX, either of which may be switched in line to the final RF power amplifier. The FT-747GX may also be used for beacon, WSPR, or PSK-31 operation. The main beacon exciter is a pair of Motorola RF Channel modems, which generate USB signals on 495.035 and 508.9, respectively. These modems take an audio signal from a CD player, which plays a repeating loop recording of the CW, QRSS, or PSK beacon signal. Only one channel modem is used at a time.

The RF from the selected exciter then goes to one of two available RF power amplifiers. The amplifier generally used is a homebrew, solid-state linear amplifier, which produces up to 300 watts PEP or CW. This amplifier was rebuilt from its original use as a 400-watt output 166.5 KHz amplifier for WC2XSR/13. The second amplifier available is a homebrew 4-400A linear amplifier that is capable of generating 800 watts of RF output PEP or CW. A two-stage solid-state intermediate power amplifier (IPA) is used to boost the exciter signal to drive the 4-400A. The IPA is built onto the chassis of the 4-400A amplifier. This power amplifier is not used often, because it is rather inefficient at the 200-watt power level required for the 20-watt power limit. This amplifier was needed for the earlier 36-foot vertical antenna, which was less efficient and required greater RF power to obtain the 20 watt radiated power level.

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