CONTINUOUSWAVE

Recently I ordered a new GAM SS-2 antenna from GAM Electronics, and chose the "Marine" model which has the whip antenna pre-cut to length for use on ship stations in the VHF Marine Band, as well as the matching coil adjusted for best match in that region of the band. In the current band plan, ship stations in the VHF Marine band transmit on the lower part of the band, using frequencies from 156.1 to 157.4-MHz.

I tested the VSWR of the GAM SS-2 when mounted on the typical L-bracket mount. For the test I mounted it on a three-foot long pine 2x4 lumber. The test was conducted on an indoor antenna test range (or what my wife would refer to as our living room). The base of the antenna was about 3-feet above a non-conducting surface that was about 10 feet above ground level that itself was about 15-feet above the level of Lake Michigan, a freshwater and very non-conducting surface. There were no metal linear conductors near the antenna that had any resonance in the VHF Marine Band.

The test configuration was as follows:

• a VHF Marine Band 25-Watt transmitter was connected by a new 22-foot-long RG-8X 50-Ohm transmission line to the input port of a directional wattmeter;
• the antenna port on the wattmeter was connected by a two-foot-long very-high-quality 50-Ohm RG400 coaxial cable to the antenna under test;
• the wattmeter was located two feet directly below the antenna, sitting on a temporary test fixture (also used as an ottoman);
• the radio was tuned to various channels in the Marine Band and switched to transmit for 1-second at various frequencies in the band;
• the wattmeter recorded the incident power, the reflected power, computed the VSWR, and measured the frequency of the transmission;
• the Return Loss (RL) was then computed (by me) from the VSWR.

The following measurements were taken or computed

CH  MHz    VSWR    RL dB
6   156.3  1.32    17.2
16  156.8  1.30    17.7
18  156.9  1.30    17.7
24  157.2  1.32    17.2 
28  157.4  1.31    17.4

Later the same antenna and transmission line were re-tested for VSWR after the antenna had been mounted on a 18-inch wood mast above the steeply sloping metal roof of a one-story building built on a pier about six feet above the level of Lake Michigan (otherwise known as the marina dock house at Northport, Michigan). Only the VSWR at 156.8-MHz (CH 16) was tested. In this instance the directional wattmeter was located at the transmitter end of the same 22-foot transmission line. The VSWR measured was 1.24:1. Note that the 22-foot transmission line was just long enough to reach from the antenna on the roof, travel down the side of the building with two drip loops included to shed rain water, penetrate the wall of the building, form into a four-turn coil of 3.5-inch diameter (to form an RF choke to suppress any antenna currents flowing on the outer skin of the outer conductor of the coaxial transmission line) and connect to a Standard Horizon VHF Marine Band radio about 18-inches from the inner wall of the building at the dockmaster's station. The transmitter was set to the 25-Watt output position. Actual measured incident power was approximately 27-Watts, giving plenty of power for good measurement of incident and reflected power.

When measuring the VSWR at the transmitter, the VSWR tends to improve (be lower) due to the influence of transmission line loss. We can estimate the transmission line loss of 22-feet of RG-8X cable at 156-MHz to be about 0.85 dB. In the measurement of the VSWR this loss affects both the power delivered to the antenna and the power reflected from the antenna, so the loss has to be considered twice in the calculation of VSWR, or as -1.7 dB.

To deduce the VSWR at the antenna, we take the return loss value for the VSWR at the transmitter, 1.24:1, and compute the apparent return loss: -19.4 dB. From that value we remove the -1.7 dB masking loss in the transmission line, and deduce the VSWR at the antenna as exhibiting an actual return loss of -17.7 dB. Converting that to a VSWR gives 1.3:1 at the antenna.

The measured VSWR of the antenna on the test range and the deduced value of VSWR when mounted in a typical elevated location are in essentially exact agreement. This tends to confirm the antenna test range as being a useful site for antenna testing at 156-MHz.

For more details about the directional wattmeter used, the dummy load, and the two-foot coaxial cable, see my article endorsing these products at

Measuring VSWR and Power Output for VHF Marine Band Antennas and Transmitters
https://continuouswave.com/forum/viewto ... =13&t=9327

Note: the loss in the transmission line was based on the assumption that the cable, RG-8X, would have a loss of -3.8 dB per 100-feet at 150-MHz (a typical published value) and thus for 22-feet would have -3.8 dB × 0.22 = 0.84 dB loss. Any loss due to connectors, insertion of the wattmeter, and the additional 2-feet of transmission line were neglected.

In this case, the antenna was installed at the dockmaster building of my local municipal marina. The change to the GAM SS-2 from the existing antenna produced a remarkable improvement in coverage for the radio installation. The previous antenna system was not working very well, with a suspected actual VSWR at the antenna of about 8:1. The range of communication was typically limited to a mile or two. Now the same transceiver can produce clear copy from ship transmissions from ten miles away or farther, and the transmitted signal is remarkably stronger.





GAM Electronics

I must also acknowledge the outstanding cooperation of GAM Electronics in ordering the new GAM SS-2.

I normally would order a GAM SS-2 antenna from a retailer like DEFENDER (who offers free shipping on orders of $100 or more), but for some reason their website listed that product as out of stock. I contacted GAM Electronics directly via telephone. Their representative, Marcie, was extremely helpful. I was able to purchase the GAM SS-2 antenna, the stainless steel mounting bracket, and a very specific transmission line directly from them.

The antenna and mounting bracket were standard products. The transmission line was a custom order. I needed a slightly longer length, 22-feet, as opposed to the standard 20-feet length. The need for the longer length was do avoid the usual problem expressed in a well-known variant of "Murphy's Law":

Any cable cut to length will be too short.

This antenna was intended to replace an existing antenna installation, and I could only estimate the cable length with some rough "eyeball" measurements. I also wanted to change the cable to be RG-8X from RG-58/U, to reduce transmission line loss.

A third customization was to change the connector on the cable at the transmitter end. This was necessary in order to pass the cable through an existing hole in the building where the antenna was to be installed. At the transmitter end I wanted an FME male connector which would be able to pass through the 3/8-inch-diameter (existing) hole. Once through the hole, then an FME to UHF plug adaptor would be installed. I wanted the transmission line FME connector to be pre-assembled so I would not have to fit the FME connector to the cable at the site. My general preference is to use FME connectors and to pre-install them in my workshop, rather to that at the site. Being able to get the FME connector installed by GAM Electronics was another great help.

After the installation, the transmission line reached the transmitter with about 18-inches to spare, and the extra length was then coiled into a four-turn "choke" balun to suppress any antenna currents that might exist on the outer surface of the coaxial shield conductor.

GAM Electronics is located in Maine and is a small business operation. The cable they supplied was marked "Made in USA." My order was shipped the next day. The shipping costs were reasonable. The custom cable was a very nice accommodation.