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Monitoring Times Reviews

February 2004

 

 

MFJ Travel Tuner

By Bob Grove

 

            The actual function of an “antenna tuner” seems to remain shrouded in mystery for most hobbyists. Most amateur radio operators seem to think it’s mandatory for every HF transceiver, and many shortwave listeners are certain that it’s the magical answer to improved reception. There is both myth and misunderstanding, but no magic.

            These simple inductance/capacitance adjusters are more correctly called “transmission line impedance matching devices” (shortened to “transmatches” by the American Radio Relay League) than “antenna tuners”; they compensate for inductive and capacitance reactance (mismatches) throughout the antenna system – antenna, transmission line and all – not just the antenna.

            The main purpose served by the transmatch is to adjustably cancel these reactances to provide an efficient transfer of power from the transmitter to the antenna without the losses caused by impedance mismatches which produce high voltages on the transmission line along the way. These periodic high and low voltage points exhibited by standing waves are measured as a ratio (VSWR, often shortened to SWR).

            Theoretically, in a lossless antenna system, the SWR wouldn’t matter, but the practical fact remains that some transmitted power does leak across the insulation in the coax, dissipating as heat, and the higher the voltage, the more power loss.

            In addition, transistors are vulnerable to burnout from high voltages, much more so than the old vacuum-tube circuits. This is why modern transceivers invoke automatic power reduction when high SWR is detected.

 

How About Receivers?

 

            Shortwave receivers aren’t concerned with transmitted power, so do they need transmatches? Generally not. Modern receivers are very sensitive, limited in weak-signal detection only by the accompanying atmospheric noise and co-channel interference. The bigger the antenna, the stronger the signal and interference levels.

            In nearly all shortwave receiving applications where an external antenna is attached to the receiver, although a signal-strength meter may show higher readings as the tuner is peaked to frequency, this peak is due to a proportionate increase in noise right along with signal. There is no improvement in signal over the noise.

            But there are exceptions. It is possible for low-cost receivers with poor RF selectivity and dynamic range, such as portables, to benefit from a transmatch when used with an externally-connected antenna. This is because a such a tuner acts as a frequency-selectable RF stage, reducing off-frequency, strong-signal interference from IF images, intermodulation and desensitization, allowing improved weak-signal reception.

 

Enter the new MFJ-902 Travel Tuner

 

            MFJ Enterprises is renowned for their low-cost, hobby-radio accessories. Recently they released a transmatch particularly well-suited for compact fixed, portable, backpack and mobile transceivers in the amateur, commercial, experimental, government and military radio services.

            With a frequency range of 3.5-30 MHz (80-10 meters) and a power limit of 150 watts, this transmatch works with virtually any high-frequency transceiver on the market. It can be used with coax-fed or random-wire antennas. Although it doesn’t have a meter for VSWR adjustment, most modern rigs have, making such an indicator often redundant and unnecessary.

            Architecturally, the 902 is a tried and proven L/C “Tee” network. It utilizes a tapped inductor wound on a 3-toroid stack and a pair of 322 pF air-variable capacitors with porcelain insulators. Two SO-239 female connectors on the rear panel invite interconnection to standard PL-259 fixtures. A banana pin plug is included for random-wire applications. A slide switch permits the tuner to be bypassed for direct antenna feed from the radio.

 

Let’s Try it Out

 

            Connecting the 902 to my Yaesu FT-100D transceiver was easy – a short PL259/PL259 jumper from the tuner to the rig, and the antenna cable to the tuner, and I was ready to go.

            But I couldn’t detect any difference in tuning, and there was a tell-tale scratching sound coming through the speaker as I rotated a control. Removing the two cabinet screws and lifting the cover, I confirmed my suspicions.

            Several plates of a variable capacitor were rubbing together; a wire passing by a soldering lug had been prepared for soldering, but never was; and a tuning knob was slipping on its shaft.

            Since no schematic diagram was provided, I had to experiment with and without a shorting clip to be sure the wire was supposed to be soldered to its nearby lug. I know that new products are often prone to initial manufacturing bugs, so I decided to repair the unit myself as a “getting acquainted” experience.

            After resetting the bent plates, soldering the ground wire, and filing a flat on the tuning shaft so the set screw could be tightened, I was ready to go.

           

How Well did it Work?

 

            It worked very well. The accompanying instructions make adjustments easy, and maximizing my rig’s power with the tuner’s controls was simple. An excursion across the HF spectrum showed that the tuner worked effectively on a test antenna from 2-30 MHz; it is not designed to work on 160 meters (1.8-2.0 MHz), and it doesn’t.

            One of the most common criticisms of tuners is their internal resistances; after all, you want all the power to reach the antenna, not wasted heating the components. In both the bypass and operational modes, the 902 adds less than 0.1 ohms to the path, not much since the series impedance of the antenna system is nominally 50 ohms.

            Heavy gauge wire on the toroid coil is a good sign, but will it make a noticeable difference to the RF path in terms of efficient power transfer? We decided to construct a simple test instrument consisting of a 100 watt light bulb to be used as a dummy load, and a metered photocell to read its brightness.

            Were there internal resonances that could sap power from the line? Not according to our measurements. The meter registered identical brightness throughout the 2-30 MHz spectrum with the transmitter delivering the same output wattage.

 

Applications

 

            I’m already envisioning taking this transmatch along with my FT-100D on my next vacation. It would be an invaluable asset for QRP or full-transceiver output into random-wire motel antennas or even electrically-short mobile and portable whips. Tiny enough to hold in your hand (4-1/2”W x 2-1/4”H x 3”D), it can be stowed in a briefcase or backpack with ease.

            MFJ-902, $69.95 plus shipping from MFJ Enterprises, PO Box 494, Mississippi State, MS 39762; 800-647-1800; http://www.mfjenterprises.com

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