THE 971 TRAIL-FRIENDLY TUNER / BATTERY PACK
By John Cumming, VE3JC
Contributing Editor, The ARS Sojourner
For a number of years, an MFJ-971 Antenna Tuner sat collecting dust on the desk in my shack. Having an automatic tuner in my main K2 rig, and possessing several compact tuners for minimalist field operating, the 971 rarely got taken along for radio adventures. This was really a shame; having about the same footprint as my Sierra, it almost looked like it belonged with that adventure-friendly rig! [Pic 1 ] And with its 6-watt scale dual-needle meter , internal balun, and wide tuning range capability, it deserved to see more action.

So I set out to make the 971 capable of carrying its weight on radio expeditions, through a series of modifications. The first mod involves the addition of a bypass switch. This permits convenient removal of the tuning network when not required. If your field operation is primarily on a single band (for which your ultra-portable antenna is resonant) but you occasionally venture onto another band, the bypass switch will be useful. When operating near resonance, the tuning network (and its associated losses) are simply switched out. The dual needle metering remains in service, to monitor power output and warn of unanticipated high SWR when the antenna falls down!

Bypassing is achieved by mounting a miniature DPDT switch directly above the Inductance knob. If care is taken in marking and drilling the mounting hole for the switch, none of the front panel lettering will be obscured, and the switch will look right at home! Wiring of the bypass switch is straightforward - the common terminal of one half is connected to the antenna output jack, while the other common terminal is wired to the output of the meter circuit. The two switched terminals for the bypassed position are shorted together, while the other pair are connected to the two air variable capacitors in the C-L-C network. [Pic 4 ]

Since both sections of the tuning capacitors are ungrounded, I decided to swap the rotor and stator connections in the tuner circuit. The reason for doing this will be apparent when an unmodified 971 is compared with the photos provided. In the modified 971, the T point, where the capacitors connect to the inductor, is now located at the back of the capacitors (closest to the inductor), while the input and output of the C-L-C tuning network are physically relocated at the front, close to the bypass switch.

Although the 971 is capable of operation up to 300 W (with appropriate internal jumpers selected), I use it exclusively at QRP levels. I therefore chose to use RG 174 coax [Pic 5 ] for the internal connections, to minimize impedance mismatch at the input and output of the tuner, and to facilitate the rewiring exercise.

I must admit that the bypassed tuner does present some mismatch on 28 MHz, so the benefit of bypassing may be minimal at higher frequencies (at least, using the construction methods employed here!)

While working on the bypass switch modification, I began to take notice of two other features of the MFJ-971: the unoccupied dead space above the meter circuit board, and the rear panel 12 VDC jack, used only to power the meter lamp. With backpack space always at a premium, the 971 would certainly be a more attractive hiking companion if it was sporting an internal battery pack!

After a bit of head scratching and rearranging, I now have a 9 AA cell battery pack inside the 971 (actually it's a 10-cell holder, with one cell shorted out, to give about 14.6 Volts from a fresh set of Alkaline batteries). A short DC patch cable from the existing jack on the 971 is used to supply the Wilderness Sierra or similar trail-friendly radio capable of operating at 15 V.

The 971 internal battery pack rests on two aluminium brackets [Pic 7 ], above the metering circuit board. You can be creative here, depending on the materials available. The main caution in mounting the internal battery pack is to assure that it will not interfere with the full rotation of the variable capacitor closest to the meter. To provide maximum clearance for the battery pack, I relocated the ground connection for the rear-panel mounted balun and shortened the balun's mounting screw [Pic 2 ]. For the battery pack holder, I used an extremely ugly galvanized fence bracket [Pic 6 ] which, when screwed on to the mounting angles, prevents the battery pack from encroaching on the space needed for the variable capacitor.

As can be seen in the photos, [Pic 3 ] a small L angle and 2.5mm DC jack have been attached to the aluminium meter support for quick connection of the battery pack. The two wires from the meter lamp are simply reterminated on to the new internal jack. (Retaining the meter lamp was not important to me. I always have my red LED camp light for late-at-night tent operating and I did not wish to risk a dead battery disaster by adding a momentary push button switch to illuminate the meter).

I hope that the pictures provided will fill in the missing construction details. A final note of caution: these modifications were performed WITHOUT removing the circuit board or tuner components. Use great care and minimum pressure when drilling the holes, to assure the circuit board or air inductor are not destroyed. Also, be sure to carefully remove all shavings and debris from inside the 971 after completing drilling and soldering.

So grab the Sierra, a battery-equipped 971, mini-paddles, earbud headphones, plus a favorite trail antenna, and you'll be ready to head to the wilderness!

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John Cumming, VE3JC, is a contributing editor to The ARS Sojourner living in London, Ontario, Canada.