Look At The Signal Enhancer

In the fifties and sixties, the best way to improve receiver selectivity at tow cost was to add one of the outboard devices popular then, such as a Q-multi-plier (which worked on the set's t-f stages) or a war surplus radio range filter {which was inserted in the set s headphone lead) Both methods left a lot to be desired.

It seems that suddenly hams have discovered that the easi est—and in some ways, most cost-effective—way of improving receiver selectivity is to add an outboard audio fitter The popularity of these so-called "active" filters has not been lost on manufacturers either— perhaps a dozen make these handy accessories, including Kantronics, Datong, Electronic Research Corporation of Virginia, Dynamic Electronics. Patomar Engineers, Waneco Radio, Autek Research and MFJ Enterprises.

MFJ has come up with several good designs, each one better than the one preceding it with each mod adding new convenience features, Their latest "maxl-filter'* is the Signal Enhancer II, a very attractive dual tunable design intended to remove interference from SSB, AM, CW, and RTTY signals. The ads for the new filter stressed its flexibility and improvements over earlier models, so I arranged with the hams at MFJ \o put one of the new filters to work in my ham shack.

The Model 752 filter arrived on a Friday afternoon, and by that evening I had digested the Installation instructions- From the instructions, I learned that the 10" x x 6' cream-and-wal-nut cabinet contains not one, but two, independent filters.

The main filter is a two-section, four-pole variable filter with four major functions: peak, notch, lowpass, and hlghpass modes, There is also a second! ^auxiliary" filter having peak and notch functions only, (The idea of having two filters is to allow you to peak or "enhance" the desired signal while at the same time you remove or "notch out" QRM.}The four-page instruction manual and the catalogue that was included in the shipping package told the facts; The main and auxiliary filters allow selectivity down to 40 Hz bandwidth, variable over the frequency range of 300 to 3000 Hz, and the notch depth goes ail the way down to 70 dB. Six frontpanel controis allow function selection (bypass, on, and SSB or CW noise limiter), main filter operating mode (peak, notch, lowpass, or highpass), and main filter selectivity and frequency adjustment. There are also two controls for the auxiliary filter, which can be operated simulta neously with the main one. The auxiliary selectivity control has a push in, push-out feature so that either the peak or notch function can be selected with a gentle push on the control knob.

The filter came complete with a 110-volt power supply; it s of the "'external power adapter" types commonly used with portable radios and small tape recorders, The filter will also run directly from any 9-18-voli source of filtered dc. The audio from the rig is connected to the filter input by means of RCA-type phono plugs; the MFJ-752 can accommodate two inputs which are selected by a rear-panel slide switch, allowing you to connect two rigs to the filter. A standard (V*") phone jack is used to hook up your regular station speaker; the speaker line is disabled whenever headphones are inserted into the headphone jack on the rear panel I should point out that this jack is of the 2-circuit (stereo) type, it's best used with stereo headphones to take advantage of the so-called "simulated stereo" feature. {When you use stereo phones, unfiltered audio directly from the receiver is fed to one earphone; the other ear gets "processed" or filtered audio. Mono headphones can be used If the stereo feature isn t wanted.

More on this later,)

The power switch is the main control. When power is off, the filter is cut out of the circuit and audio is routed directly to the speaker or headphones, in the llon,t position, the filter is In business, without the noise limiter feature. Going to NUSSB or NUCW, noise limit* ers are cut in that are designed to limit impulse noise peaks and remove background noise.

Having owned the AutekQF-i and GF-1A filters, both excellent units, I found tuning and ad* justment a bit more complicated using the Signal Enhancer II—possibly because there are more knobs to turnl Once I got the hang of it, though. I found that it did a good job on both CW and SSB in slicing through QRM and QRN. On CW, \ could zero-in on the desired signal in the peak mode and almost completely eliminate other signals by carefully adjusting the selectivity and frequency controls, I found that the lowpass mode was "super" for CW work; signais couid be boosted even more so than in the peak mode, and this mode gave more audio output to boot. (Because circuit gain rises in the lowpass mode, you have to watch the input audio level to prevent overdriving the filter with consequent distortion.)

I also prefer using the lowpass mode on SSB, although peak and notch can also be used depending on QRM conditions. If heterodyne QRM is the major problem, you can run in the notch mode, adjusting the frequency control to null out the offending signal, You can run in any main filter mode and cut in the auxiliary filter in either the notch or peak modes. For instance, you can, when working SSB. operate the main filter in the peak mode, adjusting the frequency and bandwidth controis for best audio response, and then—using the auxiliary filter—notch out any annoying heterodyne, On CW, you can kick in both the primary and auxiliary filters to yield very

"tight" selectivity with little ringing, or you can peak with the main filter and notch out another signal with the auxiliary filter The possibilities are almost endless and are really limited only by your imagination and dexterity!

t found the noise limiter circuits very useful, even though my transceiver (a Tempo 2020} has a built-in noise blanker. The limiters were especially helpful when used in conjunction with my Yaesu FRG-7, whose ANL {automatic noise limiter) works only on AM signals. The limiters did, however, cut down on the filter's audio output (particularly noticeable when using a speaker rather than headphones). and the CW limiter had to be used judiciously {as it couid be overloaded and start to 'chop" the desired signals if the receiver's audio gain were run too high). The trick is to set the audio or rf gain on your rig at the right level so that the signal is passed but the noise is blocked.

A tinkerer, f also tried the MFJ fliter with my KLM-2700 multimode 2-meter transceiver and with a JC Penney 6237 AM CB base station set. The filter worked surprisingly well with the KLM rig, especially on SSB and CW where there is no provision to install optional, sharper i f fitters. On FM, there was no noticeable selectivity improvement, although the filter s controls could be adjusted to modify and enhance the set's audio response. When using the MFJ unit with the Penney CB transceiver, a very noticeable improvement in selectivity was obtained, and it was easy to null out AM carrier heterodynes. The noise filter was effective with both units (even on FM), despite the fact that the KLM rig has a built-in noise blanker and the Penney CB set is equipped with an adjustable noise limiier

The "simulated stereo" feature took some getting used to, but has its place, particularly on CW. The idea is to have liraw" receiver audio fed to one ear and filtered or "processed" audio fed to the other. This unique feature allows you to copy off-frequency stations in one ear while you simultaneously hear "single-signal"1 audio in the other; the brain hears all the signals, but the processed signal stands out from the pack. I found that working in the simulated stereo mode cuts down fatigue as well, The kind of phones with individual volume controls on each ear work best, since you can easily strike a balance between processed and unpro^ cessed audio levels.

A couple of cautions are in

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has SWR and dual range wattmeter, antenna switch, efficient airwound inductor, built in balun. Up to 300 watts RF output. Matches everything from 160 thru 10 Meters: dipoles, inverted vees, random wires, verticals, mobile whips, beams, balance lines, coax lines.

Antenna matching capacitor. 208 pf, 1000 volt spacing.

Sets power range,

300 and 30 watts. Pull for SWR.

Meter reads SWR

and RF watts in 2 ranges.

Efficient airwound inductor gives more watts out and less losses.

Transmitter matching capacitor. 208 pf, 1000 volt spacing,

Only MFJ gives you this MFJ 941 Versa Tuner ft with all îhese features at this pnce

A SWR and dual range wattmeter (300 and 30 wans full scale) lets you measure RF power output for simplified tuning

An antenna switch Jets you select 2 coax fed antennas, random wire Of balance line, and tuner bypass

A ne» efficient airwound inductor <12 po silionsi g?ves you less losses than a tapped toroid for more watts ouï.

A 1:4 baton tor Daiance ¿mes tOOO voir capacitor spacing Mounting brackets tor mo bile Installations {not shown).

With I he NEW MFJ Versa Tuner II you can run your full iransceiver power oulput - up lo 300 wans RF powet ouîpuî - and match your

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ANTENNA SWITCH lets you select 2 coai fed antennas, random wire or balance line, and tuner bypass.

transmitter to any feedltne from 16D thru 10 Meiers wheifier you have coa* cable, balance line, or random wire.

You can lune oul the SWfl on your dipole, inverted vee. random wire, vertical, mobile whip, beam, quad, or whatever you have You can even operate all bands wiih just one existing antenna No need1 to put up sepa rate antennas for each band

Increase the usable bandwidth of your mo bale whip by tuning out the SWR from inside your car. Works great with all solid state rigs (like the Atlas) and with all tube type rigs

It travels well, too, Its ultra compact size 8x2x6 inches fit easily in a small corner of your suitcase.

This beautiful little tuner is housed rn a deluxe eggshell while Ten Tec enclosure with walnut grain sides

SO-£39 coax connectors are provided for transmitter input and coax led antennas. Quality five way binding posts are used for the balance line inputs (2), random wire input (1), and ground (t).

Continue reading here: Mfj Versa Tuner

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