There can be lots of arguments about whether its better to use one of the new high tech radios for SWL or BCB DX-ing or whether to go with a more complex table top model. My own experience has been that the table top unit gives better performance for digging out deep-buried DX but that a high tech portable in the hands of a skillful user can do very well. On the other hand, to get the most out of the table top radio also requires a certain amount of skill and getting to know the radio.
The new portables particularly have the advantage of providing much better frequency readout than many of the old desk top "boat anchors" could provide, while the older receivers can often provide more robust handling of very strong signals and deal better with connection with outdoor antennas.
The older rigs can also provide better selectivity than the new portables. They do this through the use of crystal filters and in some cases Q-Multipliers.
The issue of Q-multipliers came up on a Facebook Group of which I am a member this week and questions about its use and "whether it was working or doing anything" resulted in a short note to the poster describing the use of the device in a rather abbreviated way. I thought perhaps sharing a more detailed information about their operation and use might be in order here.
Just what is a Q-Multiplier? Well the abbreviation "Q" in the electronic sense refers to the "Quality Factor" of an inductor or tuned circuit. Without getting into the esoteric details, what it boils down to is it generally can refer to selectivity or how broadband a device might be. A tuned circuit or antenna with a High Q would have a narrower bandwidth whereas a tuned circuit or antenna with a Low Q would tend to be rather more broadband. There are many factors that go into determining the Q of a coil or antenna. In the case of coils, wire diameter, turn spacing, diameter of the coil winding and resistance of the wire or surrounding circuit are the major factors, along with any loading on the circuit. The bandwidth of antennas is a whole other issue that could take up its own entire article!
The name " Q-multiplier" then can almost be surmised as being a device that somehow "multiplies the Q" of a circuit or in this case, does something that increases its selectivity. In superheterodyne receivers, the real selectivity takes place in the Intermediate Frequency amplifiers. The superhet design basically converts all incoming signals down to one frequency where high gain, stable amplifiers are easier to design to operate in a fixed-tune mode. The concept as created in the 1920's and 30's has remained with us ever since.
Different boat anchor designs, just like different new portable designs, came with different price tags depending on the complexity of the circuits and the cost of the components placed in them. The basic design that is of most use to DX-ers is one that has at least one stage of tuned radio frequency amplification ( one R.F. Stage) and two stages of Intermediate Frequency amplification ( two I.F. stages) Most single conversion radios had a 455 kHz IF. Some dual conversion radios had a 1650 kHz First IF and 455 kHz Second IF. The triple conversion receivers often made a further conversion to 50 kHz.
Q multipliers have been around since the very creation of the superhet design as a means of improving selectivity in the Intermediate Frequency stages. It basically brings the advantage of a regenerative receiver to add to the greater advantages of the superhet design it had been determined earlier that the use of feedback or regeneration in an amplifier stage would artificially increase its Q by overcoming losses in the circuit. A small portion of the output would be coupled back to the input at a level just below that which would cause oscillation.
This can actually be done in the IF stage itself to a degree but is difficult to control because of the action of the Automatic Gain Control circuits. When the gain goes up in the absence of a signal,the stage is pushed more toward oscillation. If the internal feedback is set at this level, it does not do much good when the gain of the stage is then pushed down by the presence of one or more stronger signals, which is the exact time more regeneration is needed.
The device covered by the term Q multiplier generally refers to what amounts to a separate IF amplifier operating in parallel with those existing in the receiver. It is often connected to either the plate of the last converter tube if there is but a single IF stage or the plate of the first IF amplifier stage if there is more than one. It must be tuned to the same IF frequency as that of the receiver to which it is attached. For the " add-on" devices this was most frequently 455 kHz.
I am not sure if anyone knows who first published designs for outboard Q multipliers or extra circuits of one built into a receiver but they have been around for a long time. They were also around in abundance as add-ons in the form of kits in the 50's and 60's, most notably the Heathkit QF-1 and its descendants. Some receivers, including many of the general coverage Heathkits had jacks on the back to allow easy connection of the Q-multiplier.
There are also many construction articles on how to build one and connect it to a receiver in the archives of many magazines such as QST, CQ, Popular Electronics and Electronics Illustrated. Older editions of the Radio Amateur's Handbook published by the American Radio Relay League also have information on the theory, design and actual construction and connection of them. I am sure the same goes for Radio Society of Great Britain and organizations in other countries. Most of these articles and designs tell of how power to operate Q-multipliers may be taken from the receivers to which they are to be connected and how to set them up. Others may be built with their own internal power supplies.
Using the devices is a bit of a learned art, much as the actual tuning of a receiver is such. While instruction may be given, there is nothing like the "hands on" practice of making them work to get the most out of them.
Most Q multipliers have the same minimum controls: Tuning and some form of gain control. It might be labeled " selectivity" or in the case of the Drake Q multiplier meant to be used with the famous 2B receiver it is called " Q balance". Some many have additional controls to chose between peaking and notching signals. The Heath also had a "Broad" and " Narrow" switch. Some radios with built in Q multipliers may not have the tuning control.
I will leave the adding and/or building of a Q multiplier to other articles. Here I will cover the operation and use of one.
The first thing to do is turn the device on and set the "gain","selectivity" or whatever form the operating level control might be to about mid scale. Start with the radio in the AM mode and tuned to a quiet spot or with the antenna disconnected. Then rock the Q multiplier tuning control back and forth and listen for a "swishing" sound. If you don't hear one, run the "selectivity" control up a bit more ( clockwise) until you do. Note where on the tuning control the "swish" seems to have the lowest pitch or sounds like it is " bottoming out". This will be the spot that corresponds to the center of the receiver's passband. Note that spot for future reference. Depending on the alignment of the receiver and the Q multiplier, this might not necessarily be at the center of the control's travel. This can be corrected but should not be attempted without proper understanding of the receiver and Q multiplier circuit operation. In no instance should an attempt be made to realign the IF stages without a signal generator of known frequency calibration or proper calibration and tracking of tuning on the main tuning dial can be negatively affected!!
For tuning AM signals, it is best to start with the Q multiplier tuning set in the center of the passband. Once the signal is tuned in and centered, you can try swinging the tuning back and forth to get the best readability without slicing too much off the sidebands. If the selectivity is too tight, all the highs will be sliced off and the station will sound muffled. You can try tuning off to one side or the other with the Q multiplier tuning which can act much like passband tuning.
If there is a strong heterodyne or splatter interference on one side you can try tuning the station in with the main tuning placing its carrier on the edge of the passband placing the splatter farthest from the center, with the desired side centered in the passband. Then crank up the Q multiplier and make adjustments with its tuning.
This is where the art of using the device comes in. It is only with practice and patience that the full effects and possibilities of this device can be learned and attained.
It is on cw and single sideband that the Q multiplier can really show its maximum potential. There are probably a number of " set up " methods for cw reception with one, but here I will describe the one I have used and found to be one that at least works for me!
The first method involves a receiver with a variable pitch BFO. The first step is very similar to that with which the Q multiplier tuning is determined. Tune the receiver to a quiet spot or disconnect the antenna. Turn on the receiver's BFO ( Beat Frequency Oscillator) and rock the pitch control back and forth. you will hear the "swish" similar to what was heard with the Q multiplier tune set up. Once again the "swish" will " bottom out" or have a noticeable lower pitch when it is moved through the center of the passband. This is, of course, not where you really want it...it is where you want the desired signal to be, so at this point move the pitch control a slight amount to the right or left ( as you turn the knob) from center.
For practice, connect the antenna and tune in an AM signal that is not too strong, but which is of a fairly steady strength. Probably the best place and time to do this is in the daytime with an AM broadcast station other than a powerhouse local station. If your receiver has an S-meter. carefully center it up for the maximum meter indication. If you have no S-meter, by ear rock the tuning back and forth and with practice on a weaker signal you can hear when the signal is centered. If the receiver has fairly good selectivity to begin with you can hear the little " swish" as the carrier moves through the passband or you can hear the lessening of the high frequency sounds on the sidebands as its is centered.
Again, this ability to notice things comes with practice. The accurate and careful tuning of a receiver is much more than just dialing the tuning to a spot and finding something. There are a number of subtleties that can be recognized only with practice and that are often hard to describe in words. The receiver cannot do the work for you, no matter how complex or expensive it may be. I think I have noted before that a skillful operator can pull out DX with a simple receiver that a casual tuner cannot find with a much more expensive one. Practice, practice, practice and learn to take advantages of the capabilities and avoid the weaknesses or your receiver!
But I digress. Once you have your " test signal" tuned in, turn on the BFO and rock the pitch control back and forth. Listen for where the " Zero Beat" occurs,. The zero beat definition will be obvious as you do this. It is the point at which the pitch of the heterodyne of the beat frequency oscillator goes lower and lower in pitch and at one point becomes "zero".
Note again where the zero beat occurs on the pitch control. If your tuning has been careful, you should notice that it is at or very near the same spot as you noticed the lowest pitch " swish" when rocking it when listening to no signal.
At this point, leave the receiver tuning alone and move the pitch control to one side of zero beat or the other to obtain a pitch comfortable for you to listen to cw. Then turn the Q multiplier on and set the selectivity control fairly high. If there is a "peak-notch" or " peak-null" switch, set it to peak. Now move the Q multiplier tuning control to peak the level of the resulting beat note. Or watch the S meter for a peak. Run the selectivity control as high as you can without the device going into oscillation itself. If you keep it right on the ragged edge of oscillation you will be at the tightest selectivity point.
Now to test if your adjustment has been good, move the receiver tuning control back through zero beat and back up the other side. If the Q multiplier is doing its job and your adjustments have been proper you should notice that as you tune to the other side of zero beat, the signal should be much weaker. In other words, on one side of zero beat you will have a strong signal and on the other side of zero beat it should be much lower. The idea is to have the BFO injection offset in the receiver passband to make any signal that would have the same pitch as your desired signal to be out of the passband on the other side of zero beat.
This results in what is called " single signal" cw reception. Otherwise, as you tune through a crowded band, each signal will actually show up twice, resulting in effectively an undesired signal giving QRM to a signal that you might otherwise hear from the other side of zero beat. Effective set up of BFO pitch and Q multiplier tuning can thus for all practical purposes doubled the tightness of selectivity of the receiver! The ability to bring this about effectively comes only with practice. It is best done listening to a good steady carrier and doing the set up over and over and finding the best settings for your particular receiver.
Tuning in single sideband signals is a bit more complex. Without the Q multiplier on, tune in a good fairly steady SSB signal and practice making it intelligible. Once again, start with the BFO offset to one side or the other of the center of passband. This is why you want to remember or note the position of the control at dead center zero beat. You want to have the desired SSB signal centered in the passband and the BFO injected on a frequency offset and near the edge of the passband. Just as with single signal cw reception, you will eliminate QRM coming from " the other side" of the BFO.
The trick is the required position of the BFO pitch control is different for upper sideband ( USB) signals and lower sideband ( LSB) signals. This, again, will take practice to determine. One thing that makes it a little easier is the fact that by convention, on the amateur bands, everyone operates lower sideband ( LSB) on 160, 80, and 40 meters and upper sideband ( USB) on 20, 15,17,15,12 and 10 meters.( there is no phone operation on 30 meters).
Once you have a known LSB signal tuned in, juggle your dial tuning and BFO pitch to obtained the best overall intelligibility and note the position of the BFO pitch control. Then turn on the Q multiplier and starting with the selectivity control somewhat lower than for CW, adjust the Q multiplier tuning for most intelligible or strongest signal and run the selectivity control up until it starts to "bite" into the high frequencies of the voices and begins to make them sound muffled. Note the positions of the controls so you can always come back to them.
Then do the same with an upper sideband signal. Note those control positions. As you listen more and more, you may refine them somewhat. Note that the conventions for USB and LSB apply only to the amateur bands. While tuning in military or other utility stations you will have to make s determination of which sideband they are using. Most often aviation related stuff appears to almost always be on USB, as does military ops, but you never know....
With more advanced receivers, there may be a fixed tuned BFO that makes all this a little easier. Some receivers have function switches with USB and LSB that do all that BFO presetting for you. Then all you have to do is peak and tune the Q multiplier to tighten things up.
If you have a Q multiplier that has the " Notch", "Null" or "Reject " position, it will provide you with the ability to eliminate on possible source of interference, such as a strong heterodyne very close to the desired frequency. Tuning is pretty intuitive. Switch to "null" or whatever it is called with your unit, run the selectivity up and tune the Q multiplier tuning to get rid of the undesired signal. The downside of this is you lose the ability to tighten up the overall selectivity of the set up. For this reason I have rarely used the null feature. I am sure there are times that it might be useful.
This is only a very basic primer on Q multiplier operation. As in tuning any other portion of a receiver, it takes practice, practice, practice. Particularly with adjustments for single signal cw operation use of a steady carrier from a standard broadcast station is handy in that it provides a good, stable test signal. For SSB, you pretty much have to work with the real thing. The thing to remember is to get the desired signal in the middle of the passband and set up the BFO to give maximum intelligibility with BFO injection near one edge or the other of the passband.
The most effective Q multiplier/receiver combination I have ever seen is that of the old Drake 2B used with the Drake 2BQ Q multiplier. I have had one for over fifty years and have learned to set it up on cw such that a signal on the other side of zero beat simply does not exist.
The Q multiplier is one device or feature available on some of the older receivers or that might be added. There are also crystal calibrators and preselectors or preamps...we will look at them at another time!