Sometimes, the physical realization of a project makes all the difference between a usable project and one that lies around gathering dust.
I decided to make a portable, Q-multiplied ferrite antenna for use with a commercial, portable, pocket-sized shortwave radio.
The goal of the project was to enable good shortwave reception on the pocket-sized radio, without extending its whip antenna. This would allow easier use of the pocket shortwave radio while taking long walks, without the burden of a long whip antenna needing to be extended (which is a nuisance while walking with the radio).
The goal was happily achieved, and this has turned out to be one of the rare homebrew projects that actually finds relatively frequent use. The circuit itself is just a 2-transistor, differential-pair Q-multiplier, tuned with a varactor and connected to a ferrite rod antenna -- nothing special.
The challenging part was making this into a compact, usable form factor.
First, by trial and error, I wound some turns of enameled wire on the ferrite rod antenna and added some taps. With proper turns spacing and proper placement of the taps, I could verify that the ferrite rod could be made to oscillate with the differential-pair Q-multiplier from below 3 MHz to above 30 MHz. This is more than enough to cover the shortwave bands available on the portable receiver. The spacing on the coil was rather tricky to figure out, as it requires uneven spacing of the turns to be able to oscillate over such a wide range. When the coil taps select a widely-spaced part of the coil, then the oscillation frequency can be made relatively high; conversely, when the tapped coil range includes both the widely-spaced and the narrowly-spaced parts, the oscillation frequency can be made quite low. It's perhaps easier to consider the windings as consisting of high-L and low-L parts in series, on the same ferrite rod, and with sufficient spacing between the high-L and low-L parts to enable the low-L part to maintain its low inductance.
Next, I used 2 short wires with alligator clips to connect the main circuit to the ferrite rod antenna. The main circuit was mounted in a small plastic box.
The ferrite rod was attached to a stiff piece of cardboard. Velcro tape was attached to the ferrite rod's mounting board, to the plastic box housing the main circuit, and to the portable radio. This allows the circuit box and the ferrite rod antenna to be attached in "piggy-back" fashion onto the portable radio.
With the "piggy-back" circuit attached, the control knobs for the Q-multiplier (regeneration control that controls emitter resistance, and tuning control that controls varactor bias) are accessible as extra knobs the bottom of the receiver.
The ferrite rod is attached at the top of the receiver, so that it lies directly next to the retracted whip antenna. This way, the Q-multiplied signal from the ferrite rod generates an electric field that capacitively couples into the whip antenna. In fact, what is happening is that the Q-multiplication is forcing the ferrite rod to interact with a larger portion of the local EM field, extending its range of interaction with the incoming field -- somewhat like an "invisible antenna extension" that removes the need for physically extending the whip antenna into space!
The alligator clips allow easy selection of the proper taps for bandswitching.
The inside of the circuit box looks like this:
It works wonderfully. Naturally, the bandwidth is very narrow, which makes tuning somewhat tedious. First, the radio is tuned to the desired band. Next, the Q-multiplier is tuned (by selection of the proper coil taps, and by adjustment of regeneration and tuning on the Q-multiplier) until the oscillation Q-multiplier's signal can be heard on the radio. The regeneration is backed off to just below threshold. Then, the radio is tuned by 1 or 2 kHz, and the Q-multiplier is retuned by the same amount. It is a bit tedious, but in a crowded shortwave band with many broadcasters, it works quite well. On my concrete balcony, with the whip antenna retracted, almost no signals are audible. With the Q-multiplier, very many signals are audible with comparable signal strength to the case when the whip antenna is fully extended. Therefore, the goal was achieved: the Q-multiplied ferrite antenna allows equivalent reception to the extended whip antenna, but with the whip antenna retracted.
Additionally, the combination of a regenerated ferrite antenna plus a superhet is also quite fun to play with. One annoyance I always had with regens was the feeling that there wasn't quite enough RF gain. Especially with small antennas (like a ferrite rod antenna) and weak signals, the weak signals very often seemed to drown in the noise due to the square-law detection. But if we use regeneration for RF Q-multiplication only, and then do the RF-to-AF detection in superheterodyne fashion (with massive gain in the IF stages before the detection), we can be more or less assured that the RF-to-AF detection is no longer limiting the MDS; instead, the noise in the regenerative stage and in the ferrite antenna should now be the limiting factors. So we get all the sensitivity of a superhet (with multiple IF amplifier stages and AGC), plus the fun of peaking signals regeneratively by means of the Q-multiplied antenna. And all in a hand-sized package that can be used while walking.
The ferrite rod I used for the antenna was a "SL-45GT" rod intended for AM BCB use (reference: viewtopic.php?p=73842#p73842). I could confirm that this ferrite rod can boost signals all the way up to 22 MHz, which is the highest frequency receivable by the portable shortwave radio.
As I mentioned, the most tedious part is figuring out the correct turns spacing and tap positions on the ferrite rod that will allow oscillation over your desired frequency ranges. I used double-sided tape on the ferrite rod to allow me to easily unwind and rewind the turns to adjust the spacing. The double-sided tape then holds the turns in place. Once the turn spacing has been decided, you can then use a bit of hot melt glue to more firmly hold the turns in place.
Also, it takes some practice to be able to tune the Q-multiplier to the same frequency as the shortwave radio. The problem is that the shortwave radio, due to the superheterodyne mixing process, will generate spurious signals and will also respond to the image frequency, so there are multiple (not just one) tuning settings of the Q-multiplier that will cause an apparent signal to be generated on the shortwave radio. If you are listening to a spur or the image signal, instead of the properly-tuned Q-multiplier signal, you will not be able to peak the received signal strength since the Q-multiplier is tuned to the wrong frequency. It's particularly confusing at higher frequencies (say 18-20 MHz) because then the image frequency is so close to the desired frequency (455 x 2 kHz away). So it's easy to make a mistake and tune the Q-multiplier to the image frequency instead of the desired frequency. The easiest way to fix this is to extend the whip antenna temporarily and locate a strong station. Then retract the whip antenna, such that the station is barely audible or even inaudible. Then, without touching the radio tuning, try to tune the Q-multiplier to peak the signal strength. It may take some time, but it should always be possible. Then, once the Q-multiplier has been properly tuned in this way, incremental tuning is easier -- nudge the radio tuning up or down a little bit, and then nudge the Q-multiplier tuning just a little bit in the same direction, to peak the signal or noise on the radio. If you tune the Q-multiplier too fast, then there is again the risk you will mistune the Q-multiplier to a spurious frequency or image frequency.