Small Antennas Review

Small Tuned Antennas for Medium Wave DXing Portable receivers are capable of hearing a surprising amount of stations. However, when one wants to listen to distant domestic stations or transoceanic broadcasts, the internal ferrite bar antenna often needs some assistance from an external antenna in gathering up more signal. There are numerous antenna choices available, and you can select a directional or omnidirectional antenna, a broadband or tuned model, portable or stationary, and so on. Because I often DX with small portable “Ultralight” radios, primarily indoors but also at remote "DXpedition" locations, I prefer an antenna that is small and portable (so I can take it anywhere), tuned (for additional sensitivity and selectivity), and directional (for nulling of interference). Fortunately, there are several commercial models which meet these criteria, both air-core and ferrite-core loops, from which to choose.

Front row: Quantum QX Loop, Quantum Q Stick, C Crane Twin Coil Back row: Super Select-A-Tenna, Terk AM Advantage This article reviews and compares several of these antennas, pictured above, which are generally representative of the ready-to-use antenna products that are available today. I have ranked them in terms of the actual signal-to-noise improvement that they provide when coupled to a small portable receiver, although each product has unique characteristics to recommend it. The data upon which the results were derived is included at the end of this article. 1. The Super Select-A-Tenna (Model 541S) The Super Select-A-Tenna (Super SAT, right) is an approximately 11-inch air-core loop, available from Universal Radio for $179.95. Unlike the regular Select-A-Tenna, the Super SAT has a regenerative “Q multiplication” circuit which the maker claims increases the signal by 40 decibels as well as dramatically improving selectivity. Jacks are provided for connecting an external antenna to the Super SAT and for connecting its output directly to a receiver or to a coupler. As far as I know, this is the only commercially-available air-core regenerative antenna; the slightly larger Kiwa Loop was also an air-core regenerative antenna, which provided for bearing and azimuth control (which the Super SAT lacks, but see below).

As can be seen in the data at the end of this article, the Super SAT provided the best gain/signal capture of all the antennas reviewed. It was noticeably better on the low end of the band, and somewhat better on the upper band, than the Quantum QX Loop. This is consistent with previous comparisons of the Kiwa loop and the Quantum Loop – the larger capture area simply provides more gain. Being an air-core loop, the Super SAT was also a bit quieter (more resistant to local electronic noisemakers) than the ferrite-based QX Loop, and has a very generous “sweet spot” surrounding it in which to position a portable receiver for passive coupling. Placing the receiver too close to the Super SAT hinders the antenna’s ability to regenerate; I usually keep the two about 4-6 inches apart. For more details about placement and use of passive antennas, check out the Using Passive Loop Antennas article at Dxer.ca. The Benefits of Regenerative Q Multiplication As I understand it, Q multiplication does not change what the loop itself receives; rather, it changes how that signal is presented to the receiver. By increasing the level of the incoming signal and restricting the bandwidth, the resulting signal seen by the receiver will be much further above its internal noise floor (overcoming the noise and losses from the internal circuitry) and less prone to being attenuated due to the receiver’s AGC responding to nearby strong signals (which are no longer as strong compared to the desired signal). Near oscillation (maximum Q multiplication), the bandwidth is approaching essentially zero, and finetuning the frequency of the antenna can allow you to select the sideband you want to favor, as well as where in that sideband the receiver is centered. As a result, even a good communications-grade receiver like my Icom R75 can often benefit from a regenerative antenna and the pre-selection that it offers. However, in the case of portable receivers, which may have high noise floors and/or average sensitivity and selectivity, the difference can be profound. Compared to the regular SAT, I found that the Q multiplication on the Super SAT provides another 20-25 decibels of gain, depending on how close I got to to the point of oscillation. (Note: with the Q multiplication control at its minimum setting, per the manual, the Super SAT's performance is essentially identical to that of the “regular” Select-A-Tenna). For example, using weak daytime signals, the following shows how the Q multiplication improves reception on a Sony M37V modified with a LTM-450G filter: Super SAT With and Without Regeneration Frequency

Without Regeneration

With Regeneration

530

Mumbling audio

100% word recognition

800

Barely readable

Easily readable

1340

50% of words recognized

Easy recognition

1650

Battling interference, weak readability

Reduced interference, good readability

Split-Frequency DXing When chasing "split frequency" (9 khz spaced) foreign broadcasts, a regenerative antenna is even more valuable. For example, when using the Tecsun PL-380 set to its 1 khz filter, one can generally tune 1 khz away from a station and still have readable audio. In other words, to listen to 729 khz under the shadow of a domestic on 730 khz, one could tune to 728 khz and still hear it fine, avoiding 730 khz as much as possible. However, tuning down to 727 khz results in the garbled "Donald Duck" audio associated with AM reception of SSB ham broadcasts, since the Tecsun's 1 khz filter has sliced off the 729 khz carrier. With a regenerative antenna, this problem is solved. First, tune the radio and antenna to the garbled sideband, and increase the antenna's regeneration control to near oscillation; then detune the frequency control a bit toward the carrier frequency. This results in the injection of the carrier frequency by the antenna, allowing the desired sideband to be converted into readable audio. In the example above, you would first tune the radio and antenna to 727 khz and increase Q to near oscillation, then tune the antenna a bit up-band towards 729 khz until audio is obtained. By tuning the radio to 727 khz rather than 728 khz, you are now 50% further away from the interference on 730 khz!

I have found that this approach allows the DSP filtering to meet and often exceed the performance of expensive Murata narrow filters that have been transplanted into various Ultralights. In some cases, I have been able to detune another khz away (i.e., to 726 khz) and still readable audio. How does the Super SAT compare to a much larger tuned passive antenna? I compared the Super SAT with a 48-inch diagonal tuned box loop (right), using weak daytime signals and Tecsun PL-380 receiver. In theory, the much larger loop should provide a 7-8 dB signal-to-noise advantage over the Super SAT, owing to the former’s much larger capture area, and this was generally the case: no surprise there! However, I found that in many cases the Super SAT was as good as, and sometimes better than, the much larger box loop. For weak signals, where the challenge is to get enough signal to the receiver to get above the receiver's noise floor, in some instances the increased output of the Super SAT "noisier" signal more than made up for the 7-8 dB signal-to-noise difference, allowing the actual end result to be better. For signals suffering from adjacent interference, especially at the high end of the band, the Super SAT's vastly superior tuning sharpness allowed it to pull a station out of the muck that the larger box loop was sometimes not even able to find. Finally, the difference in actual signal-to-noise between the two antennas appears to be less pronounced in the upper band, meaning that the sheer size of the box loop is not such a substantial factor up there. So, while the large box loop certainly has a signal-to-noise advantage due to its size, there are other variables to consider when deciding which antenna to use. Using the Super SAT The Super SAT has silky-smooth varactor tuning, and a separate fine-tuning control, which made it the easiest to tune of all the products reviewed, especially at the high end of the band. One caution: the fine-tuning knob is close to the case, meaning that hand capacitance can be an issue. As such, I often have to turn the knob about 1/8 turn counter-clockwise past where I want to be, so that when I withdraw my hand it's at the proper setting. This problem can be solved, though, by pulling the knob a bit off of the shaft (perhaps 3/8” away from the case) and using just a single finger tip to operate the knob, which minimizes the amount of “you” that the antenna picks up. The Super SAT's case is well sealed, and the all-in-one design makes it a great choice to take to the beach or other DXpedition adventure. I would note that the side of the case is slightly angled, such that it tends to lean backwards if on a soft surface; this can be rectified by putting a rectangular piece of plastic or wood on the bottom of the unit. The unit automatically powers off after approximately 30 minutes; I like the fact that it helps conserve the power of the 9-volt battery on which it runs (claimed battery life is 100 hours), but I wish the time period was 60 minutes or so. You can periodically flip the switch in the “on” direction while it is operating, which appears to reset the 30 minute clock. Connectivity To use the Super SAT with a communications receiver, or to feed a portable radio via a coupler, an output jack is available on the back which taps off of the main loop. However, connecting to the output jack loads the main coil and partially “spoils” the Q to some extent, such that the Q multiplier circuit will not reach maximum Q (i.e., just prior to oscillation) all the way down to 530 kHz. Specifically, I found that with a communications receiver connected I could get oscillation down to about 600 kHz, whereas using the supplied coupler with a portable radio I could only get down to about 750 kHz.

This problem with Q spoiling can be rectified using the supplied ferrite coupler, placing it just in front of the unit in order to reflect part of the signal back to the loop. If you try this, note that the coupler has to be in-phase with the main loop; in the picture at left, if the red plug end of the coupler were on the right, it would not work. It's a simple matter of seeing which orientation is correct: if placing the coupler near the loop has no impact, simply flip the coupler over. Using a Y-adapter, a separate line is run to either the communications receiver or, in this case, another coupler to feed a portable receiver. Connection of a low-impedance external antenna to the jack on the front of the Super SAT also substantially “spoils” the Q, to the point that it is difficult to get much tuning sharpness at all. This also happens with the Quantum QX Loop, so it appears that one needs an actual regenerative preselector to provide Q multiplication to a low-impedance external broadband antenna (it may be that the Super SAT and QX Loop only accept high-impedance antennas in order to work properly as couplers). Providing Azimuth Control One thing you cannot do with the Super SAT is adjust the azimuth of it (i.e., tilt one way or the other), short of tipping is slightly and physically holding it there. The Quantum Loop (below) can do this, as could the Kiwa Loop mentioned earlier, allowing for pin-point nulls. If you do want to adjust the azimuth of the Super SAT, there are at least a couple choices. The first is to attach it to a camera tripod; I tried this and realized that my tripod was not generally not designed to handle the sheer weight of the Super SAT, plus the range of adjustment was limited. A second way is a pivoting cradle made out of 3/4" PVC pipe (below). This is a very simple set-up, but works quite well. By ensuring that the Super SAT is well-centered with the respect to the pivot points, it stays where I tilt it, and getting precision nulls is a snap. Velcro holds the Super SAT securely in the cradle.

Super SAT in Azimuth Cradle

PVC Cradle Detail

In summary, I was very impressed with the Super SAT! With the above modifications to allow for azimuth control and full Q multiplication when connected to a communications receiver, I essentially have the equivalent of a Kiwa Loop in a compact package and at a fraction of the price.

2. The Quantum QX Loop The Quantum QX Loop is available from Radio Plus for $200-240. The on-board regenerative Q multiplier, as with the Super SAT, provides dramatic signal and selectivity improvements; as far as I know, this is the only commercially-available regenerative ferrite-based antenna. The base unit tunes the plug-in ferrite loop head, which is infinitely steerable in bearing and azimuth. Other loop heads are available for longwave and shortwave. The QX Loop's dual power scheme is also very convenient for portability. External antennas can be connected to the loop head, and the output is via an SO-239 jack in the back of the base unit. For use with portable radios without an antenna jack, Radio Plus sells an aircore coupler, although any coupler will work; for more information, check out the Coupling Antennas to Portable Receivers article at the DXer.ca web site. For signal-to-noise performance, the QX Loop was consistently behind the Super SAT in terms of providing readable audio on weak signals, especially at the low end of the band. At the high end of the band, the results were less distinguishable between the two antennas, at least in terms of gain and signal-to-noise. However, I still prefer the Super SAT, since the fine-tuning control made it much easier to use; tuning on the high end of the band can be difficult on the QX Loop since the stations are bunched together and the tuning needs to be quite precise. The good news is that the QX Loop oscillates to maximum Q across the whole band whether driving a communications receiver or a coupled portable, while the Super SAT needs an extra coupler to do this on the low part of the band. It appears that the Super SAT was intended for passive coupling with portable receivers, while the QX Loop was designed primarily with communication receivers in mind, so trade-offs are inevitable. An important feature of the QX Loop is that, with all of the available antenna heads, you can quickly and easily vary the direction and azimuth: the QX Loop is unique in this regard. This allows for pinpoint nulls, which with the Super SAT would require you to physically hold the whole unit in a certain orientation unless using a cradle, tripod or other device. A final feature of the QX Loop that I have found to be very valuable is that you can hook up any number of loops, allowing the base unit to tune and control much larger antennas. For example, a standard office filing crate (left) can be used to create an antenna with more signal-pulling ability than the Super SAT, and I use this antenna at the beach. For more information on this and even larger designs, see the Active Passive Loop article at the Dxer.ca web site.

3. The C Crane Twin Coil Antenna This active antenna system, available for about $80 to $100, includes a large ferrite-core loop antenna unit which can be located some distance away, plus a control unit. It is called a Twin Coil antenna because there are two coils, one on each end of the ferrite; this may produce an extra decibel or two of signal, but generally it seems to be a negligible performance factor. For connection to a portable radio without an antenna jack, a ferrite coupler is supplied. The Twin Coil can be operated with the supplied adapter or on a 9-volt battery. Other ferrite-based active antennas of this type are the Palomar Loop Antenna, Palstar LA30 and the Kaito KA35, which all look like the Quantum QX Loop with their steerable ferrite heads, but do not have Q multiplication or (with the exception of the Palomar Loop) azimuth adjustment, and therefore are likely to be similar in performance to the Twin Coil. On the control unit, the inner control knob is both an ON/OFF selector and fine tuning control, while the outer dial is for coarse frequency adjustment. I found the controls to work very smoothly, and generally could zero in on the desire frequency fairly easily. The antenna unit can be located away from the base unit by using a PS/2 extension cord, which can be valuable if you are in a metal-frame building or your listening post is near noisy electronic equipment; this ability to remotely use and tune the antenna is unique among the products reviewed. Placing the antenna unit on a small camera tripod allows for precise azimuth nulls. As far as signal gain, the Twin Coil was not able to keep up with the Super SAT or QX Loop units, primarily because of the lack of a Q multiplier. However, it still provided good improvements on weak signals that were otherwise unreadable on all of the test receivers, including the Sony ICF-S5W. The tuning on the Twin Coil is fairly sharp, although again not comparable to the Q-multiplication units above. Creating Regeneration with the Twin Coil Antenna Fortunately, there are simple methods by which you can turn the Twin Coil into a regenerative antenna, increasing both signal level and tuning sharpness. Using these, I was able to obtain the same gain and selectivity as with the Quantum QX Loop. The first way is to simply move the coupled receiver close to the antenna unit, as shown at right. To effect Q multiplication, first tune the base unit to the proper frequency, then move the receiver/coupler close to the antenna unit until you start to hear what sounds something like a whooshing or screeching sound. The coupler must be in phase with the antenna unit; if you move the coupler/ receiver up next to the antenna unit Regeneration with the Twin Coil antenna and nothing happens, flip the coupler over and it should work fine. For example, in the picture at right, flipping the coupler would place the plug end on the right of the coupler; you could also flip the Twin Coil's antenna unit upside-down if that is more convenient. Also, bear in mind that the two coils in the antenna head are at either end, so the ferrite/radio combination will have to be placed at one end or the other (even though the picture at right shows them centered).

Next, re-adjust the base unit tuning for maximum signal – the gain and selectivity should have improved substantially. Move the receiver/coupler closer to the antenna unit to increase Q, and further away to decrease Q; if you move in too far, the circuit will go into oscillation, in which case just move the receiver/coupler away a little. At maximum Q, the gain will skyrocket and the bandwidth will narrow such that treble in the audio will largely disappear, similar to what happens if you use a narrow IF filter on a communications receiver. The receiver/coupler may have to be a little closer to the antenna unit in the lower part of the band. This sounds complicated, but becomes quick and easy with a little practice. A more convenient way to create regenerative Q multiplication is to use another ferrite coupler and a Y-adapter, as shown in the picture at left. The output of the Twin Coil's base unit is sent to both couplers, one coupled to the receiver and one to the antenna unit (remember, the coils are at either end of the ferrite). The same procedure as above is used to create regeneration, only here you move just the regenerative coupler (the one attached to the red cable) closer to the antenna unit until Q multiplication starts.

Two Coupler Regeneration

With this set-up, the antenna unit can be placed away from the receiver and tuning unit, using a regenerative coupler feed cable the same length as the PS/2 antenna unit cable. A drawback to this approach is that you will generally still have to adjust the distance between the antenna unit and the regenerative coupler every time you switch stations.

For even greater convenience and control, placing an in-line potentiometer on the regenerative coupler's feed line would allow you to adjust the Q without having to change the relative position of the antenna unit and coupler. Simply place the regenerative coupler right next to the antenna unit, and then slowly decrease the resistance of the potentiometer to bring the circuit towards oscillation, adjusting as you tune around the band. In the picture at right, I strapped a larger (8inch) ferrite with about 10 turns of wire to the antenna unit for the regenerative coupler to increase the coupling. The small black case with the knob takes the input from the base unit and sends it directly to the receiver coupler (red line), while there is a 10 kohm potentiometer to control the output to the regenerative coupler (yellow line) – it really works well! Drop me an email if you would like more details on this fairly easy project.

Two Couplers Plus Level Control

4. The Terk AM Advantage This is an approximately 9-inch tuned air-core loop, widely available for around $30-40. At this price, it is an excellent value. This is perhaps the most basic type of loop, a capacitor-tuned air core loop. In my experience, this antenna is the equivalent of the regular Select-A-Tenna (models 541 and 541M) and the Kaito/Tecsun/Grundig AN100 and AN-200 products, which are also 9-10 inches in diameter and operate on the same principle. Somewhat larger air-core loops are available on eBay from sellers tquchina (the 11x11 inch TG39) and cygnus4444 (the 14-inch diameter Magical Traveller), and kits like the 18x18-inch loop from MTM Scientific are available. While genreally falling short of the Twin Coil's signal augmentation, the Terk provide a good gain boost to all of my portable radios, Ultralight to full-size, including the newer Tecsun DSP Ultralights with tuned front ends. For effective coupling, I found that the radios needed to be much closer to the Terk than with the Super SAT, owing to the lesser “output” of the Terk. Also, the cylindrical tuning control on the Terk sticks up into the loop area, meaning that you can't easily put a radio inside the loop. While this is often not an optimum position anyway (depending on the radio), the control is more difficult to see and operate; as such, some prefer the Kaito/Tecsun/Grundig models. There is an output jack for connecting the Terk loop to a receiver with an antenna jack. Since this is a passive device, the signal to the receiver is not that powerful. However, it is still head and shoulders above the small air-core loops that come with many home stereos and radios today, and the Terk is marketed for those applications as well as for passive coupling with a portable receiver. Additionally, using a simple broadband RF preamplifier on the output, like the $20 model at left from eBay seller Orient100, turns the Terk into a nice amplified antenna with similar performance to the Twin Coil when connected to a tabletop receiver. 5. The Q Stick The Q Stick is an 8-inch ferrite-core passive antenna, available from Radio Plus for $52. The Q Stick provides a decent boost to many portables, both Ultralights and larger units such as the Sony 7600GR. However, it does not work with some radios, including many of the newer Tecsun DSP Ultralights, nor does it provide any significant improvement for some full-size portables such as the Sony ICF-S5W. Of the products tested, where the Quantum Stick coupled with the receiver, the derived boost in gain was consistently the smallest. That said, I often take the Q Stick along when I travel, depending on which receiver I grab, as it is easily the most compact portable antenna I own. The availability of two bands (LW and MW) are unique, and with a suitable receiver (I like the Sony 7600GR with the Q Stick) makes an excellent DX setup for traveling. The Q Stick can also be used as a coupler for longwire antennas, and it has a built-in jack just for this purpose; as with the Super SAT and QX Loop above, in my experience this jack does not successfully accept a low-impedance broadband antenna feed.

Summary Which antenna is “best” likely depends on what you need: I think that all of these antennas have their own niches. • The Super SAT has the best weak signal enhancement, has Q multiplication and is able to be finetuned easily, can be easily adapted for azimuth and Q control, and its all-in-one design make it desirable for taking to the beach. • The Quantum QX Loop is right behind the Super SAT in terms of gain, connects well to communications receivers, has directionality and azimuth control, and can accept any number of external loops. • The C Crane Twin Coil provides good gain, means for Q multiplication, and offers the unique option to place the actual antenna portion well away from your listening post. • The Terk AM Advantage is a cheap and easily-operated antenna that is compatible with all portable radios, and has some external connectivity as well. • Finally, the Q Stick is a two-band ultra-portable antenna that allows for improved DXing on the fly. With all of these products, additional benefits such as notching, phasing and other tricks are possible – check out the article Using Passive Loop Antennas at Dxer.ca for more details. (Note: the various Dxer.ca articles referenced herein are also available on the UltralightDX Yahoo Group site). Kevin S., Bainbridge Island, WA satya(at)sounddsl.com December 2010

Reception Test Results What the ratings mean: 0. Nothing heard 1. Mumbling audio 2. Occasional words recognizable 3. Weak intelligibility 4. Good intelligibility 5. Loud and clear Tecsun PL-380 - Weak Signal Reception Comparisons Freq

Barefoot

Super SAT

QX Loop

Twin Coil

Terk

Q Stick

530

1

4

3

2.5

3

3

800

0

3.5

2.5

2.5

1.5

0

1340

1

4

3.5

2

2.5

1

1650

0

3.5

3

2

0

0

The Tecsun PL-380 is one of the most sensitive Ultralights radios available, and has a tuned front end, meaning that an external passive loop must have appreciable signal available to overcome the internal tuning and make any real difference in reception. While the Q Stick had a surprising boost on 530 khz, it generally was not even noticed by the PL-380. Of particular interest are the results on 1650 khz, a weak TIS target that is sandwiched in between two strong local stations on 1590 and 1680 khz whose transmitters are only 3 miles away from me. In addition to gain, the antenna also had to provide some selectivity in order to receive 1650. I believe this is because the front end of the PL-380 was so inundated by signal that it needed some help separating it all out, despite the sharp DSP filtering. The two antennas with Q multiplication (the Super SAT and Quantum Loop) had no problem getting readable audio, and the Twin Coil gave it a shot, but the other two were of no help in this situation.

Sony SRF-M37V - Weak Signal Reception Comparisons Freq

Barefoot

Super SAT

QX Loop

Twin Coil

Terk

Q Stick

530

1

4

3

1.5

2.5

1.5

800

0.5

2.5

1.5

2

1

0.5

1340

0.5

3

2.5

2

2

0.5

1650

0

3.5

3

1

1

0

The results with the Sony M37V were similar to those with the Pl-380: the Super SAT always was doing better than the Quantum Loop, with the Twin Coil and Terk battling for third place. Again, success on 1650 was limited to the two antennas with Q Multiplication, since the M37V's front end also needed a lot of help.