TWO POPULAR AM
Loop antennas have been in use since radio's infancy. Some of the earliest radios employed set top loop antennas for times when an outdoor "aerial" was not practical. Many radios of the 40's and 50's had simple loops wound around the perimeter of their back covers. In the 50's and 60's as compact ferrite rod "loopstick" antennas became common, wire wound loops faded from common use to the point where the average person may have never seen or heard of a loop antenna. It's easy to forget that these ferrite rod "loopsticks" are actually loop antennas using ferrite cores to increase their efficiency.
Loops come in all sizes and are still used in many applications where their efficient signal capture makes even relatively small loops amazingly effective at AM and SW frequencies, where traditional wire antennas often run from 30 to perhaps hundreds of feet. They can be either passive or active (amplified). Broadcasters often use larger, specifically-tuned loop antennas to insure a strong stable signal for monitoring purposes, and there are also large loops which can cover SW/AM/LW bands. These may be "tuned" or "untuned" designs suitable for remote mounting. (With a Tuned Loop you have to adjust the tuning as you change frequencies on your radio, so these are also known as "proximate designs" they are generally used right next to the radio). The larger loops are generally constructed of pipe or tubing and are thus single-turn designs, while smaller loops have several windings. Finally, there are also specialty direction-finding loops which are designed for designated frequency ranges.
Loop antennas in general also have an ability to reject certain kinds of electrical interference compared with wire antennas. This is partly because a loop antenna reacts to the magnetic portion of the RF energy received and ignores the electrical component, where much of the noise is concentrated. A loop also has the ability to be directionally aimed to maximize signal pickup, and likewise, noise sources can often be nulled by careful placement and aiming of a loop antenna. Sometimes you'll be lucky enough to experience miraculous relief from local noise compared with a wire antenna, but be aware that the ferrite rod "loopstick" antenna inside your radio possesses the same directional and noise-nulling capabilities, although being smaller it gathers less signal.
For this review I compared two of the most popular passive tuned loops available for AM (Medium Wave), the Select-A-Tenna (Models 541 and 541M), and the Terk AM Advantage Loop. It pays to shop for price. The Two Select-A-Tennas range from $59.95 to $69.95 for the Model 541M and from $49.95 to $64.95 for the 541. (One popular on-line retailer shows one of these models with a higher price than "List" as given on the Select-A-Tenna Home Page). The Terk lists at $49.95 but is generally available for $39.95 and I found it at an on-line electronics store for $27.95)! The Terk was previously also available as the Radio Shack AM Loop. The difference between the two Select-A-Tenna (or SAT) models is that the "M" model adds an input/output jack the Terk also has this jack. It's a standard 1/8" mono mini-plug and is connected to a secondary loop wound next to the primary loop and can be used to connect the antenna directly to a radio with an antenna input jack or terminals. Alternatively you can feed the signal from, say, a random wire antenna into these antennas, then use the antenna to couple the signal to your radio, with a pre-tuner thrown in. Many users will never use the jack feature, but it can add to the antenna's flexibility in some circumstances as we shall see. Otherwise the two SAT's work identically. There is also an active model the SAT Model 541S for $199.95 which adds an amplifier for additional gain and control. I did not test that model.
These are both "proximate" designs, meaning they are intended to be used near the radio. These are "tuned" loops, meaning you must tune them to peak each frequency received. Although it is an extra step when you are tuning stations it is easy and seems like a small price to pay for the reception boost these loops can provide. They are also relatively small; the Select-A-Tenna is a solid disc about 2 1/4" deep and approximately 10 3/4" in diameter with its tuning dial right at its center. The Terk is about 9 1/4" diameter, 1 1/2" deep although at its base it is about 2 ¼" deep and is an open loop with its tuning control in its base. The Terk also has a rubber bottom to protect the surface it sits on. Side by side there is little doubt that the Terk was designed to look good in a home setting while the SAT is more utilitarian in its appearance.
These two antennas are primarily used via inductive coupling; that is, with no direct connection to the radio whatsoever. One merely places the radio near the loop and the loop's field is picked up by the radio's built-in ferrite rod antenna. This is somewhat amazing if you've never seen it before and will allow you to impress your friends with your wizardry! One bit of advice: Initial setup up and testing of these antennas can be confusing if done at night. During nighttime reception, there are many more strong signals than during the daytime. When you increase the level still further with an external antenna, the radio's AGC circuits reduce the gain to compensate, so you may not hear a difference. This is often miss-construed to mean that these loops don't do much at night, but believe me, nothing could be further from the truth. The improvements gained through their use at night is a bit different than what you may get during the day but sometimes will be very helpful.
If your radio has no internal ferrite rod antenna (such as the Eton E-1 or vintage radios with wire-wound loops on their back panels), inductive coupling won't work so you will have to use the direct connection. Both of the mini-jack equipped models (the SAT Model 541 M and the Terk) come with an adapter cable terminating in bare wires. Neither of these antennas furnish much information about output impedance, but when I tested them on a radio equipped with both high and low impedance antenna inputs (nominally 50 ohms and 500 ohms), both of the loops showed slightly higher signal levels with the high impedance connection, although they worked acceptably into either impedance so matching your particular radio's input shouldn't be a problem. However each antenna showed markedly stronger signal levels when used with inductive coupling so you may want to experiment to see which setup works best for you.
DO THEY COMPARE?
These loops also tout their use with AM radios which already have simple external loops, such as home Hi Fi systems and some table radios. I tried them with one such radio and got results somewhat better than with the supplied untuned loop antenna.
As I previously mentioned, that mini-jack on the SAT 541M and Terk can also be used as an input although Terk does not mention this in their literature. If you are in a shielded location, such as an apartment, office building or an RV, you can use these loops to not only inductively couple an outdoor wire antenna to your radio but to also use it as a pre-selector which could help prevent overload and interference. I did not test them in this configuration but see no reason why they wouldn't work as advertised.
WITH THE LOOPS
BUT I HEARD THESE LOOPS DON'T WORK WITH MODEL "XXX" RADIO
These are the most sensitive AM portables I have and they showed virtually no improvement with either loop. The loops still increased the signal levels a bit but there was little if any improvement in reception quality these two radios are already near the theoretical limits of signal-to-noise ratio on weak signals and the additional gain had a subtle effect at best.
But just to show how radio-individual these results are, my GE Superadio III and CCRadio Plus, which are still quite sensitive but which both have a slightly higher noise floor than the two radios above, showed some improvement with the loops. The CC's LED Level Meter rose from ¼ to full scale on some signals from my readings on S-Meter-equipped radios I would guess this equates to something around 6 db of gain. Listening to several stations 100 - 130 miles away (daytime) the background hiss was noticeably improved with the loops. I got similar improvements with other good AM portables too, like the Grundig Satellit 800 & 600, Eton/Grundig S350/Tecsun BCL2000/3000 series. Slightly less sensitive (generally smaller) radios showed even more improvement, such as the Eton E-5/Degen DE-1103 series, Sony 7600GR and any number of smaller radios I happened to have around. The smaller internal antennas and less sophisticated front end circuits in these radios limit their sensitivity to the point where either of these loops can make a huge difference. Weak signals can literally rise from faint whispers in the noise to full-blown signals that are pleasantly listenable.
THEY WON'T DO?
One other thing these loops won't do is match the performance of more expensive/sophisticated antennas. Both the C.Crane Twin Coil Ferrite and Quantum & Wellbrook Loops (see my reviews) can provide substantially stronger signal levels and more control with better noise rejection, but of course, you have to pay for that performance ($100, $200 and about $370 respectively). With my Eton E-1 I routinely use both the C.Crane and the Quantum to great effect they make a killer combo, but I would not recommend either the SAT nor the Terk for this radio the signal level just won't be there for you. And of course, truly dedicated AM hobbyists may be able build larger loops with more signal capture or to erect outdoor antennas that may outperform any indoor antenna, such as 1000 foot and longer Beverage wire antennas or K9AY loop antennas there's no limit to what you can do if properly motivated.
Comments or questions? Email me at firstname.lastname@example.org
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