Loop Antenna (A3015)

© 2006-2022 Kevan Hashemi, Open Source Instruments Inc. © 2022 Nathan Sayer, Open Source Instruments Inc.


Cable Properties
Impedance Matching
Antenna Combiners


[24-NOV-22] The Loop Antenna (A3015) is designed to receive telemetry signals in our Subcutaneous Transmitter System and transmit commands to our Implantable Stimulators. The antennas operate in the unlicensed 902-928 MHz Industrial, Scientific, and Medical (ISM) frequency band. We connect up to A3015C antennas to an Octal Data Receiver (A3027) to detect signals from implanted Subcutaneous Transmitters (A3028). We connect the same A3015C to a Command Transmitter (A3029) and use it to transmit commands to Implantable Stimulator-Transponder (A3041).

Figure: Damped Loop Antenna (A3015C). The cable is 50-Ω RG-58A/U. The antenna is a 2.5-mm wide trace on the printed circuita board, 320 mm long. The A3015C is equipped with a stand made of two brackets, which allow it to be placed on the floor or shelf of a Faraday enclosure. For top view see here.

Most radio-frequency antennas are designed to maximize sensitivity in their most sensitive direction. When we set up two stationary antennas for communication, we make sure the most sensitive directions point to one another, so that we can obtain the most powerful signal from each. The A3015, on the other hand, must communicate reliably with antennas implanted in freely-moving antennas. The animal will turn, stand up, roll over, and ambulate around its cage. Unlike traditional antennas, the A3015 is designed to maximize sensitivity in its least sensitive direction. That is: we want to raise the minimum sensitivity of the antenna over the set of all possible directions and polarizations of radio waves arriving from a freely-moving transmitter.

Figure: Damped Loop Antenna with Zip Ties and Cable (A3015D). The A3015D fastens to the struts of a shelf or rack.

The damped versions of the A3015 include an attenuator between the antenna and the coaxial cable. The attenuator absorbs roughly half the power arriving at the antenna. The attenuator damps radio-frequency oscillations that will otherwiswe arise in the antenna cable. The attenuator greatly improves the performance of the antenna in highly reflective environments such as Faraday enclosures. See the Impedance Matching section below for measurements.

The following versions of the A3015 exist. The A3015A and A3015B are obsolete. Undamped antennas, such as the A3015A and A3015D, do not perform well in Faraday enclosures, as we describe below.

Version Name Description
A3015A Loop Antenna 320-mm wire loop with stand
A3015B Damped Loop Antenna 320-mm wire loop with stand
A3015C Damped Loop Antenna 320-mm copper trace with stand
A3015D Aquatic Loop Antenna 50-mm wire loop with connector
A3015E Damped Loop Antenna 320-mm copper trace with zip-ties
Table: Versions of the A3015. The "damped" antennas have a 2-dB or 3-dB attenuator at the base of the antenna.

Different antenna shapes are sensitive in different directions and to different polarizations of the incoming radio-frequency wave. Their behavior is also affected by their interaction with the cable and the circuit at the other end of the cable. Long cables attenuate the signal. Short cables carry reflections back from the other end. A perfect match between the cable and antenna makes the antenna more sensitive in some directions, but less sensitive in others. For robust reception of telemetry signals from freely-moving animals, we try to increase the minimum efficiency of the antenna, so that the antenna will have no blind spots. For measurements of reception from live animals and test sources, see our Reception Page.


S3015_1.gif: Antenna circuit diagram showing connector, attenuator, and layout.

A301501C.zip: PCB Gerber Files for the A3015C Loop Antenna.

A301501B.zip: PCB Gerber Files. Six A301501B PCBs plus one A3021A combiner. A combination circuit board to be cut up with shears.

A302101B.zip: PCB Gerber Files. Six A301501C PCBs plus one A3021B combiner. This circuit board moves the attenuator closer to the connector center.

Figure: A3015 Schematic. Sketch is of the A3015B.

[06-DEC-21] We have performed quality control on the A3015C in many ways over the years, but today's test is as described by Nathan Sayer. "My process for testing antennas went as follows. I hung the antenna being tested in front of one of the absorbers with no attenuators attached to the cable. I then moved a transmitter all around the enclosure and made sure that I saw around 75% reception. Then, I attached a 30dB attenuator to the antenna being tested and made sure that it got reception when the transmitter was close to the antenna. The transmitter had to be within maybe 15cm (estimate) in order to get reception with the 30dB attenuator attached."

Cable Properties

[12-MAR-22] The following table summarizes the properties of common coaxial cables. We use pre-assembled RG58C/U cables with BNC connectors, purchased from L-com. Their RG58C/U data sheet is here. When we tried cut-price coaxial cables, we found their attenuition to be three times higher than the RG58C/U specification requires.

Type Impedance
A 100 MHz
A 900 MHz
A 2400 MHz
RG-58/U 50 5.0 0.11 0.34 0.61 Solid Conductor
RG-58A/U 50 5.0 0.18 0.70 1.3 Stranded Sn-Coated Conductor
RG58C/U 50 5.0 0.16 0.67 1.2 Stranded Sn-Coated Conductor
RG-59/U 75 5.9 0.08 0.25 0.40 Solid Conductor
RG-59B/U 75 5.9 0.11 0.37 0.65 Cu-Coated Steel Conductor
RG-213/U 50 10 0.06 0.25 0.46 Stranded Conductor
RG-214/U 50 10 0.06 0.25 0.46 Double-Shielded
Stranded Ag-Coated Conductor
RG-142B/U 50 5.0 0.10 0.4 0.7 Double-Shielded
Teflon Dielectric
Solid Ag-Coated Conductor
RG-223/U 50 5.4 0.13 0.45 0.8 Double-Shielded
Solid Ag-Coated Conductor
RG-174/U 50 2.8 0.30 0.94 2.5 Stranded Conductor
Table: Coaxial Cable Properties by Type. We give attenuation, A, in dB loss per meter, at three frequencies: 100 MHz, 900 MHz, and 2400 MHz. We obtained our data from diverse sources. Sometimes we were forced to interpolate between existing data points to obtain our attenuation values, or to use an on-line calculator. By default, all conductor materials in the table are copper.

Our older laboratory BNC cables are RG-58A/U. As shown in the table above, these cables attenuate by 0.7 dB/m. A 1-m antenna cable gives us plenty of opportunity to place the antenna between two animal cages. Nevertheless, we ordered some longer RG-213/U cables, and compared their performance to those of RG-58A/U cables.

We used the same power-measurement apparatus we describe in our RF Combo Manual to measure the attenuation of cables. Our source of RF was −4 dBm at 910 MHz, provided by an A3016MT. We connected this source of power to the RF input of a ZAD-11 mixer with our test cable. For our LO we used a +10 dBm 864 MHz A3016SO. The IF amplitude and power are given below. For the raw data, download this.

Figure: Intermediate Frequency Power vs Cable Type and Length. Blue graph is for RG58C/U, pink graph is for RG213/U. RF source is −4 dBm at 910 MHz. Mixer is ZAD-11 from Minicircuits, with 7-dB insertion loss at 910 MHz. LO is +10 dBm at 864 MHz.

There must be more than just attenuation taking place in the cables. Our IF power is lower for our 12-inch RG58C/U cable than it is for our 36-inch RG58C/U cable, and almost equal at lengths 12 inches and 132 inches.

Nevertheless, we conclude that we can expect a power loss of no more than 3 dB as a result of inserting a 96-inch RG58C/U cable between the mixer and the antenna. Given the flexibility and range of the 96-inch cable, and the small power loss, we see no reason to use a shorter cable with the A3015.

Impedance Matching

The coaxial cable we use to connect the antenna to our receiver or transmitter circuit presents its own impedance to the antenna. The most common values for coaxial cable impedance are 50 Ω and 75 Ω. If we know the polarization of our incoming signal, and the signal is weak, we benefit from matching our antenna and cable impedance closely. The close match allows us to extract more power from the antenna, or deliver more power to it. But if we do not know the polarization of the incoming signal, a poor imedance match makes the antenna less discriminating. It's maximum sensitivity drops, but its minimum sensitivity rises. In the case of loop antennas, such as the A3015A or A3015B, the mis-match between the cable and antenna dampens the asymmetric resonance of the loop in the vertical direction, which would otherwise render the loop insensitive to horizontally-polarized waves. We present measurements of the maximum and minimum sensitivity of various antennas here.

Antenna Combiners

[20-FEB-16] An antenna combiner takes the signals from several antennas and adds them together to produce one combined antenna signal. The simplest combiner we can use is a BNC T-junction, like this, to join two antenna cables together. The T-junction is inexpensive and readily available. But the T-junction antenna combiner has two disadvantages. When we connect two cables together directly, we lose at least half of our signal power. And we can destabilize the antenna amplifier as well, because the signals propagating along the cables reflect from the T-junction. Our Data Receiver (A3018) has one antenna input, a deficiency of gain, and a tendancy to oscillate. The T-junction combiner does not work well with its antenna amplifier.

Better than a T-junction is a passive combiner that adds the two antenna signals together withoug loss and without reflections. The Antenna Combiner (AC4A) is a four-way combiner made our of a ZB4PD1-2000, which is a four-to-one passive power combiner, two BTRM-50, which are 50-Ω terminators, and a short coaxial cable.

Figure: The AC4A Four-Way Antenna Combiner. Any unused inputs to the power combiner we terminate with the fifty-ohm terminators.

Each of our faraday enclosures contains at least one antenna. We can connect four such antennas to a single receiver input with the AC4A antenna combiner. Or we could use a ZAPD-1 to connect two antennas to one receiver input. These passive combiners attenuate our antenna signal less than 1 dB. But if we are going to have a circuit combining antenna signals, we can add amplification to the combiner as well, which iw shat we have in the Active Antenna Combiner (A3021B), which amplifies each of four inputs before adding them with a 4-way passive combiner.

Figure: The A3021B Active Four-Way Antenna Combiner.

The Octal Data Receiver (A3027) has eight antenna inputs, each of which provides ample gain and is unconditionally stable. We can combine antenna signals with T-junctions and see no significant degradation of reception. We present a comparison of passive, active, and T-junction combiners in the antenna chapter of the A3027 manual. The implication of these tests is that a T-junction joining two antenna cables is just as good as any other combiner, but three T-junctions joining four antenna cables is significantly less effective than an AC4A or A3021B combiner. Thus we should feel free to connect up to 16 antennas to an Octal Data Receiver (A3027) using eight BNC T-junctions.


Using the power-measurement apparatus described above, we mixed 864-MHz LO with 910-MHz RF to obtain a −14 dBm IF signal. We passed the RF signal through a 240-cm cable on the way to the mixer. We unplugged the cable from the RF source and inserted an 80-mm whip antenna into its BNC socket output. We plugged an A3015A Loop Antenna into the free end of the cable that had previously been connected to the RF source. At range 1 m, we observed transmission from the whip antenna to the loop antenna. We rotated the A3015A loop antenna in all directions, and our IF signal varied from −46 dBm to −56 dBm. Transmission and reception across 1 m presents a minimum loss of 32 dB with respect to direct connection. In other words: we receive 0.1% of the power available at the RF source.

We discuss transmission efficiency here. To the first approximation, our loop antenna gathers all RF power that enters its effective aperture. At range 100 cm, the power radiated from our quarter-wave antenna is distributed over a ±45° solid angle perpendicular to its length. This solid angle has surface area roughly ½4π1002cm2 ≈ 60,000 cm2. The effective aperture of a well-terminated loop antenna is roughly equal to its diameter squared, which in the case of our 900-MHz A3015A loop, is 100 cm2. We expect the power loss going from the RF source to the antenna to be roughly −28 dB.

But our loop antenna is not well-terminated. It is terminated with 50Ω when its own impedance is 100Ω. We discuss poorly-terminated omni-directional antennas here. Our loop antenna is 3 dB less sensitive in one direction than a well-terminated loop, but 6 dB more sensitive in the perpendicular direction. With our poorly-terminated antenna, we expect the one-meter transmission loss to be −31 dB. We observe −32 dB.


[17-NOV-10] Our original A3015A loop antenna had no attenuator between the antenna and the coaxial cable. This antenna perfomed well out in the open, but reception was intermittent when we used it inside faraday cages. It took us a year to identify the cause of poor reception, but when we did so, our observations were repeatable and compelling.

We had been going into the OSI office every morning to work for an hour on intermittent data corruption and reception problems. One this particular morning, we arranged the cables and observed poor reception from the faraday cage. We exchanged the data receiver, moved the transmitter within the cage, re-seated the cage lid, and removed the antenna combiner, but always we observed poor reception from within the cage. We began a series of experiments with coaxial attenuators, which we can insert into the antenna system at any BNC junction.

FE2B Lid Off, antenna-coax-wall-coax-receiver69%
FE2B Lid Off, antenna-coax-wall-3dB-coax-receiver94%
FE2B Lid Off, antenna-3dB-coax-wall-coax-receiver94%
Empty FE2A, antenna-6dB-coax-wall-coax-combiner-receiver
Transmitter in Open, antenna-3dB-coax-combiner-receiver59%
Table: Effect of Attenuators Upon SCT Reception. We acquire the signal from a single A3013A in various configurations of antenna, antenna combiner, and of two faraday enclosures: an FE2A and an FE2B. The receiver is an A3018C. When we give a value in dB, we are indicating a coaxial attenuator. When one row is a repetition of another, we are repeating our earlier measurement. The transmitter is in the FE2B enclosure.

These results convinced us that we should add an attenuator to the A3015 circuit board. We see that 3 dB is always adequate. Any larger attenuation would weaken our signal without any benefit from better impedance matching.

[14-DEC-12] We purchased ten 1.8-m RG-58/U BNC cables from Jameco, part number 205-527BK. We connected five of these together to make one 9-m cable. We applied +22.0 dBm (8 V p-p) of 146 MHz from our Command Transmitter (A3023) to one end, as measured by our 300-MHz oscilloscope. The signal at the other end was 18.5 dBm (5.3 V p-p). We lose 0.39 dB/m, which is three times the 0.11 dB/m we expect for RG-58/U from our table of cable properties. We replace the 205-527BK with 1.8 m of our Amphenol RG-58C/U and get 20.9 dBm out the other end, for 0.12 dB/m, which is slightly less than the 0.16 dB/m we expect for this type of cable.

[19-FEB-16] An antenna that detects transmitters only up to ranges of one or two centimeters would be useful as part of an animal tracking system. A video camera above the animal cage can monitor movements of blobs, each blob being one or more animals moving or resting together. When a blob is near the short-range antenna, we receive data from the transmitters implanted in the animals, and so we can identify which animals are in the blob. We took an A3015B and cut the wire back until it was a loop of diameter 10 mm. We measure reception versus range with three different attenuators placed in series with the antenna cable. All measurements are within an FE2F faraday enclosure. In our first test, we place the transmitter in 50 ml of water to simulate imlantation in a 50-g rodent. The antenna is vertical.

Figure: Reception versus Range for Short-Range Antenna. The transmitter is standing up in 50 ml of water. We have various attenuators in series with the cantenna cable. We move the transmitter straight back (0d) and sideways (90d).

Reception does not always increase as range decreases: reception dead spots cause drops in reception even when the transmitter is near. We pour the water out of our 50-ml beaker and repeat, making sure the transmitter is still standing up and oriented identically.

Figure: Reception versus Range for Short-Range Antenna. The transmitter is standing up in air. We have various attenuators in series with the cantenna cable. We move the transmitter straight back (0d) and sideways (90d).

[20-AUG-19] When our transmitters are operating within an aquatic chamber, we use shorter antennas to receive their signals. We applied +0 dbm of 915 MHz to an A3015C antenna vertical on our bench top and measured the power received by various antennas in air and water using our handheld spectrum analyzer.

Figure: Prototype Aquatic Antennas. From left to right: 20-mm Whip, 50-mm Stiff Loop, 50-mm Stranded Loop (A3015D), and 33 nH Inductor.

We summarize our measurements in the table below.

AntennaOrientationPower In Air (dB)Power Water (dB)
BNC Plug OnlyVertical−70−57
A3015C Damped LoopVertical−27NA
A3015D Aquatic LoopVertical−32−39
A3015D Aquatic LoopAll−41..−54−38..−51
20-mm WhipVertical−55−40
20-mm WhipHorizontal−46−54
50-mm Stiff LoopVertical−41−42
50-mm Stiff LoopAll−38..−43−41..−48
Table: Power Received by Various Antennas in Various Orientations and Medium for +10 dBm applied to A301C at range 30 cm.

The Aquatic Loop (A3015D) placed in a beaker of water gives us robust reception of transmission from an A3028E at range 30 cm in a Faraday Enclosure. When operating in an aquatic chamber to receive signals from an aquatic animal, we recommend the A3015D.

[14-MAR-22] We place an A3015C loop antenna on center of the floor of an FE2A enclosure. We support another A3015C antenna outside the enclosure, in front of the door, at a range of 100 cm from the first antenna. We connect +10 dBm to the external antenna through 2 m of coaxial cable. The two antenna are facing one another. The internal antenna is upright, as defined by its mounting brackets. At first, the external antenna is at 90° to the internal. The internal antenna we connect to the back wall of the enclosure, and then to our hand-held spectrometer. We apply 400-3000 MHz and for each frequency measure power received by the internal antenna with the FE2A door open. We repeat with the external antenna at 0° (also upright).

Figure: Power Received (dBm) versus Frequency (MHz). Two A3015C separated by 100 cm facing one another, rotated by 0° and 90°.

In the 900-930 MHz band we see −27 dBm with the antennas aligned, and as little as −40 dB with them un-aligned by 90°. This difference of 12 dB suggests that the A3015C efficiency perpendicular to its face varies by 6 dB as we rotate about the axis of the loop. At its best, we see −25 dBm at the receiving antenna. Given that we have a 3-dB attenuator in both antennas, we have at most 7 dBm radiated by the external loop and at least −22 dBm at the receiving loop. We are receiving 1/800 of the power transmitted. The effective area of our receiving antenna is of order 1/800 of 13 square meters = 12 cm diameter, which is roughly the diameter of the A3015C.

[17-MAR-22] We construct the following prototype Filtering Loop Antenna out of an A3015C, a CBPFS-0915 in-line filter, SMA-BNC adaptors, a 1-m RG58C cable, and a BNC elbow.

Figure: Prototype Filtering Loop Antenna.

[21-MAR-22] We compare the filtering loop antenna to the damped loop antenna in our Faraday canopy. Nathan writes, "I ran some tests comparing the loop antenna performance with and without the saw band pass filter. I found that on both the ODR and the TCB the saw band pass filter being connected to the loop antenna made no difference in reception." At Marsaille, where we are suffering from a 1 kW LTE base station ten meters away on the roof, which transmits in 1850-1910 Mz as well as 832-862 MHz. We find that the Damped Loop Antenna attached to an Animal Location Tracker's auxiliary input provides dramatically improved reception compared to the coil array alone. Reception rises to 97% in one enclosure and 92% in another. When we switch to the Filtering Loop Antenna, by inserting the CBPFS-0915 in the antenna cable, reception improves further to 98% and 94%.