Loop Antenna (A3015)

© 2006-2016 Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Design
Antenna Combiners
Cable Properties
Cable Attenuation
Impedance Matching
Reception
Efficiency
Short-Range Antenna

Description

The Loop Antenna (A3015) is a circuit board with a BNC socket, support brackets, an antenna, and an attenuator. In the A3015A and A3015B, we make the loop antenna out of stiff, insulated wire that we solder into the holes on the circuit board. In the A3015C, the loop is integrated into the printed circuit board. We designed the A3015 for use with our Subcutaneous Transmitters. Most antennas are designed to have the greatest sensitivity in their most sensitive direction. But the A3015 is designed to have the greatest sensitivity in its least sensitive direction.


Figure: 915-MHz 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 A3015 provides mounting brackets that serve to hold it vertical when resting on a horizontal surface, or allow it to be screwed to any other surface. The photograph below shows the mounting brackets of the A3015C as seen from above.


Figure: A3015C Top View, Showing Mounting Brackets.

The A3015 provides an attenuator between the antenna and the coaxial cable. By default, this is a 3-dB attenuator. It absorbs half the power picked up by the antenna and allows the other half to proceed to the coaxial cable. A 3-dB attenuator ensures that the antenna presents a well-behaved load to the antenna cable. The attenuator greatly improves the performance of the antenna in highly reflective environments such faraday enclosures. See the Impedance Matching section below for measurements.

We have the following versions of the A3015. The A3015A is identical to the A3015B, but with a 0-dB attenuator instead of a 3-dB attenuator. The A3015A has problems operating within faraday enclosures, as we describe below.

VersionNameShapeImpedance
A3015A915-MHz Loop AntennaCircumference 320 mm, No Attenuator102 Ω at 915 MHz, reactive or inductive otherwise
A3015B915-MHz Damped Loop AntennaWire Loop 320 mm, 3-dB Attenuator70 Ω at 915 MHz, close to 150 Ω otherwise
A3015C915-MHz Damped Loop AntennaPCB Track 320 mm, 3-dB Attenuator70 Ω at 915 MHz, close to 150 Ω otherwise
Table: Versions of the A3015.

The different antenna shapes are sensitive in different directions and to different polarizations. Their behavior is dictated not only by their shape, but also 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 impedances makes the antenna more sensitive in some directions, but much less sensitive in others.


Figure: A3015 Schematic. The Sketch is of the A3015B.

The A3015A uses two L-shaped brackets with holes to provide support on a level surface, and to allow bolting to any flat surface. We use part numbers 15275A51 and 15275A53 from McMaster-Carr.

In the following discussions, we concentrate upon the use of the A3015 as a receiving antenna for our 915-MHz Subcutaneous Transmitters. The transmitters reside in the bodies of animals, and transmit out of the animal and the animal cage to an antenna connected to a receiver such as our Demodulating Receiver A3005C.

Design

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.

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.

Cable Properties

The following table summarizes the properties of common coaxial cables.

Type Impedance
(Ω)
Diameter
(mm)
A 100 MHz
(dB/m)
A 900 MHz
(dB/m)
A 2400 MHz
(dB/m)
Comment
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
RG-58C/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, but in each case we give a link to one of the 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.

Cable Attenuation

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.

[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.

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 A3013A or A3013B), 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.

[17-NOV-10] Our original A3013A loop antenna had no attenuator between the antenna and the coaxial cable. This antenna perfromed well out in the open, but had problems when we used it inside faraday cages. It took us a year to track down these problems to the antenna, 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 intermittend 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 connector.

ConfigurationReception
antenna-coax-wall-coax-receiver81%
antenna-coax-wall-coax-2dB-receiver97%
antenna-coax-wall-coax-3dB-receiver99.6%
antenna-coax-wall-coax-6dB-receiver99.0%
antenna-coax-wall-coax-8dB-receiver99.8%
antenna-coax-wall-coax-9dB-receiver100%
antenna-coax-wall-coax-receiver84%
antenna-coax-wall-3db-coax-receiver100%
antenna-coax-wall-coax-receiver86%
antenna-coax-wall-2db-coax-receiver100%
antenna-coax-wall-3db-coax-receiver100%
antenna-coax-wall-6db-coax-receiver100%
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%
antenna-3dB-coax-wall-coax-receiver100%
antenna-coax-wall-coax-receiver88%
antenna-3dB-coax-wall-coax-combiner-receiver
Empty FE2A, antenna-6dB-coax-wall-coax-combiner-receiver
100%
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 gain in impedance matching.

Reception

[18-NOV-10] Today we discovered the importance of shielded cable for the connection between the Data Receiver and the LWDAQ Driver. An unshielded cable picks up interference and greatly aggravates the system. We switched to a shielded LWDAQ cable and performed tests upon this system. We have transmitter No7 in the lower enclosure, close to the antenna, so we are certain of a strong signal. We start with an A3015A with a 3-dB attenuator and record the number of messages in channel No7 each second, as well as the number of bad messages per second. After 200 s we remove the attenuator and record for another 200 s.


Figure: Effect of Attenuator. We remove the 3-dB attenuator in the antenna at time 200 s. Accepted messages in channel No7 are plotted on the left axis. We expect 512 per second with no interference. Bad messages, which are those received in other channels when no such transmitters are active, are plotted on the right axis.

In the graph we see interference coming and going as some external source of 915-MHz power turns on and off. Interference generates bad messages. When we remove the attenuator, we see ten times as many bad messages during interference, and ten times as many such messages being accepted into the transmitter data.

The following series of tests explore the effect of attenuators, terminators, and interference. We start with the system shown here, with 3-dB antenna attenuators. In each case, we calculate reception by taking the average of ten one-second intervals. When we show the number of bad messages, we wait until the bad message rate jumps up and record at that time.

TestConfigurationReception
1No7 in FE2A near antenna99.3%
2Disconnect FE2B97.1%
3Terminate open socket99.3%
4Remove 3-dB antenna attenuator100%
5Restore 3-dB and rotate antenna loop97.2%
6Move transmitter99.7%
7Remove terminator99.6%
8Reconnect FE2B99.7%
9Remove 2 terminators on unused combiner sockets94.9%
10Restore 1st terminator99.3%
11Restore 2nd terminator99.9%
12Remove 1st terminator99.5%
13Remove 2nd terminator94.4%
14Restore 2 terminators99.9%
15Remove 2 terminators and 3-dB antenna attenuator99.8%
16Move No7 to FE2B99.9%
17Restore 2 terminators99.9%
18Remove 3-dB atenna attenuator99.9%
19Move transmitter until reception is weak76.6%
20Put 3-dB attenuator at cage wall0%
21Remove combiner and connect directly to data receiver99.7%
22Remove 3-dB attenuator99.4%
23Restore 3-dB attenuator to cage wall99.2%
24Remove 3-dB attenuator, restore combiner67%
25Replace combiner with 6-dB attenuator68%
26Restore combiner, unshielded root cable42%
27Add 3-dB attenuator to antenna, unshielded root cable0%
28Remove combiner, unshielded root cable15%
29Remove 3-dB attenuator, unshielded root cable90%
30Move transmitter close to antenna, unshielded root cable88% + 500 Bad/s
31Add 3-dB attenuator to antenna, unshielded root cable97% + 40 Bad/s
32Remove 3-dB attenuator, unshielded root cable90% + 800 Bad/s
33Restore shielded root cable100% + 100 Bad/s
34Add 3-dB to antenna99.7% + 5 Bad/s
Table: Effect of Attenuators, Shielded Data Cable, and Other Factors. The transmitter is No7. The root cable is the cable from the data receiver to the LWDAQ driver.

Tests 1 to 18 show that when our signal is strong, unused combiner sockets should be terminated. In one case we see reception drop from 99% to 94% because of poor termination. In tests 19 to 25, the transmitter signal is barely strong enough for detection by an antenna without an attenuator. Adding any form of attenuation in the antenna cable causes reception to drop. The combiner affects the signal like a 6-dB attenuator.

In tests 26 to 29, we have a weak signal and an unshielded root cable. Between 24 and 26 the only change is removing the root cable shield. Reception drops from 67% to 42%, which may or may not be significant. But we see no dramatic increase in the bad message rate when our transmitter signal is weak and we remove the root cable shield. With no attenuator and no combiner, we get 90% reception.

In tests 30 to 34 we move the transmitter close to the antenna so that we have a strong signal. We start recording the bad message frequency. The source of these bad messages is suppressed both by the 3-dB attenuator and the shielded cable, and most of all by both acting together.

We conclude that a 3-dB attenuator in the antenna will improve reception when the transmitter signal is strong. When the transmitter signal is almost too weak to receive, the additional attenuator will degrade reception. We plan to overcome the degradation caused by the 3-dB attenuator in the future with the help of an active antenna combiner: the A3021.

The above experiments are closely related to our experiment with shielded and unshielded LWDAQ cables in the presence of a 3-dB antenna attenuator. A shielded cable decreases the bad message by a factor of ten. From the above experiments we see that a 3-dB attenuator on the antenna also decreases the bad message rate by factor of ten. When we remove the attenuator and the shield, the bad message rate increases by a factor of a hundred.

[19-NOV-10] Today we measured reception with random movement on the end of a stick within our FE2A enclosure. Our objective was to determine how the 3-dB attenuator affects reception and to better understand the source of bad messages in the OSI office. We have two transmitters: an A3013A without encapsulation, No7, and an A3019A with encapsulation, No1. We move and rotate the transmitter inside the cage for a minute and calculate the average reception, the robustness, and bad message rate.

TestConfigurationReceptionRobustnessBad Rate
35A3013A, 0-dB antenna, no combiner93%90%<1/s
36A3013A, 3-dB antenna, no combiner93%89%<1/s
37A3013A, 3-dB attenna, combiner84%74%<1/s
38A3013A, 3-dB attenna, combiner, FE2B added87%70%<1/s
39A3019A, 3-dB attenna, combiner, FE2B added78%53%<1/s
40A3019A, 0-dB antenna, no combiner92%82%<1/s
41A3019A, 3-dB antenna, no combiner94%92%<1/s
42A3019A, Turn absorber grey side up88%92%<1/s
43A3019A, Absorber black side up,
put absorber fragments in receiver box
94%93%<1/s
44A3019A, Repeat84%78%108/s
45A3019A, Repeat91%80%234/s
46Remove absorber from receiver. Turn off transmitter.0%0%500/s
47Disconnect FE2A0%0%0/s
48Restore FE2A, 6-dB attenuator at receiver box,
3-dB at antenna
0%0%0/s
49Remove 6-dB attenuator0%0%400/s
50Remove antenna leaving 3-dB attenuator0%0%0/s
51Remove 3-dB attenuator0%0%0/s
52Restore antenna0%0%100/s
533-dB at antenna enclosure0%0%0/s
54Remove 3-dB from antenna enclosure0%0%10/s
Table: Reception and Bad Message Rate Under Various Conditions. Note that the bad message rate varies with time, regardless of the configuration. We see no bad messages until test 44. The transmitter is always in the FE2A enclosure, which has a hole in it through which we can run a stick to move and rotate the transmitter in the enclosure volume. The LWDAQ cable is shielded and only 1-m long.

We see the bad message rate going up and down as we watch without altering the system. We try to wait until we see the rate go up before we make our measurement. Today we see high bad message rates even when the transmitter is off. The bad messages stop when we disconnect the antenna. They stop if we put 9 dB of attenuation between the antenna and the receiver, but not when we insert only 3 dB. Our observations are consistent with powerful interference entering the faraday enclosure and resonating between its walls. The poor performance of the FE2A is evident in our earlier experiments. The FE2A absorber is half as thick as the ones we settled upon for the FE2B, and is on the floor instead of glued under the lid.

[31-DEC-10] We left two transmitters running in two faraday enclosures for three hundred hours. Antennas are A3015Bs with 3-dB attenuators installed. Instead of an antenna combiner, we use a BNC T-junction at the Data Receiver. We obtain the following reception, averaged over each hour with the help Neuroarchiver Analysis.


Figure: Reception with T-Junction Combiner and Two A3015B Antennas. Transmitter No7 is in the FE2A enclosure, No9 is in the FE2B.

We also use analysis to search for periods when reception drops below 90%, and find that this occurs during only one twenty-second period during the three hundred hours. During this twenty-second period, reception drops to zero for both transmitters.

[10-JAN-11] We incorporate an Antenna Combiner (A3021B) into our system. This combiner has amplification before combination. We plug the antennas into the combiner. We do not terminate the unused combiner inputs. We obtain the following reception. We note that No9 has not moved since the T-Junction experiment.


Figure: Reception with A3021B Combiner and Two A3015B Antennas. Transmitter No7 is in the FE2A enclosure, No9 is in the FE2B.

We see that reception is substantially improved with the introduction of the active antenna combiners. We searched for periods of poor reception and found two. We examined the raw data in the Neuroarchiver and found that both eight-second periods of poor reception contain thousands of bad messages.


Figure: One of Two Reception Failures During 100 Hours. Transmitter No7 is in the FE2A enclosure, No9 is in the FE2B. For the other failure, see here.

We see that the antenna attenuator, the active antenna combiner, and the shielded root cable together reduce the frequency of these bad-message failures from once every few minutes to once every few days.

Efficiency

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 RF through a 240-cm cable on the way to the mixer. We unplugged the cable from and insterted an 80-mm whip antenna into the BNC socket on the RF source. 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.

Short-Range Antenna

[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).