Antenna (A3015)

Contents

Description

The Antenna (A3015) is a circuit board with a BNC socket and angle brackets that supports an antenna. The board provides two holes for the signal and ground connections of the antenna.

Figure: 915-MHz Loop Antenna (A3015A). The cable is RG-58A/U.

We define the following 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.

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.

Cable Properties

The following table summarizes the properties of common coaxial cables.

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 attenuition values, or to use an on-line calculator. By default, all conductor materials in the table are copper.

Cable Attenuition

Our standard 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 attenuition 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 attenuition 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.

The Loop Antenna will be poorly matched to both these cables. When using the Loop Antenna to receive signals from freely-moving transmitters such as our Subcutaneous Transmitters, the mis-match between the cable and the antenna dampens the asymmetric resonance of the loop in the vertical direction, which would otherwise render the loop insensitive to horizontally-polarized waves.

We base our statements upon our own work with antennas, which we describe in detail here.

Reception

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.