Section 300 Content Consideration
This section contains examples of nearly every type of antenna and transducer used in not only the EMC arena but also other disciplines. When choosing your antenna be aware that they need to be supported by some form of mount which can be found in section 800. Even if you have your own mounts do check that the coupling interface is compatible. This can also mean checking to make sure that your antenna can be swung through ninety degrees for a polarisation change without the elements striking some part of the mount. Having decided on whether you require an antenna to just receive or transmit or do both, then read the full description and specification of that antenna on our website or in the main catalogue/CD. In the description of each antenna, we describe the type of mount interface. If you have no (or unreliable) coaxial cables, we provide a comprehensive list of varying lengths & appropriate types & adaptors in Section 700. Most are supplied with calibration data for insertion loss. Generally, the end user need only be concerned about loss when working above 30MHz as a rule.
Section 300 is broken down into five subsections. The first of these is Low-Frequency Magnetic Field covering from DC up to 30MHz. Many of the devices in this subsection will have the capability of handling a certain amount of injected power and then radiate mainly for immunity and shielding effectiveness testing. These types are likely to have an impedance matching switch to reduce the mismatch from the 50O o/p Z of the power amplifier to the very low i/p Z of the loop. All loop antennas are clearly described as to whether they can be used to transmit or not. Of the receiving group you have the choice between active and passive loops. Be aware of the orientation of the loop to give a maximum. Although all magnetic readings are read as a voltage across the 50O i/p of a receiver, the antenna factor will convert this to amps per metre. Generally, all loops are shielded against the electric component by enclosing the conducting loop within an all-metal shield. This ensures that it is just the magnetic component of the radiated energy that is responsible for producing an output voltage. Due to the relatively long wavelengths in this band, and hence your distance from the source, the 377 Ohm relationship to the “E” field will not hold true and you may have to measure the “E” field separately with a high impedance rod. However, the magnetic component of an EM field is a fairly steady component to work with being somewhat unaffected by moving conducting (non-ferrous) bodies in the vicinity. In both the LF magnetic as well as the electric group, sets of transmit & receive antennas can be hired as a single item.
The Low-frequency Electric Field similarly covers the range DC to 30MHz. The DC coverage is provided by the 2ft Crawford Cell which has a useful range up to 300MHz, otherwise, most devices are in the form of 41 inch whip rods and parallel elements. Again, decide if you are going to transmit or receive. If receiving, decide on whether the antenna is to be active which has the effect of reducing the antenna loss factor to a few dBs or whether it is necessary to have straight coupling via a matching balun. Be aware of the overload problem of active antennas and some baluns. Very few of the antennas will have a full bandwidth from 10kHz through to 30MHz, generally they are switched at about 5MHz. This, unfortunately, prevents an automatic program making a full uninterrupted sweep as the antenna would have to be switched mid-way. If a series of readings are being taken over different terrains or locations then use a ground-plane (counterpoise). This can be attached to the base of the rod. The RVR-25A/30 series were designed for the EMC-30 receiver and have a coupling cable to the receiver which switches the matching baluns by octaves. However, they can still be used independently and switched manually to cover the ten-octave bands. There is no polarisation change to make with 41 inch rod antennas, they are all operated in the vertical position.
The HF-VHF sub-section deals basically with biconicals but also embraces the combination of Biconicals & Log Periodics known as “Bilogs”. The frequency embraces 26MHz-3GHz. Be aware of power handling capabilities and mounting fixtures. Although all devices in this range are passive some are specifically for transmitting & others for receiving. Included in this group and the microwave group are sets of two antennas of the same type which can be hired under a single item number making it more economical. Generally, all the antenna factors supplied in the handbooks are for a 3 metres range over a conducting ground plane. Some of the “bilogs” have a set of 3 metre free space figures for FAR’s. See individual specifications for more details. All the bicons come with collapsible elements which assist in packaging and shipping. A similar sub-section VHF-UHF contains mostly dipole sets covering up 1GHz.
Finally, the UHF-Microwave sub-section. This embraces frequencies from 200MHz upwards, the section houses all the Log Periodics, Double Ridge Guide and horn antennas. When selecting microwave antennas you may well need to hire the appropriate length of microwave cable otherwise losses will be considerable. A selection of microwave adaptors are also available (Section 700). In previous years we offered a group of “Site Attenuation” antennas with the appropriate calibration data. This has unfortunately been withdrawn as we could no longer rely on the maintenance of the UKAS calibration after unverifiable usage & carriage. However, under section 900 we now offer this service using the RSM procedure involving scanning the NPL groundplane and transferring this to your site for comparison.
Included with this section is an “E” Field “sniffer” with a range from 10kHz to 1GHz. This is used for finding leaks around shielded doors & I/O panels.