LCL, Radiation, and Radiation Resistance

Transcription

1 LCL, Radiation, and Radiation Resistance Some thoughts on how to determine antenna or cable radiation, ideas which may not necessarily represent the opinion of IARU At first we should have a look at Fig 1, showing the so-called T-ISN, a circuit to measure the LCL of telecommunications lines. The attenuation between the symmetrical voltage between points a1 and a2 and the voltage at the measuring output BNC1 (-9.6 db) is the LCL and is expressed in db. A high LCL indicates that the line currents are highly symmetrical, therefore any radiation from the line should cancel. But when we employ a dipole for the termination of the line, as shown in Fig 2, the line will not radiate either, but the total system will! What is the difference? In Fig 1 the terminating resistors are the usual ones converting the line current into heat. In Fig 2 the terminating resistor is the so-called radiation resistor of the dipole which converts the line current into electromagnetic radiation! And what will happen if the line itself also contains radiating resistors? Then the line will radiate too, in spite of the symmetrical currents at its input! Even if a power line may be treated as a symmetrical line, for HF it can be assumed that its equivalent circuit diagram will contain a mixture of normal resistors and radiation resistors, as illustrated in Fig 3, to explain its radiation capabilities. Therefore we feel that the application of a T-ISN (or other ISNs which have appeared in the work of CISPR/I for PLC) for the assessment of line radiation should be restricted to cases where radiation resistances within the line can be neglected (e.g. to old homogeneous telephone lines only).

2 How we can explain radiation of a power line? Fig 4 shows a computer simulation to demonstrate how a stub in the mains line will change the phase relation within a power line from pure symmetrically to equal phases, forming an effective radiator. Fig

3 4 makes also clear that a T-ISN at the input of a mains line should be absolutely unable to detect any radiation-efficient structures inside the mains line. Fig. 4, see text Radiated power from an antenna can be expressed as radiation resistance times the square of the antenna current. The same relation can also be used to describe the conversion of field strength into antenna current in a receiving antenna, because there is a reciprocal relationship between receiving and transmitting antennas! Therefore we can also use communication lines like receiving antennas. This test will show their RF capabilities over their total length, not their behaviour at the input only. The better reception of a specific transmitter will be, the higher is the radiation resistance of the line. The following pages shows adaptors necessary to connect the lines to the measuring equipment and also some preliminary measuring results compared to a broadband receiving antenna. Measurements were done in a rural one-family house built around As expected the receiving properties of telephone lines are rather poor (with some exemptions possibly due to the specific house telephone installation). But the attenuation between the broadband antenna and a mains line as antenna have been found to be 25 db +/- 13 db and 20 db +/- 8 db respectively! The variation of +/- 8 to 13 db is in line with Fig 4 showing that some frequencies are received with rather low attenuation depending on the phase shift action of the stubs in the mains line. These measurements should make clear that power lines are moderate radiators, and therefore any LCL measurements should be abolished and a radiation resistance or another realistic term should be assigned to them. Otherwise the old AMN or LISN

4 should be used again to assess their radiation, which has limited mains radiation rather effectively over the last decades. This attenuation of about 20 db average between an outside antenna and the mains line has indirectly been confirmed by a document from ETSI PLC on dynamic notching [1]. In this document a comparison has been made between the external receiving field strength of 67 broadcast stations in db(uv/m) and their so-called HF ingress power in dbm in the mains line of buildings. A direct comparison between dbm and db(uv/m) is not possible, of course. But from the fieldstrengths given the receiving power which a half wave dipole will deliver into a load can be calculated and expressed in dbm and used for comparison. Note: For exact measurements the coaxial switch used to change between different antenna sources should have good attenuation or screening between inputs!

5 First measurements: Attenuation between broadband antenna, mains antenna, and telephone line antenna. The signals received are mainly broadcast carriers which could be received at the ends of the 500-kHz-ranges of an amateur transceiver in CW mode. A certain inaccuracy due to fading has to be accepted; measurements were always aimed to be made at the maximum of the signals. Receive Freq. khz Antenna S-level Mains S-level Attenuation db Telephone S-level 1539 S9+5dB S6 23 S S9+5 db S7 17 S6 23 Attenuation db 3855 S9+10 db S8 16 S S9+30 db S S7 42 db 4005 S9+5 db S7,5 14 S2-3 ~ S9+15 db S7 27 S S9+15 db S7 27 S S9+20 db S6 38 S S9+10 db S7 22 S2-3 ~ S9+30 db S8 36 S S9+10 db S6 28 S S9+20 db S8 26 S S9+10 db S6 28 S3-5 ~ S9+10 db S5-6 ~41 S2-3 ~ S9+30 db S9+5 db 25 S7-8 ~ S9+15 db S6 33 S1-2 ~ S9+10 db S6 28 S2-3 ~ S8-9 S S S9+5 db S9 5 S1 53 S9 = -73 dbm 1 S-step = 6 db More measurements: Attenuation between broadband antenna and mains antenna only, measured with a commercial level meter of 50 ohms input. All db figures are relative. Frequency khz Antenna db Mains db Attenuation db

6 References : [1] ETSI PLT44_15r1 pages 18 and 19