The referenced web site makes the well-known point that the S meter of a receiver gives only a relative reading of field strength, and not the actual signal strength. It is not surprising that S units are not absolute signal strength readings, since they are based upon a subjective scale in the RST signal reporting system. S1 means a barely readable signal, and S9 means a very strong signal.
It is, nevertheless, useful to have a receiver with an S meter, or even a bar graph that represents signal strength. By calibrating the receiver using a signal generator, the receiver becomes becomes a very sensitive and selective RF voltmeter. If the relationship between the field strength of a signal, and the voltage induced in an antenna by that field strength, is known, the receiver becomes an approximate field stength meter.
The antenna with a known relationship between field strength and induced voltage, combined with a receiver with a calibrated S meter, would not be good enough for qualifying the performance of a Part 15 transmitter (such as at 13.56 MHz), but it, at least gives the operator some idea of what his field strength is. I use an old Boonton 102C signal generator to calibrate the S meter. The inexpensive field strength meters based on rod antennas, and costing only a few hundred dollars, also do not meet FCC requirements. They are only good for "pre-qualification testing," as is the scheme I am describing here.
I use a shielded loop antenna to pick up the signal. The shield is necessary to reduce sensitivity to the electric field, which would make the induced voltage higher than it should be. The shield needs to have a thin break to prevent it from being a shorted turn coupled to the loop antenna. The loop antenna must be tuned to the operating frequency. There is a formula that relates the terminal voltage of the loop antenna to the field stength, which is:
V = (2*pi*E*N*A)/lambda,
where V is the voltage (in volts) induced by the loop, E is the field stength in V/m, N is the number of turns in the loop, A is the area of the loop in square meters, and lambda is the wavelength in meters.
The weakness of the field strength measuring scheme I am describing is that it is difficult to know what the effect of the input circuit of the receiver has on the induced voltage of the antenna, and the output of the signal generator generator used to calibrate the S meter. So, if, for example, using it for pre-qualifying a transmitter used on 13.56 MHz, it is necessary to set the transmitter to a substantially lower measured field strength than is allowed by Section 15.225. To set to the full field strength, a real, professional quality, field stength meter is necessary, which is not cheap.
Using this method in the AM broadcast band presents a special problem because of the internal loop antenna that is used with most broadcast receivers. The receiver must be made specifically for connection to an external antenna. I have used an ancient general coverage receiver (Lafayette HE-10, which I think is actually a Hallicrafers receiver manufactured for Lafayette) for this purpose.
I remember reading where someone was tuning his Rangemaster on a tower. She was a long ways away with a mobile ham rig watching the S-Meter while chatting with him on a cellphone. He thoulght the results of tuning this way was best.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
The method John cited sounds good.
Here, I use a method which I have used for many years to peak things regarding AM tuning. My xmit ant. is in the basement near my lab bench. I stapled two short wires, about 18" each to the rafters over my bench forming a dipole. In the center, I soldered a RF choke (value not critical, short to 60 Hz., open to RF) to supress 60 Hz. noise and connected my scope tip and ground across the choke. I am near enough to the transmitting antenna that I get many tens of millivolts RF displayed on the scope. It is easy to peak the transmitter and antenna tuning this way, plus I can easily check the modulation on the waveform.
Unlike the description of the calibrated S meter, this method is only relative and doesn't give any clue to the actual field strength.
Neil
"I remember reading where someone was tuning his Rangemaster on a tower. She was a long ways away with a mobile ham rig watching the S-Meter while chatting with him on a cellphone. He thoulght the results of tuning this way was best."
Sounds like the long gone WSJL in Villas NJ run by Bill Blew. I have one of his Hamiltons (Master) My friend who was recently visited has one of WSJL's Hamiltons (Slave) and i don't know what happened if anything to the other 3 slave units that WSJL had.
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree
I think now that mention it, it was him.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager