This is an excerpted comment from the August 2014 QST review of the Tecsun PL-880 portable radio. Those who are ARRL members can look at the whole article. NOTE: The over-all review was very favorable with the only major knock on the metering for signal strength and S/N which makes me wonder about the versions commonly refered to here. See the below comments:
"The signal strength was optimistic, not close to S-9 with a -73 dBm signal applied. The signal to noise (S/N) meter readings were pessamistic with a 10 dB S+N/N input signal resulting in a 0 dB indication on the display."
The statement "The signal strength was optimistic, not close to S-9 with a -73 dBm signal applied" doesn't seem to make any sense. The PL-880 displays signal strength on a scale of 1 to 99 in units of "dBµ". It doesn't display in S units. The manual makes no claim as to the accuracy, so the implication is that it is simply a relative measurement despite the seemingly precise use of the "dBµ" label.
Even if you forget about "S-9" and use the definition that S-9 equals -73 dBm, how did they apply a -73 dBm signal when the input impedance of the PL-880 is not specified in the manual? It's most certainly not 50 ohms because the manual recommends attaching an unspecified length of wire to the antenna jack. This implies that the antenna jack is intended for a long wire antenna, not an antenna cable of any specific impedance.
Is there any more information in the article that would help to clarify this?
... how did they apply a -73 dBm signal when the input impedance of the PL-880 is not specified in the manual? It's most certainly not 50 ohms ...
The "S" meter of many communications receivers is calibrated to show about S9 when the power measured across two terminals in the receiver is -73 dBm -- considering whatever impedance exists across those two terminals.
A -73 dBmW (73 dB below one milliwatt) signal produces 50 µV across 50 ohms. But that 50 µV is not a measurement of the arriving field intensity, which commonly is measured in units of µV/m.
Measuring field intensity accurately requires knowledge of the gain of the receiving antenna at the frequency of interest, which is an unknown in all but professional field intensity measuring equipment costing thousands of dollars.
If for a frequency of 1650 kHz the loopstick antenna in a Tecsun or similar radio had a gain 50 dB below that of a 1/2-wave dipole (about realistic?), then the E-field intensity arriving at that loopstick antenna would need to be about 657 µV/m in order for an S meter circuit in a receiver to show S9.
That is an error of 22 dB between the "50 µV" S9, and the arriving field intensity needed to produce that S9 reading on the receiver.
A website catering to people interested in "Ultra-Light" radio receivers such as the Tecsun PL-310 has a post describing an interesting test of the linearity of the dBµ ("signal strength") indication shown on the front panel display of such receivers.
The poster took the case off the radio and disconnected the wires from the loopstick antenna at the circuit board. Then he connected those terminals to an external signal generator having a calibrated attenuator at its output.
The poster found that the linearity of the display on the receiver tracked very well with the changes he made in the attenuator of the signal generator. IOW, an input level change of 10 dB produced a display change of 10 dB on the Tecsun, etc.
That is useful information, but does not mean that such receivers display the true value of the arriving radio wave, in decibels with respect to a field intensity of 1 µV/m.
Good Afternoon Friend Phil,
I forwarded your comments to the engineer that performed the test at the ARRL Lab. As soon as he gets back to me I will share his comments with the group.
John
Rich: I shared your observations as well.