17 December 2005

asking the experts

Although I have learned much about shortwave listening, there are still things I haven't figured out entirely. I compiled a list of questions and am seeking answers from my knowledgeable readers. You're also welcome to ask additional questions in the comment section. Later on I will try to update this entry with the best questions and answers, and I'll credit each contributor. Be sure to leave your name, call sign, or alias if you wish to be credited.

Note that I'll be away for the last two weeks of December 2005. See you next year!

1) For introductory purposes, how long have you been listening to shortwave radio?

2) Can you describe what chuff means? This word is used in shortwave receiver reviews, and involves tuning a digital receiver, but I can't determine the exact meaning.

3) How should fading be accounted for when determining an SIO code? Is fading caused by interference?

4) How are the electronics of a dual-conversion receiver different from a single-conversion receiver?

5) Can you explain how images (reception of a signal on the wrong frequency) occur? What bands can be affected by images?

6) Why is it that a single sideband signal needs to be fine-tuned?

7) Can someone describe the difference in the sync detect between the Sony ICF-2010 and the SW7600GR?

8) Is there any way to determine how many atmospheric bounces a received signal has made? I know that an atmospheric bounce will introduce attenuation, so a signal that has bounced more will be weaker. Are there any other identifying characteristics?


Anonymous said...

1.Too many years: the '60's to start.
2 Chuff is by my understanding, the sound made by going up or down with a button control on a digital receiver.
6.Because 2 Ssb signals operate on one freq.(say 28:335)it splits in half,unlike one signal,with Am. That's why it needs to be clarified.

Tom VE3MEO said...

1) intermittently for over 40 years
2) receivers that use a digitally controlled (that does not mean fingers, rather microcontroller or microprocessor controlled) oscillator are typically muted on each step change in frequency. If not muted, there may be a click. Because there is always noise present in the received spectrum, even if there is no desired signal, the audio output seems to puff or chuff like a steam engine as the receiver is scanned or tuned.
3) No answer re SIO except to suggest you should report an average observed over a minute or so. In that case the S average would account for fading. Fading is caused by absorption variations and by interference between the multiple paths between transmitter and receiver.
4) The electronics are very similar except that the double conversion receiver has at least one additional oscillator and mixer and one oscilator may be fixed frequency.
5) When a broad spectrum is heterodyned with the local oscillator, the output of the mixer will contain the original spectrum, and that shifted up by the oscillator frequency (sum) and that shifted down (difference). The following IF filter frequency will match a source frequency of the same frequency, one that was shifted up and one that was shifted down. The ideal receiver would have filters that pass only one of these but practical radios allow some of the unwanted ones through. Of the two shifted frequencies that come out at IF, one is the desired one and the other is the image. All bands can be affected by images but the intensity of interference depends on the strength of the image frequency signal before any filtering and the attenuation of the filters. Generally, attenuation of image frequencies is better at lower frequencies than higher because receiver bandpass filters of a given complexity are inherently narrower at the lower bands and better attenuate the image frequency energy.
6) Most SSB signals are transmitted with the carrier suppressed. Suppressed carrier signals are not correctly demodulated by an envelope detector (the normal and most common AM receiver mode) in the absence of a carrier. Switching to SSB mode turns on an internal oscillator at or near the Intermediate Frequency that is mixed with the SSB signal to produce difference frequencies with the sideband in the audio frequency range. The sideband and the Beat Frequency Oscillator must be aligned so that the BFO frequency is close to where the missing carrier would be, relative to the sideband, to "clarify" the audio, i.e., minimise frequency distortion. Because the error must be less than a few tens of Hertz for intelligibility, a "fine tuning" or "clarifier" control is required.
7) No can answer
8) No, or not without control of the transmitter and special modulation. One might estimate, based on assumptions about the probable elevation of the probable reflecting layer of the ionosphere. Note that these are not atmospheric reflections or refractions but ionospheric. There are ionospheric sounders that operate like radar to measure the altitude and frequencies of vertical reflections. There are also computerised propagation models that attempt to predict what paths may be open for different frequencies and estimate path loss for different hop lengths. A given path may support multiple hop lengths (e.g., 1, 2, 3... hops) and that gives rise to the multipath interference discussed earlier and consequent fading. There is one transmission mode and decoding software that I am aware of that give a fascinating representation of the delay between the primary and lesser paths - the mode is DRM and the software is Dream.