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March 16, 2013

Comments

Gary

This is related to FM capture effect. With FM, you're only supposed to hear the stronger of two signals on the same frequency. This is in contrast to AM, where many signals can be heard at the same time on the same frequency.

There is a limit to the FM capture effect, and it's measured by the capture ratio, which is the number of dB that one signal must be stronger than the other for only the stronger signal to be heard. If the signals are closer in strength than the capture ratio of the receiver in use, then both will be heard. It sounds like this is what's happening in your case, and it may not be easy to resolve with a different inexpensive portable radio.

If you move location slightly, this will probably change the strength of the two signals enough for one or the other to become dominant.

Ante

Jeff,
Maybe the new CC POCKET radio would do the job? I heard it has better selectivity than SANGEAN DT 400?

herculodge

Yeah, I'm thinking of getting one.

Gary

The CC Pocket may have better selectivity on FM than the DT-400W, but this is not relevant for separating signals on the same frequency. Selectivity relates to separating signals on adjacent frequencies. For this particular situation, what is needed is a lower capture ratio, not better selectivity.

herculodge

Gary, can you recommend a portable lower capture radio?

Gary

Jeff - Unfortunately, capture ratio doesn't seem to be mentioned in the specifications or reviews of portable radios today. If the two FM signals you're hearing are very close in strength, then it will be very hard to separate them. With a stationary radio, the solution is a directional FM antenna, but this is of no help with a walk-around portable.

In the past, capture ratio was specified by manufacturers for their component FM tuners and receivers. For example, this Pioneer ad in the September 1975 "Popular Science" indicates a capture ratio of 1 dB, which is quite good.

http://books.google.com/books?id=RwEAAAAAMBAJ&pg=PA17&lpg=PA17&dq=Pioneer+%22capture+ratio%22&source=bl&ots=aIN094HpaY&sig=tE6UMEYq8Qts4nF4ZYcDJ6x_s0g&hl=en&sa=X&ei=NxlGUeTRNonnqgHspIHoDQ&ved=0CDEQ6AEwAA#v=onepage&q=Pioneer%20%22capture%20ratio%22&f=false

Gary

Jeff - I had sent a more detailed post, but I guess it got lost somewhere. I don't know which portables have a lower capture ratio, as this isn't something that seems to be indicated in the specifications or mentioned in reviews these days. Perhaps one of the DSP radios would do better in this regard, but I'm not sure. I get the impression that the CC Pocket is DSP-based, so it might work better than what you have. I guess the only way to know for sure is to try one.

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