FM band (Band II) images on short wave on Grundig Yachtboy 400

Short wave receiver discussion
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FM band (Band II) images on short wave on Grundig Yachtboy 400

Post by KPC850 » 27 Sep 2016 15:03

I notice the YB400 receiver I have (radio dating from the 1990s) is prone to 'images' in HF (in the 16MHz-to 26MHz area) from strong FM stations, most prevalent from stations at the high end of the FM band.

The image will of course be a distorted mess (WFM on an AM receiver) - unless the station's modulation was very low, and fine tuning either side of the carrier it may become more listenable.

The effect is most pronounced with frequencies at the high end of the FM band, powerful transmitters from several miles away at the high end of the FM band can easily appear as an image in HF. However stations at the low end of the FM band have to be significantly stronger (nearer) to have the same effect.

considering 55.850 MHz is the 1st IF (intermediate Frequency) for this radio when on HF (and below).

How to calculate where FM images will fall on HF on YB400:
Subtract 55.850 from FM band frequency, then divide by 2.

So taking a strong FM band station on 107.8 as an example:
(107.8-55.85) / 2 = 25.975 MHz

In reverse, if you hear such an FM image on HF on this radio and to work out the FM band frequency:
multiply the HF frequency by 2 , then add 55.85.

The above happens with the internal telescopic aerial, I have not tried the strength of images with the external antenna connection. When using the latter a lowpass filter could be used if desired.

Whilst images are usually undesired, the above info might be useful though if you were trying to find the location of an FM band transmitter, you could leave this radio tuned to the calculated image, and retract the aerial and/or detune the image as the 'image' gets stronger.

The above YB400 was later replaced by the YB-400PE. Also uses 55.85 for 1st IF, but I never tried one to see if it is less prone to these images or not?

I also tried a Sangean ATS 909 (also badged as Radio Shack DX 398 ) radio for FM band images . It was less prone than the YB400 to strong station FM band images, though they were there with sufficiently strong nearby signals again especially from the high end of the FM band. Its uses a slightly different frequency for 1st IF, 55.845 MHz, so substitute 55.845 for 55.850 in the above calculations to be exact.

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