UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Is there a proper way to use a telescopic aerial on a portable FM radio?

Mr 1936 · 27th Jun 2020, 8:13 pm

If you are indoors, where most portables are used, there are usually multiple reflected signals from all the walls and objects in the house. In this situation the field strength and polarization can vary considerably between two positions less than a metre apart. The main thing is to avoid a local weak spot, so you can either shift the radio or, more conveniently, reposition the aerial using its base hinge.

The complete aerial comprises the whip as one arm of an asymmetric dipole, with the radio chassis, speaker, batteries etc forming the other half. The whip is normally chosen to be about a quarter wave long when fully extended. However, the tuning for a receive aerial is generally not very critical, and I doubt you would ever notice the slight worsening of SNR if the whip was shortened by say 20 percent.

Ribbon twin feeder is popular in the USA and Japan, and can conveniently be used to make a folded halfwave dipole. These were often supplied with FM tuners so that you could get them going as soon as you unpacked them ! In principle they can be adjusted to the exact length to suit a chosen FM frequency (shorter length = higher frequency). In free space a length change of plus or minus 20 percent will have such a small effect that it's probably not worthwhile altering. However, if you fix the dipole indoors straight onto a wall, the material the wall is made of can tend to tune it downwards in frequency due to the dielectric loading. In this case there could be merit in shortening it a bit to bring it back on frequency.

27th Jun 2020, 8:13 pm	#5
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Is there a proper way to use a telescopic aerial on a portable FM radio? If you are indoors, where most portables are used, there are usually multiple reflected signals from all the walls and objects in the house. In this situation the field strength and polarization can vary considerably between two positions less than a metre apart. The main thing is to avoid a local weak spot, so you can either shift the radio or, more conveniently, reposition the aerial using its base hinge. The complete aerial comprises the whip as one arm of an asymmetric dipole, with the radio chassis, speaker, batteries etc forming the other half. The whip is normally chosen to be about a quarter wave long when fully extended. However, the tuning for a receive aerial is generally not very critical, and I doubt you would ever notice the slight worsening of SNR if the whip was shortened by say 20 percent. Ribbon twin feeder is popular in the USA and Japan, and can conveniently be used to make a folded halfwave dipole. These were often supplied with FM tuners so that you could get them going as soon as you unpacked them ! In principle they can be adjusted to the exact length to suit a chosen FM frequency (shorter length = higher frequency). In free space a length change of plus or minus 20 percent will have such a small effect that it's probably not worthwhile altering. However, if you fix the dipole indoors straight onto a wall, the material the wall is made of can tend to tune it downwards in frequency due to the dielectric loading. In this case there could be merit in shortening it a bit to bring it back on frequency.