Is there a proper way to use a telescopic aerial on a portable FM radio?

[David Gardener]

This may seem like a dumb question but I was wondering if there is a proper scientific way to achieve the best reception on a given fm radio station by adjusting the length and angle of the telescopic aerial? Usually I would extend it to its maximum length then move it around to get the best reception by trial and error. I have read that T shaped ribbon aerials can be tuned to get better reception on certain stations by shortening them. If I remember correctly the higher up the FM frequency the shorter the aerial should be. Does this principle apply to telescopic aerials or is it simply a case of the longer the better and adjusting the angle and position using trial and error?

[HamishBoxer]

Once FM radios came with the T aerial and all I know is they are directional for best results.

[G6Tanuki]

Polarisation has varied with history. Horizontal, Vertical, or Slant? Back in the days whr "FM" was the unique provenance of the BBC they expected you to have a horizontally-polarised multi-element beam in order to 'get the best' from their [mono] broadcasts of the 'latest' Third--Program[me] symphony by Sir Neville Mariner.

Time moved on - and we got cheap-and-cheerful FM radios in portable and in-car flavours. Commercial radio appeared, too - they wanted to appeal to the most-profitable audience - so the real-world moved towards 'slant' polarisation which worked well with portable- and car-radios which used vertical whip-antennas.

These days, unless there's one specific remote 'DX' transmitter you want to receive [in which case you need a directional antenna with appropriate polarisation] truth is, you should wiggle your portable's antenna to get the best signal, because in the real-world it will have been refracted/reflected by lots of unpredictable-and-unquantifiable things.

If using one I wiggle it to the worst signal and then go to the opposite position, seems to work well. I used to try making it a 1/4 wavelength long, made no audible difference.

[Mr 1936]

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.

[David Gardener]

Thanks for the interesting and comprehensive replies So basically I should have the aerial fully extended and due to the way radio waves bounce around unpredictability in a home environment the position and angle of the aerial is best achieved by trial and error. I like the tip one poster gave of pointing the aerial in the worst place then moving it to the opposite direction.

[paulsherwin]

Yes, you just do whatever works. This is even true of fixed aerials in places like lofts. Reception is very unpredictable where there are lots of reflective and screening surfaces like walls and roof tiles.

[simpsons]

Notwithstanding all that has been said, the choice you have really depends on how long the aerial is.

Should the radio have an aerial that extends to 75cm then this is equivalent to a 1/4 wave dipole tuned to 100 Mhz.

The math is this: MId-band 100 Mhz is equivalent to 3 metres wavelength.

Should the radio have a shorter length aerial then mathematically a 37cm or 1/8 wave dipole is the next best thing. But, each time the aerial is shortened, the signal pick up is reduced and possibly multipath reflections more of a problem. In this case just have it extended as far as it will go.

Chris

[regenfreak]

The whip antenna is referred as a "dipole" that the ground plate acts like a mirror reflecting the image of the vertical monopole. I would have thought the efficency of antenna is poor if n is not an integer and less than 1:

antenna length = n x wavelength/4