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FM antenna, quirky?

PaulMB

PaulMB

pfm Member
I've made a DIY internal dipole antenna for my recently purchased Uher tuner, and following all the information on the web I connected it to the 300 Ohm input on the tuner. On my favourite station (100.30 FM) I was getting 3 out of 5 "tuning lights". Just for the hell of it, I tried an old bit of scrap electric wire, about 75 cm long, connected to the central terminal of the 75 Ohm coaxial input, and left hanging behind the stereo amid a mass of power cables, speaker cables, etc. Well, with that, which according to all the internet information should receive and sound like shit, I'm getting 4 out of 5 "tuning lights."
So the question is: is it normal for FM antennae to be quirky like this? Or being in a big city there is just a very strong signal bouncing around everywhere so antenna theory does not hold true?
 
Hi Michael, thanks for your interest.

The DIY dipole is made thus: two wires about 2 cm apart, 120cm long, joined at the ends, attached horizontally along the upper back edge of the piece of furniture that holds all the stereo stuff. So it is horizontal and about 80 cm from the floor.
The centre of the lower run is open, and the two wires go down for about 40cm to the two 300 Ohm connectors on the tuner.

The antenna that works better, surprisingly, is just a 70cm bit of old wire connected to the central pole of the 75 Ohm coaxial connector, and hanging down the back haphazardly but more or less vertically.

Both sound fine, but I was just curious to know if it is normal that all the theory just flies out the window.
 
RF is funny stuff! The field strength can vary greatly just by moving a short distance, so there's no way of comparing a piece of wire down the back of the kit, or an improvised dipole. The 300 ohm and 75 ohm inouts will have different sentivities, and if the dipole isn't exactly broadside on to the transmitter, the recieved signal will be smaller. A vertical wire is omnidirectional in the horizontal plane.

Having said that, I would make an improvised dipole from a piece of two-core mains wire, stripped of outer insulation for 75cm. Seperate the two cores so that the far ends are 1.5m apart and mount either horizontally, broadside on to the transmitter, or vertical. Connect the other end of the mains cable to the 300ohm balanced input, or 75 ohm input. For an improvised antenna, it won't matter much, whichever works better. The 75 ohm input is usually 3dB more sensitive due to the transformer action of the balun.

S.
 
The single bit of wire dangling behind the kit will have a "virtual" other arm, so that may explain why you are getting reasonable signal. It is also just about the right length for a single arm. Your folded dipole isn't right dimensions, I think, for FM. Size matters. As does height - have you tried putting it higher up? If not, why not?
 
Thanks everyone. So it is very much a try-and-see field, as I suspected.
I can't put it higher up without having wires visibly strung across the walls. My wife would not mind at all, but I refuse to ruin the already dubious aesthetics of the "drawing room" with antenna rigging. Maybe out of curiosity I'll try a temporary antenna mounted on a big camera tripod.
But for practical purposes, the 75cm dangling wire is working fine, with 4 out of 5 tuning lights coming on.

Thanks again.
 
PaulMB said:
Thanks everyone. So it is very much a try-and-see field, as I suspected.
Click to expand...

Not really. It is very well understood with excellent theory behind it. The only required trial and error is the actual physical location of the antenna for best reception, and even that is reasonably predictable depending on where you are and any obstructions between you and the transmitter.

You haven't yet said where on all the internets you found your information.
 
The most obvious fault your DIY dipole has is that you arranged it horizontally, whilst FM radio is polarized vertically. False polarization results in a mathematic loss of 3dB. In reality it will be a bit less, but still in the area of 50%, and most definitely in the region of the losses you describe.
 
FM is mostly either vertical or mixed polarity, and as all of the BBC's big transmitters are mixed that means that for the majority of the population a horizontal antenna is not only perfectly fine but also offers some significant advantages.
 
topa said:
The most obvious fault your DIY dipole has is that you arranged it horizontally, whilst FM radio is polarized vertically. False polarization results in a mathematic loss of 3dB. In reality it will be a bit less, but still in the area of 50%, and most definitely in the region of the losses you describe.
Click to expand...

Polaristion is either mixed or circular. I don't know of any which are only horizontally polarised any more, or any that were ever only vertically polarised.

The benefit of a vertical receiving dipole is that it is omnidirectional in the horizontal plane, and this will pick up on the vertically polarised component of a mixed or circularly olarised transmission. A horizontal dipole has to be broadside-on to the transmitter for maximum pickup, and this can be limiting in the case of an improvised receiving antenna.

S.
 
Michael J said:
FM is mostly either vertical or mixed polarity, and as all of the BBC's big transmitters are mixed that means that for the majority of the population a horizontal antenna is not only perfectly fine but also offers some significant advantages.
Click to expand...

I had always assumed that FM transmissions were horizontally polarised as it was supposed to give better immunity against various types of interference - going back to the 1950s - have I always been wrong or has thinking changed over the years? I seem to remember that VHF TV transmissions were vertically polarised, system dating from before the war.
 
signal strength isnt everything , that signal may be made up from reflected signals which a simple aerial doesnt reject
 
This is all fascinating. I think I'll un-nail my horizontal DIY antenna, tape it to a length of plastic pipe and try holding it vertically in various positions around the tuner.
 
am using a bit of solid core wire also, hung vertical, if i got an FM dipole thingy with a balun, the aerial would need to go vertically, not horizontally?
 
Barrymagrec said:
I had always assumed that FM transmissions were horizontally polarised as it was supposed to give better immunity against various types of interference - going back to the 1950s - have I always been wrong or has thinking changed over the years? I seem to remember that VHF TV transmissions were vertically polarised, system dating from before the war.
Click to expand...

Before FM radios were common in cars or on portable radios, indeed most FM was horizontally polarised, however this changed as more cars and portables offered FM reception. In 1979, the decision was taken to re-engineer BBC stations for mixed polarisation. A small vertical whip antenna as is fitted to cars or on portable radios won't work efficiently with horizontally polarised transmissions, so antenna structures were changed. Polarisation can be mixed, slant or circular, but they all have seperate vertical or hoizontal components that a receiving antenna can access.

There's a good explanation here. http://tx.mb21.co.uk/gallery/wrotham/mixedpol.php

S.
 
Andrew.B said:
am using a bit of solid core wire also, hung vertical, if i got an FM dipole thingy with a balun, the aerial would need to go vertically, not horizontally?
Click to expand...

Doesn't matter, as the transmissions are mixed polarisation. If you put it horizontal, then it has to go broadside-on to the transmitter. If it goes vertically, then it's omni-directional in the horizontal plane.

S.
 
Just tried a 150cm dipole, using the traditional pinkish tape-shaped antenna wire, taped to a 6-foot length of wood and connected to the 300 Ohm terminals. Moved it around a bit. But in the best position only as good as the original bit of scrap wire hanging down the back, in other positions much worse.
So I'll keep my 75cm of mains wire, thankful that I get excellent reception without all the rest.
Probably there are strong signals bouncing around everywhere in my flat.

Andrew: Maybe I'll try a piece of solid core too, unwound from a coil.

Thanks again to everyone.
 
sergeauckland said:
In 1979, the decision was taken to re-engineer BBC stations for mixed polarisation. A small vertical whip antenna as is fitted to cars or on portable radios won't work efficiently with horizontally polarised transmissions, so antenna structures were changed.S.
Click to expand...

That probably explains why the first car radio I had with FM back around 1980 was absolute sh*te whereas for many years now they have been fine.
 
I recently did a test where I compared a 17 element dedicated FM antenna to a 'mystery' antenna. The latter did get surprisingly good signal strength and at times very low noise and good sound compared to the main antenna that has maybe 12 dB more gain and much much higher rejection of multipath signals.

http://www.pinkfishmedia.net/forum/showthread.php?t=111165&highlight=antenna

I did have one ripped .wav sample in this thread where I alternated between these two antennas without revealing which one was A and which one was B, or the identity of the mystery antenna.

Well, now I will say the mystery antenna was a 5m length of NACA 5 strewn on the ground. I was somewhat taken aback how decent a signal it gave and how it allowed the NAT01 to produce some very fine sounds. But as good as it was the sound quality obtained from the G17 was still better in all ways...more dynamics, tonal textures and instrumental separation.

In the past I also have found that the T-dipole antennas are sometimes worse than just sticking your finger in the antenna socket.
 
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