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Old 9th Jun 2020, 10:27 am   #1
ronbryan
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Default Indoor folded dipole FM aerial impedances

Folded dipole indoor FM aerials are widely available and are made from flexible twin feeder with plastic bits on the ends for mounting and to make the folding connections. The same basic aerial seems to be made with different terminations e.g. two tags, a bi-pin plug or a Belling-Lee style TV co-ax plug.

Having looked up folded dipoles in the ARRL Antenna Book, it confirms that the 75 ohm impedance of a normal dipole is transformed by the folding (which provides two conductors in parallel for half of the current to flow in each conductor) to 300 ohms.

Is the 75 ohm co-ax connector provided on some versions just mismatched, (quite apart from providing an unbalanced connection), or does the 75 ohm plug have a minute balun squeezed inside?

Ron
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Old 9th Jun 2020, 10:37 am   #2
paulsherwin
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Default Re: Indoor folded dipole FM aerial impedances

They are just mismatched I think. It only makes a few dB of difference.
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Old 9th Jun 2020, 11:06 am   #3
Electronpusher0
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Default Re: Indoor folded dipole FM aerial impedances

I have had a couple of these aerials made of twin feeder with TV style co-ax plugs and the plug has just been connected to the feeder, no balun, just mismatched.
These type of aerials are better than nothing but the receiving elements are too narrow and too close together to be really effective.

Peter
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Old 9th Jun 2020, 11:12 am   #4
Scimitar
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Default Re: Indoor folded dipole FM aerial impedances

The reason for folding and putting the impedance up is for Yagi use. As you add elements, the impedance falls back to the nominal 75R.
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Old 9th Jun 2020, 11:15 am   #5
Hartley118
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Default Re: Indoor folded dipole FM aerial impedances

An indoor antenna is inevitably coupling into a complex standing wave system rather than into a travelling wave in free space. I guess that changes the maths.

Martin
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Old 9th Jun 2020, 11:21 pm   #6
TrevorG3VLF
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Default Re: Indoor folded dipole FM aerial impedances

I use a folded dipole made from 300 ohm ribbon. The loop is connected across at 4.5 inches from each end. The total length is 60 inches.
One side of the loop is cut at mid point and 300 ohm feeder is connected here. The aerial is held verticlly, suspended with a drawing pim.

The impedance is changed to 75 ohm by using a balun as used to mount a loop on a portable monochrome TV. This is plugged into a TV/FM distribution amplifier so that three outputs have TV and FM available.
I also have a four element horizontal Yagi in the roof, pointed at Sutton Coldfield but do not use it. I need to get my ears refurbished.
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Old 10th Jun 2020, 4:24 am   #7
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Default Re: Indoor folded dipole FM aerial impedances

The 300 Ohm figure for a folded dipole is valid only for one suspended in free space. Most get pinned to walls, doorframes and anything handy, so the impedance can change rather a lot depending on dielectric constant, lossiness of walls, as well as whatever reflections are around it.

Many tuners have dual inputs, a pair of terminals for a balanced input from twin feeder, going into a balun which suits 300 Ohm impedance, or going in through a 75 Ohm coax 'Belling-Lee' connector.

Despite mismatches etc, they work in areas with good signal strength.

David
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