75 ohm Antennas

roashru

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Jul 28, 2005
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hello everyone, this post is all about 75 ohm antennas if you dont know what a real 75 ohm antenna is please dont make a comment. no transformers are used.
 

roashru

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Jul 28, 2005
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picture is a made 50 ohm dipole with a ground plane made for microwave frequencies. the only difference in tv is length of the main antenna. then there is the coax velocity factor.
 

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Titanium

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A transformer balun is typically only used if the impedance on connected devices do not match or to distribute using a different impedance feed line. Baluns are used to convert between balanced and unbalanced signals. If both the antenna and the receiving antenna are the same type and impedance, the transformer is not necessary.

Do you have a question or observation about using a vertical dipole antenna for receiving television?
 
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KE4EST

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That is actually a 1/4 Wave vertical with a ground plane. A di-pole has two 1/4 wave elements at 180 deg end to end, known as col-linear.
I would not use a vertical for TV reception as the antennas on a TV tower are horizontally polarized. Going vertical you can lose several dB of gain.
If you are close to the tower you will not notice but any distance out and you will see a big difference.

Very easy to build a 75 ohm Yagi antenna, yes.
Years ago when twin lead was king, antennas were made to match the impedance of the twin lead, thus 300 ohm.
Still a lot of people using twin lead, and the fact that lazy manufacturers have to re-design their machines to make a 75 ohm feed point........
So TV antennas mostly still come with a 300 ohm feed point.
You lose about 3dB gain with a balun to convert 300 to 75.
However, machine made antennas at a factory or much better than home brew, unless you have the equipment, so the loss is less than a home brew.
I know I have tried, as I have built many yagis for radio use, the commercial stuff(if made my a good manufacture), has always done better. :)
 
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FaT Air

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Why does the title of the thread say
75 ohm antennas
when you demonstrate a 50 ohm antenna?

Where does
coax velocity factor
factor into a simple ground plane antenna?
Answer: It doesn't. It 'factors' when the 'build' is a coaxial vertical collinear.
But even those are terrible choices for OTA TV.

For TV stick to a bowtie or yagi as your DIY project,
If you want to 'be rid' of the transformer, consider a gamma or delta matching section on the yagis.
Have a good dsy.
EDIT: Transformer loss- Use a Higher quality transformer
 
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harshness

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picture is a made 50 ohm dipole with a ground plane made for microwave frequencies. the only difference in tv is length of the main antenna. then there is the coax velocity factor.
Specifically what frequency range are you looking to work with (or wavelength if that's how you roll)?
 

KE4EST

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a yagi is not 75 ohm
Not sure what the purpose of your thread is, but a Yagi can be designed for 75 ohm quiet easily.

Is English your second language?
 

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