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g86
Joined: 21 Jan 2003
Posts: 208
Helped: 4
Location: On top of antenna :))
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 09 Mar 2003 12:37 Which system is better 50 or 75 Ohm |
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Earlier 75 Ohm systems were used. Most probably it was a Russian standerd. Not sure though. But now 50 Ohm has become standard almost in all applications. My question is which system is better?
I feel 75 Ohm is better because easily connected with dipole or yagi antennas. Suitable for edge fed Microstrip as the edge impedances are typically closer to 75 Ohm. Probe feed diameters are become thinner and thus can be usefull for higher frequencies. Power loss due to termination will be smaller. And more ..
What is your opinion?
  
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vanderspunk
Joined: 01 Jan 1970
Posts: 26
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| 09 Mar 2003 14:02 |
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75 ohm was optimized for low loss in air-filled coaxial transmission lines, but not for power handling. 50 ohm was a compromise.
See the attached document for more info.
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ME
Joined: 14 Mar 2002
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| 09 Mar 2003 14:21 |
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why50pdf.pdf can be downloaded from this site: http://my.ece.ucsb.edu/bobsclass/134/Handouts/why50ohm.pdf
Please don't upload files that can be downloaded for free elsewhere, just post a link instead.
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g86
Joined: 21 Jan 2003
Posts: 208
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Location: On top of antenna :))
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| 09 Mar 2003 14:25 |
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A very nice topic. A bag full of thanx for you.
| vanderspunk wrote: |
75 ohm was optimized for low loss in air-filled coaxial transmission lines, but not for power handling. 50 ohm was a compromise.
See the attached document for more info. |
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flatulent
Joined: 19 Jul 2002
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Location: Middle Earth
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| 09 Mar 2003 17:08 old standards |
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These values go back to at least 1940. The RG prefix in cables such as RG8 and RG11 are US military nomenclature. Back then there was only solid dielectric and air dielectric with spacers supporting the inner conductors.
During the war equipment was shared between nations and that is why you found the 75 onm cable so common. Most of the equipment that used higher frequencies needed low loss cable. The lower frequency equipment used open wire transmission lines.
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sick_man
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| 09 Mar 2003 17:27 |
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the 50 R system was devised too allow for silicon too give better transfer charicteristics over long runs of cable
the original system for 75 R was an open wire ladder feeder
first used for reception antennas but only at that time for 1/2 wave dipoles = 1.1 SWR @75R
where as a 50R feeder needs a 50 antenna load and a transmitter load
to allow for full power delivery
a 75 R system has a bigger bandwidth and several nice features of groundwave systems
missing or not fully exploited by 50R systems
a dipole in free space will always have an impedance of 75R too give 1.1 ratios
50R systems are also lots more tollerant of missmatch
as the system runs on a perminant "balanced" missmatch
the transmiter wont heat up too much unless there is a short
bit like a zobial network in an audio amp
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ME
Joined: 14 Mar 2002
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| 09 Mar 2003 17:48 |
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| Are 75 ohm used for other applications than radio, TV and satellite systems?
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flatulent
Joined: 19 Jul 2002
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| 09 Mar 2003 21:46 two things |
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Video base band systems frequently use 75 ohm cable.
Good thought Monkey Business. Dipole antennas are 72 ohms impedance. The use of an identical impedance feed line produces the best transmission of energy. The twin lead was used for four reasons. It has lower loss than coax. It has lower weight than coax and put less stress on the antenna to hold it up. It was balanced just like the antenna. It cost less than coax.
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donda
Joined: 11 Jun 2002
Posts: 11
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| 10 Mar 2003 0:18 Re: Which system is better 50 or 75 Ohm |
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| g86 wrote: |
Earlier 75 Ohm systems were used. Most probably it was a Russian standerd. Not sure though. But now 50 Ohm has become standard almost in all applications. My question is which system is better?
I feel 75 Ohm is better because easily connected with dipole or yagi antennas. Suitable for edge fed Microstrip as the edge impedances are typically closer to 75 Ohm. Probe feed diameters are become thinner and thus can be usefull for higher frequencies. Power loss due to termination will be smaller. And more ..
What is your opinion?
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You must practicing or learninng or alse,so beware us from dummy quest.,
First you must know-it is free(vacuum)-wawe-resistance.it is aprox. 278 Ohm,so 75 Ohm cable is best for impedance transforming without heavy looses,when you are building downlink.
Another hand,signal supression is optimal in 51 Ohm fider,and it is very important to transfer high power UPLINK with min. looses,it is better to brought signal directly to antena with 50-Ohm line and then transform to 278 ohm.
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flatulent
Joined: 19 Jul 2002
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Helped: 292
Location: Middle Earth
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| 10 Mar 2003 5:03 one more thought |
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Here is some more information.
Many types of antennas have lower impedances. Verticals worked against a ground plane are 36 ohms. Parasitic antennas like the Udi (Yagi) have lower feed impedances.
Impedance transforming networks are broader band and less lossy when the do less drastic transformations. Going from 36 to 50 is better than 36 to 75.
As a result of this, most transmitters operating below about 30 MHz were designed for 50 ohm line way back around 1940 or so. By 1960 all of them were.
As time went on, VHF antennas used with low power transmitters were mostly ground plane and Udi types which motivated VHF transmitters to be designed for 50 ohm cable.
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superluminal
Joined: 31 Jan 2002
Posts: 95
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Location: Inside a quantum well
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| 10 Mar 2003 5:50 |
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| There is a nice appendix in Thomas Lee RF book about this
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