Uhf/vhf/fm coax connectors for home made hd antenna

Discussion in 'General Electronics Chat' started by swope36, May 25, 2013.

  1. swope36

    Thread Starter New Member

    May 25, 2013
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    Ok im a cheap ass and have tons of electronic componets what I wanna know is it possible to build a uhf/vhf/fm converter for a digital antenna I constructed. Can Anyone help please? can you step dowt the ohms by adding a resister and by passing it all together and just solder it? Please help?
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Exactly what are you trying to convert?

    I assume you will be using standard RG-6 type cable?

    You can get the standard splitters for a few bucks. You need a balun transformer to do it yourself and those are not typical stuff.
     
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  3. swope36

    Thread Starter New Member

    May 25, 2013
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    rg59 wont carry a digital signal l only thing rg6 will/ I m lain off and have no money but have compentents to build a matching transformer for the coax hook up
     
  4. swope36

    Thread Starter New Member

    May 25, 2013
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    I built an hd antenn but need a way to fasten antenna. can I cut rhe coax and solder it
     
  5. swope36

    Thread Starter New Member

    May 25, 2013
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    Thannks for the helf friend...yI need to start hanging here more to learn from you pros :) Take care
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You will need a 75 Ohm-300 Ohm matching transformer. They are about $1 at Frys.

    The impedance of "free space" is roughly about 300 Ohms so most well designed antennas have an impedance of about 300 Ohms to get best signal recovery. The impedance of coax is 75 Ohms, so you need a 300-75 transformer on the cable.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    120\pi ohms is close to the exact value. ;)
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    That's 120*pi ohms.

    http://www.setileague.org/askdr/imped.htm
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    No you can't do stuff like that and expect it to work. Characteristic impedance of a coaxial cable is not even remotely related to the DC resistance of resistors. Soldering stuff to cables without a defined plan is also unlikely to yield acceptable results. You should be trying to find a way to raise revenue instead of fooling around with soldering irons.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The * is implied. So always write 2*\pi never 2\pi in equations? ;)
     
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I hate Latex... ;)
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I know. I said roughly 300 Ohms so I wouldn't get asked why all antennas are designed to be 300 Ohms......if the correct value of impedance is 120 pi which is something like 377.
     
  13. JMW

    Member

    Nov 21, 2011
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    300 is close enough. At any rate why not design your antenna for 75 ohms? The TV has an input. And what makes you think RG-59 will not work for digital? Please don't tell me you are one of those 99.999% virgin copper cable guys. If the run is under 100Ft or so RG-59 will be fine. If you go for 300 ohm antenna, twin lead has the lowest loss, and the $1 adapter will work fine.
    There are a number of antennas on the market, and we have road side vendors selling them for $20. They are mounted in 1/2 inch PVC conduit.
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Because best power transfer occurs when source impedance is same as antenna, and free space impedance is 377 Ohms so 75 Ohm antenna would give very poor results.


    "Digital" frequency range extends up to about 1000 MHz, which formerly was called "UHF". RG-59 has a lot of signal loss in the UHF range, RG-6 has a lot less signal loss.

    If you say so... it's only "fine" if you think a 21.5 dB loss is "fine"... see below:
     
  15. JMW

    Member

    Nov 21, 2011
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    Let us examine your reply. Why not build and design a 377 ohm antenna and TX line. Communications systems for the most part are designed around 50 ohms, ya saying they don't work? TV used to have 300 Ohm inputs, why didn't they round to 400 Ohms? TV's, VCR's, Sat TV, DVR's are all 75 Ohms, no matter what, they all have some sort of 377 to 75 Ohm baluns, whether by design or add on device. According to you, all of these are inefficient.
    1000 MHz is still UHF.
    Digital signals extend to 1GHz really? Says who? Have you checked with the FCC, they might be interested in that? The 800 MHz channels and above were transferred to LMR in the '80's and 90's. Much of the 700 MHz has also been transferred. No new channels above 50 are being allocated.
    Finally let's look at the OP's parameters, he is "cheap", "has a ton of parts" and is cheap. 20% of the channels are VHF, he will have to do a zip code check to find out what is available in his area. That being said, I stand by my statements.
     
  16. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Or, just try reading it. Antennas are built around 300 Ohms to get a decent match to free space (377 Ohms). In the past (when TV channels were almost all VHF), 300 Ohm "twin lead" was used to run the signal but it's lossy and prone to interference. 75 Ohm coax (unbalanced) is very good for running signal, but it needs a balun to match the antenna's 300 Ohm impedance. If you think you gain something by building a 75 Ohm antenna to "match the TV", you lose more because of how poorly the antenna matches the signal it's trying to receive.

    PERIOD.

    You don't want a 75 Ohm antenna because it will give VERY CRAPPY signal gain because of impedance mismatch to free space.

    You don't want RG-59 because most broadcast channels, not to mention cable and the rest, have migrated into upper UHF range where signal loss from RG-59 is horrific.

    You pay a little extra $ for RG-6 and save a whole lot of $$$$$ for the size and cost of the antenna you would otherwise need to overcome the big signal loss the RG-59 would cause.

    It's not really all that complicated.
     
    Last edited: May 29, 2013
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I really have no idea what you are trying to prove here. Even if you have your channels of interest limited to 600 MHz, you can try reading the chart:

    a 100 ft run of RG-59 will knock that signal down by about 15 dB.

    And you think that's OK?

    Whatever.

    Most very good roof antennas only have "gain" in the range of about 8 - 10 dB at that frequency, which means thanks to the RG-59, your signal will be less than half of what a two wire dipole would give you. Which is basically nothing. That's why people use RG-6.
     
  18. JMW

    Member

    Nov 21, 2011
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    Twin lead is lossy and subject to interference? Insert Artie Johnsons line from Laugh In. A quick Google will show that it is better than RG 11. As for interference rejection; balanced twin is far better than coax for common mode rejection. Antennas do not need a baluns. Briefly the characteristic impedance depends upon the type of antenna, length of radiator, feed point, even the angle of ground radials will affect it.
    While RG 59 is not the best, if he has it amongst his collection, it is certainly worth trying.
    Ya gotta go with what the OP asked, he has a lot of stuff and wants to pay little.
     
  19. vk6zgo

    Active Member

    Jul 21, 2012
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    I agree that RG59 isn't too good at UHF,especially the pretend "RG59" they sell now,but RG6 isn't all that great,either.

    It was originally designed for Cable TV use,where the main worry was keeping
    the Cable signal out of other services--hence the concentration on shielding,as against minimising losses.

    The old "TV" coax with the semi-air insulation was considerably better than either in terms of loss.
    The downside of that cable was that it was flimsy,& prone to damage.

    RG6's big advantage is in its rugged construction,which combined with compression type "F" connectors,makes for a reliable system,albeit with poorer loss figures.
    300Ω ribbon, when new,has lower losses than any flexible coax cable,but it is difficult to route,as lack of care here can cause unbalance of the line.
    In a recent rewire of my TV antenna system,I fed RG6 down between the bricks of my wall & a built-in cupboard,then under the house to a couple of splitters,then to 3 separate room outlets---you couldn't do that with 300Ω ribbon!

    By the way,many antennas have feedpoints other than 300Ω---much of this depends upon the antenna design.
     
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