We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissable in your locale.

Author Topic: What kind of cabling to use 75 pr 50 ohm?  (Read 3713 times)

Offline Jamezyy

  • Newbie
  • *
  • Posts: 4
    • View Profile
What kind of cabling to use 75 pr 50 ohm?
« on: November 21, 2014, 1331 UTC »
Hello guys i am new to this forum :) im sure this will be a good experience for me in meeting enthusiasts. I am from Malta in europe and i have just bought a 7w pll fm transmitter, now on the roof i already have standard 75ohm cabling for my tv antenna, could i be able to use this same cabling for my pll transmitter? What will it affect? Will it damage my transmitter? As it says to use 50ohm cabling only. Any help would be appreciated.

Offline Jamezyy

  • Newbie
  • *
  • Posts: 4
    • View Profile
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #1 on: November 21, 2014, 1333 UTC »
Btw i bought a cze-7c transmitter

Offline Jari Finland

  • Full Member
  • ***
  • Posts: 220
    • View Profile
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #2 on: November 21, 2014, 1401 UTC »
If it says explicitly 50 ohm, I think that is a sign that you should better use 50 ohm cable.

However, in practical life on FM radio frequencies that doesn't matter much. You just can't pull out all the power through wrong cable to antenna. Due to mismatch you will have probably a bad standing wave ratio, which means that the energy your transmitter creates, bounces inside cable, warms it a bit and in serious case possibly damages the transmitter if the heat is too much. I say this in principle, knowing nothing about the particular transmitter and the coax and antenna. Possibly you would be safe, because 7w is not much, but low power is not a reason to be careless.

If you insist using that cable, you need a impedance transformer that changes 50 ohms output into 75 ohms.

More info is needed. If the tv antenna cable leads indeed to a TV antenna, you are adding problems to your setup. You really would need a FM antenna for 88-108 MHz. Otherwise you have a mismatch again.

Also, if you are talking about receiving central antenna system of apartment building and several tv receivers are connected to that system, transmitting watts into that system is a absolute no-no.

In the first phase one could test just with a telescope antenna of an old radio, place tx by the window and go out with a pocket radio to see what happens.
« Last Edit: November 21, 2014, 1408 UTC by Jari Finland »

Offline Jamezyy

  • Newbie
  • *
  • Posts: 4
    • View Profile
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #3 on: November 21, 2014, 2025 UTC »
Thankyou for answering, i am sure its a 75ohm coax as i stated thats its a standard tv cable, i already have a spare cable set up, its nkt connected to anything, i was gonna eith buy a 1/4 wave gp amtenna or make a diy dipole antenna, the wiring is my problem and im worried i might burn my transmitter

Online redhat

  • DX Legend
  • ******
  • Posts: 1228
  • USA
  • Music is my drug.
    • View Profile
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #4 on: November 23, 2014, 2317 UTC »
You can build an impedance matching section to match 50 ohms to 75 ohms.  Assuming the antenna is 1:1 SWR, using 75 ohm cable should only raise your SWR to 1.5:1.  You might be ok at those power levels.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Jamezyy

  • Newbie
  • *
  • Posts: 4
    • View Profile

Offline John Poet

  • Hero Member
  • *****
  • Posts: 881
  • Lansing, Michigan, USA
  • I survived the Mad Lagomurph
    • View Profile
    • Free Radio Cafe forums
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #6 on: November 25, 2014, 0529 UTC »
 I think you're better off investing in fresh 52-ohm coax that already matches your equipment.

Power losses in coaxial cable are high at FM frequencies, even at perfect SWR, so you want to keep your cable run as short as possible, and maybe use the heavier coax, RG8 rather than RG58 if it has to go very far.

You also should have a low-pass filter designed for FM broadcast, in between the transmitter and your cable to reduce harmonics which can interfere with licensed services at higher frequencies.

And yeah, running the transmitter into your TV antenna could/would burn up the finals due to SWR mismatch.
« Last Edit: November 26, 2014, 0833 UTC by John Poet »

John Poet

"A treasonous voice of dissent"

The Crystal Ship Shortwave
Free Radio Cafe Forums

Offline Pigmeat

  • DX Legend
  • ******
  • Posts: 4347
    • View Profile
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #7 on: November 25, 2014, 0547 UTC »
RG-8 or RG-8 mini coax have always been my choice when it comes to coax cable. If you can find the Belden brand of coax where you live, get it.

Online redhat

  • DX Legend
  • ******
  • Posts: 1228
  • USA
  • Music is my drug.
    • View Profile
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #8 on: November 25, 2014, 2158 UTC »
A pad means a device that attenuates the signal.  If you have one on each end, your losing almost 11dB, that means 10 watts in, .8 watts out.  Plus, you'll burn up the attenuators.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline John Poet

  • Hero Member
  • *****
  • Posts: 881
  • Lansing, Michigan, USA
  • I survived the Mad Lagomurph
    • View Profile
    • Free Radio Cafe forums
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #9 on: November 26, 2014, 0835 UTC »
Ugh.  Thought those were some simple impedance transformer, but didn't spend too much time looking.

John Poet

"A treasonous voice of dissent"

The Crystal Ship Shortwave
Free Radio Cafe Forums

Offline Jari Finland

  • Full Member
  • ***
  • Posts: 220
    • View Profile
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #10 on: November 26, 2014, 1715 UTC »
Well, simpliest impedance "transformer" would be a 7w cheramic 22 ohm resistor in the feedline. Costs about as much as 10 metres of 50 ohm coax. And after soldering that resistor you don't need to wonder any more where all your power disappears. You know the answer.  ;D

Offline Witchdoctor

  • Newbie
  • *
  • Posts: 2
    • View Profile
    • Email
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #11 on: December 14, 2014, 1345 UTC »
http://www.pasternack.com/50-ohm-bnc-male-75-ohm-bnc-female-matching-pad-pe7006-p.aspx

So this would work wonders right?

new to forum....but not radio.  FYI I have found pasternack to be grossly overpriced at best, ridiculous on average, and I avoid them if possible.  Please please shop around BEFORE going through those guys.  Your best bet if you are going to be doing any antenna work is to go with MFG, specs  (50 ohm) and research the type of antenna you wish to use.  a simple dipole can be effective, or if you really like tinkering, build a FM band version of the ringo ranger, a 5/8 wave vertical...  Be warned, antenna building can get addictive  ;D
But research is always best before starting.  Peruse some of the ham sites for ideas, then modify to fit your frequency needs...best of luck  :)

Offline Pigmeat

  • DX Legend
  • ******
  • Posts: 4347
    • View Profile
Re: What kind of cabling to use 75 pr 50 ohm?
« Reply #12 on: December 15, 2014, 1909 UTC »
You said it, Witchdoctor. I've built so many types of antennas over the years it's a wonder the coax and wire haven't wrapped around me like a python ad squeezed the life out of me. It's a nasty habit.  ;)

TV line is an often overlooked material for FM antennas. Folded dipoles, J-Poles if you need a little gain, etc....

Give the ham sites a look and check out the DIY antenna designs for 6 and 2 meters. They're easily adaptable to the FM bands. Even a simple ground plane design can give excellent results. At FM frequencies you can build one out of scraps of wire and the female connector that matches up with the male connector on your coax. It takes a couple of hours,at most,to throw together.