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Author Topic: Coax impedence  (Read 289 times)
KG6THI
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« on: July 03, 2004, 12:23:58 AM »

What physical factors affect the impedence of coaxial cable and what would happen if I were to use a section of 75 ohm feedline in a 5 watt base station on 2 meters?
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KZ1X
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« Reply #1 on: July 03, 2004, 12:45:22 AM »

primarily dielectric characteristics and the ratio of center conductor diameter to overall diameter, and probably nothing
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N6AJR
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« Reply #2 on: July 03, 2004, 01:56:38 AM »

ya mite end up with a "best match" of 1.5 :1 but ok for low power.  if it is rx cable like for CATV it may not work with high power.
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KG6THI
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« Reply #3 on: July 03, 2004, 03:31:15 AM »

It is receive cable for CATV and I'll be putting 5W into it. At what point would it become an issue?
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K0BG
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« Reply #4 on: July 03, 2004, 08:30:04 AM »

No. It'll actually handle a 100 watts or so. Just don't let the SWR get too high.

Alan, KØBG
www.k0bg.com
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K1CJS
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« Reply #5 on: July 03, 2004, 10:33:27 AM »

You'll have a mismatch between your radio and the cable and again between your cable and the antenna--depending on the type antenna you are using.

What will happen is the radios finals will start to heat up if you were to transmit at high power, even for just a few minutes.  If you continue, you may cause the finals to fail--it doesn't take much when they get hot.

I blew out the finals of an old two meter set doing that.  In a pinch, and if you cut down your output power, you'll probably be OK, but if you intend to set up a 'permanent' base station, get yourself some 50 ohm cable.  In the long run, you'll be way better off.  You'll hear stations you wouldn't have heard using the 75 ohm stuff, and you'll probably be able to contact (talk to) stations that would be out of your range if you use the 75 ohm stuff, because the radio, cable and antenna will be matched.

Think of it this way.  If you needed to connect a 3/4 inch hose to another 3/4 inch hose but only had a 1/4 inch connector to do it, it would work.  Less water would get through than if the hose and connector were both 3/4 inch, but some water would get through.

You would cut down on the signals getting through the same way if you used 75 ohm cable the way you want to.

The 75 ohm cable would be better off being used for receiver sets only, such as scanners or shortwave sets, although 50 ohm cable would be better for that also.
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AA4PB
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« Reply #6 on: July 03, 2004, 05:00:24 PM »

75 Ohm cable in a 50 Ohm circuit will give an SWR of 1.5:1. It is not going to overheat the finals or burn anything up unless you run more power than the cable is rated for. It's also going to have an insignificant impact on the amount of radiated power or the received signal strengths. As a mater of fact, good quality 75 ohm CATV cable may just have less loss than some run of the mill 50 ohm cable.

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K1CJS
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« Reply #7 on: July 03, 2004, 08:01:46 PM »

SWR adds up--mismatches cause increased SWR.  It is entirely possible for finals to be burnt out by the cumulative effects of reflected power.  An ideal situation doesn't exist, the ideal 1:1 match is just that, an ideal.  The use of 75 ohm cable in a 50 ohm system may cause a severe enough mismatch to damage the finals in a radio.

The radio is looking for a 50 ohm load, the cable is 75 ohms, the antenna is another value, or its frequency mismatch is causing increased SWR, the mounting spot of the antenna has an effect, etc., etc.

Do yourself a favor.  Put your hand on the heatsink area of your radio after you've been transmitting for a while and see if it is hot.  If it is and you're using 75 ohm cable, plan on getting another radio soon, you are slowly destroying the output circuitry of the radio.

If you rather get the best preformance and least trouble from your station, use what the radio manufactures made the radio to use--50 ohm cable.
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AA4PB
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« Reply #8 on: July 03, 2004, 08:15:55 PM »

Well, if you do the numbers you'll see that a 50 ohm load on 75 ohm cable gives an SWR of only 1.5:1. Any decent, modern radio will handle that without a problem. Many radios even have antenna tuners built in that will convert that to an ideal 50 ohm load for the finals. Those that don't will reduce their power output if the SWR goes above 2:1.

Lots of people routinely work with a 1.5:1 SWR. In fact, you are generally wasting your time to put a lot of effort into getting it below that.

If you put up a dipole high above the ground in terms of wavelength, it will actually have an impedance of about 70 ohms, a closer match for the 75 ohm cable than for 50 ohm cable.

As a practical matter, I've run 100 watts using 75 ohm cable with several rigs and never had any "cooked" finals because of it.
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N6AJR
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« Reply #9 on: July 03, 2004, 09:25:40 PM »

BTW  I buy  spools of rg8X 50 ohm coax on ebay from a guy named GSSEC for $85 for a 500 foot spool and put my own connectors on the ends..  keep it short (under 30 -50 feet for uhf/vhf  and put the hf stuff farther out..  cheep and it works for me..
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W8JI
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« Reply #10 on: July 04, 2004, 05:23:08 AM »

by AA4PB on July 3, 2004  Mail this to a friend!  
Well, if you do the numbers you'll see that a 50 ohm load on 75 ohm cable gives an SWR of only 1.5:1. Any decent, modern radio will handle that without a problem. Many radios even have antenna tuners built in that will convert that to an ideal 50 ohm load for the finals. Those that don't will reduce their power output if the SWR goes above 2:1.>>>>>

While it is true the SWR on the 75 ohm cable wouod be 1.5:1, that isn't what the radio sees.

75 ohm coax causes an SWR in a mayched 50-ohm system of anywhere from 1:1 to 2.25:1 SWR, not 1.5:1.

Remember you have to normalize impedance transformations to 50 ohms.

If the antenna impedance is less than 50 ohms, and that sometimes happens, the SWR can be made much worse by using 75 ohm cables!! So you have to be careful.

75 ohm cables will handle a surprising amount of power. When matched RG6 will handle at least 350 watts on 6 meters, and easily 1500 watts CW on 80 meters if in open air. RG-59 about half that.

73 Tom
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AA4PB
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« Reply #11 on: July 04, 2004, 10:42:21 AM »

I believe the SWR on the feed line is determined by the difference between the load (antenna) and the coax impedance so indeed it will be 1.5:1 if the antenna is really 50 ohms and the feed line impedance is 75 ohms. You are correct however that the complex impedance at the transmitter end will vary dependent on the length of the line and the frequency. This is also true with 50 ohm cable if the load is not exactly 50 ohms.

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