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 R959 COAX. 
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hello steve, would it be posible to use RG59 FILOTEX 75 OHM COAX CABLE on a 150 foot run from a hf multiband vertical antenna to a yaesu ft-817 qrp radio. only asking as been given a brand new roll of the cable that a mate was going to be use for vidio use and now will not use, regards david


Mon Feb 07, 2011 9:22 pm
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 R959 COAX. 
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David,

What's the antenna?

Steve G3TXQ


Mon Feb 07, 2011 9:29 pm
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 R959 COAX. 
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hi steve and thanks for the fast reply, antenna well havent got one at the moument just wanted to no if the coax could be used with an hf antenna before purchasing one, regards david.


Mon Feb 07, 2011 9:34 pm
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 R959 COAX. 
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David,

The only problem with using 75Ω coax is the impedance transformation it can cause.

Say your antenna had a perfect 50Ω feedpoint impedance. Fed with 50Ω coax there would be no mismatch and the impedance at the radio end of the coax would be 50Ω whatever its length. Fed with 75Ω coax there is a 1.5:1 VSWR line mismatch and the impedance at the radio end will be somewhere between 50Ω and 112Ω depending on the length of the coax.

I don't know the FT-817 well, but I'd guess that if it sees an impedance higher than about 85Ω (VSWR=1.7:1) the protective circuitry would begin to reduce its output power.

In summary, if you have low VSWR antenna and you expect to connect the coax straight into a radio with no internal tuner, you could have a problem with some lengths of RG59.

Hope that helps.

Steve G3TXQ


Mon Feb 07, 2011 9:51 pm
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 R959 COAX. 
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thanks steve, ill have to suck it and see with a borrowed antenna, regards david.


Mon Feb 07, 2011 10:00 pm
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 R959 COAX. 
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David,

A quick look at a Smith Chart shows that if you feed a 50Ω antenna with different lengths of 75Ω cable, for half of all possible lengths the VSWR seen by the radio will be under 1.7:1 and for the other half of all possible lengths it will be above 1.7:1.

Assuming the radio begins to reduce power at about 1.7:1 VSWR, it looks like there'd be a 50:50 chance of success !

Steve G3TXQ


Mon Feb 07, 2011 10:09 pm
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