That is what I use. I use it nearly exclusively for my complete station. Have to use crimp on connecters because it has aluminum shield. If it gives me any mismatch I just trim the length. Remember if the feed line is cut to an electrical half length the input impedance equals the output impedance. The stuff I use I purchase from a local hardware store. It is sold for satellite TV hookup and is rated for up 2.5 GHZ. Cost locally is about .10 cents a foot. Just do not try and use high power while transmitting with it.
Fred (KF0AK)
----- Original Message ----- From: "Fred A Parker" fparker@paulbunyan.net To: "amsat-bb" amsat-bb@amsat.org Sent: Tuesday, May 22, 2007 4:29 AM Subject: [amsat-bb] 75 ohm CATV Cable on 70 cm
If it gives me any mismatch I just trim the length.
Hi Fred, KF0AK
Supposing that the impedance of your load i.e. the antenna is 50+j0 ohm and the characteristic impedance of your transmission line is 75 ohm the SWR in it is 75/50 = 1.5 Because of the above SWR your 75 ohm transmission line make an impedance transformation in any point of it as you can see looking at the 1.5 SWR circle in to a Smith Chart. The only effect to trim the lenght of your transmission line will be that to transform the 50+j0 ohm impedance of your load to another R+/-jX impedance that as seen by your TX i.e. the 50+j0 ohm generator is in condition to transfer the maximum possible power in your mismatched transmission line but in any case the SWR in to the 75 ohm line is still 1.5 in any point of it.
Remember if the feed line is cut to an electrical half length the input impedance equals the output impedance.
It is correct as shown by one complete turn over the SWR circle of the Smith Chart and if the impedance of your load i.e. the antenna is 50+j0 ohm than your TX i.e. the generator will again see an impedance of 50+j0 ohm but the SWR in to the 75 ohm line is still 1.5
Fred (KF0AK)
73" de
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