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## [TowerTalk] Finding roof beams, determining impedance of coax cable.

 To: [TowerTalk] Finding roof beams, determining impedance of coax cable. jaf00@aag.com (Flanders, Jim) Thu, 26 Jun 1997 14:09:22 -0500
 ```All you have to do is change the resistor value to 72 ohms. Actually, a variable Resistor would be great if you can find a noninductive variable. Jim Flanders W0ooG -----Original Message----- From: Edward W. Sleight [SMTP:k4sb@worldnet.att.net] Sent: Friday, June 27, 1997 3:17 AM To: David O Hachadorian Cc: towertalk@contesting.com Subject: Re: [TowerTalk] Finding roof beams, determining impedance of coax cable. David O Hachadorian wrote: > >Take a half wave length of cable and terminate it with a non-inductive > > > >resistor such as 50 ohms. > >Place a swr bridge at the TX end. Tx into the cable with just enough > >power to get a reading on the meter. > >If the swr is 1 to 1 the cable is the impedance of the resistor. Note > > > >that the MFJ antenna analyzer will also > >do this for you, but not all of us have one, and we all should have a > >swr bridge and a 50 ohm dummy load. > > Actually, it should be almost any length of cable EXCEPT a half > wave. A half wave will present the 50 ohm termination, > regardless of the cable's characteristic impedance. > > A better way to do the test is to hook up the 50 ohm resistor to > a random length of cable and then vary the transmitter frequency > over a wide range of frequencies. If the swr is flat over a wide > range of frequencies, then it is 50 ohm cable. If it's not 50 > ohm coax, experiment with various 2 watt carbon terminating > resistors until you find the proper one that makes the swr curve > flat. > > Dave Hachadorian, K6LL > k6ll@juno.com > Sorry, but this won't work either on cables other than 50 ohms, UNLESS the bridge is set up for 72 ohm operation. ( or whatever the subject coax actually is ) You might flatten the SWR curve in sucha a manner, but in actuality, all you are doing is another version of changing the length of the coax. And all you are doing here is fooling the bridge ( or transmitter ) which is actually OK, unless you're dealing with very high SWR figures. ( what you are doing in such a case is trying to find a point on the line where the outgoing wave is 180 degrees out of phase with the reflected wave. ) Best Ed -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [TowerTalk] Finding roof beams, determining impedance of coax cable., Flanders, Jim [TowerTalk] Finding roof beams, determining impedance ofcoax cable., David O Hachadorian [TowerTalk] Finding roof beams, determining impedance ofcoax cable., k4sb@worldnet.att.net [TowerTalk] Finding roof beams, determining impedance of coax cable., k4sb@worldnet.att.net [TowerTalk] Finding roof beams, determining impedance of coax cable., Flanders, Jim [TowerTalk] Finding roof beams, determining impedance of coax cable., Flanders, Jim <=