I wonder what you were using to measure the impedance of the cable with below .5 MHz? Some cable especially rg59 types have copper clad steel center conductors. If the copper clad is very thin low frequencies can penetrate the copper clad and get into the steel where the loss can go up substantially. If you are using that cable to transform an impedance the additional lose can make the impedance transformation something other than expected. The impedance will be closer to the characteristic impedance of the cable rather than the expected transformation impedance. But to have the characteristic impedance fall apart at .5 MHz would be a mystery. 75 ohm cable is used extensively in video base band applications where flat low frequency response is needed.
73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Thursday, August 30, 2007 10:48 AM > To: [email protected] > Subject: Re: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > Gary, > > I've measured RG59 cable terminated into a 75 Ohm resistive load with a > variable freq impedance meter. We found the coax stopped being 75 Ohms > below about 0.5 MHz. The cable manufacture also verified this. Other > engineers in our department knew of this as well. > > We were designing security systems using video and the vertical and > harizonal sync signals became very distored over long, 2500 ft. RG59 > cables and this was the major reason. We had to design circuits that > corrected this, but the cable had the problem. > > I am sure different RG59 cables have different low freq bandwidths. RG11 > would also be different as well as cable TV cable. > > All coax has a lower and upper frequency range. Since we deal with radio > this is not much of a factor until one gets real low or GHz levels. > > Coax also has the problem of a upper freq limit due to it's outer shield > becomes large enough to act as wave guide. One will see upper freq specs > will be lower the larger cable. > > 73, ron, n9ee/r > > > > >From: Gary Schafer <[EMAIL PROTECTED]> > >Date: 2007/08/29 Wed PM 09:23:57 CDT > >To: [email protected] > >Subject: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > > >As far as bandwidth goes,,, where do you get this .5 MHz for rg59 cable > as a > >lower limit? > > > >Open wire lines begin to radiate as frequency is increased to the point > >where the line spacing becomes an appreciable portion of a wave length > due > >to the time it takes for propagation of fields between wires. > > > >73 > >Gary K4FMX > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. > > > > > > > > Yahoo! Groups Links > > >
