Gary, To measure the impedance of the RG59 I used an HP impedence meter which displayed Z and phase. I use to use it to determine where caps became resonant as a demo for many caps look inductive above a given freq. Mica caps did pretty good, but still hard to find a cap at 1000 pf that was a cap above 25 MHz. These become issues in bypass caps and also for resonant circuits trying to get higher Qs where the C is large.
In a good lab one often has tons of test equipment for making measurements, even spectrum anal that go down to tenths of Hz and to many GHz. I've had the previdlege of working in such places and some was for my use in my work. I know RG59 is a most commonly used cable in video. However, one does not have to go far before it really affects video especially color where the phase is so important. Also the syncs get torn up so bad monitors loose sync on the veritical retrace and a portion of the picture is torn at the top. Many manufactures make line amps that not only compenstate for loss, but varied freq response and some for sync...the better ones do sync also. The vertical sync is at about 60 Hz and horiz at 15734 Hz which is in the audio freq where the signal is not really propergating like in RF. Many things change. Of course for a run of couple hundred feet this is not a problem, but long runs it becomes one. 73, ron, n9ee/r >From: Gary Schafer <[EMAIL PROTECTED]> >Date: 2007/08/30 Thu PM 07:39:21 CDT >To: [email protected] >Subject: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > >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 >> >> >> > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
