Are you sure that the impedance meter you used was speced for operation below .5 MHz?
Yes all capacitors have inductance. Lead length is particularly a problem. 15 KHz can be treated as RF or audio it all depends on what transducer you are using it to couple it with. Use a speaker and it is audio. Use an antenna it is RF. All RF propagates the same on a transmission line. 15 KHz or even 1 KHz propagates as RF just like any RF signal does through the air and even thru the ground as in the case of low frequencies. Read about what some of the VLF guys are doing. On a video cable remove the termination on the far end of the cable and look at the reflected energy. It has the same effect at those frequencies as it does at HF or VHF. Yes long runs of video cable can be a problem. Long runs of cable in the catv industry have the same problems of frequency roll off. They call it "tilt" and their amplifiers have compensation for cable attenuation in order to make the system "flat". I have an HP signal level meter that measures RF from 10 Hz to 30 MHz. I can feed an audio oscillator set to 1 KHz or 1 MHz into the same input as I feed a 1 MHz RF generator into. The signal level meter handles it the same. Only difference is the output impedance of the audio oscillator is 600 ohms rather than 50 ohms. The instrument doesn't know or care if we want to call it audio or RF. As far as it is concerned it treats it as RF. I have an audio amplifier that has just about a flat response from around 5 Hz to 1 MHz. Is that an audio amplifier or an RF amplifier? :>) 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Friday, August 31, 2007 7:12 AM > To: [email protected] > Subject: Re: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > 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. > > > > > > > > Yahoo! Groups Links > > >
