Ron, Maybe you could tell us why coax cable has a lower frequency limit? You claim that it does but have not explained why or how.
Why does the impedance change significantly at lower frequencies? 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Saturday, September 01, 2007 8:49 AM > To: [email protected] > Subject: Re: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > Gary, > > Yes the HP meter was spec'd to go below below 0.5 MHz, it went down to 100 > kHz. > > I don't know where the confusion is...all coax and feedline has a upper > and lower freq limit. Might try to learn something about this. > > I know about low freq RF. Worked on a Navy program that used 18 kHz, a > C130 aircraft with 30,000 ft of wire hung out the back as a platform to > talk to surmerged submarines. Ran over 250 kW. It was called TACMO. Due > to the weight the wings kept falling off...well they were continously > inspected and replaced before they fell off, but the aircraft was > deffinitly over loaded. Had generators on all 4 engines to get the power > they needed. Now that was a repeater. > > However, AC power distribution is not trying to radiate power, but > transfer it with widly varing loads. Totally different engineering. > > At low frequencies such as 1 kHz little radiation takes place. Far less > at 60 Hz. The EMF returns to the radiator, wire, before the next cycle > can force it out. This is a problem in some applications, but since most > do not want radiation it is not. > > 73, ron, n9ee/r > > > > > >From: Gary Schafer <[EMAIL PROTECTED]> > >Date: 2007/08/31 Fri PM 05:59:28 CDT > >To: [email protected] > >Subject: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > > > >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 > >> > >> > >> > > > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. > > > > > > > > Yahoo! Groups Links > > >
