[AMRadio] RE: r390a, sx17, t4xb for sale

[Gary Schafer]

Most RF coax cable has a capacitance anywhere from 26 to 30 pf per foot.
So if you had 15 feet of cable you had about 390 to 450 pf of parallel 
capacitance.

The preamp output and amp input may be higher impedance than you 
realize. If you are feeding the output from the plate of the preamp it 
will be fairly high.

The difference between audio and RF service is that in RF the cable is 
usually operated at its characteristic impedance. In the case of this 
cable 50 ohms. The capacitance and inductance in the cable work together 
as many tuned circuits. The loss is then limited to the AC resistance of 
the wire in the cable.

The characteristic impedance of the cable does not come into play until 
it is some significant part of a wavelength.

If you used a very short length of the same cable at RF it would look 
more like a capacitor. Just like it does at audio.

At audio frequencies the cable would have to be several hundred feet 
long before its characteristic impedance would start to have any effect. 
  So the dominant effect at these frequencies is the parallel 
capacitance that is in the cable.


If the cable is used in a low impedance circuit then the 300 to 400 pf 
of capacitance may be insignificant. But if the same cable is used in a 
high impedance circuit, several thousand ohms, then the 300 to 400 pf of 
capacitance forms a good high frequency attenuator.

73
Gary K4FMX


Brett Gazdzinski wrote:
> I know the D104 is very sensitive to the cable used, on mine,
> only the cable that came with the mike works well at all.
> This was on a line level link between the output of the mike
> preamp and the power amp, lowish impedance I thought.
> I measured the capacitance of the cable and got something like 
> 10 PF? I think.
> 
> And how would a cable be an efficient transporter of RF but not
> audio frequencies?
> Surely capacitance or inductance would impair RF more than audio???
> 
> Resistance is not a problem in rg8m...
> 
> 
> Brett
> N2DTS
> 
> 
>>-----Original Message-----
>>From: amradio-admin at mailman.qth.net
>>[mailto:amradio-admin at mailman.qth.net]On Behalf Of Gary Schafer
>>Sent: Tuesday, January 13, 2004 12:04 PM
>>To: amradio at mailman.qth.net
>>Subject: Re: [AMRadio] RE: r390a, sx17, t4xb for sale
>>
>>
>>High capacitance per foot on the cable. In the high impedance mike 
>>circuit a little capacitance will kill the highs.
>>
>>73
>>Gary  K4FMX
>>
> 
> 
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