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: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Gary Schafer
Sent: Tuesday, January 13, 2004 12:04 PM
To: [email protected]
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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