Topband: Re: Coax capacitor losses

[Carel, pc5m]

Is not a rule of thumb to use:

Just use the dB/m loss on the frequency of interest the actual length of the
coaxial stub ?

Example RG8, @1Mhz loss 0.5 dB/100ft (@10MHz: 1.5 dB)
Length stub = 20 ft, will give al loss of  0.1 dB (0.3dB)

Assumed is that the impedance of the vertical 36 Ohm ? Is almost equal to 50
Ohm ...

Don't think 0.1 dB (0.3 dB) is something to worry about....

Or, is this too much simplified....

Rg,

Carel, pc5m

-----Original Message-----
From: topband-bounces at contesting.com [mailto:topband-bounces at contesting.com]
On Behalf Of John Kaufmann
Sent: Saturday, December 18, 2004 4:10 AM
To: richard at karlquist.com
Cc: topband at contesting.com
Subject: RE: Topband: Re: Coax capacitor losses

John Kaufmann said:

> Let me update my earlier post about losses in coax lines, where I 
> cautioned people about using them as reactance elements (capacitors or 
> inductors).  If you are familiar with the Smith Chart, you can 
> calculate the loss resistance in a section of line, based on its 
> length and its loss specs (dB/ft or dB/m).  Then you can determine 
> whether or not you will

Rick Karlquist said:

> You also need to know how much of the loss is due to dielectric loss 
> tangent vs conductor resistance.  For a matched 50 ohm line at 1.8 
> MHz, the dielectric loss is negligible.  However, it may be 
> significant in a capacitor made out of coax.



When I refer to "loss resistance" in my earlier post, I am really referring
to an equivalent loss resistance that accounts for power dissipation from
all physical mechanisms.  It models as a resistance in series with the
desired reactance (capacitive or inductive) and is what you would measure
with an impedance bridge connected to the end of the line.  This series
representation of reactance and resistance also represents the equivalent
circuit that the antenna would see if you connect the line in series with
the antenna input.  You can use a Smith Chart to calculate it based on
transmission line behavior of the line as my earlier post also indicates.

73, John W1FV

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