Re: [amsat-bb] Coaxial's "Z" request

["i8cvs" <domenico.i8cvs@xxxxxx>]

Hi Sil, ZL2CIA

If you can make an "educated guess" of  the dielectric constan K of the
insulating material than the equation that you mention is correct provided
that Dshield is the inside diameter of it i.e. the outer diameter of the
insulating material.

If the inner conductor is a round wire it will be easy to measure the
outside diameter i.e. dcentre of the equation using a micrometer as you
suggested

If the inner conductor is a stranded copper wire it will more difficult to
get the exact dcenter

Alternatively if you believe that the characteristic impedance is Zo=92 ohm
and if you can fix the type of  the insulating material to get K than you
can cut any odd numbar of 1/4 electrical wavelengts of it for a frequency as
low as possible and 28 MHz is OK

Connect the above matching line to the output of  any VSWR meter designed
for 50 ohm (Zi)  and terminate the matching line with a carbon resistor Zu
wich value can be calculated with Zu = Zo^2 / Zi  = 92^2 / 50 = 170 ohm

Three 47 ohm 1 watt carbon resistors connected in series with short leads to
one of 27 ohm standard values is a good choice to get 168 ohm +/- 5 % and if
the measured VSWR  is close to 1 than Zo of your cable is very close to
92 ohm

An output power of 5 watt for a few second in to a 3 watt dummy load like
the above is adeguate enought for a quick measurement using simple
instrument and the already carbon resistors in any junk box.

If you suspect that your carbon resistors are inductive like the spiralized
type than it's inductive reactance can be partially compensated  foldind
over it a soft foil of copper as a capacitor and soldering it to only one
lead of this resistor and a suitable copper foil comes from the outer
conductor of many TV coax cables.

>From the above equation you can derive Zo= SQR ( Zi * Zu ) and
Zi = Zo^2 /Zu so that using the output of your 50 ohm VSWR meter as Zi
and using a 1/4 electrical wavelengt long matchin line for Zo you can
experiment with many combinations of  know and unknow factors of
your coax cable

Have fun

73" de

i8CVS Domenico

----- Original Message -----
From: "Sil - ZL2CIA" <zl2cia@amsat.org>
To: "Amsat-BULL" <amsat-bb@amsat.org>
Sent: Friday, April 07, 2006 11:53 AM
Subject: Re: [amsat-bb] Coaxial's "Z" request


> Hi,
>
> I have a similar question.
>
> Like Mak, I have a coil of unidentified coax. I believe it to be 92 ohm.
> It was offered to me for free and seems to be in good condition. I have
> used RG62 to make matching sections in the past, so it seemed like a
> good idea to accept it (= how your attic fills up with junk).
>
> It seems to me that if I carefully measure the diameter of the centre
> conductor and the braid diameter (using, say, a micrometer), I should
> then be able to calculate the characteristic impedance using the formula
> Zo=(138Log10(Dshield/dcentre))/SQRT(K)?
>
> (from Radio Data Reference Book - G. R. Jessop)
>
> The dielectric constant K can be obtained by making an "educated guess"
> about the insulating material.
>
> Will this work?
>
> Accurately measuring the diameter of the centre conductor (dcentre)
> would seem (to me) to be the problem. Small errors here will
> significantly change the result.
>
>
> Sil - ZL2CIA.
>
>
> SV1BSX wrote:
>
>  > Hi all,
>  >
>  >  I bought a Coaxial-cable that mentioned  Z=120 Ohms.
>  > (I thought that they will be very useful for "stubs").
>  >
>  > The type of coaxial (written on plastic cover) is:
>  > 2002  HCE  02 0798
>  > Unfortunately, looking for specifications ( VF etc) is impossible to
>  > find anything about it. Seems to be completely unknown coaxial
>  > cable. So now I'm wondering also about the impedance of it !
>  >
>  > Is it possible to measure the "Z" into an unknown coaxial with simple
> instruments which are available in our Shacks in order to confirm if the
> impedance is 120 Ohms?
>  > Any suggestion or idea is welcome.
>  >
>  > Thanks in advance.
>  >
>  > 73, Mak SV1BSX
> ----
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