Hi All,
Yes to coax not being perfect. And it has problems too. And it's not a
stupid question.
The variable no one has ever brought up is "coaxial cable structural return
loss" eg: How well can a manufacturer make a 50 ohm coax truly 50 ohms? I
have been bitten by this in the past because an HP8753D/E Network Analyzer
or other instruments removes the cable's imperfections when calibrating.
Then you connect the antenna and ask what happened to the SWR?
It is not uncommon to find return losses of around 25 to 30 dB in EBay
cable, swapmeet stuff, etc. Good cable should be 40 to 50 dB. The caveat is
you need to have a 50 ohm load that is about 55 dB return loss itself. The
HP909 N Female from the cal kit is about the best you can hope for unless
you can find a NIST certified unit.
My 2 centavos....
73 N7RT
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
jimlux
Sent: Wednesday, June 01, 2016 4:29 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Stupd question about measuring Z and SWR
On 6/1/16 1:20 PM, Hans Hammarquist via TowerTalk wrote:
> When you want to measure the "real impedance" at the antenna, isn't it
important that you have a feeder (coax) that has the correct Zo=50 ohms all
the way?
>
>
> I guess that the bridge will not read a correct antenna Z if the coax is
not perfect.
>
>
actually, what you need is a feedline that has a *known* Z and length.
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