[TowerTalk] How to cut 75 Ohm lambda/4 phasing lines (with 50Ohm testequipment)?

[jeremy-ca]

You can use that same method with 75 Ohm cable. You are looking for the 
null, not an absolute power value.

The same holds with the NA, the null frequency will remain the same but the 
return loss will vary depending upon the lengths of the connecting cables.

Ive used a NA, sig gen, noise bridge and even a sweeper to a spectrum 
analyzer over the decades to cut coax. In a pinch a receiver with antenna 
tuned to a noisy band will work. The human ear can readily detect the point 
of maximum null.

Carl
KM1H
----- Original Message ----- 
From: "Carsten Esch" <carsten.esch at appello.de>
To: <towertalk at contesting.com>
Sent: Tuesday, August 14, 2007 11:32 AM
Subject: Re: [TowerTalk] How to cut 75 Ohm lambda/4 phasing lines (with 
50Ohm testequipment)?


> Hi,
>
> I want to cut lambda/4 phasing lines (RG-11 foam) for some 4-Squares using
> hybrid couplers and have the following question ...
>
> In the past I did cut some 50 Ohm phasing lines with a signal generator
> going to a T-connector, one side of the T going to a precision wattmeter
> (with internal 50 Ohm load) and the phasing line (open at the end) went to
> the other side of the T. With this set up it was very easy to cut the 
> cable
> to the desired length (minimum power in the load).
>
> Can I use the same set-up to cut 75 Ohm lines or would it be necessary to
> use a 75 Ohm load?
>
> How big is the error if a networkanalyser (with 50 Ohm input) is used to 
> cut
> the length?
>
> Thanks for any insight ;-)
>
> 73ss
>
> Carsten, DL6LAU
>
> _______________________________________________
>
>
> ... maybe I found the answer myself after writing the mail and thinking it
> over ;-) ...
>
> Could it be that the set-up with the T-connector is independent of the 
> used
> impedance of the cable because it will create a short at the T when the
> lamba/4 lenght of the line is found??
>
> Please correct me if I am wrong (or say yes you are right ;-).
>
> Thanks!
>
> Carsten
>
> _______________________________________________
>
>
>
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