# ARRL

## AC7LX

 Joined: Tue, Feb 12th 2019, 16:31 Roles: N/A Moderates: N/A

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Electrical Quarter and Half Wave coax length? Jul 26th 2019, 17:11 6 4,014 on 29/7/19

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Electrical Quarter and Half Wave coax length? AC7LX on 27/7/19
Zak,

Electrical Quarter and Half Wave coax length? AC7LX on 27/7/19
Zak,

First, Thank you for the response.

However, the fundamental answer to the question was not provided.

Not that I disagree with with anything you've stated. In a perfect world all of these factors would be taken into consideration.

It's just that I am looking for a more basic answer here.

Assume I don't have access to a TDR or analyzer, although I do agree one could resort to basic VSWR, something most amateurs should be capable of doing.

Further assume, for the sake of practicality, that a given piece of cable does meet specifications.

Given the above, before one gets to verifying/analyzing a cable, one needs to have a starting point, ergo the calculated electrical length.

Variances in coaxial cable and connectors notwithstanding, but set aside for the moment... once I've calculated a given electrical length, where/how do I measure to apply that calculated length when cutting the coaxial cable?

Again, I am looking for basic, fundamental, answer here as it pertains to a given calculated electrical length. Where is the calculated length measured?

It's logical to assume that there will be some trimming of the cable to accommodate soldering to either another cable or some end point. Further assume that the trimming to accommodate a connection is as minimal as possible.

Therefore, does not determining where/how to cut, include knowing where to measure... before cutting?

Do I measure the calculated electrical length referenced to length of the cable shield or, do I measure the calculated electrical length referenced to the length of the inner conductor?

:D
Electrical Quarter and Half Wave coax length? AC7LX on 26/7/19
Calculating the electrical length of a quarter or half wave seems pretty straight forward. Or so it initially seems.

Say, for example, I wish to make an electrical quarter wave section of coax to convert a 100 Z ohm antenna feed point to 50 Z ohms to match my feed line.

I take a length of RG-59, calculate the length (in feet) using 983.6 (speed of light) divided by the frequency (145 MHz) and multiply that by the cable velocity factor (in this case 0.78). This gives me a length of ~1.3227 feet.

OK great, right? Not Really...

My question here is, where is this calculated length actually measured?

The Shield length?
The Center conductor length?
How about when connectors are installed on both ends?

Any clarification would be appreciated.