Monday, January 11, 2010

Impedance of Twisted Pair Wire / Cable

A recent morning discussion by our group on 75 m included the use of twisted pair wire or cable for certain HF antennas. There were questions about the impedance of twisted pair wire at HF. Since Jack KE4ID is building a 2-conductor Beverage antenna using "field phone wire", Bill KB4PYR suggested that he use a noise bridge to estimate its impedance at the HF range of interest. A good article about Beverage construction is this one by W8JI.

Today Jack advised that his noise bridge measurements show the impedance to be about 100 ohms in the 80 m frequency band. The results were the same whether or not a current balun was included at the source. The 100 ohm value of impedance at radio frequencies is consistent with many web pages that can be found using the search string "twisted pair cable impedance". One example source is the article Twisted pair cables.

4 comments:

Jorge Luis - ki4SGU said...

What about the foil shield?? I'm thinking of CAT5, and maybe his cable does not have any foil, but if it did -- Would that be a show stopper as an antenna ??

wa5mlf said...

The cable mentioned by KE4ID is unshielded, single-pair telephone 'field' wire. I think there was also discussion of using garden variety electrical zip cord, also unshielded.

Carl Ferguson said...

Is the whole purpose to get more "surface area" radiating?

wa5mlf said...

As I recall, part of KE4ID's reason for the single-pair wire was to implement a 2-element beverage as noted in the Wikipedia reference:
"Some Beverage antennas use a two-wire design that allows reception in two directions from a single Beverage antenna."
For use as dipole elements, a pair of wires would provide the virtues of greater surface area and less resistance.