Thursday, January 21, 2010

NVIS Antenna Resources

NVIS antennas have been a very popular topic among our members recently. A very interesting view of NVIS potential is found in the Hourly Area Predictions web page shared by ki4sgu. Use the pull-down menu under General Charts For Global HF and select Atlanta, then click the display button. The resulting color-coded chart indicates recommended frequency ranges for HF radio communication with the Atlanta area. It seems to conform fairly well with our morning QSO experiences on 75 and 40 m.

There are many published resources about the design and performance of NVIS antennas. Here are a few that we discussed recently on the air:
We look forward to additional reports from members who are experimenting with these antenna designs.

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.