130' vs 110' for Multiband Doublet Antenna

See this document for a first pass analysis of the antenna choices that W4BXI is considering.

W1TP Telegraph & Scientific Instrument Museums

Friday's snow in Birmingham was not sufficient to keep folks away from an excellent presentation by Bell Ringer #263 about the history and technology of the German Enigma cipher machine. A reference in Keith's presentation contains a video that reenacts the use of the machine during WWII. 

A visit to the W1TP home page reveals a treasure trove of information and photos of interest to collectors and historians. The focus is on telegraph equipment and scientific instruments and the Enigma. One of his web pages contains some nice photos from the 2009 Dayton Hamfest.

Enjoy!

75 m Loop Antenna at W4UOA

Corrected version -- original had results for double the intended loop length.
Here is some analysis of a loop antenna proposed by W4UOA. The following slide show presents impedance vs frequency for several bands. Resistance and reactance are plotted in the upper half of each slide, and impedance magnitude and phase are in the lower half. Click on any slide to view it in the source album. Additional magnification is available using the magnifier icon in the Picasa album view.



The radiation patterns below show the typical progression in frequency, with more lobes and lower angle of radiation as frequency increases. There is a large amplitude difference between the peaks and nulls in the patterns at higher frequencies.



The antenna elements and support wires are the silver lines. The feedline is blue. The color-coded radiation pattern was left at original scale to show its relationship to the antenna structure. The program allows zooming the pattern much larger, such that the antenna structure is engulfed. Viewing at any azimuth and elevation angle, in increments of 5º, is available. Standard 2-D plots are also available.

The predicted performance of this antenna seems consistent with other published analyses of loop antennas, and does not appear to be adversely affected by un-grounded metallic support wires.

Radio Estate Sale for Oren Anderson, W4HDF

KE4ID is in the process of posting items for Oren's estate sale. You can view available items on this page of the Birmingham Crystal Radio Group web site. Check back often to see additional items as they are added.

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:

• The Cloud Warmer NVIS Beam - A nice article found by kb4xx.
  
• A Short Study of NVIS - A concise presentation of the design and performance. Includes antenna radiation patterns to show effects of various design choices.
  
• Some Notes on NVIS Antennas  - This is a link to several very detailed articles by L. B. Cebik. A free web account provides access to a treasure trove of antenna articles by this author.

We look forward to additional reports from members who are experimenting with these antenna designs.

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.

Bell Ringers Map

Here is a snapshot of a map (updated 2/18/2010) that includes most of the Bell Ringers who appear on 40 or 75 m.  Click on the image to magnify or to download. In this view I've left off the details of cities and roads to provide a less cluttered view of were our members are located.

The placemarks are defined in a KML file that can display in either Google Earth or Google Maps (for viewing in a web browser). Let me know if you'd like to see a different view or if you want to make some yourself using a copy of the KML file.