Feeding a Multi-band Dipole Antenna

During recent on the air discussions we revisited the design options for a multi-band dipole antenna with balanced feeder. Design guidelines for a slightly-shortened dipole antenna are published in the document Choosing the Correct Balun by W8JI. The recommended lengths of balanced feedline are based on odd multiples of 1/8 wavelength on the lowest frequency of operation. This is covered on pages 7 & 8 of W8JI's document.

Some of our group wondered if the same feedline recommendations would apply to a standard half-wave dipole as they do to the slightly-shortened dipole. To explore this topic I used the 4nec2 antenna modeling software to analyze a half-wave dipole of total length 134 ft (12 ga copper) at 30 ft above rocky ground. This length is close to resonance at 3.5 MHz. The shortened dipole in the article, to cover 80 m and above, has length 110 ft.

I collected data for the feed point impedance at one frequency on each of six bands. I also expanded the model to use a 450-ohm feedline of length 30.48 ft (close to the 1/8 wave length of 30.9) and collected the impedance data at the end of the feedline for the same 6 frequencies. The results are shown in the graph and table below.

The graph clearly shows the wide variation of impedance (orange bars) at the feedpoint, while the impedance at the xmt end of the feedline (blue bars) shows that the impedance variation is considerably narrowed. This supports the comments on page 7 of the W8JI document.  At 7 MHz the dipole becomes a full wavelength, with very high impedance, but the feedline -- 1/8 wavelength at 3.5 MHz -- becomes 1/4 wavelength at 7 MHz, thus transforming to a much lower impedance.

The table below gives the data used in the graph. I did not include the resistance and reactance data, but they are readily available.

f (MHz)	Z (xmt)	Z (fp)
3.5	338	56
5.3	539	1156
7	105	5970
14	839	2433
21	453	1637
28	225	1180

This analysis covers only the 1/8-wavelength feeder case, but any other lengths (e.g 3/8, 5/8) can be analyzed easily.

John

VoIP Patch: Links

As background, here are some links (in reverse date order) to blog postings about the topic. Several of us morning regulars, especially W4BXI and W4UOA, have put together and tested various configurations over the past five years. The configurations have been successful for the great majority of the time, but testing has revealed some configurations that work better than others or are easier to operate than others.

There are many variations of PC hardware and different ways to use the available VoIP services. 

http://thebellringers.blogspot.com/2014/02/voip-patch-discussion.html

http://www.w4uoa.com/2013/03/dual-skype-one-computer-skype-patch.html

http://www.w4uoa.com/2013/01/w4bxi-napkin-patch_25.html

http://wa5mlf.blogspot.com/2012/03/sound-card-controls-for-voip-patching.html

http://wa5mlf.blogspot.com/2012/03/voip-patch-update.html

http://thebellringers.blogspot.com/2011/07/voip-phone-patch.html

http://thebellringers.blogspot.com/2009/09/latest-w4bxi-system-audio-drawing.html

http://thebellringers.blogspot.com/2009/08/voip-hf-radio-bridge-in-qst.html

http://thebellringers.blogspot.com/2009/04/w4bxi-system-audio.html

http://www.w4uoa.com/2008/08/voip-hf-bridge.html

102nd Titanic Special Event Station

Here is info about the special event stations that will commemorate the sinking of the Titanic:

W0S  
Apr 12-Apr 13, 1400Z-1400Z,
Frequencies (MHz): 21.265, 14.265, 7.265, 3.865.
Contact: N0SAP David Beckler,
1137 W Crane Dr, Nixa, MO 65714.
n0sap@yahoo.com

W4S
Apr 12-Apr 13, 1200Z-0100Z, 
Frequencies (MHz): 21.335, 14.235, 7.265, 3.985
Clinton, TN.
Contact: KC4RD Jim Womack,
617 Black Oak Rd, Clinton, TN 37716.
kc4rd@arrl.org

Map of morning QSO regulars

It's been 4 years since I produced a Google Earth map showing where members of our HF radio group are located. Below is a screen shot of a new map that shows the locations of most stations who have participated in our morning QSOs on 75, 60 and 40 m.  Some are on the air daily while others are occasional participants. Click on the map to see a full-sized copy. This updated version includes several more stations.

At this scale, some stations that are close together do not have their callsigns displayed. Below is another view that is zoomed in on the area with the majority of AL, GA, TN and NC stations.

Both maps give a good view of the relative distances and directions among stations. The first map shows which distant stations (like mine) experience fading signals on the lowest bands as the sun moves higher each morning.

John
(updated 3/19/2014)

BirmingHAMfest Photos

Photos from the event, taken by KC0ONR, can be viewed here.

Using Windows XP after April 8

Page 13 of this 40-page report discusses the use of Windows XP after the end of Microsoft support next month. It provides a list of recommended actions to reduce the exposure to future exploits if you plan to continue using XP.

The report was a small part of episode 446 of the Security Now netcast on March 11.

For those who are interested, the episode's major topic of iOS Security is based on this report from Apple.


73,

John

VoIP Patch Discussion

Our group has tested many variations of hardware-based and hybrid hardware-software-based phone patches. Several variations of patches based on VoIP computer applications seem to work well, enabling the radio operator to be on a full-duplex VoIP conversation with the distant phone user. One way of accomplishing this is to use two PCs: one to provide the radio interface and another to provide the control operator's audio paths. This has also been done with a single PC when the sound card hardware or radio interface provide some needed mixing or monitoring features.

Today's discussion moved toward understanding the minimum required configuration to accomplish the patch while still using VoIP. The drawing below (click to expand) is my attempt to identify the functions and interfaces that are needed. It is not a solution, but is intended to stimulate further discussion that may lead to the best solution. One of the desired features is to enable the control operator to use a high quality microphone for all of his speaking needs.

Comments are welcome, and together we can test some additional configurations to find what works best. I applaud W4BXI for his persistence in testing and using so many alternative approaches. A simplified approach will make the patch capabilities accessible for a greater number of our radio friends.