Antenna Analysis

This morning's discussion on 75 m covered the characteristics of a commercially-available doublet antenna of 130 ft total length, center-fed with 75 ft of 450-ohm open line. The following additional parameters were used in analysis performed with the 4NEC2 antenna modeling software:

• 12-gauge copper conductors
• height above ground 30 ft
• average ground characteristics

Below is a sequence of horizontal radiation patterns, computed at one frequency for each of the 9 HF bands. The patterns for 80-40 m are very similar in shape. Starting at 30 m, additional lobes appear. You can click on the figure to select and examine any slide more closely.



Using the program's 3D features, the following plot shows the complexity of the antenna's radiation pattern at 28.5 MHz. This is an overhead view, with antenna oriented from left to right. Click on the figure to enlarge.


A frequency sweep from 1.8 to 29.7 MHz was made to investigate the behavior of the antenna's impedance at the transmitter end of the feed line. The following figure shows the calculated values of resistance, reactance, impedance, and phase at increments of 0.2 MHz. Numerical values are available in the program's output file, and one set of numbers (at 7.2 MHz) are shown on the figure. Click on the figure to enlage.


It can be seen that the antenna's impedance (green curve) is fairly high (several hundred ohms) at many of the ham band frequencies, confirming the need for a tuner to achieve matching to a 50-ohm transmitter. The program includes a feature for calculating the needed matching components. This could be used to predict the ability of a given tuner to achieve a match at any given frequency.

John WA5MLF

Poll Results for Saturday Net Operations

After 7 days the web poll on this topic has closed. Thanks to all who expressed their interest in future operations of the Saturday Net by voting.

The choices, from which a voter could choose one OR more, were:

1. Use 75 m exclusively until sunspot activity enables a return to 40 m.
2. Use 75 m with optional Skype and/or phone patch connections for stations who are too distant from the core 75 m participants.
3. Use 60 m
4. Use 40 m exclusively, with the aid of relay stations. Remain on 7230 kHz while evaluating the effect of changing back to standard time on November 1.
5. Implement an RF bridge between 75 m and 40 m, enabling stations to select the band that gives them the best ability to hear and be heard.

The poll recorded 33 participants, with a total of 44 choices selected. Here is a summary of the results, which can also be viewed on the poll gadget, although you must scroll horizontally to see the numbers.

1. 75 m exclusively -- 8 votes - 24% of 33
   
2. 75 m with patching -- 3 votes - 9%
   
3. 60 m -- 1 vote - 3%
4. 40 m with relays -- 20 votes - 60%
5. 40 m - 75 m bridge -- 12 votes - 36%

Below is a pie chart showing the results based on the votes for each choice divided by the 44 total selected choices. Click on the image for a magnified view.

Thus, most responders favor the use of 40 m for the Saturday net. There is also clear indication of a desire to have the best of both worlds, combining the long range of 40 m that favors more distant members with the short range of 75 m that favors the Alabama-Georgia core of participating members. With today's technology we have several choices beyond traditional relaying to help bridge the 40 m and 75 m propagation gaps and enable all members to participate. These include a Skype-based VoIP HF Bridge and a Cross-Band Internet Bridge. These may not be available for every Saturday net, since they involve the presence and extra efforts of two members who are equipped to implement these services.

Based on our October 26 conference call, our goal in the near term is to resume a formal Saturday morning members' net with roll-call check-ins, using whatever tools are available. Weekday mornings have and should continue to hold lively, informal QSOs using two bands.

Feel free to post comments below and/or send email to me or to W4BXI.

John WA5MLF

Saturday Net Operations

In response to John's (W4BXI) call to conference about possible ways to improve our Saturday Bell Ringers Net, the following Bell Ringers met:
John (WA5MLF), John (W4BXI), Carl (W4UOA), Gary (N4OLN), Jorge (KI4SGU), Larry (KB4LWT), and Phil (KB4XX). After discussing our 40 meter propagation problems, here are some ideas we thought might make our Saturday Bell Ringers Net better:

1. Use 75 m exclusively until sunspot activity enables a return to 40 m.
2. Use 75 m with optional Skype and/or phone patch connections for stations who are too distant from the core 75 m participants.
3. Use 60 m
4. Use 40 m exclusively, with the aid of relay stations. Remain on 7230 kHz while evaluating the effect of changing back to standard time on November 1.
5. Implement an RF bridge between 75 m and 40 m, enabling stations to select the band that gives them the best ability to hear and be heard.

We invite your feedback about these choices and any others that may be relevant. Please use the poll feature at the upper right corner of this blog to capture your votes for one or several of the choices. One of our goals is to resume a formal Saturday net with check-ins by member number.

The alternatives are listed in brief form in the poll to enable better visibility of the results after your votes are entered. You'll still need to use the horizontal scroll bar to see results. (The poll feature is one that I have not previously used.)

You may also post comments directly to this blog posting for other members to read.

An updated list of Net Control Stations is posted on our web site.

Thanks for your interest and inputs!
John WA5MLF

Map of 75 m Net: 1994-1997

Following yesterday's discussion of the Net's past operation on 75 m, I looked at my logbook records and found that the Saturday morning net used 3920 kHz in the 1994 - 1997 time frame. Making note of the member stations that checked during that period I produced this Google map showing their locations to reveal the Net's viability on 75 m during the low period of the sunspot cycle. The map supports our understanding of the membership as being concentrated mainly in LA, MS, AL, GA. The highest member number among check-ins at that time was 272.

Click on the link below the map to get a larger view with more navigation features. Many of the member locations shown are only at the city level of detail, not street address. It is interesting to compare this map with the one produced earlier this year that reflects participation in the 2008-2009 time frame.

John WA5MLF


View 75 m Net 1994-1997 in a larger map

VFO Controlled Low Power Transmitter

The Rockless, a VFO Controlled Low Power Transmitter, is featured in the November 2009 issue of QST. Jimmy K9JWJ contacted the author and was directed to The Rockless QRP web site for more info. Thanks, Jimmy, for the link.

ON8PO Radio Museum

See this amazing museum of radio equipment! Thanks to W4BXI for the link!

Early telephone multiplex systems & Bell Labs

During a recent morning QSO on 3968 we talked about early frequency division multiplex technologies for carrying multiple voice channels on individual metallic circuits. Probably the earliest example was the O-carrier system deployed on open wire routes. A few photos are shown at this web site.

Information about these FDM systems is sparse on the web. The above referenced article on FDM does include a link about L-carrier systems deployed on coaxial cable.

N-carrier systems were used on twisted-pair cables starting about 1950. According to two books in my library, N-carrier was used for routes ranging from 15 to 250 miles. They used two frequency bands: 36 to 140 kHz (low group) and 164 to 268 kHz (high group) in opposite directions, on two cable pairs. At successive repeaters the signals were translated between the low group and high group to promote equalization across the entire route. Depending on the version of the system, either 12 or 24 channels were carried on 2 cable pairs.

Searching the web for more information about these carrier systems also yielded a nice article about Bell Labs. Scroll down to find entries grouped by decades.

The Bell Ringers web site also includes some good historical links on the Links page.