Lineman's Test Set from 1940's

From WA9JNM:
You might like these pictures of my grandfather's telephone lineman box. He worked as a lineman during the 1940's in southern Indiana. The darn thing still cranks out the
voltage around 30 volts. 73's Steve

What is that single sideband?

Today's discussion on the air was good for stimulating the brain cells as we considered what a single sideband RF signal really consists of. The components of a classic AM signal example using a single, steady audio tone are familiar and are well-represented in the ARRL Handbook and numerous textbooks. In my copy of the Handbook, the chapter on Mixers, Modulators and Demodulators derives the result of mixing (multiplying) a carrier and a modulating frequency. The result is shown as:

AM signal =

sin 2f_ct + ½ m cos (2f_c - 2f_m)t - ½ m cos (2f_c + 2f_m)t

where: f_c is the carrier frequency, f_m is the modulating frequency

You recognize the first term as the carrier, the second term as the lower sideband and the third term as the upper sideband.

A simple view of single sideband would discard the carrier term and one of the sidebands. This could be implemented, for example, using a sharp filter. Examining the term that is left shows a constant sinusoid (cosine function) at a frequency above or below the original carrier frequency by an amount equal to the constant modulating frequency.

During our discussion I postulated that if I transmit a pure (single frequency) audio tone on my SSB transmitter and did not tell you where I was tuned (e.g. 3740 kHz), you could not tell, by tuning your receiver, what audio tone frequency I was transmitting. This is supported by an illustration in the Modulating Sources chapter of my copy of the Handbook. It shows a spectrum analyzer display with a single peak and an oscilloscope view of a constant amplitude RF envelope. The caption labels it as “an unmodulated carrier or single-tone SSB signal”.

Another way of saying this is: Suppose another ham tunes up with a carrier at 3738 kHz. What do you hear at 3740 kHz on your SSB receiver on LSB? You hear a 2 kHz audio tone. Now suppose I transmit a 2 kHz audio tone on my SSB transmitter on LSB on 3740 kHz. If you are listening on LSB on 3740 kHz you hear a 2 kHz tone. The effect is the same.

In the absence of more rigorous analysis, I maintain that a constant pure audio tone transmitted on SSB is equivalent to an unmodulated carrier. Of course, the real world equipment generating such a signal will add some distortion, making it not precisely identical to an unmodulated carrier. Also, a voice waveform is highly complex, with multiple varying frequencies and amplitudes.

The more rigorous treatment of SSB (example) uses math that is equivalent to the phasing method of generating SSB. It is a notch up in level of complexity compared to what is presented in the Handbook. It is also the basis of many communications systems that we take for granted today: broadband Internet access, digital TV, cell phones, etc.

Comments?

John WA5MLF

Net Manager - WA5MLF

WA5MLF (Bell Ringer #17) has assumed the duties of Net Manager for the Bell Ringers Net, effective 11/15/09. He succeeds W4BXI, whose leadership in this role is much appreciated. Any questions or suggestions about the Bell Ringers Net should now be directed to WA5MLF.

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