Late spring brings with it the start of a new severe weather season. As we saw this week in the Dayton area, severe thunderstorms and tornadoes can cause damage and threaten life. The Hara Arena, the former site of Hamvention was destroyed by a tornado.
National Weather Service forecasters track storms carefully using radar and automated weather observations, but their forecasts can be aided by Skywarn reports from trained observers. These eye-witness observations can validate radar data and help forecasters make decisions on whether to issue additional warnings.
I recently attended a Skywarn training session conducted by the Norton, MA National Weather Service office. The class was held at the Milford, MA Fire Department and lasted about two and a half hours.
Rob Macedo (KD1CY), the Norton office’s Skywarn amateur radio coordinator, taught the class with the assistance of an NWS forecaster. Rob started the training with an overview of how often southern New England has severe thunderstorms and tornadoes. He discussed a few of the tornadoes that have occurred in New England, and the role that Skywarn spotters played by informing NWS forecasters of conditions on the ground.
Rob taught that moisture, instability, and lift are the ingredients needed to form a thunderstorm. He also told us about the stages of a thunderstorm, thunderstorm types, and cloud identification.
The instructors spent quite a bit of time on thunderstorm types and cloud identification. Properly identifying clouds is key to spotters providing accurate reports to the forecasters.
The NWS forecaster concluded the training with a discussion on challenges of weather spotting in New England and the criteria for reporting weather phenomena.
This was the fifth time I’ve taken the course over the last 20 years. I was impressed by how the training materials use videos and photographs from New England to help prospective spotters better understand what they’ll see here. My first training I took in the late 1990s was done with a slide projector showing photographs of perfect examples of thunderstorms in the mid-west, things we’d never see here.
Skywarn training is a wonderful opportunity for any weather-lover, and a great way to fulfill our obligation to use amateur radio to benefit the public. Learn more about local Skywarn programs at the NWS Norton office, and at the WX1BOX website.
In the first RockMite article, I described the receiver of the radio. I identified the place in the receiver that accepted the output of the oscillator (marked B going into pin 6 of the mixer) but skipped describing the actual oscillator. In this article, we’ll examine the oscillator and how it drives both the receiver and the transmitter.
The RockMite Overall
As a reminder, here is the whole schematic for the RockMite.
Schematic for the RockMite transceiver (from QRPMe.com)
Figure 1. The Schematic for the RockMite Transceiver
The part of the schematic we need to analyze for this article is near the word “Transmitter” in the big schematic. I’ve cut it down in the next figure (and removed some of the distracting elements).
Figure 2. The RockMite Oscillator
Colpitts Oscillator
The RockMite has within it a Colpitts oscillator. The transistor Q4 is the amplifier and the two capacitors C10 and C11 form the voltage divider. This design employs a crystal (Y2) that is band-dependent, of course, as are C10, C11, and C12. The output of C12 leads to another layer of transistor amplification that enlarges the voltage swing.
Note point B between R13 and R14. This signal leads to the input of the mixer U1 and serves as the beat frequency oscillator (BFO) that detects the received signal.
Silicon Tuning Diodes
Before I can describe the rest of the circuit I need to introduce a part that I thought was almost magical when I learned about it. The part number for these devices is MVAM109 (in the center of Figure 2). The symbol looks like a cross between a diode and a capacitor. Indeed this is exactly what this part is: a diode and a capacitor combined in a very special way. The varactor (tuning) diode acts like a capacitor, but the capacitance depends on the reverse voltage across the diode. I’ve included a plot from the Motorola data sheet below.
Figure 3. Capacitance versus reverse voltage
You can see that as the reverse voltage goes up the capacitance exhibited by the part goes down. Change the voltage and you change the capacitance. We use this in the part of the circuit described in the next section.
Tuning One of Two Frequencies
The oscillator uses Y2 (a crystal) to help determine the frequency. But, the frequency can be altered by adding or subtracting capacitance. MVAM109 (D6) will be the device that changes the capacitance.
Examine the circuit beginning with Q2 (2N7000). When the transistor is turned on there is a completed circuit between the source and drain (S and D in the diagram). In this case, the source is connected to ground. So, when the transistor is turned on the point in the circuit after D is brought down to zero volts.
Check out R9, a 4.7K Ohm resistor that connects to the drain of Q2, and to a Zener diode D5 (which is also band dependent, as it turns out). The Zener is used as a voltage regulator. Say the Zener is a 3.9-volt model. Since we are connected (through R9) to V+, the voltage at the point where D5 connects to R9 will be limited to 3.9 volts (or whatever the Zener is rated) and R9 is the current limiting resistor. This is assuming the transistor switch Q2 is open (not conducting).
In this situation we have D6 being reverse-biased to the voltage determined by the Zener diode. That much reverse bias will cause the varactor to exhibit some amount of capacitance.
If the switch Q2 is closed (and it conducts) then that point in the circuit is brought down to zero volts and the varactor is no longer reverse biased (or to a voltage below what the D5 part would supply) and we get a completely different capacitance.
The gate of Q2 is controlled by the UI PIC from a line called “Shift”, and it is used to switch between these two frequencies (by turning Q2 on or off). This is how the RockMite offers two different crystal-controlled frequencies with the push of a button.
Next Time
We have already gone through three-quarters of the schematic of the RockMite. Next time, in the last installment, I will walk through the transmitter and the filtering used by the radio.
We have lots of great activities coming up this summer for young people and for new and recently upgraded Hams. You can click on one of the links below to find out more about a specific activity that you are interested in or just read on. Our planned activities include:
All of these activities will be great fun for folks who are exploring Amateur Radio or have just gotten their Technician or General Licenses.
High-Altitude Balloon Launch
HAB-3 Students and Teachers – Ready to Launch
We are hoping to launch our High-Altitude Balloon for the fourth time (HAB-4) on an upcoming weekend in June. The launch will take place at the Winchester School at around 11 am. You can use your smartphone, tablet, or computer to track HAB-4 to the edge of space and back. You can learn more about our HAB, our launch plans, and tracking our HAB here.
We are currently monitoring the weather and the jetstream conditions to determine when we are GO for launching HAB-4. Please monitor our Youth Forum for up to date information on HAB-4 and a final decision on when we will launch.
We will be holding our 2019 Field Day at the Hudson Memorial School in Hudson, NH on Saturday, June 22nd and Sunday, June 23rd. We will be building a 14 transmitter station with 4 radio towers starting on Friday and we’ll be on the air from 2 pm on Saturday to 2 pm on Sunday. Everyone is welcome to join us for a tour, to do a Fox Hunt, and to Get On The Air and talk with other Hams. You don’t need a license to participate.
13 Colonies – A Chance To Operate in a Special Event
Each year during the July 4th week (July 1st, 9 am Eastern Time to July 7th, midnight Eastern Time) Amateur Radio operators around the world participate in the Thirteen Colonies Special Event. The idea is to work at least one K2x special event station in each of the states that grew from the original Thirteen Colonies.
The Nashua Area Radio Society operates the New Hampshire stations using the K2K special event call sign during this event. NARS members and guests operate from their home stations as K2K. This is a great way to have fun on the air, learn to operate using voice or digital modes, and practice your operating skills.
We are making our station AB1OC/AB1QB for guests to operate during the Thirteen Colonies Special Event. If you’d like to operate or just come by and listen in and learn, please contact Anita, AB1QB at [email protected].
We hide small radio transmitters during Field Day and on weekends so that folks can track them down using their radios. This activity is called Fox Hunting. All you need is a basic Handy Talky (HT) radio. You don’t need a license to try Fox Hunting.
We will have all the gear as well as experienced Fox Hunters available at Field Day to help you get started. We can also help you learn about and try Fox Hunting during any weekend that our Foxes are out.
Abby, AB1BY, and Jamey, AC1DC Operating during our Chat Net
We hold a weekly Repeater Chat Net every Sunday evening at 7:30 pm Eastern Time. Everyone is welcome to check-in to our nets, hear about upcoming activities, and answer our weekly question. Our question is usually about Ham Radio and gives each person who checks in a chance to share some information about their interests and activities.
NARS Operators and Friends at Mount Washington SOTA/POTA
We have several Summits On The Air (SOTA) activations planned for this summer. These events involve taking our radios to the top of a mountain peak in New Hampshire or Massachusetts to put them on the air and contact other Hams. A 5W Handy Talky (HT) radio is all you need to talk to Hams all over New England from a mountain top!
We will be again holding a Summer Ham Camp and Technician License Class at Hudson Memorial School in Hudson, NH. Our 2019 Summer Ham Camp will take place Monday, July 29th through Thursday, August 1st – 9 am to 3 pm.
Summer Ham Camp provides a combination of hands-on Ham Radio activities and classroom prep to enable you to earn your Technician Amateur Radio License. The planned activities include:
A Fox Hunt
HF Digital and Voice Operating with Hams around the world via a remote station
Making Contacts on Local Repeaters using an Emergency GoKit
Building a Portable HF Radio Station and Antenna and using it to Make Contacts around the world
A Practice Repeater Chat Net on a Local Repeater
Building and Tuning an HF Antenna
And more
2019 Summer Ham Camp Details
Our Summer Ham Camp includes classroom instruction to prepare you to earn your Technician Amateur Radio license at the end of the week. We provide scholarships for students and teachers which covers a license training book and the class fees. The only cost is a $15 fee to cover the FCC license exam (which we administer) at the end of the week.
We will hold our Annual Picnic on Saturday, August 24th (rain date Sunday, August 25th) a Greeley Park in Nashua, NH. This is a family activity that gives us a chance to meet everyone’s family members and get to know each other better.
Ham radio fun includes a Fox Hunt and making some satellite contacts with a simple handheld antenna and an HT.
Members bring food, drinks, and/or a desert and we share in a great pot-luck meal.
Everyone is welcome to attend. Please contact Jamey, AC1DC at [email protected] for more information.
We will be hosting a display featuring hands-on elements that will introduce you to many aspects of Amateur Radio.
Boxboro 2018 – HF GOTA Station N1T
We will be providing multiple GOTA stations including a Computer Controlled Satellite Ground Station, and HF Shortwave Station, and a Repeater GoKit Station.
Boxboro 2018 – Karen KC1KBW – BGHS Teacher Building a Kit
Our display will also include Kit Building activities. NARS members will provide tours of the display, help using the GOTA stations, and guidance to help you learn to solder and build a kit!
Something New at HamXposition – Ham Bootcamp!
Ham Bootcamp
Do you already have your Technician License and wondering how to use it to get on the air and have fun? Maybe you’ve recently upgraded to General class and would like some ideas on how to go about building an HF station and operating on the shortwave bands? Perhaps you don’t have a license yet and you are interested in trying out Ham Radio?
We’ve created an activity just for you! Its called Ham Bootcamp and we’ll be offering a one-day version of it as part of the Northeast HamXposition at Boxboro. We’ll spend a morning together helping you to get your station together and get on the air.
Ham Bootcamp will include several hands-on activities including a chance to use the GOTA stations and Kit Building elements of our display to learn with help from experienced NARs members. We will also provide a guided tour of the HamXposition Vendor area and the Nashua Area Radio Society display to help you learn about Ham Radio Gear and activities.
This activity will be free to a limited number of HamXposition attendees on a first-come-first-served basis on Saturday Morning, September 7th from 9 am until noon. Also, the Kit Building Activity will be available at a discounted price for Ham Bootcamp attendees.
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