Standing Waves – The Great Bay Radio Association Newsletter
January, 2014
Upcoming events
INSIDE THIS ISSUE
January Meeting, Monday January 13
1
Prez Sayz
2
Last Meeting's Minutes
2
NESMC 2 Meter band plan
3
Find the Impedance of your multimeter
4
Build an RF voltage probe
7
Shorts – Musical Morse
8
Club officers / contact information
ARRL sponsored contests
1-4 - 1-5
RTTY Roundup
1-5
Kids Day
1-18 – 1-20
January VHF
2-10 – 2-14
School Club Roundup
From the desk of the President
Thank you to all that made it to the Christmas party last month even though mother nature did not help. Also a
thank you to all that helped out during the three parades (Dover, Rochester and Somersworth). The Banner was in
the Rochester and Somersworth parades this year and I heard many comments about how they had forgot about
amateur radio.
Next meeting will be on PR for the club. I have a few options for shirts and jackets that I think you will like as well
as a source for good signs, banners and business cards. We will have a few options as well for safety vests.
Please bring anyone’s information that you have to attend the tech class. I am looking for Name, address, phone
number, email address. Looking at a start date in February, you can send the info to me at n1xuq@metrocast.net if
you would like to as well. 73
Jeffrey, n1xuq
GBRA Newsletter Page 1
Website located at http://www.w1fz.org
GBRA Minutes 12-09-2013
Meeting called to order: 7:00 pm and immediately adjourned.
Christmas party followed.
Of interest ...
NESMC proposes new band plan for 2 Meters
The New England Spectrum Management Council is proposing a trial change to the 2 Meter band plan. This change would
create ten new repeater channel pairs. These pairs are:
Input
144.91
144.92
144.93
144.94
144.95
144.96
144.97
144.98
144.99
145.00
Output
146.41*
146.42
146.43*
146.44
146.45*
146.46
146.47*
146.48
146.49*
146.50*
*9K00 bandwidth or less
The six narrow-band channels would be restricted to digital modes, such as Dstar and DMR.
Current repeater operators in the 146.4 – 146.6 / 147.4 – 147.6 range will be given the opportunity to move to one of the
new channels. No other coordinations will be made on these channels during this trial period, and registrants will be asked
to verify, and provide evidence, that these frequencies are not in use.
The trial period will last for one year, and onn January 1, 2015, NESMC will hold a second vote to make the changes
permanent.
While these changes may be good for those with pending coordination applications, not all amateurs are happy with more
pairs being allocated to repeater use. Among those who use simplex FM and other modes on 2 Meters,
Part of the problem is that the proposed input frequencies are in a segment of the ARRL band plan dedicated for weak
signal, simplex and data, while the output frequencies are in a segment identified for use as simplex frequencies. In the
ARRL band plan, currently, about ½ of the 2-meter band is already dedicated to repeater use, and those who use 2 meters
for other purposes are feeling somewhat squeezed.
From a management viewpoint, as expressed by NESMC, there are already grandfathered and uncoordinated repeaters
within these segments, and they are hoping to be proactive, and to get ahead of an already existing problem.
So why is there such a contention for frequencies and frequency pairs?
Part of the reason is very simple – 2 meter radios are incredibly inexpensive. It's possible to operate 2-meter FM for less
than $50.00. With an inexpensive HT, and a local repeater, a newly licensed ham is easily on the air, and communicating
with others. In some areas, the increased traffic on repeaters might leave some users looking for a more quiet home. One
solution that some operators have implemented is to close repeaters to only members of a family or a private club.
GBRA Newsletter Page 2
Website located at http://www.w1fz.org
But this solution only leads to more contention for repeater pairs.
At least for the time being, it seems that there isn't an easy solution for the congestion on 2 meters. Its popularity, coupled
with the low cost of equipment make it a very valuable resource – something like ocean-front property in the Amateur
bands.
For those interested in learning more, NESMC maintains a Yahoo group located here:
http://groups.yahoo.com/neo/groups/nesmc/info
And their website, which has information regarding the changes, is located here:
http://nesmc.org/
Tech Talk
Determining the impedance of your multimeter or voltmeter
One tool that is indispensable for the amateur is a decent multimeter. Costs for these instruments are decreasing rapidly,
and quality is on the rise. And multimeters are available at ham-fests, electronic shops and even hardware and automotive
shops.
Every now and again, we might need to make a measurement in a high impedance circuit, and it can be essential that we
don't overly load that circuit. Or, we might wish to measure a voltage that is supplied through a high value resistor.
The value read on the scale of a meter isn't always the value of the voltage source. We need to consider the impedance of
the voltage source, any series resistance, plus the internal resistance (impedance) of the meter itself. The voltage at our
meter's terminals is expressed as: VM = VS *[(Rin)/(RS + Rin)] where VS is the voltage being measured, Rin is the
impedance of our meter, and RS is the combined source impedance, including any series resistor.
If we knew the impedance of our multimeter, we could calculate the actual voltage using this formula, but if we don't know
what that impedance is, we're really out of luck.
To make measurements accurately, especially in high impedance circuits, it will be necessary to know the impedance
(internal resistance) of our multimeter. While the specifications for the meter might provide a value – it may be in the form
of “< 500 M ohms”. While this might help us to know if we will overly load a circuit or not, it doesn't give us a sense of
accuracy when measuring the voltage on the load side of a high value resistor.
For example, suppose that we have a 100 M-ohm resistor, and with our multimeter, we measure 0.917 Volts on the loadside of that resistor – how close to the real value on that side of the resistor is our multimeter reading?
One of the meters on my bench is an Extech 430. It's fairly accurate, but the manual lists the input impedance as >7.5 Mohms, but this doesn't tell me a great deal. I decided to see if I could narrow it down a bit more.
So, how can we determine the input impedance of our digital multimeter?
What we'll need is a power supply, and a high value resistor – 100 M-ohms would work well for most meters. If the volts
scale of the meter is fairly accurate, and we have a resistor with decent precision, we stand a good chance of understanding
the impedance of our meter to a fair tolerance.
The first step is to set a known voltage out of the power supply – 10V will make the math fairly easy while providing some
resolution. Measure it with your multimeter and insure that it reads 10.00.
GBRA Newsletter Page 3
Website located at http://www.w1fz.org
Then connect the 100 M-ohm resistor in series. In my case, the result was the above-mentioned 0.917 V, meaning that
9.083V was being dropped across the 100M-ohm resistor. This means that the remaining 0.917V is across the multimeter
terminals, and the problem is resolved to a simple voltage divider.
We can use a ratio and cross-multiplication to solve the voltage divider problem here.
100 M /9.083=x /10
Solving for x, we have 1000M = 9.083 * x. Dividing 1000 M by 9.083 gives us 110.09 M-Ohms, or about 110 M-Ohms.
Subtracting the 100 M-ohms from the resistor leaves us with a total of just about10 M-ohms input impedance. This is
important because had I used the value of the 7.5M-Ohms listed in the manual, in a high impedance circuit, I could have
been off rather significantly in my calculated measurements.
This is certainly within the specifications for our meter, and now I realize that when I'm making voltage measurements in
circuits with meg-ohm impedances, I have some idea what the effects of this meter might have on those measurements.
Homebrewing
A simple RF probe
Hams have a long history of homebrewing our own equipment. Sadly, today, this is a tradition that many think is
disappearing. Part of the reason for this is that many of the components used in modern equipment are much smaller than
those often used in the past. Surface mount components often require different techniques and tools than were used in the
past. But this doesn't mean that we shouldn't be able to build much of our own equipment. Sometimes surface mount
components actually make construction of some projects a great deal easier. In future articles we will look at surface mount
components, but for this article we'll consider something that is quite simple and useful – just for the purpose of getting into
the habit of building useful things.
This month we're going to build an RF voltage probe. This project will make use of the technique we discussed earlier, of
measuring the impedance of our multimeter. The RF probe that we'll build is a simple and classic design that can be used
for tracing signals through various parts of transmitters and receivers. It blocks DC and low frequency energy, and will
directly measure RF energy in RMS volts.
Components:
C1, C2 .01uF 1000V ceramic disk cap.
D1 1n34 Germanium diode
R1 4.7 M-Ohm resistor.
GBRA Newsletter Page 4
Website located at http://www.w1fz.org
Probe – short length of #10 or #8 solid wire, flattened on one end to solder to PCB
Aligator clip and short length of wire for ground
Flexible, small coaxial cable such as RG178 and BNC connector for output.
Spring for PCB – case connection
C1 is the DC blocking capacitor. There will likely be a DC component to many of the voltages we will be looking at –
especially in tube-type circuits. The voltage rating of this capacitor should be chosen with care, to be high enough for use
in whatever circuits we may be using this device on.
D1 is a 1N34 diode. These are germanium diodes, and, unlike the silicon diodes we're familiar with, have a forward voltage
drop of about 0.3V. This makes them especially useful for measuring relatively low voltages, and they also work well at
RF frequencies.
C2 filters the output of D1and this is where we measure our voltage.
R1 is ½ of our voltage divider. The RMS voltage will be 0.707 of the voltage across C2, and this resistor will drop about
29.7% of that voltage. So, where is the second resistor in our divider? It is our multimeter – and this is the reason that in
our other technical article this month we looked at the impedance of our multimeter.
10 M-Ohms is about standard for a typical digital multimeter, so if you haven't measured yours, or if you don't know the
specifications, it's a good round-number to use. For the sake of this discussion, we'll use this number – if you want more
precision, make sure to measure yours. Remember that if your multimeter's input impedance differs significantly from 10
M-ohms, your readings will be quite different from the actual RMS voltage. If your multimeter's impedance is significantly
lower, your readings will be lower
We know that we need to know that in our voltage divider, about 70.7% of the voltage must be across the multimeter to
read the RMS voltage, which means that 29.3% will be present across R1. So, our multimeter's impedance must be 70.7%
of the total resistance.
We can use a simple ratio to figure out the total resistance.
10M /.707=x / 1
.707 x 10M
X =14.14 M −Ohms
So the Total resistance is 14.14 M-Ohms, and we'll subtract the 10 M-ohms of our meter's input impedance, leaving a 4.14
M-ohm resistance. 4.14 M-ohms isn't a typical value, but 4.7 M-Ohms is commonly available, and will work for this
application. If we need greater precision, we could always find a more precise resistor – but in my case, this is what was
available in my parts drawer.
The RF probe that I built was contained in one of those aluminum waterproof containers available in many hardware stores
and other locations. Below are some photographs of the project.
This is one of the aluminum containers that was
used to build the RF voltage probe. The first step is
to remove the ring and the bail. Then the knob on
the cap is sawn off, and holes are drilled in each
side to fit a grommet. The grommet at the narrow
end will be used to pass the probe through, and the
hole in the cap, where the knob was sawn off will
be for the output cable and the ground cable
GBRA Newsletter Page 5
Website located at http://www.w1fz.org
This photo shows the construction of
the RF probe. The prob tip was made of
a piece of copper wire – probably about
8 Ga. One end was flattened with an
anvil and hammer to facilitate soldering
to the PCB. The other was filed to a
point. The capacitor visible here is C1.
Also visible is the 1N34 diode and the
4.7 M-Ohm resistor. To the right, you can see the grommet through which the ground wire and the coaxial output cable are
being fed.
The double-sided PCB is cut to fit in the approximate center of the container, and the edges of the non-ground side of the
PCB are filed so that the copper will not contact the aluminum case. The small cut-outs on the right side of the PCB are
present because the inner diameter of the cap is smaller than the diameter of the rest of the case. The board fits snugly in
the case between the two grommets.
Testing the probe:
Because I used a 4.7M-Ohm resistor instead of a 4.1M-Ohm resistor, I expected that more voltage would be dropped across
it, and, indeed, that is what I see. When I generate a 9.9V RF signal, my meter reads about 3.2V instead of the expected 3.5
– or just about 10% less than the intended value.
But given that I'll be using this probe more for troubleshooting and signal tracing, that isn't a serious deficiency, and I know
that I can easily add about 10% to measurements and be in the general ballpark.
In this photo, we see C2, and a spring
which is not indicated on the schematic.
The spring is soldered to the ground side
of the PCB, and maintains contact with the
aluminum container to keep it at ground
potential. The spring from a battery holder
may be used for this.
This photograph shows the completed RF
voltage probe. Visible here is the
grommet through which the probe tip
passes.
Use of the RF voltage probe.
This probe can have many uses around the shack – for troubleshooting, and, with a length of wire on the probe, it can be
used as a field-strenght meter.
GBRA Newsletter Page 6
Website located at http://www.w1fz.org
Shorts
Musical Morse
CW in a song?
Slim Gaillard called out CQ in a piece of music called "Communication"
Check it out here: http://www.youtube.com/watch?v=iYqYbK6LfJw
And another:
http://www.youtube.com/watch?v=Q19_CIDycWg
More CW music
Phil Tulga has an interesting tool where music can be made with Morse Code. He has a Shockwave app on his website that
lets you type words and play them back as drums, voices or tones. You can find Phil's page here:
http://www.philtulga.com/morse.html
And not to be left out ...
The Canadian band Rush has a song called YYZ which you can see here: http://www.youtube.com/watch?
v=5nmOMo4OPi4
YYZ is the airport code for the Toronto Pearson International Airport, which is near the band's home town, and hearing the
airport's VOR CW ID, the rhythm stuck with the band.
GBRA Newsletter Page 7
Website located at http://www.w1fz.org
Do you have questions related to Amateur Radio?
Do you need help servicing your own gear?
Do you want to know how to properly construct, install, match or ground your antenna?
Send in your questions, and we'll try to answer them here, or we'll post them to see if anyone else has an answer.
Or would you like to submit an article for publication yourself?
Email your questions, comments or submissions to deirdre@deesigned.net
GBRA OFFICIALS
President Jeffrey A Zajicek,
N1XUQ
Vice President Jason Jasper,
K1FDP
Secretary Deirdre Hebert AB1ST
Treasurer George Whitehead
W1BOF
PIO Larry Inman K1SRJ
Webmaster Jason Jasper K1FDP
ARRL APPOINTEES
Strafford County EC
Jason Jasper K1FDP
Community Liaison [Dover]
George Whitehead W1BOF AEC
Community Liaison [N Strafford]
Bill Nelson KA1PTW AEC
Community Liaison [Rochester]
Jeffrey Zajicek N1XUQ AEC
Net Manager
David Allen KB1NZG AEC
Technical Coordinator [Digital]
Fred LaVallee K1ACL AEC
FlDigi technical advisor/ setup
guru
Mike Lavoie N1AMD
Secretary
Erin Zajicek N1YGH
Great Bay Radio Association Information
The Great Bay Radio Association is a nonprofit
public service organization, with the purpose of
education, service and the advancement of Amateur
Radio.
General Business Meetings are held the
SECOND Monday of each month, at 7:00 pm, in the
Rochester Community Center located in Rochester,
NH. Visitors are always welcome and membership is
open to anyone having an interest in Amateur Radio,
whether a licensed amateur or not.
Dues are $ 20.00 per year [individual],
$ 25.00 [family] and $ 10.00 [student].
GBRA Newsletter Page 8
Website located at http://www.w1fz.org