Where did the “1.2” come from?

On question E8C06 and E8C07 the formula uses 1.2. Where did the 1.2 come from and what does it represent?

I wasn’t sure what he was referring to since my study doesn’t mention how to calculate that value at all. Instead, it reads:

The bandwidth needed for ASCII digital transmissions increases as the data rate increases. The bandwidth necessary for a 170-hertz shift, 300-baud ASCII transmission is 0.5 kHz. (E8C06) The bandwidth necessary for a 4800-Hz frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz. (E8C07)

I e-mailed him, asking him, “”Is the formula you’re referring to perhaps in another license manual? If so, and if you can send that to me, perhaps I can explain it to you.”

He replied, “This was out of the Gordon West Extra Class book page 81.” He attached a copy of the page, and it did indeed refer to the formula:

BW = baud rate + (1.2 x f shift)

Now, I had never run across this particular formula, but I decided to do a little Googling. What I turned up was interesting. It appears that the 1.2 number basically comes from some version of the ARRL Extra Class License Study Guide. Where they got it from I don’t really know.

In my Google search, I turned up one source that simply says that:

BW = baud rate + the frequency shift

Perhaps someone along the line said, “Well, that’s the theoretical value. Practically, if we increase that by 20% to 1.2 times the frequency shift then the signal will definitely fit in that bandwidth.” I’m just guessing here. I’m not really sure.

I told my reader that, for what it’s worth, there’s a lot of this in amateur radio. The formula used to calculate the length of a half-wave dipole antenna is perhaps the biggest example of this. There’s no real science behind the formula length in feet = 468 / frequency in MHz. It’s just a rule of thumb.

While it may be disappointing that the science behind this is perhaps a bit shaky, the good news is that using these rules of thumbs produce circuits and systems that generally work.

From my Twitter feed: diode ring mixers, TAPR news, SDR SA

Dr. NVR @Stefano_NVR
How a Diode Ring Mixer works | Mixer operation theory and measurement: youtu.be/junuEwmQVQ8

Jeff Davis
Summer 2014 TAPR PSR Journal Available –> tapr.org/psr/psr126.pdf #hamradio

EDN.com @EDNcom
RT @measurementblue: Michael Dunn tries The \$11 spectrum analyzer & SDR ubm.io/1q5JI5W @EDNcom @EDNMichael ow.ly/i/645Dp

rtl-sdr.com @rtlsdrblog
BeagleBone Black Image File with RTL-SDR + GNU Radio + More rtl-sdr.com/beaglebone-bla…

If goTenna can encrypt, why can’t hams?

I’ve written here before about encryption and whether or not amateur radio operators should be allowed to use encryption. I’d like to throw another log on the fire.

I just read an article in RadioWorld that describes the goTenna, a device that uses Bluetooth and the Multi-Use Radio Service (MURS) to allow users with smartphones to text one another even if there’s no WiFi or cellphone link. The goTenna device communicates with the smartphone via Bluetooth and then transmits in the MURS band (151 – 154 MHz).

The goTenna manufacturers claim a range of  a half mile to three and a half miles. That’s probably reasonable. In the city, you’ll get a half mile or so range, while out where it’s more open, you’ll get more range.

Among the “key features” are the following:

• Automatic message retry & delivery confirmation
• Individual & group messaging
• ”Shout” broadcasts to anyone within range
• End-to-end encryption (RSA-1024) & self-destructing messages

I have often thought that handhelds should include some kind of text-messaging feature. I suppose you can send text messages using D-STAR, but it seems like an awful expense to do that. It seems that adding this functionality to something like a BaoFeng would make it very appealing.

Also note that this device encrypts the messages. If goTenna can encrypt, why shouldn’t hams be allowed to do so? I’m really not convinced by the arguments put forth by those who are anti-encryption on my previous blog post. I think that someone—someone more knowledgeable about the topic than me—should petition the FCC to allow encryption in certain situations.

NASA Looking for Out-of-This-World Mars Communications Services

As you may or may not know, the ARRL has an award called the Elser-Mathers Cup that is to be awarded to the amateurs that complete the first amateur radio contact between the Earth and Mars. It’s been sitting on a shelf at ARRL HQ since 1928.

Well, apparently, NASA isn’t waiting for hams to make this happen. A story in Network World reports that NASA has issued a Request for Information that explores options to buy commercial communications services to support users on Mars.

NASA’s current Mars relay infrastructure is aging, and there is a potential communications gap in the 2020s, which is why NASA wants to explore alternative models to sustain and develop the Mars relay infrastructure. Mars landers and rovers are constrained in mass, volume, and power, all of which contribute to a substantial restriction in the data rates and volumes that can be communicated on the direct link between Mars and Earth.

To address the limitation in direct-to-Earth bandwidth, the Mars Exploration Program has developed a strategy of including a proximity-link telecommunications relay payload on each of its Mars science orbiters. The relay payloads establish links with landers and rovers on the surface, supporting very high-rate, energy-efficient links between the orbiter and lander.

From my Twitter feed: SDR, SWL, kits

sdrsharp @sdrsharp
You want more and better rtl-sdr tools? Consider helping this initiative from Kyle Keen indiegogo.com/projects/a-mon…

Ulis K3LU @UlisK3LU
Western radio broadcasters tuning out (excellent article on the demise of #shortwave broadcasters) via Straits Times shar.es/LgFv9

From my Twitter feed: SDR, KX3 news, IoT

SWLing @SWLingDotCom
Designing a truly portable SDR goo.gl/fb/zCsyPI #shortwave #swl #dx

QRZ Now @QRZnow
Elecraft Says It’s 2 Weeks Away! #hamradio fb.me/3xncqJtBg

Internet of Things @iot4all
Pinoccio: Mesh All The (Internet Of) Things hackaday.com/2014/07/15/pin… via @hackaday #IoT #IoE

From the trade magazines: free design tools, DSP, education

Top Free DIY Tools Ever Engineer Needs. We’re seeing a relative explosion in free tools for engineering electronics. Some of those online tools prove to be worthless, and it’s back to blind searching or some paid tool. To help sort out the nonsense from the useful online tools, check out this list.

What Do You Know About DSP? Louis Frenzel, who is a ham by the way, reminisces about his experience with digital signal processing (DSP) and recommends a new book for those just learning about DSP.

Outrageous! Experience is no qualification to teach EEs. This is another column by Louis Frenzel. He writes:

It is obvious that the colleges do not value industry experience when it comes to hiring professors….It seems to me that professors with real world experience could teach the fundamentals in context and to explain what is really important and what is simply nice to know. Experienced teachers would be able to teach students things they ordinarily do not teach in school. They could tell their design war stories and explain that troubleshooting is just as important to know as design. I think that an MSEE with ten good years of experience is more qualified to teach than a no-experience PhD.

From my Twitter feed: wire splicing, SDR, BCB loop

Jerry Taylor @kd0bik
How-To: Splice Wire to NASA Standards fb.me/3BMl1CsL0

<b”>rtl-sdr.com @rtlsdrblog
Using and RTL-SDR and RTL_433 to Decode Various Devices rtl-sdr.com/using-rtl-sdr-…

Jeff Davis @ke9v
@AA7EE goes mad scientist with his Tuned Loop for BCB –> aa7ee.wordpress.com/2013/11/17/a-t…

dusty_s @dusty_s
EchoHam (formerly EchoMac) update in Mac App Store” <- glad to see an update! feedly.com/e/G5x1odQW

Dangerous Prototypes @dangerousproto

Brian (VA3BCO) @VA3BCO
Are we Getting Closer to a Touchscreen SDR HT? >#hamr #hamradio wp.me/p4GMgg-28

2215 problem found

A couple of days ago I reported that I’d let the magic smoke out of my Tek 2215 oscilloscope. I was playing around with it last Friday evening, heard some arcing, and then saw a puff of smoke exit the back of the instrument. Oddly enough, the scope still seemed to work, though.

To get some help, I joined the TekScopes Yahoo Group and described my problem. Almost immediately, I got a couple of responses. One guy suggested:

I would take a look at surge suppressor VR901 and line to neutral capacitor C901 at the AC input before the preregulator. Either could have failed without significantly affecting operation of the oscilloscope.

Another said:

I’ll wager that your Tek 2215 has a failing AC-inlet filter. Visit the following site: http://www.vintage-radio.net/forum/showthread.php?t=88137, which discusses failures of Schaffner AC-inlet EMI filters due to a certain type of capacitor manufactured by Rifa…http://www.vintage-radio.net/forum/showthread.php?t=69128

One commentator mentions incompatible rates of expansion between the epoxy encapsulant and the capacitor innards. Another explanation (which I like better) is that microcracks in the encapsulant allow moisture to get into the metallized paper that forms the capacitor’s innards. The innards expand, which applies stress and makes more cracks, and the deposited metallization corrodes and forms conductive compounds with the moisture. Before long…pffffft!

Neither was completely correct, but they both were close. See the photo and schematic below:

It looks like the problem is indeed a capacitor in the filter section, but my scope does not have a Schaffner AC filter, like they used on some of the earlier (and more expensive) scopes. Instead, this filter is right on the main board. I’m guessing that the 0.068 uF cap shorted out,which led to the resistor burning out.

One odd thing about troubleshooting this problem is that while it looks pretty obvious from the photo, it wasn’t that obvious right off the bat. As you can see from the schematic L925 is not on the board. Instead, it’s mounted in a little housing that sits right above these two components. I had to remove that housing before I could see them.

At any rate, now I just have to find a suitable replacement capacitor and solder it all back together.