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By Dan KB6NU, on September 5th, 2010
Here in the U.S.—where we still measure length in feet—468 is a magic number. Why? Well, the formula for calculating the length, in feet, of a half-wave dipole antenna is:
L (ft) = 468 / f (MHz)
If you do the math, a half-wavelength is actually 492/f, so where did the number 468 come from? The explanation most often given these days is that a radio wave travels about 5% slower in wire than it does in free space, so the distance that a radio wave would travel in a wire is about 5% less than it would travel in free space.
Now, I don’t know about you, but while I’ve used this formula for building dipoles, I’ve never had one tune up perfectly using that number. There are a number of reasons for this, the main one being the height above ground of the dipole. What I’ve found is that the elements of the dipole are usually longer than they need to be.
I sometimes joke that whoever came up with that number did so so that hams wouldn’t cut their dipoles too short. After all, it’s much easier to make a length of wire shorter than it is to make it longer.
Ward, N0AX, wasn’t satisfied with any of the common answers to where the number 468 came from. In the latest issue of QST, he consulted the materials in the ARRL library and found the answer. The October 1926 issue of QST included an article titled, “The Length of the Hertz Antenna.” (“Hertz antenna” was the name most commonly used for a dipole in the early days of radio.)
The author of that article constructed nine different dipoles and measured their resonant frequencies. He then calculated a value, K, by which you’d multiply the wavelength to get the wire length. If you multiply that number by 300, you’d get values ranging from 423 to 471.
The number 468 first appeared in the 1929 ARRL Handbook.
For this article, N0AX did a number of simulations of a 20m dipole at various heights, ranging from 1/8 wavelength to 2 wavelengths. He came up with numbers ranging from 466.4 to 483.4. This is somewhat at odds with my experience, although I must admit that I’ve never been able to get my dipoles up that high. That’s my guess for why my dipoles are almost always shorter than 468/f.
At any rate, this article is certainly worth reading.
By Dan KB6NU, on September 3rd, 2010
Here’s a press release that I received today:
FOR IMMEDIATE RELEASE: August 25, 2010 ?News Media Contact: Moira Vahey at 202-482-0147 or press@ntia.doc.gov
BOULDER, CO — The Institute for Telecommunication Sciences (ITS), a division of the National Telecommunications and Information Administration (NTIA) of the U.S. Department of Commerce, today released a collection of online training and educational videos for public viewing on ITS’s website. These videos cover telecommunications topics ranging from an easily understandable review of the fundamentals of radio spectrum—such as defining decibels using common logarithms—to in-depth explanations of complex engineering issues like resolving signal-interference problems.
In announcing the public posting of this video collection, Al Vincent, Director of ITS, stressed that this is the first of what he expects to be a valuable educational series. “The publication of these videos reflects ITS’s goal to be not only a technical resource for telecommunication standards development, but also a trusted, impartial informational resource for industry, students, the general public, and other government organizations,” said Vincent. “This unique web-published video series represents a significant expansion in the accessibility of ITS’s research findings for the public, and we certainly hope to continue to make information on additional topics available in this way.”
ITS provides a variety of useful information and tools on its website, including technical reports, radio propagation data and models, and various ITS-developed software tools. Since its founding over 70 years ago, ITS research has been fundamental to the development of realistic and achievable standards for telecommunication protocols and equipment.
Video presenter Frank Sanders, a physicist and electrical engineer who leads ITS’s Telecommunications Theory Division, regularly authors ITS contributions to the International Telecommunication Union (ITU). The ITU is a United Nations agency that deals with telecommunications and technology issues.
The Institute for Telecommunication Sciences is the research and engineering laboratory of NTIA, which serves as the executive branch agency principally responsible for advising the President on communications and information policies. For more information about ITS, visit the ITS website. For more information about NTIA, visit the NTIA website.
I just watched Episode 15: Resolving RF Interference: RF Frontend Overload Problems, and it provided a very good explanation of front-end overload. Although ITS deals mostly with radar systems operating in the GHZ range, most of the presentation also applies to ham radio systems operating at HF, VHF, and UHF.
One of the things that made this presentation so good, is that Sanders is a very good draftsman. Even though he was moving right along, the graphs that he would draw on his whiteboard were very nicely done, and I think that made the presentation a lot more understandable. That’s a good lesson for all of us ham radio instructors.
By Dan KB6NU, on September 2nd, 2010
While I enjoy all the technology we get to play with as amateur radio operators, I enjoy just as much the people you meet on the bands. In the last week and a half, I’ve had three QSO that are worth blogging about.
 Carl, N1EVO. I worked Carl on August 25. He was running QRP, from his home in Fort Kent, ME, which is just south of the Quebec/Maine border. He mentioned that he was the manager of a small hotel on US 1, which is 2,390 miles long and runs from the Canadian border all the way down to Florida.
We had an interesting chat about managing the hotel during tourist season, and Carl said he would send me a QSL card and a “Mile 0″ sticker. Yesterday, in the mail, I received his QSL card and two stickers. “Fort Kent,” he writes, “can be the end or the start of US 1.” - Joe, KI4GAP. I worked Joe a couple of days ago. On his QRZ.Com page, he mentioned growing up in the suburbs of Detroit. I mentioned to him that I was an “east sider,” meaning that I grew up on the east side of Detroit. He also grew up on the east side of Detroit, and as it turns out, is the same age as I am. While it doesn’t appear that we knew each other way back when, we did have fun comparing notes.
Joe is now a staff photographer for a newspaper down in Virginia. he mentioned that he was being sent to the coast to take pictures of Hurricane Earl. Just now, I’m listening to a news report of the imminent landfall. Stay safe, Joe.
- Mark, VE3CPK. Last night, I had a nice long ragchew with Mark. Again, I pulled up Mark’s QRZ.Com page, which contained a link to another page with more info. As it turns out, Mark is the Vice President, Advanced Technology for Research in Motion Limited (RIM), makers of the BlackBerry. He is also 55, but has been a ham for 44 years!
You can bet that ham radio played a big part in Mark’s life and career. I told him that I thought hams should be doing more with microwave technology, and he readily agreed. Someone really needs to come up with a killer ham app for microwave systems and then build some radios to implement it. Why shouldn’t these radios be as common (at least among the ham community) as 2m HTs?
By Dan KB6NU, on September 1st, 2010
One of the items making the rounds on the HamRadioHelpGroup mailing list is this video on the Sea Water Antenna:
At first, I thought, “What a novel idea!” Of course, as the Bible says, there’s nothing new under the sun. PA1AP noted, “Reinventing the wheel I would say. There is prior? art for this and cannot be . . . → Read More: Sea Water Antenna?
By Dan KB6NU, on August 30th, 2010
I’m happy to report that I’m now on 17m. I don’t know really what took me so long—especially since I enjoy working 30m so much—but I finally put together a dipole for 17m and hung it up yesterday.
Yesterday evening, the band seemed to be hopping. The first station I copied was CE3FZ at about an S5. . . . → Read More: 17m!
By Dan KB6NU, on August 27th, 2010
I’ve blogged about finding hams in your area before:
How Many Hams Are in Your Neighborhood?
How Many Hams in Your Grid Square?
A recent thread on the HamRadioHelpGroup mailing list prompts me to revisit this topic, though. The replies list four different websites that you can use to find out this information:
FCC Universal Licensing System – Advanced Search,
QRZ.Com. . . . → Read More: Find Hams in Your Area
By Dan KB6NU, on August 26th, 2010
A couple of weeks ago, I spent a week in a cabin on Elk Lake, MI. Over 25 members of my family congregated for this annual event., including my three sisters, a passel of nieces and nephews, their spouses, and kids. It’s a beautiful spot, and we have a great deal of fun.
As I have for . . . → Read More: Vacation Pics
By Dan KB6NU, on August 22nd, 2010
Yesterday, down at the museum, we got a whole pack of Cub Scouts on the air, thanks to Ovide, K8EV, my ever-ambitious “kid wrangler.” No sooner had I managed to make a decent contact than he lassoed a group of five Cub Scouts from Detroit. Fortunately, conditions held out so that I could give them all . . . → Read More: WA2HOM Introduces Cub Scouts to Ham Radio
By Dan KB6NU, on August 20th, 2010
The ARRL recently posted the article, “K2BSA: Amateur Radio Fun in the Warm Virginia Sun,” which discusses amateur radio activities at this year’s National Scout Jamboree. It reports:
…nearly 6000 youth scouts — 13 percent of the total Jamboree attendance — received thorough exposure to ham radio, touring the K2BSA station and getting on the airwaves. Six . . . → Read More: Ham Radio at This Year’s National Jamboree
By Dan KB6NU, on August 20th, 2010
This is from the August Michigan Section News, by Dale, WA8EFK, Section Manager:
The year 2010 will mark an important anniversary in the History of Amateur Radio: The birth of the first public service net and it happened here in Michigan.
Before the implementation of a net concept, radiogram traffic and emergency communications activity was conducted on a . . . → Read More: The QMN: A Celebration of the First Traffic Net.
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