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 . . . → Read More: 468: Ham Radio’s Magic Number]]> 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.

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?]]> 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 patented, they should do a little more homework and look around before making such claims… google for ‘Ionic Liquid Antenna’.”

I did just that and found a few interesting references. Apparently, N9ZRT did much of the early work on this type of antenna, and his work is online. In March 2005, some researchers published an academic paper on this topic. Unfortunately, you have to pay for this paper. Another good article can be found on the Highfields (UK) Amateur Radio Club website.

Someone from SPAWAR, the research center that produced the video, replied that in their opinion, this design is unique in that it uses a pump to produce a column of water to form the antenna. This feature makes it patentable. I’m not a patent attorney, but they may have a point here. At any rate, I’m guessing that hams should still feel free to experiment with the antenna.

Yesterday evening, the band seemed to be hopping. The first station I copied was CE3FZ at about an S5. . . . → Read More: 17m!]]> 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. I tried calling him a couple of times, but after no response, I went hunting. I found PY7WC pounding in at S9. After a couple of calls, he became my first 17m contact.

I tried calling a couple other stations, but without success. This led me to believe that my antenna was far from optimal. It is kind of low, but so is my 40m dipole. The 40m dipole works pretty well, and I was hoping the 17m dipole would work well, too.

This morning, however, I had a quite different experience. Even though it was quite early in the morning (1230Z), I punched the 17m button on the IC-746PRO. There wasn’t much activity, and what I could hear was kind of weak, but I tuned around until I heard EA1ARV calling CQ. He was barely moving the meter, but I gave him a call anyway. Not only did he hear me, but we had a decent contact.

So, I guess the upshot of all this is that the dipole is not in an optimal position (which I knew anyway), but it does work, and I can make contacts. I’ll have to play around with getting it up higher, maybe in an inverted-V configuration. It seems like a fun band to operate, though.

Well, doh! Any amateur radio operator could tell you that you probably don’t want to touch your antenna while . . . → Read More: iPhone Problems Show Apple Could Use Some Hams!]]> Some purchasers of the latest Apple iPhone have been experiencing reception problems. Apparently, the band around the phone is the phone’s antenna, and when held in a certain way, the antenna doesn’t function as well as it should.

Well, doh! Any amateur radio operator could tell you that you probably don’t want to touch your antenna while you’re transmitting and probably not while you’re trying to receive, either. I guess there aren’t many hams on Apple’s engineering staff.

For more technical information, listen to this IEEE podcast with antenna engineer Spencer Webb.

Before I could even do that, I had to hack a path through the undergrowth to the tree . . . → Read More: Back on the Air]]> I was off the air for nearly a week. I had to take down my antennas so that some tree guys could come in and take down a couple of elm trees that finally succumbed to Dutch Elm Disease.

Before I could even do that, I had to hack a path through the undergrowth to the tree supporting the far end of my 40m/30m fan dipole. I had been negligent about clearing the buckthorn and honeysuckle, and it took me about a half hour just to get enough of that out of the way to take down the antenna.

I also had to take down the 80m random wire. To do that, I had to get to the mulberry tree that was supporting the far end of the wire. This was a chore as in the past year or so, several thick strands of poison ivy had climbed up the trunk. It took a fair amount of time to carefully pry the vines from the trunk and carefully them.

It took the tree guys three days—Wednesday through Friday—to take down the two trees. On Saturday, I had other things to do, and besides it’s been very hot here, so I wasn’t motivated to do a lot of work outside. On Sunday, though, I decided to finish the job of clearing the buckthorn and honeysuckle and then get the antenna back in the air.

I seriously misjudged the amount of work. I worked from 9 am until noon cutting branches, digging out saplings, and bundling it all up for pickup. By the time noon rolled around, I still hadn’t finished the bundling, but it was already quite warm, so I retreated inside and turned on the air conditioning. Just before dinner, I went back out and finished the bundling, but had other plans for the evening, so once again put off setting up the antenna.

Yesterday, I did go out and finish the job. This time, instead of running the 30m legs at an angle to the 40m legs, I decided to run them along the same plane. The antenna seems to like this orientation for some reason. To do this, I simply connected a 12-ft. piece of cord to the end of the 30m legs and tied the other end to the end insulators of the 40m legs. With this arrangment, the 30m legs droop down below the 40m wires.

I hooked it all up and everything seems to work just fine. I made four contacts last night, including two DX contacts: DL7UKA on 30m and PY3XAT on 40m.

I do have a couple of observations on how the antenna has weathered the elements:

• The splice in one of the 40m legs that I made three years ago seems to be holding up quite well. I expected it to fail by now.
• The fancy, UL-resistant dacron rope that I bought when I first put up the antenna is weathering quite well, but so is the cheap nylon rope I used on the 30m legs.
• This spring I noticed that the adapter in the PL-259 that plugs into the balun had somehow worked its way out and had slid a couple of feet down the coax. I thought that was kind of amusing. I screwed it back in before I hoisted the antenna up again.

So, I’m back on the air now, but it really is time to do some more antenna tinkering. I really need something better for 80m, and I do want to try 17m. I just have to get off my butt and do it.

Rig came back this past Monday, a day early. The auto tuner now appears to be working correctly. Unfortunately, the invoice simply says “Could not duplicate complaint.”

Maybe being manhandled by UPS or being poked and prodded by the . . . → Read More: Wish I’d Have Thought of This…]]> On the Ten-Tec Omni VII mailing list, Bert, W0RSB, reported on his recent experience with Customer Service. He said,

Rig came back this past Monday, a day early. The auto tuner now appears to be working correctly. Unfortunately, the invoice simply says “Could not duplicate complaint.”

Maybe being manhandled by UPS or being poked and prodded by the service tech bounced something into place. At any rate, this exercise cost me nearly $200; if the problem comes back, I can buy a decent external auto tuner for little more than that.

I had a similar problem with my Icom IC-746PRO. I sent it in, and the service tech could not duplicate the problem, and when I got it back, it worked just fine.  My guess is that it was a sticky relay that got unstuck with all the jostling during shipping.  My bill was about $170, but for that, I at least got them to fix a minor problem with the LCD backlight as well as install all of the factory updates.

I never even considered buying an external tuner instead of getting the internal tuner fixed, but this is a really good idea.  At Field Day, we used an LDG Z-11Pro  that plugged right into the IC-746PRO and was tuned by hitting the TUNE button on the radio.  Worked like a charm.  Universal Radio sells them for $170. They handle a wider SWR range, too, than the internal tuner on this Icom. If the internal tuner ever goes out again, I’m buying an LDG.

Engineered Metamaterials Enable Remarkably Small Antennas