KB6NU's Ham Radio Blog

KB6NU's Ham Radio Blog

Antennas

Random Links to Interesting Stuff

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Here are a bunch of quick links to interesting stuff that I’ve learned about, either from the ham radio lists I’m subscribed to or from friends who’ve sent me links to thing they think would interest me. I try to save these and do a more complete entry on them, but when they start piling up, I just cover them in one fell swoop like this:

  • RUMtrol746PRO. This Mac program is designed to control an IC-746PRO or IC-7400 via the CI-V interface. It’s the only program I know of that runs on a Mac that will do this. To use this, I’m going to have to either buy a SignaLink USB to interface to my radio or get a serial port A/B switch so I can switch the USB-serial port adapter between the Buxcomm interface (when running PSK) and the CI-V adapter.
  • Pneumatic Antenna Launching Systems. This site has a ton of information about making devices which shoot tennis balls over tree limbs so that you can then get ropes up there to support antennas. Most of them are made from PVC tubing. I like the idea of using tennis balls instead of the lead sinkers that the EZ-Hang uses. Kits are available.
  • Mini Mill. This site shows you how to build a mini-windmill to “light a LED, small light bulb, or even charge a couple of NiCad batteries.” It uses a motor from an old floppy disk drive or printer.
  • Hammond Museum of Radio.We got to talking about this museum at the last event at the Hands-On Museum. It’s located in Guelph, Ontario, not too far from where I live in Southeast Michigan. This might be worth a drive sometime this summer.
  • CQTube. A ham radio version of YouTube, this site currently only has 17 videos on it, but they are ready, willing, and able to accomodate any ham radio-related vids you might want to upload.

Feedline Loss Calculators

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On the HamRadioHelpGroup mailing list, a fellow wrote about the measurements he made on some RG-8U coax. He connected a 60 W, 440 MHz transmitter to one end of the cable, a 50-ohm dummy load to the other end, and measured the power. With 60W in, he only measured 1 W out.

Now, that seemed like quite a bit to me, so I did a quick Web search and found the OCARC Line Loss Calculator and a couple of RG-8U data sheets. Belden specified their 9913 cable, the RG-8U equivalent, at 4.3 db/100 ft. Plugging all the numbers into the line loss calculator, it would appear that the power measured at the other end of coax should have been more than 22 W, not just 1 W. My guess is that he’s got some other problem with that piece of cable.

Mark, K5LXP, pointed the list towards perhaps an even better online resource: VK1OD’s RF Transmission Line Loss Calculator. This calculator not only calculates the loss of a line at a particular frequency, but also takes into account the mismatch between the line and the load. The loss figures given in the data sheets are accurate only when the line and load are matched.

When there’s a mismatch, the loss goes way up. For example, 100 ft. of Belden 9913 has a loss of approximately 2.8 dB at 440 MHz and an SWR of 1:1, according to VK1OD’s calculator. At an SWR of 5:1, however, is about 4.8 dB.

Last Weekend at KB6NU

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The weekend started out innocently enough. I had made arrangements with a guy to go over to his house and put up an antenna. Sounds simple enough, doesn’t it? Especially when you consider that the antenna we were going to put up was a 40m dipole.

Well, we had problems right off. I had intended on using the EZ-Hang, which is basically a combination fishing reel and sling shot. The idea is that you shoot a lead weight attached to some fishing line over likely branches, then attach a rope or string, then reel it in. It is supposed to work like a charm, but I’d never used it before, and we ran into several problems.

The first problem that we had with it is that a bunch of the line had somehow gotten tangled up inside the reel and wouldn’t feed out. We tried untangling it, but after about 15 minutes, decided to just cut it off and be done with it.

After we got that working, we tied a lead sinker to the line, pointed it up into the tree, and let fly. It was a beautiful shot. The sinker sailed right over the branch we wanted it to. The only problem was that it was no longer attached to the fishing line.

After some searching, we actually managed to find the sinker. We tied it back to the fishing line, pointed, aimed, shot, and exactly the same thing happened again. Damn. We searched around, found the sinker, tied it back up, then tried another test shot. The line broke again.

At that point, we gave up on the EZ-Hang and resorted to the weighted tennis ball. Fortunately, my friend has a better arm than I do, and with only a few practice throws, we were able to get a line over a branch about 30 feet up. We duplicated this feat on another tree about 70 feet away. We now had ropes over branches, but we had consumed about half the time available for this task.

We then proceeded to build the antenna. That went pretty smoothly until we started attaching PL-259s to the coax. My friend did not have a vice to securely hold the connectors while we applied the solder, so after a botched attempt, we decided to drive over to my house to do this. We got one PL-259 on, but then basically ran out of time.

My friend had to get going, so we called it quits. After four hours, we were still only about half done with the antenna project. Chalk it up to Murphy’s Law in action.

RTFM
That evening, still miffed that we didn’t get the EZ-Hang to work, I searched through my file cabinet for the documentation that came with the EZ-Hang. This is, of course, what I should have done before we tried using it.

What the instructions said was to tie the sinker to the line using a Palomar Knot. The Palomar Knot uses a double thickness of line, which should make a much stronger knot. From what I’ve read on the ‘Net, it’s a very common fishing know. Not being a fisherman, though, I had never heard of it before.

At Least I Can Still Operate
Something did go right for me Saturday. My first contact was VK6HCI on 30m. I then QSY’d to 40 and worked FM5LD. My last QSO of the day was a PA station, and we had a nice ragchew. I also worked a couple of stations doing the PA QSO Party.

On Sunday, I worked a few more stations in the PA QP, including N3BUD and W3SO, adding to my collection of stations whose callsigns spell words. Somewhere in there, I also worked WB4DAD, making it three new QSLs to add to my collection. So, while I did feel kinda bad about the antenna fiasco, the DX and new QSLs for my collection helped me get over it. :)

Unstick Coax-Seal

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The September 5, 2007 ARRL Contester’s Rate Sheet has the following tip:

To remove stuck-on Coax-Seal from connectors and cables, Carl KM1H recommends Brake Clean if you’re working on the tower. On the ground, use Lacquer Thinner or Acetone which are harder to use aloft. Steve K7LXC also notes that professional grade vapor wraps leave much less residue than Coax-Seal.

A New Antenna for My KX-1

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Since I’ve owned my KX-1, the only two antennas that I’ve ever connected to it are the 40m dipole in my backyard and the 28-ft. wire (with three radials) described in the KX-1 manual. The 28-ft. wire is the only antenna I’ve ever used when operating portable.

While I’ve had good success with the wire, I knew I could do better, So, last night, at our club’s AMP Team meeting—where AMP stands for ARROW Mobile and Portable—I tried something new. I built a 67-ft. doublet from some 30-ga. wire I have and fed it with some twisted-pair wire. Theoretically, I figured that if I made the twisted pair a half-wavelength, then I’d have a relatively low impedance at the rig.

I cut the two elements, and then had a brain fart and cut the feedline to the same length. Now, theoretically, that should be bad, but I decided to hook it together and see what it would do.

I stripped the wires, and using a dogbone insulator as the center insulator, used wire nuts to connect the feedline to the antenna elements. At the end of the elements I just looped the wire and then tied a piece of string to the loop. Using a tennis ball on the other end of the string, I got the string over a branch and the wire up into the trees. The antenna was up only about ten feet, and I got some ribbing from one of my friends about it making a good NVIS antenna, but I pressed on anyway.

I connected the feedline to the binding posts on the BNC-binding post adapter, turned the KX-1 on and hit the antenna tuner control. Despite having only a quarter-wavelength of feedline, the tuner tuned it up just fine.

The real test was whether or not I could make any contacts. Tuning around a bit, I was very much encouraged. There were lots of signals, many of them S7 and above. Just for kicks, I called CQ around 7040, but when I didn’t get a response, I tuned around a bit.

Doing so, I nabbed Jozef, WB2MIC, in Wells, VT around 7058. He had a fine 599 signal, and he gave me a 559.

After a nice QSO, I tuned around some more and heard N5DY, Stillwater, OK calling CQ at 7036. He came right back to me, and although he wasn’t copying me as well as Jozef, we had a nice QSO. He was running a 4 W homebrew transmitter.

Finally, I worked Greg, AA8V, in Frostburg, MD. He gave me a 459 report.

All in all, I’d say my new antenna is a big success. Being made from 30-ga. wire, it’s very light and I got it up into the trees pretty easily. And it also turned out to be a good performer.

I’ll be trying it out again at the next AMP Team meeting. Next time, I’ll take my antenna analyzer along to actually measure the impedance.

NIST Antenna Calibrations Extended to 60-110 GHz

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From the 5/24/07 issue of NIST Tech Beat:

NIST engineer Katherine MacReynolds prepares a new NIST “tabletop” range for characterizing high-performance antennas, such as horn antennas (small gold pyramids) operating at 94 Gigahertz. The surrounding blue foam cones absorb electromagnetic fields to reduce scattering from nearby objects, thereby improving measurement accuracy.Credit: © 2006 Geoffrey Wheeler.

The National Institute of Standards and Technology (NIST) has developed a new “tabletop” sized facility to improve characterization of antennas operating in the 60 to 110 gigahertz (GHz) frequency range. This extended frequency capability serves needs for advanced civilian and military communication and radar systems.

Many electronic systems are moving to higher frequencies to attain higher channel capacity, better spatial resolution and other advantages. The new measurement facility will help accelerate development of technologies such as automobile collision-prevention radars, which operate at 94 GHz and require antennas small enough to be integrated into car bumpers. Improved NIST antenna calibration capability also helps to assure the accuracy of many systems. “NIST is the start of the measurement traceability chain,” says Perry Wilson, leader of the Radio Frequency Fields Group. “For instance, we calibrate the probes used by aerospace companies to calibrate instruments launched on satellites and other critical systems. Weather satellites are an example; improvements in antenna accuracy mean better data for weather models, resulting in better weather predictions.”

The new facility continues NIST’s history of innovation in antenna measurements, building on the “extrapolated gain” technique developed several decades ago. The original extrapolation range and techniques made it practical for researchers to accurately compute an antenna’s far-field characteristics based on near-field measurements. By making the range compact, costs are significantly reduced. In addition, the extrapolation technique uses over-sampling and averaging techniques to minimize the effects of scattering and range imperfections.

The tabletop extrapolation range is used to measure the gain (increase in signal power) and polarization (orientation of the electric field) of high-performance antennas. To make measurements, one antenna is fixed on the table and a second is moved along a rail. A laser tracker is used for alignment and positioning. The laser tracker is capable of following a moving target with less than 20 micrometer uncertainty at 1,000 points per second. The range is arranged on an optical table to provide the mechanical isolation and stability necessary to achieve low uncertainties at short wavelengths of radiation. Typical measurement uncertainty for certain types of antennas in the 60 – 110 GHz range approaches that of NIST’s existing calibration facilities for antennas operating at lower frequencies (less that 60 GHz).

For more details on NIST antenna calibration services, located at the institute’s Boulder, Colo., campus, see the NIST Antenna Near-Field Measurements page.

Splicing Antenna Wire

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Three months ago, my 40m dipole came down in an ice storm when a big branch fell on top of one of the elements. The weight of the branch plus all the ice actually broke the FlexWeave wire.

Today, I finally decided to fix it. Being the cheapp guy that I am, I thought that I could just splice the wire, using a short piece of FlexWeave to connect the two. I couldn’t have been more wrong.

I knew the wire was dirty, but I thought I could clean it sufficiently by sanding it down. That didn’t work at all. After twisting the wires together as specified by an app note written by Joe Carr that I found online, and heating it up with a soldering gun, the new wire gleefully absorbed the solder, but it just blobbed up on the old wire.

Rummaging around in my supplies, I came up with a can of Brasso. I tried cleaning the wire with Brasso, but while I got a lot of dirt off the wire with the Brasso, it didn’t solder any better.

Oh well. At this point, I probably should just give up and cut a new 33-ft. element. I’ve got more than an hour invested in this now, though, and I’m determined to see this through just on principle. Any suggestions would be gratefully accepted.

Don’t Like MFJ? Here Are Some Antenna Analzyer Options

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On the Elecraft mailing list, there’s been a flurry of e-mail discussing antenna analyzers. The most popular antenna analyzers is the MFJ-259B. As is often the case when MFJ equipment is discussed, there was much wailing and gnashing of teeth. Fortunately, the thread did not turn into simply a “bash MFJ” item. Instead, several guys suggested alternatives:

  • miniVNA. This analyzer is available from WiMo in Germany. This unit has several nice features, including a frequency range of 0 – 180 MHz and computer integration. Unfortunately, the latter is also a drawback, as this analyzer cannot be used without a computer. Cost: 255 euros (about $350 USD).
  • VK5JST AERIAL ANALYSER. This is a kit sold by the Adelaide Hills Amateur Radio Club, Adelaide, Australia. This looks like a nice unit, and as the guy who suggested it says, “The price is good and you get the added bonus of making it yourself.” Cost: about $110 USD, including shipping.
  • AMQRP AA-908. This is a project of the American QRP Club. This is built on top of the Micro 908 board. The price is kind of steep—$230—if all you want is an antenna analyzer, but you can also make the unit into a PSK modem, DSP audio filter, and an intelligent signal generator.
  • N2PK VNA. This is a real homebrew project. Printed circuit boards are available, but no complete kits are available. Larry, N8LP, commented, “The N2PK is an awesome piece of gear for the money.”
  • Autek RF1, VA1. I suggested looking at either the Autek RF1 or VA1. They fit in the palm of your hand, are inexpensive ($150 for the RF1 and $200 for the VA1), and work pretty well. They’re certainly accurate enough for most amateur radio antenna work. Disadvantages include the lack of a computer port, finicky tuning, and the lack of an analog meter.
  • Tenna Dipper. If you’re really on a budget, get a Tenna Dipper from the 4 States QRP Club. It only outputs one frequency per band, and you can only use it for 50-ohm systems, but only costs $25.

There are other options from Kuranishi, TimeWave, and AEA, but they’re very expensive. They may be worth the money, but are probably overkill for most amateur radio applications.

The “Almost Random” Wire Antenna

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I’m still amazed at the success I’m having with my random wire antenna on 80m. Yesterday, I worked a guy who said he was running 500W. He was, of course, 599 here, but he gave me a 589. There’s now no doubt in my mind that a random wire can be an effective antenna.

On the HamRadioHelpGroup mailing list, there was a related discussion about the “almost random” wire antenna. The idea is that some lengths just won’t work well on one or more bands. In general, you want to avoid choosing a length that is a half wavelength, or multiple of a half wavelength of a band that you want to operate.

Tim, N9PUZ notes:

I have used the 49.2 ft (15 Meter) length of wire on vacation with both automatic tuners and an MFJ Model 16010 tuner with good results for 40, 20, 17, and 15 Meter operation. This is not as good an antenna as a dipole or doublet in my opinion but it’s a workable antenna.

Moe, AB8XA, added:

With all the bands we have at our disposal, finding an end-fed antenna length that’s not near 1/2 wavelength on some bands is tough, impossible for me so far. As far as dodging 1/2 wavelengths, 86.9′ may be the best compromise for most bands. Using 95% velocity factor, I find 86.9′ roughly 1/3 λ on 80 m, 2/3 λ on 40 m, 4/3 λ on 20 m, 5/3 λ on 17 m, 7/3 λ on 12 m, and 8/3 λ on 10 m (2.60 – 2.73 λ). But it is exactly 1/2 λ on 60 m, and 0.94 λ, almost 3/3 or 2 half λ, across 30 m, and 1.95 – 2.00 λ, or 6/3 and 4 half λ, on 15 m.

My IC-718 doesn’t have 60m, so slightly shortening the 86.9′ may make it possible to tune 30 m and the lower, CW end of 15 m. But that might bring the lower end of 10 m too close to 2.5 λ.

It appears to me, with an integrated tuner in the radio, which may not have the tuning range necessary for some bands, or even with a wide-range tuner in the shack, the end-fed brings the antenna inside and close to the operator. A remote automatic wide-range tuner, such as the Icom AH-4 or SGC SC-230, between the end of the antenna and lengths of coax and control cable to the shack, appears to me might resolve that problem.

One of these tuners mounted well away from the shack, and close to the ground to feed the required radials, also offers the opportunity to turn an almost all horizontal wire into an inverted-L. From what I’ve seen playing with some modeling software, adding some vertical portion to the antenna can slightly ease the cloud-burning effect of a relatively low horizontal run on lower frequencies, as well as significantly reduce the depth of the nulls of a long horizontal wire on higher frequencies. But I’m sure no antenna expert. It’s always been my toughest subject.

My antenna is 85-ft. long, so I bet I can get mine to load on nearly any band. My next project is to connect it so an SGC-239 automatic antenna tuner that I have and just see what I can do on bands other than 80m.