Understanding Antennas For The Non-Technical Ham

A free PDF antenna book, available from Hamuniverse at: http://www.hamuniverse.com/basicantennas.pdf

OCFD experiment

Homebrew Off Centre Fed Dipole

I’ve used a 66′ doublet for a couple of years now, fed with 450 ohm twin feeder. However, I’ve noticed recently that pickup from QRM has increased. Balanced antennas should be just as immune to interference pickup as coax fed antennas, but only if its properly balanced. When I put it up I just added two lengths of 33′ wire but didn’t do any cutting/checking to see if this was exact. Also, the doublet was used as an inverted V and the twin feeder had to run near the house (no other route). So I decided I would try a OCFD (Off Centre Fed Dipole) again. Last time I built one, I fed it with 300 ohm feeder and it worked OK. This time, I built with the standard 22’/44′ wire into a 4:1 balun. This is designed for 7-30Mhz operation. To include 80m, the lengths need to be doubled. This is then fed with coax. About 10′ down from the balun, I formed an unun (unbalanced-unbalanced), sometimes called a 1:1 balun. This is usually used to prevent common-mode currents on the coax feeder. In my case, where I predominantly listen, I figured it will also help to reduce QRM. This is just an assumption.

To mount the 4:1 balun box, which is just a small ‘tupperware’ container, and bring in the 2 ends of wire, I cut a t-shaped piece of upvc I left over from a cladding job. This stuff is easy to cut to any shape and appears to be RF proof. Also, its uv proof too (well for about 25 years!). The coax is also secured to the long part of the T before entering the balun box. The T-piece was the tye-wrapped to a short piece of 20mm pvc pipe. I have a 5m extending decorators pole, so I secured the pipe to the tope of this using, you guessed it, Tye-wraps. The pole was secured to a fence with bungee cords, about 40′ from the house and the coax run to the house and in through the window. I directly connected it to the FT-450D. Both antenna wires sloped down to bushes either side, so ends were about 5′ above ground.

Although I seldom transmit of the HF bands, the first thing I tried was to see if the built-in autotuner would work. On all bands 40-6m, the autu tuner worked. So in the event I ever get the urge to TX again, I know it will tune up OK.

General listening followed on both ham bands and broadcast bands over the next 7 days. The QRM is definitely down, even in the sub 5Mhz region where before it was excessive. I’ve logged all the usual broadcast stations with ease, along with many hams on 6m (lot of E’s openings at the moment), 10m and 20m. This antenna is a keeper!

The next stage is to move it from the pole to the usual place on the gable end of the house. This was the plan for the latter half of this week. Well, 28+ degree heat kept me from doing that. The next couple of days is going to be windy, now its cooled off, so hopefully after that I can get it up in the air. Once this is achieved, I shall report back results.

Can I recommend the OCFD? Most certainly. The cost to me was zilch, as I had everything, including the FT-140 toroid for the balun (good junk box!)

A RARE WAVE IN EARTH’S MAGNETIC FIELD

From SpaceWeather.com

When a stream of solar wind hits Earth, magnetometers around the Arctic Circle normally go haywire, their needles swinging chaotically as local magnetic fields react to the buffeting of the solar wind. On Nov. 18th, however, something quite different happened. Solar wind hit Earth and produced … a pure, almost-musical sine wave:

Rob Stammes recorded the event from the Polarlightcenter, a magnetic observatory in the Lofoten Islands of Norway. “A very stable ~15 second magnetic oscillation commenced and persisted for several hours,” he says. “The magnetic field was swinging back and forth by 0.06 degrees, peak to peak, with the regularity of a metronome.”

Imagine blowing across a piece of paper, making it flutter with your breath. The solar wind can have a similar effect on magnetic fields. The waves Stammes recorded are essentially flutters propagating down the flanks of Earth’s magnetosphere excited by the breath of the sun. Researchers call them “pulsations continuous” — or “Pc” for short.

“A very sensitive magnetometer is required to record these delicate waves,” says Stammes. “I use a mechanical magnetometer with bar magnets suspended from a special wire. LEDs and light detectors in an isolated dark box record the motion of the magnets, while vanes in oil damp out non-magnetic interference.”

Pc waves are classified into 5 types depending on their period. The waves Stammes recorded fall into the range 15 to 45 seconds–that is, Pc3. Researchers have found that Pc3 waves sometimes flow around Earth’s magnetic field and cause a “tearing instability” in our planet’s magnetic tail. This, in turn, can set the stage for an explosion as magnetic fields in the tail reconnect.

A quartet of NASA spacecraft recently flew through just such an explosion. Last week, researchers from the University of New Hampshire reported that four Magnetospheric Multiscale (MMS) spacecraft spent several seconds inside a magnetic reconnection event as they were orbiting through Earth’s magnetic tail. Sensors on the spacecraft recorded jets of high energy particles emerging from the blast site. One jet was aimed squarely at Earth and probably sparked auroras when it hit the upper atmosphere.

Stammes has recorded many Pc waves in the past, “but this is the first time I have detected category Pc3,” he says. “This was a very rare episode indeed.”

Going back to HF for digital comms

I came across this via a link from the Extreme Shortwave Listening group and posted from the blog of nw7us.

There is an ITU paper which discusses the problems of reliable ship-to-shore comms using only satellites, and a design for moving them back to HF. The paper includes architectural design for the system, along with explanations of the type of transmission, in this case, OFDM or Orthogonal Frequency Division Multiplexing. No, me neither.

A few quotes from the paper:

The NAVDAT HF system can use a simple time-slot allocation similar to the NAVTEX system which could be coordinated by IMO.

That NAVDAT HF system can also work on single frequency network (SFN) as described in Annex 4. In this case, the transmitters are frequency synchronized and the transmit data must be the same for all transmitters.

The NAVDAT HF digital system offers a free broadcast transmission of any kind of message from shore‑to‑ships with possibility of encryption.

That last one is telling “a free broadcast transmission”. That reads to me that one of the reasons to return to HF is the cost of renting satellite time, whereas HF is “free to air” with no ongoing costs other than maintenance.

Its not a large paper, but worth the read. The link to the pdf file is here: NAVDAT