Antennas

I used to work for Jon Titus, KZ1G, when he was editor of Test&Measurement World magazine. In addition to being a fine editor, he is the developer of the Mark-8 Minicomputer, which some folks consider to be the first personal computer. He published an article on how to build this computer in the July 1974 issue of Radio-Electronics.

In addition to being a personal computer pioneer, he’s also been a ham radio operator for a long time. At one point, he acquired the patent from the inventor of the Slinky antenna and sold them for a number of years. The patent has since expired, but he still owns the trademark.

I found out about this one day while we were talking about antennas. He was so enthusiastic about the antenna, I said that I’d have to try it for myself. He said that even though he no longer sold the antenna, he thought he still had some of the parts, and that he’d look for them when he got home. A couple weeks later, a package arrived containing two Slinkies, a center insulator, two aluminum clips, and an instruction sheet.

The Slinky dipole antenna, for those unfamiliar with it, uses two Slinky coils instead of wire for the dipole elements. The Slinkies used for the antenna have a bigger diameter than the Slinky toys. This allows you to use fewer turns than if you used the Slinky toys for your antenna. Jon had these coils specially made.

When stretched, the Slinkies form are helical coils. To keep the coils horizontal, you must string a rope through the Slinkies and attach the rope to the antenna suports. I use a pulley attached to my house as one antenna support and a tree branch for the other.

The length of a dipole made with Slinkies as the elements is much shorter than a dipole made simply of wire. In fact, the instructions recommmend that the overall length of the antenna be no more than a quarter wavelength and can be much shorter. You tune the antenna by shorting out a number of coil windings with the aluminum clips.

A Long Time Coming
I hate to admit this, but it took me years to actually build the antenna. What finally prompted me to do so was that our club set up a multi-op station to work the Michigan QSO Party. I figured that the Slinky would be easy to set up and tune, and beause it’s a short dipole, could fit in a small space if need be.

As it turned out, it was all of that. At first, we decided not to use the antenna. Our host had erected an 80m bazooka antenna, and one of the other guys had brought a multi-band trap dipole. Because we only planned to operate two transmitters, we figured that we’d use a tuner to load up the bazooka on 40m, and use the trap dipole on the higher bands.

As it turned out, however, we weren’t able to get a good match on 40. So, we scrambled and put up the Slinky, tuning it to 40m.

At first, we thought there was something wrong with the antenna. While Tom and I struggled to erect the thing, Sam was reading the SWR with my antenna analyzer. At one point, Sam said, “The SWR looks pretty good now,but it’s jumping around.” I asked myself what could be wrong– bad solder joints, bad coax?

I went downstairs to see for myself. The measurements were indeed jumping around, but as I puzzled over this, I soon noticed that the SWR jumped whenever the operator of station #1 began sending. What we were reading was not a high SWR, but rather RF being emitted by the first transmitter!

After solving that little teaser, we hooked it up to the Drake Twins we were using on 40, and it worked solidly, if not spectacularly for the rest of the day. When we decided to pack it in, it took us only 15 minutes to take it down.

Last weekend, I strung it between two trees in my front yard so that a friend and I could work two transmitters in the Fists Spring Sprint. Despite being only 15 feet off the ground, it worked well despite poor band conditions.

The next day, I took it down and put it up in my backyard, tuning it to 30m instead of 40m. I’m working almost everything I can hear, and getting good signal reports in the process.

One thing I notice about the antenna is that it doesn’t like being wet. The SWR goes from about 1.1:1 to almost 1.4:1 when it rains. Maybe that has something to do with the supporting rope.

Despite it working so well on 30, I think I’m going to put it back on 40. There just isn’t enough activity on 30 for my taste. More about 30m later.

Links
Here are some links describing the Slinky Antenna and variations on the theme:

• The Helitrix Antenna
• My home-built Slinky dipole antenna for the 10 – 80 meter bands
• Slinky and Loaded Beverages
• Beverages: How Short Can They Be?

My biking and ham radio buddy, Mark, came over Monday evening. We had three objectives:

1. measure the SWR of J-pole I built a couple of months ago,
2. measure the SWR of the 2m/440 antenna I just purchased from Purchase Radio, and
3. build a 2m J-pole for Mark

To measure the SWR, Mark brought over his MFJ 259 Antenna Analyzer. In some ways, it’s much nicer than the Autek Analyzer that I have. For one thing the display is much more informative. The Autek display consists of simply a five-digit numeric display, capable of displaying only one parameter at a time. The MFJ is a full alpha-numeric display and displays the test frequency, the measured SWR, and some other information simultaneously.

The MFJ analyzer’s frequency range is also much greater. It covers the 2m and 440 MHz bands, while the Autek only goes up to 32 MHz. Mark doesn’t think it’s very accurate at 440, though.

The first thing we did was measure the SWR of the J-pole I’d built. I’ve been very happy with the antenna, so I was a little surprised that the first reading was slightly over 2.0. I did have it hanging very near to a fairly massive new barbecue grill we have, so surmising that the grill was detuning the antenna, I moved it away from the grill and the SWR did go down to about 1.5. Respectable in my opinion.

Next, we measured the SWR of the dual-band antenna. This antenna is a 5/8 wavelength antenna that I had mounted on a mast. Its SWR measured about 1.5 near the ARROW 2m repeater frequency of 14.96- MHz. I was happy with that, so we moved on to the antenna construction.

We built the J-pole from plans I found on the Net (shown above). You can build J-poles from 300 ohm twinlead, 450 ohm ladder line, and 1/2-in. copper pipe. The dimensions for each of these versions is slightly different, as the velocity factors are all different.

We opted to build ours with the ladder line because we each already had a hunk of it and it’s much easier to carve up some ladder line than solder up copper pipe. It’s not as sturdy, but it’s certainly sturdier than 300 ohm twinlead.

When I built mine, I did not have an SWR meter to tune the antenna with. So, I just built it to the recommended dimensions, and as mentioned earlier, it turned out to work pretty well. Since we had the analyzer when we built Mark’s we played around with the tuning stub, and it appears that he was able to get a slightly better match than I was.

Overall, it was mission accomplished.