Alleyway Antenna Analysis: The Ever-Present Trials Of The Urban Ham

It certainly feels a bit like I am going back to my roots lately with scrappy little antenna projects strung up along the fence in my alley. Yup, that is a giant trash barrel I am using as a table top. A lady ham’s gotta do what she’s gotta do, ya know?

If you are an astute observer, you will notice this toroid is not wound in the same manner as the now ubiquitous 49:1 unun meant for the end-fed halfwave antenna. Instead of two primary – to – fourteen secondary windings (i.e., a 1:7 windings ratio) necessary to make a 49:1 impedance transformer, I used three primary – to – twenty-one secondary windings (still a 1:7 windings ratio, and a 49:1 transformer). The core is composed of two 140:43 toroids superglued together, and then wrapped with glass tape. Yes, I am quite aware that most people say not to use the tape. I also used an overly thick 14awg Thermaleze magnet wire, but again, it was my best choice with what I had.

The primary winding is wrapped together with the secondary winding for the first three turns. The primary and secondary windings are soldered together at the start, and this bundle is attached to the chassis ground. The other end of the primary winding is attached to the center pin of the bulkhead SO239 connector. And, because it was all that I had available to me at the time, that is a 250pF capacitor rated at 3kV secured between the ground and center pin leads. The end of the secondary wire is attached to an unknown length of speaker wire, probably about 60 feet in length, and secured along a 6 foot fence.

When I tested this on the bench using a variety of resistances (from 2000 ohms to 10000 ohms) connected between the output and the ground of the transformer, I was concerned that the bandwidth was much too low, with the lowest SWR centered around the 160M band with very high SWRs at the HF frequencies higher than 14MHz. Unfortunately I did not take a picture.

But, when I attached an actual antenna, I was much more pleased with the results. The lowest SWR was falling in around 6MHz, so a bit electrically long for the 40M band. The 20M band looked acceptable. And the 15M band had a higher SWR, but certainly usable, and also a bit electrically long. This typical 40M EFHW pattern of resonances at 40, 20, and 15) is encouraging, and hopefully will show nicely with a properly hung radiator. This unun does not seem to be very useful for frequencies above 15M.

So, the alleyway antenna analysis is over now, and I am sharpening my CAD skills as I work up a sketch for a custom enclosure.

I hope you like it.

KM1NDY