I finally finished my attic shortwave antenna a few days ago, after puzzling over how to do it for almost four years. It was both easier and harder than I thought. The project took up most of my spare time for a week, and required me to practice tossing a tethered tennis ball around up there between the two attic hatches.<\/p>\n
I had to get the antenna up in the attic, above the walls of the house here, because the walls are stucco-coated chicken wire and thus form a very effective shield can. Even a 40' dipole in my workshop could pull in only some of the strong local AM broadcast stations. The new antenna works extremely well, and scanning the bands on Wednesday night with the Icom 736 brought in all the usual suspects from Europe at 9 MHz, along with an amateur station in Costa Rica at 7220 and another one that was (I think; copy on that one was poor) in Argentina. The quiet sun means that the bands above 14 MHz are basically dead, but assuming that we're not headed into the next ice age, they'll be back in a couple of years.<\/p>\n
The wire was cut for 7200 KHz, and the measured SWR minimum was at 7130 (with another potentially useful one at 21400) so I got pretty close. Seeing if I could get a signal out was the next test. I tuned around on 40M to find a quiet spot, pressed push-to-talk, opened my mouth…<\/p>\n
…and the fire alarm went off. I tore upstairs to find Carol in a panic and QBit barking furiously at the cold-air return where the siren lives. I didn't assume that the transmitter was at fault, but took a quick run around the house and garage to make sure nothing was burning, and by the time I reset the siren, the alarm system had already called the fire department. Nothing was burning, and with a red face I had to tell the firemen who came up in a truck (not a huge one, fortunately) that my transmitter had triggered a false alarm.<\/p>\n
The garage smoke detector is perhaps 5' below the south leg of the dipole, and I may have to have the company that installed the alarm system run shielded cable to it. We think that the dipole was inducing sufficient current in the smoke detector cable to trigger the system, so the shielded cable may be enough. If the dipole is inducing currents in the smoke detector itself, the detector may have to go into a Faraday cage of some sort. The fact that the vulnerable detector is in the garage is fortunate. Out there a Faraday cage would be almost stylish; but maybe not so stylish on our livingroom ceiling.<\/p>\n