Sunday, January 29, 2012

Winter Field Day and a Magnetic Loop Antenna

Last Field Day I spent a fair bit of time taking photographs and video which made for an entertaining record of the event when uploaded to YouTube. This time I was determined to do something more in tune with the Field Day theme and actually get my hands dirty ... or possibly burnt, cut or crushed (Physical coordination not being my strong point). Fortunately I only sustained one small scrape in the course of constructing a magnetic loop antenna for the 30 - 10 meter bands (A frequency range can be measured in meters since wavelength is directly proportional to frequency, a frequency of 7.49 million cycles per second has a wavelength of 40 meters at the speed of light). I had finished the antenna construction Saturday evening but the 20M band had fallen silent as ionospheric conditions favored the longer wavelength bands like 40M ... which the antenna didn't cover.

On Sunday morning the 20M band opened up again and I was able to make contacts all over the US using 10 - 25W. If you stop to think how far the light from a 25W light-bulb can reach it puts the fact that I could talk all over the US, using the same amount of power, into perspective!
My loop antenna is a simple resonant circuit: Consisting of a loop of copper connected to a variable capacitor. A smaller matching loop inductively links the transceiver to the larger copper loop so the radio see the right load.

The devil is in the details of course but I'll save a discussion of theory and construction for another post. There was a fair amount of interest in the loop as other folk also live in similar antenna restricted neighborhoods and are prohibited from putting up large antennas for the HF bands. Another ham brought out a very fine commercial loop antenna with a flexible loop that could be packed into a netbook sized bag. Such an antenna could be just the thing for trekking with a radio and operating in the great outdoors.
Seems I didn't learn my lesson with cheap coax ... The short piece that connects the wire matching loop to the longer cable run was a piece of RadioShack brand with a PL-259 connector conveniently attached. As I started started to transmit I noticed the SWR (Reflected power ratio) start to rise the longer I transmitted. As soon as I stopped for a moment it quickly dropped so I knew that whatever was getting upset had to have a fairly small thermal mass ... it wasn't the tuning capacitor for sure! I'll have to swap out the short piece of coax for a SO-239 socket and run my good coax directly to the antenna. While I was testing and making a few field day contacts I kept the power level low which kept the SWR under control.

Tonya added her baked creations to the Field Day menu and I suspect we may have beaten last years time in consuming everything apart from one slice of apple pie which lasted until Sunday morning. We were extremely well fed with delicious food thanks to many contributions with special thanks to Mike N5TGL and his wife who hosted the event and tolerated our impromptu lectures and animated discussions of ham radio miscellany for the two days of Winter Field Day.
The Society for the Preservation of Amateur Radio is the organizing body that schedules this event for the last full weekend each January. The object of the event is familiar to most Amateur Radio operators: set up emergency-style communications and make as many contacts as possible during the 24 hour period. The rules encourage as many contacts on as many bands and modes as possible, because during a real emergency, the most important factor is the ability to communicate, regardless of band, mode or distance.