Restricted Space 40-10 Meter Antennas

40-10 Meter Antennas in an Antenna Restricted Community

Over a period of several months prior to the completion of my attic antenna suite I operated with a borrowed AEA Isoloop antenna. This magnetic loop antenna covers the HF band from 10-30 Mhz. The only current commercial supplier of a similar loop is MFJ Enterprises MFJ-1786. I found the loop to be an effective antenna when operated horizontally (horizontal dipole mode) at a reasonable height above ground (30 ft. in my case). The theoretical pattern is omnidirectional. I found no significant evidence that my pattern was any different.

Most of my operation with this antenna was QRP (3 watts) PSK using a PSK-20 (Small Wonder Labs, but no longer in business) on 20 meters. During this time I was able to work over 45 countries. I recommend a small magnetic loop when either space is unavailable for full size antennas or regularly retuning is not objectionable.

My final antenna suite while in the townhouse consisted of 2 sets of HF dipoles and a J-pole for 2 meter operation. The figures below shows the configuration of these antennas. These views are looking at the wall of the attic area facing the living space

All of the wire antennas used insulated hookup wire. Lightweight scrap plastic is used as end insulators. End support is provided by nails into the roof rafters.

The first dipoles started as standard 20, 15, and 10 meter dipole combination. All three dipoles were fed from a single 1:1 balun. All dipoles were full size but as is normal with this kind of configuration they were experimentally trimmed for resonance within each band. The 20 meter dipole extends the full length of the space and the last 3 feet or so is drooped vertically at the end wall. The 20 and 15 meter dipoles worked fine with VSWR under 2:1 across the band. The 10 meter antenna however, was very narrow. To remedy the narrow bandwidth I used a bowtie dipole antenna for 10 meters. My version of the fan was constructed of 1/2 inch copper pipe tee with 14 AWG house wire connecting elements. VSWR is under 2:1 for nearly the entire 10 meter band

The second dipole set (lower on the roof rafters) started as a 40 meter only "slinky" antenna. This antenna simply uses 2 standard metal slinky toys connected to a 1:1 balun and stretched to the corners of the attic space. A non-metallic messenger line provides support for the slinkys. This antenna is a linear inductively loaded short dipole resonant on 40 meters. Overall, the on-air performance was ok but certainly not great. It did allow me operation on 40 meters.

Some literature suggests that this 40 meter half wave slinky can be operated as a 3/2 wave antenna on 15 meters. I was unable to get an acceptable (<3:1) VSWR on 15 meters. Thus, I choose to use a separate 15 meter dipole even with the potential interaction between the antennas.

After setting up the 40 meter dipole, I decided to add WARC band coverage to my antenna suite. 12 meter and 17 meter dipoles were added to the 40 meter dipole. Thus, in this limited attic space I now have 6 operating HF bands and instant band switching!

In all cases of setting up these dipoles the exact location of the dipole was not critical. The initial dipole length was set by

468 / frequency (Mhz) = Length in feet

The dipoles were then trimmed in INCREASING frequency order i.e. 20m then 15m then 10m. If at all possible, use of an antenna analyzer such as an MFJ 259 is recommended for antenna measurements. Using an analyzer changes the trimming task from hours to minutes (especially important in an attic on a hot summer day) and eliminates radiating test signals.

30 meters was not included for space reasons and I'm not much of a CW operator anyway.

It should be noted that while many of the dipoles show very good VSWR it's not perfect solution. All the antennas have VSWR at or below 2:1 and all loaded into my TS-570D. Antenna patterns in such close confines of the house and other antennas are not really predictable. My on-the-air experience with low power has been acceptable.

Though my attic antennas were developed on a trial and error basis, antenna modeling can give significant insight into the antenna performance. here is a web site that illustrates how to go about modeling attic antennas. Computer Assisted Low Profile Antenna Modeling