80 meter antenna

We built several Cricket 80a kits, which operate in the 80 meter amateur band.  Since we don't have any antennas specifically for this band, I decided to make one.  The design is a basic dipole fed with coax and a current balun at the feedpoint.

Here are the feedpoint parts: 

The coil is bifilar wound with 13 turns of 22 AWG speaker wire through a toroid.  It measures around 350 microhenries.  The extra loop is for a support cable to lift the feedpoint.  The enclosure is a watertight plastic container that has holes drilled for the SO-239 connector and mounting hardware.  I found that it was easier to control the drilling by hand (rather than by power drill), and a step drill made the holes cleanly deburred.

The two wires attach to the coax center and shield, while the other leads attach to the antenna.  It made sense to do the electrical work first...

... and then install it in the container.  I applied plastic epoxy to each of the pass-throughs before installing hardware in an attempt to seal out any water.  After the hardware was installed, I screwed in the antenna connections tight.

Here is the final enclosure, ready for the radiating wires.

Connecting from the hooks to the radiating wire segments is done with a short feeder segment of stranded wire.  Loops are soldered in the stranded wire while it's installed on the enclosure.  To avoid melting the plastic, I gripped a hemostat onto the plastic side of the junction to draw the heat.

Here are the feeder segments ready for the radiating wire segments. 

The next step was to design the radiating wire segments.

For the radiating structure, we are constrained by the feedline length (25 feet of RG-58) and the confines of our lot.  We're lucky that the antenna basically runs the length of one side of our property, basically touching the ground on one end due to ground slope.  Here is what NEC seems to suggest:

• Height of antenna above ground: 2 meters
• Length of each leg: 20 meters = 65 feet

Here is the impedance and VSWR according to NEC:

Here is the radiation pattern according to NEC, which clearly indicates that the main beam is vertical, which should be good for near vertical incidence skywave (superimposed on the antenna structure):

Given this plan, I measured out two runs of 67 feet each; better to cut long and trim than the other way around.

The wires are attached to the feeders.

Ready for installation!

Then I strung the works into position.  This took a while; after trimming off the excess wire, I got a near perfect SWR around 3.560 - 3.580 MHz, right where I wanted it.  As NEC predicted, the antenna seems to degrade higher in the band, with a 2.5:1 SWR around 3.800 MHz.

Loop antenna

Since HF antennas tend to be rather large, I wanted to try a small resonant loop.  After doing some research, I found a few sites that seemed rather detailed.  This one seems about the best.

I constructed mine from 3/8" copper tubing, and it is mounted on a camera tripod base.

It is fed using a gamma match.

The tuning is accomplished by a scavenged broadcast FM tuning capacitor and a homebrew air-variable butterfly capacitor.

The antenna can be tuned on the 20 meter through 10 meter bands.  Here I am using it on 20 meters...

Omnidirectional sonar antenna

Because I'm interested in collecting indoor sonar data that has lots of multipath, it is useful to have an omnidirectional antenna either for reception or transmission.  Here's a simple design for an antenna that radiates omnidirectionally in the azimuthal plane, but rejects sound from above and below.  Roughly, it consists of two round disks with a speaker or microphone placed in the center of one of them.  This acts like a cylindrical waveguide.

Before building this, I wrote a simple boundary element method simulator in GNU Octave to test the radiation pattern.  This was helpful to choose dimensions for the disks and the spacing between them.  The simulator used a dense grid of boundary points (blue below) and propagated to a dense slice of points (green).

After a bit of tweaking, I settled on a separation of 20 cm and a diameter of 60 cm.  This yielded a radiation pattern that's roughly flat for equatorial elevations, and drops off sharply beyond that.

Here's a near field plot as well, showing the standing wave structure between the disks.

I constructed the antenna from 1/4" plywood, using 1" dowels as standoffs between the disks.  To ensure the standoffs were reasonably precisely made, I faced off their ends in the lathe, and used the lathe to drill pilot holes for mounting screws. 

I also cut a tripod mounting bracket using the router.

The speaker is mounted in the center of the top disk. 

Here is the finished antenna.

The antenna seems to reject sound from above and below pretty noticeably, but as could be expected from an omni, its overall gain is not very high.