for 20-30-40 meters
July 2, 2005
My favorite antenna for portable use is the end fed halfwave or EFHWA. I have documented some of the earlier ones I built in my pedestrian mobile pages.
Here's two new projects from the NØLX workbench. One is an EFHWA for 20-40 meters, and the other is a matching unit for the antenna. This all came about because of another project: I'm building a Rock-Mite CW transceiver and needed a 40-meter antenna.
...
July 2, 2005
My favorite antenna for portable use is the end fed halfwave or EFHWA. I have documented some of the earlier ones I built in my pedestrian mobile pages.
Here's two new projects from the NØLX workbench. One is an EFHWA for 20-40 meters, and the other is a matching unit for the antenna. This all came about because of another project: I'm building a Rock-Mite CW transceiver and needed a 40-meter antenna.
The antenna is 33-feet tall and uses
stranded 20-ga insulated wire in three sections. The center is 20 feet
long and the two outer ends are 6-1/4 feet each. You can see a picture of the antenna in use on my boat. It is held vertical by an MFJ fiberglass pole.
The antenna is a physically shortened
halfwave, and its electrical characteristics are maintained by
two loading coils. The coils have taps to allow operation on different
bands. I used 18-ga stranded, insulated wire wound around a short
section of PVC tubing and covered with electrical tape. An alligator
clip allows for shorting the different taps. Each coil uses 11 turns
for 30 meters and 23 total turns for 40 meters. Bypassing the coils
completely gives a full-size antenna for 20 meters. Based on tests
made later, if I were to make another I would add a turn for both
bands, making it 12 turns for 30m and 25 total for 40.
Matching Unit
The photo below, on the left, shows the
inside of the tuner. It consists of an AM radio tuning capacitor and a
toroid coil with five secondary windings to match 50 ohms input. A
rotary-dial switch, seen in the photo below, right, selects the
secondary taps. The antenna plugs into the red banana jack on top,
while the black banana jack on the side is for later
counterpoise/ground experiments. For now, I use no counterpoise, and it
seems to work fine. The toroid coil is a T68-2 iron powder from Amidon.
There are 34 turns in the primary, for an inductance of 6.5 uH, and 5
tapped turns in the secondary. The capacitor has a measured capacitance
of 6-135 pF.
On the right you can see the thumb wheel rotary switch I use to select the taps of the secondary windings that transform the high antenna impedance to 50 ohms.
On the right you can see the thumb wheel rotary switch I use to select the taps of the secondary windings that transform the high antenna impedance to 50 ohms.
How well does it work? I tested it on 40
SSB. With 5 watts from an FT-817 and the antenna sandwiched between the house and two trees in my very
small front yard, I was
heard in Oregon and Indiana from here in Golden, Colorado. The funny
part: the Oregon station, running 800 watts, had just commented to his
friend on how, "it would take some real power to make it through my
noise level tonight." That's when I broke in and worked him.
Later, I again set up in the front yard, but this time on 20
meters. Same 5 watts, sideband. My first contact was Jack, XE1/W5FG in
Mexico, then Pat, XQ1VLY 5,500 miles away in Chile! A final
confirmation that it all works....