[ 2018-03-29 ]
20m Quarter Wave Vertical Antenna
A cheap 20m quarter wave vertical antenna that works surprisingly well at least for receiving
Looking for an efficient antenna with moderate space requirements I stumbled upon the design of a quarter wavelength vertical antenna with elevated radials.
I decided to build the antenna for 20m as the vertical element is only roughly 5 meters long.
The antenna was simple to build and performed quite well considering I didn't go to any great lengths of tuning or otherwise optimizing the antenna.
At the feed point there is a 1:1 current balun (common mode choke) which is made of 10 bifilar turns of speaker wire on an FT200-2 core. Both the core and wire gauge are oversized for my QRP operation but I had those materials readily available from another project.
One turn of the choke connects the vertical element to the center conductor of the BNC connector while the other turn connects the radials to the body of the BNC connector. The common mode choke is supposed to prevent the feed line from radiating due to common mode currents on the center conductor and braid of the coax feed line.
The antenna elements were cut to 5.5 meters in length, with the vertical wire being folded back onto itself to shorten it to 5.3 meters. The resulting SWR with 10m of RG-58 feed line was so low that the (crappy) meter in the FT-857D hardly indicated anything.
The choke is housed in a small plastic distribution box. The antenna elements were terminated with ring terminals on one end which are then attached to the feed box using M4 bolts and wing nuts. The connection from the choke to the bolts is also made using ring terminals. The other side of the choke is soldered onto the BNC connector.
Below is a screen shot of the wsprnet.org propagation map. As you can see I got out quite well, much better than I ever achieved with a magnetic loop from my apartment.
So far I managed to make only 6 FT-8 QSOs, mainly due to lack of practice and low power (10W max.) operation. I had to turn down the RF gain of my FT-857D quite a bit because the stations were coming in quite strong and overdriving the front end. Either those OM were running very high power or maybe I'm just not used to having a well performing antenna connected to the transceiver.
I may be able to build a similar setup with a base loaded vertical for 30m and put up a QRSS grabber or WSPR receiver with a Raspberry Pi.