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Antenna system

Antenna system

I have always been a QRP operator, but recently I thought it would be nice to add another 16 dB or so to my 3 watt QCX transceiver. After getting an FT-450D transceiver I realized some upgrades of the antenna system could be in order. I now have a somewhat more stable random length dipole made out of stranded speaker wire, split at about 8 meters for a total wire length of about 16-17 meters. Using plastic bag sealing clips I improvised some open-wire feeder of about 2 meters, connected directly to a homebrew T match tuner (see picture below). The load capacitor has inadequate place spacing, but I only observed arcing on the 20m band (at 50W), where the elecrical length of the antenna is close to a full wavelength and the feedpoint impedance very high.

The tuner has resistive SWR bridge with a red LED, following the Sudden ATU from GQRP. See its manual for a schematic. Instead of 50 ohm resistors I use 9 parallel-connected 1/4 watt 470 ohm resistors because I had plenty of them, and this is plenty for tuning up with a 5 watt carrier. It even survived 100 watts for about ten seconds, before I noticed a strong smell and saw thick smoke rising from the resistors. Turns out the 6m and HF power settings are independent on the FT-450D, and the default is full power. Now the color code of the resistors is black-black-black-black, but it works all the same.

I can tune the antenna to all bands from 80m to 6m, but for some bands it is unable to match all the way to 50 ohms. This is when the narrow-range automatic tuner of the FT-450D comes in handy, and in the end I got a (mostly) decent setup for 80 through 6 meters. Of course, a 16-meter dipole 5-8 meters up is not optimal for the lower bands, and running parallel to a wall of reinforced concrete for most of its length probably does not help either. But any antenna is better than no antenna, and this gets me on the air (and I've worked most of Europe already, even at QRP levels). The total cost should be a few tens of dollars, assuming you can find a good deal for some used variable capacitors.

Homebrew antenna tuner

The tuner is built on a breadboard (we actually used it for cutting bread earlier) with a front of plexiglass. 1.2 mm bare copper wire is used for most wiring, and the coil is constructed with 1.4 mm lacquered copper wire (30 turns, 50 mm diameter, tapped at 12 places with a maximum of about 20 uH). Suboptimal component placement meant wiring became messier than necessary.