EFHW Matchbox
The purpose of this post is to share the efhw matchbox I built, and the SWR curves I obtained.
I commonly use a shortened efhw antenna. My 10/20/40 antenna is made of #10 m wire, followed by a #25 microH coil, -acting as a choke on 20 and 10m- , followed by # 2m wire. This antenna is working as a half wave on 20 m, a full wave on 10 m, and a shortened half wave on 40 m.I also have a 10/18 MHz antenna working using the same concept, haff wave on 18Mhz, shortened on 30 m.
I usually feed my antenna with a 81:1 balun, to “adapt” the high impedance antenna to the 50 ohms impedance of the transceiver.The impedance varies with the height of the antenna, the soil, the antenna configuration, etc- The impedance is different for each band.The 81:1 transformer response is not “flat”. The impedance matching is “acceptable for all bands ”, but far from perfect, and varies.
I wanted to explore the possibility of an “impedance matchbox”, for better matching. As an impedance matching circuit, I essentially built a parallel resonant LC circuit, which has a high impedance when tuned to resonance. On the TX side, this high impedance is “transformed” to adapt to #50 ohms.The circuit is simple. A toroid (red, I believe T130-2 or T 150-2), a variable capacitor,# 250 pF, a few connectors, a box. For the convenience of the settings, I used a multi-turn vernier adjustment knob, and a 6 positions switch, to select the best suitable tap on the coil. Everything was available on hand, in my drawers,
The adaptation was initially checked with my mini 1300 antenna analyzer. Of course, the range of the adjustment of the matchbox will depend on the components. essentially L values/range, max capacity, residual capacity. My matchbox is usable with efhw antennas, from 40 to 10 m, for an impedance discrete range of # 1000 to 5000 ohms, with SWR < 1.5, most of the time lower than SWR 1.2
To use the matchbox, once the best tap is selected for a given band, just adjust the capacitor for max signal/noise, by ear, or visually on Smeter…and transmit. The matchbox is less convenient than a broadband transformer, since you need to adjust for each band, but it is very fast to do. The circuit has really high selectivity, nice for RX, and gives a much better impedance adaptation vs a broadband transformer. Yield, efficiency … .I do not know how to accurately measure, so I won’t comment. HF contacts really work well with qrp power.
73 / 72 de Pierre F5MOG
A few pictures below, schematics, the matchbox, SWR curves obtained in my garden, using the above described antennas, installed as inverted L, about 5 m vertical, the remaining part horizontal.
