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Portable L-Match with SWR indicator

Being portable with a CW QRP rig is great fun. However, a small and lightweight portable antenna is needed. Ideally, it is some sort of an end-fed antenna, as these antennas will only need a single elevated point, as the feed point can be close to ground. There are many options, the simplest is a so-called random wire.

A random wire is just a piece of wire with some (no so) random length. The actual (electrical) length of the wire (keep the shortening effect of the wire insulation in mind) is not critical, you only need to avoid multiples of λ/2 at the operating frequencies. Typical lengths are 10.6m or 12m (35ft or 40ft) allowing for 40m, 30m, 20m, 17m, 15m, 12m and 10m bands or 21.6m (71ft) allowing for 80m, 40m, 30m, 20m, 17m, 15m, 12m and 10m bands. Not being multiples of λ/2 and therefore not multiples of λ/4, implies that these antennas are never resonant with the operating frequencies, but also never be high impedance (usually a couple hundred Ohms) on these bands. This allows for some easy matching, using a 9:1 UnUn and an L-match.

Alternatively, you may actually head directly for multiples of λ/2. In this case, you will get a so-called end-fed half-wave antenna (EFHW). For example, a simple 21m (69ft) long wire will be approx. λ/2 on 40m, λ on 20m, 3λ/2 on 15m and 2λ on 10m. On these frequencies, the antenna will have a very high impedance but will be resonant (real impedance). Hence a 49:1 or 64:1 UnUn can bring this high impedance down to near 50Ohm. Moreover, a 21m wire will be near λ/4 on 80m and near 3λ/4 on 30m and thus will have an impedance near 50Ohm by itself on these frequencies. The required wire lengths, however, will never match exactly on all bands, hence a L-Match will still be needed to match these antennas exactly.

An L-match consists of a variable series-inductance from TRX to the antenna and a variable shunt-capacitance at the antenna side to ground. This L-match then matches high impedances to 50Ohms. You may also reverse the L-match to match low impedances to 50Ohm. Such an L-match is not complicated to build and they are very popular with QRP and portable operators due to their simplicity and small size. So why having a further site on the L-match here? Just to give a tip for constructing the variable inductor:

Get one of these cheap 12-step rotary switches from RND or LORLIN for PC through-hole mounting. There is a single contact in the center of the switch and 12 contacts on the outside. A T80-6 core fits nicely between these contacts. The outer diameter of the core is about 20.2mm, while the contacts are arranged in a circle of 22.2mm diameter.

Just wind 24 (or 36) turns on the core using a reasonable thick wire (e.g., 0.5mm-1mm) and distribute them equally. Then, scrape away the coating of every second (or third) winding. Place the core between the inner and outer pins of the switch and solder the outer pins to every second (or third) winding. This trick allows for a compact, stable and RF-friendly connection between the switch and the inductor.

Circuit of the LED-VSWR bridge.

Finally, a simple SWR indicator is needed as I do not want to carry a real SWR meter with me. I only want to know whether a reasonable match is provided. This SWR bridge will be inserted by a switch between the TRX and the L-match. The voltage divider R5 & RV1 can be used to set the SWR at which the LED starts to glow. For all other resistors use 0.6W or even 1W metal-film types, as they need to dissipate at least 0.625W each at 5W input. Do not use 1/4W types!

Board layout of the LED VSWR bridge.
The circuit can be build easily on a tiny (18 x 15 mm) perf-board.

The complete bridge circuit can be fitted on a small perf-board which will likely fit into the L-match box.

Best & 73,
Hannes, DM3MAT