Choosing a Portable Antenna for a Mountain SOTA Activation
You plan a Summit On The Air activation on 20 m CW from a 12,000 ft peak. Your radio is a 10 W QRP transceiver. You can carry either a 20 m end-fed half-wave (EFHW) with a 49:1 transformer and 33 ft of wire, or a 20 m linked dipole with a 25 ft mast. The summit has rocky ground, strong afternoon winds forecast at 30 mph, and no trees. You will be on the summit for about 3 hours. Your feedline is 25 ft of RG-174. Propagation forecasts show the MUF over the path is near 18 MHz and the A index is 15.
1. Compare the two antennas for this activation in terms of efficiency, deployment, and wind survival.
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EFHW with 49:1 transformer:
- Efficiency: The 49:1 transformer has some loss (typically 0.5-1.5 dB), and the high impedance at the feedpoint makes it sensitive to counterpoise and ground conditions. On rocky ground without a counterpoise, common-mode current can reduce efficiency and cause RF in the shack.
- Deployment: Needs a support (mast or rock), and the transformer must be elevated; wind loading on a 33 ft wire is significant.
- Wind: A long wire in 30 mph wind needs secure anchoring; the transformer is a failure point.
Linked dipole with 25 ft mast:
- Efficiency: A center-fed dipole is inherently balanced and efficient; no transformer loss. A 25 ft mast puts the center at about 0.4 wavelength on 20 m, giving a low-angle pattern useful for DX.
- Deployment: Requires guying in wind; more hardware to carry.
- Wind: A mast can be guyed with three lines; the dipole elements flex.
Given 10 W QRP and a marginal MUF, every dB matters: the dipole is generally the better choice if you can guy the mast.
2. Given A=15 and MUF near 18 MHz, what operating strategy maximizes contacts on 20 m?
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A=15 indicates disturbed conditions, with higher absorption and degraded low-angle propagation. The MUF near 18 MHz means 20 m is near the upper limit and may be open only to certain directions and times.
Strategy:
- Operate during the best window: late morning to mid-afternoon local time, when the MUF peaks.
- Aim for NVIS/short-skip if high-angle radiation is favored; a dipole at 25 ft gives a mixed pattern.
- Call CQ on the QRP calling frequencies (14.060 MHz) and spot yourself if possible.
- Be flexible: if 20 m is dead, switch to 30 m or 40 m, which may support regional contacts despite absorption.
- Use the lowest loss feedline (RG-174 is acceptable for 25 ft at 20 m, about 0.5 dB).
- Keep power at 10 W and rely on efficient antennas and good operating practice rather than power.
3. How do you mitigate RF exposure and wind safety on the summit?
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RF exposure: At 10 W on 20 m, the controlled-environment MPE is easily met at distances beyond a few feet from the antenna. Keep people at least 10 ft from the antenna during transmission, and do not transmit if anyone is within that zone. The EFHW transformer and feedpoint should be away from the operating position.
Wind safety:
- Guy the mast with three lines at 120° intervals, anchored to rocks or stakes.
- Lower the antenna if winds exceed safe limits; a fallen mast is a hazard.
- Wear eye protection and keep the operating position upwind of the mast.
- Secure loose gear and have a planned descent route if weather deteriorates.
- Carry a shelter and monitor forecasts; 30 mph winds on a summit can become 50 mph gusts.