- Why does my dipole use 468/f instead of 492/f (half of 984)?
- A theoretical half-wavelength in free space would be 492 / f(MHz) feet. Real wire antennas have an end effect and a velocity factor slightly less than 1 (typically 0.95), which shortens the resonant length. The widely-used constant 468 already bakes in these corrections. Your mileage will still vary with wire diameter, nearby objects, and height above ground, so always trim and measure with an antenna analyzer or SWR meter — use this result as your starting cut length, never your final one.
- What is a good VSWR for a ham radio station?
- Most transceivers will operate without fold-back at VSWR 1.5:1 or lower (return loss ≥ 14 dB, reflected power ≤ 4%). VSWR up to 2:1 (return loss ≈ 9.5 dB, reflected power ≈ 11%) is generally acceptable with a built-in ATU. Above 3:1 you may trigger the radio's protection circuit and the coax loss increases significantly. Professional repeater installations aim for 1.2:1 or better. Note that the impedance you type here is the antenna's feed-point resistance — for a purely resistive mismatch to a 50Ω line, a 75Ω load gives VSWR 1.5:1, a 25Ω load also gives 2.0:1.
- How much does coax cable loss matter in practice?
- At HF (below 30 MHz) with short runs, coax loss is usually under 1 dB and barely perceptible. At VHF/UHF (144 MHz, 432 MHz) with longer feedlines, loss can be significant. For example, 100 ft of RG-58 at 144 MHz loses about 5.5 dB — that's roughly one-third of your transmit power dissipated as heat in the cable before it even reaches the antenna. LMR-400 at the same frequency loses only about 1.5 dB per 100 ft, making low-loss cable a far better investment than a power amplifier for receive performance and transmitted EIRP. The "Coax Loss" tab lets you compare cable types directly.
- How do I use the LC resonance calculator for antenna traps or matching networks?
- Antenna traps (parallel LC circuits) present a high impedance at their resonant frequency, effectively shortening a trapped dipole for higher bands while allowing it to be full-length on lower bands. Enter your target trap frequency and either the inductance or capacitance you have on hand; the calculator will give you the missing component value. For a series LC matching network (such as a beta match or L-network section), you want the circuit to resonate at your operating frequency — the same formula applies.
- What is Friis free-space path loss and when is it useful?
- FSPL describes how much signal is lost simply due to the spreading of radio waves in open space, even with perfect isotropic antennas and no obstacles. It increases 6 dB every time you double the distance (or double the frequency). Ham operators use it to estimate link budgets for simplex VHF/UHF contacts, satellite links (OSCAR, ISS), and moonbounce (EME). Real-world signals suffer additional losses from terrain, foliage, atmospheric absorption, and Faraday rotation — FSPL is a best-case lower bound, not a real-world prediction.