The Mobile Install That Interferes With Itself
A ham installs a 2 m/70 cm mobile antenna on the trunk lip of a car using a lip mount, with the coax routed through the trunk and along the sill to the dash. The radio is a 50 W dual-band unit. On 2 m the SWR is 2.5:1; on 70 cm it is 1.3:1. The operator hears alternator whine that rises with engine speed, and other stations report a raspy signal on 2 m. The antenna is about 40 cm from the rear window glass, and the trunk lid is bonded to the body only through the hinge. The operator wants to know whether the antenna is defective and whether the whine is a radio problem.
1. Interpret the SWR difference between 2 m and 70 cm. What is the likely cause?
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The 70 cm SWR of 1.3:1 is acceptable, while the 2 m SWR of 2.5:1 suggests a ground-plane or mounting problem that affects the lower band more. A trunk-lip mount relies on capacitive coupling to the vehicle body; if the trunk lid is poorly bonded (only through the hinge), the mount's ground reference is inconsistent, and the effect is worse at 146 MHz than at 440 MHz because the electrical dimensions differ. Proximity to the rear window glass (40 cm) can also detune the antenna. The likely cause is insufficient or inconsistent grounding and nearby objects, not a defective antenna. Fixes: bond the trunk lid to the body with a short braid, ensure the mount contacts bare metal, and move the antenna away from the glass. Re-measure SWR afterward.
2. Explain the alternator whine and the raspy transmit audio. Are they the same problem?
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They are related but distinct. Alternator whine on receive is usually conducted or radiated noise from the vehicle's charging system entering the radio through the power leads or antenna system; it rises with engine speed because the alternator's ripple frequency rises. Fixes include wiring the radio directly to the battery with fused leads, adding filtering, and improving antenna grounding. Raspy transmit audio reported by others can result from RF feedback, poor grounding, or the same ground-plane deficiency causing the radio's RF to couple back into the audio path. Both point to installation quality rather than a faulty radio. The judgement is to treat the vehicle as part of the antenna system and fix bonding, power wiring, and antenna placement before condemning the equipment.
3. Rank the fixes and justify the order.
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First, bond the trunk lid to the body and verify the mount's ground contact - this addresses the 2.5:1 SWR and the likely RF feedback behind the raspy audio. Second, move the antenna away from the glass if bonding alone does not bring 2 m SWR near 1.5:1. Third, fix the power wiring: fused leads direct to the battery, with the negative lead kept short, and add filtering if whine persists. Fourth, re-check SWR on both bands and confirm audio reports. The order follows the principle of fixing the antenna system and grounding first, since those affect both SWR and RF feedback, then addressing conducted noise. This avoids replacing equipment that is probably fine.