Coax Cable for FM Antenna: Matching Matters More Than Loss
Quick answer / TL;DR: Short answer: use 75 ohm coax with a balun at the antenna, keep the impedance consistent through every connector, and mount the antenna as high and as clear of metalwork as possible. Cable loss at FM frequencies is small, so a modest RG6 run is ample; the problems that matter are a missing balun, a 300 ohm ribbon section left in the path, and multipath from nearby structures.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.An FM antenna is a balanced element and a coax feed is unbalanced, so the connection between them requires a balun and a consistent 75 ohm impedance throughout.
At around 100 MHz the cable loss is modest, which makes matching and interference the dominant issues rather than attenuation.
This article covers the balun, the impedance rule, and the multipath and interference problems that produce poor FM reception on a technically sound installation.
The antenna's driven element is usually a dipole, which is balanced, while coax is unbalanced with the shield at ground potential.
Connecting one directly to the other produces a current on the outside of the shield, which turns the feedline into part of the antenna and distorts the pattern.
A balun fixes this by transforming between the two, and its absence is a common cause of reception that is worse than the signal level suggests.
Matching and Loss
| Element | Requirement | Consequence if wrong |
|---|
| Impedance | 75 ohm throughout | Reflections, reduced signal |
| Balun | Required at a balanced element | Pattern distortion, noise pickup |
| Ribbon feeder | 300 ohm, needs a balun | Mismatch at any transition |
| Cable loss at 100 MHz | Small, about 2 dB per 30 m | Minor contributor |
| Connectors | 75 ohm type | Cheap 50 ohm types cause reflections |
The connector row is a detail that is often wrong in domestic installations, where 50 ohm BNC or PL259 connectors are sometimes fitted to 75 ohm cable.
The mismatch is small at FM frequencies but it adds up across several joints, and the reflections show as a slightly weaker, noisier signal.
Multipath and Interference
Multipath occurs when the same station arrives by more than one path, and the signals interfere at the receiver.
The symptom is distortion or hiss that changes when the antenna is moved, even though the signal strength reading is high.
Directional antennas reduce multipath by rejecting signals from the side, which is why a simple dipole in a city often performs worse than its gain suggests.
Interference from nearby electronics is a separate problem, and shielding plus a balun at the antenna reduces pickup on the feedline.
Installation Practice
- Fit the balun at the antenna rather than at the receiver
- Keep 75 ohm impedance through every connector
- Route the cable away from mains wiring
- Mount clear of metalwork where possible
- Ground the mast where local practice requires it
Real-World Applications
A dipole is connected directly to coax. Reception is noisy and the pattern is unpredictable, and fitting a balun improves both.
A ribbon feeder is joined to coax without a balun. The transition mismatches, and a proper balun at that point resolves it.
A station is strong but distorted. Multipath from a nearby building is the cause, and a directional antenna reduces it.
A 50 ohm connector is fitted to 75 ohm cable. The mismatch is small but measurable across several joints, and replacing them improves the signal.
Standards and Verification
Coax impedance and loss are specified by the cable manufacturer, and antenna matching practice follows established radio frequency convention rather than a single consumer standard.
Verification is a listening test on a weak station plus a signal strength reading, ideally with the antenna rotated to check for multipath.
Rotate the antenna as a diagnostic. A station that improves when the antenna is turned is showing multipath rather than a weak signal.
Antenna Choice and Siting
A simple dipole is adequate in a strong signal area, and a directional antenna is better where stations are distant or multipath is present.
Height matters more than gain for line of sight, and moving an antenna a few metres can change reception more than changing its type.
Keep the antenna clear of metalwork, because nearby metal distorts the pattern in ways that are hard to predict.
Where the antenna is in a loft, the roof structure and any foil-backed insulation will affect the result.
Test in the intended position before fixing anything permanently, since loft installations are difficult to adjust afterwards.
Record which direction gives the best result for the weakest station, because that is the alignment worth keeping.
A rotator is rarely justified for FM, and a fixed direction chosen by test is usually sufficient.
Where two stations require different directions, a second antenna and a combiner is the cleaner solution than a compromise position.
The combiner introduces loss, so it is worth confirming the budget before adopting that arrangement.
Where the receiver has a 300 ohm antenna input, a balun is required at the receiver end as well as at the antenna.
Using a 75 ohm cable into a 300 ohm input without a balun produces a mismatch that is audible as reduced sensitivity.
Related reading: Coax cable signal loss — blogs/news/coax-cable-signal-loss
Related reading: Coax cable bend radius — blogs/news/coax-cable-bend-radius
Related reading: Coax adapter — blogs/news/coax-adapter
Frequently Asked Questions
Do I need a balun for an FM antenna?
Yes, where the element is balanced. Without one the shield carries current and the pattern distorts.
What cable for FM?
75 ohm coax, such as RG6. Loss at FM frequencies is modest.
Why is a strong station distorted?
Usually multipath. A directional antenna reduces it.
Does connector impedance matter?
Yes. Fitting 50 ohm connectors to 75 ohm cable causes small reflections at each joint.
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