Quick answer: A flat coax cable solves a mechanical problem, not an electrical one. Its thin conductor and flat dielectric have far higher loss per metre than RG6, so it is suitable for a short window or door crossing and not for a long run. Keep it under 1 m where possible, expect 2 to 4 dB of loss, and never use it behind a MoCA or satellite link that is already marginal.
The use case is real: a tenant cannot drill a wall, a listed building forbids new penetrations, or a temporary office needs a feed for a week. A flat cable passes under a door frame or through a window seal and the job is done. The mistake is treating that short jumper as a substitute for the rest of the run, because the attenuation curve is nothing like the coax it replaces.
Why flat coax loses so much
Geometry
In RG6 the centre conductor is about 1.0 mm copper clad steel inside a 4.6 mm foam dielectric, which holds a constant 75 ohm impedance along the length. A flat cable uses a thin strip conductor maybe 0.15 mm thick separated from a foil shield by a thin plastic film. The impedance is maintained only approximately, and the dielectric loss per unit length is several times higher because the field is concentrated in a lossy plastic instead of foamed polyethylene.
Measured effect
At 1 GHz, typical RG6 loses 5 to 6 dB per 30 m, while a flat jumper loses 2 to 4 dB in 0.5 m. At 2 GHz, which is where satellite intermediate frequency and some MoCA bands sit, the flat cable’s loss roughly doubles again. A link that had 10 dB of margin before the jumper may end up with none.
Where it works and where it does not
| Application | Frequency | Suitable | Reason |
|---|
| Indoor TV aerial | up to 700 MHz | Yes, short | Low loss at UHF |
| Cable modem return path | 5 to 1000 MHz | Marginal | Upstream power rises |
| Satellite IF | 950 to 2150 MHz | Risky | High band loss |
| MoCA 2.0 | 500 to 1650 MHz | Risky | Rate drops before link fails |
| Temporary antenna | up to 700 MHz | Yes | Short duration |
Installation details that decide success
The door gap
A standard interior door has a 3 to 5 mm gap under it, which a flat jumper passes easily. An exterior door with a weather seal does not, and forcing the cable under it crushes the dielectric and changes the impedance at that point. Use a window feedthrough designed for the purpose, which brings the cable through a small panel that closes the gap.
Connector strain
The F connectors on a flat jumper are the weak point. The centre conductor is a thin strip and the crimp or compression ring has very little material to hold. Support the cable within 100 mm of each connector and avoid pulling it tight across the gap, because a jumper under tension pulls the strip out of the connector over a few months.
Return loss and why the connector matters
Impedance discontinuities reflect energy back toward the source. In a flat jumper the strip conductor and the foil shield form a transmission line with poor dimensional control, and the F connectors at each end are crimped onto a conductor that is a fraction of the thickness of RG6. Reflections show up as standing waves that change with cable position, which is why moving the jumper can change the signal quality.
A short jumper with well seated connectors is usually acceptable for terrestrial television. The same jumper in a satellite path may work in dry weather and fail in rain, because rain fade removes the margin the reflection had already consumed.
Practical scenarios
- Tenant with a wall mounted TV and no permission to drill: a 0.3 m flat jumper under the balcony door plus a normal RG6 run inside is acceptable for terrestrial TV.
- Satellite receiver in a rental flat: measure the receiver’s signal quality before and after; if the quality drops below 60 percent, replace the jumper with a window feedthrough panel.
- MoCA between two rooms: flat jumpers usually cause the MoCA rate to halve rather than fail, which looks like a slow network rather than a cable problem.
- Trade show booth: a flat jumper is ideal for a two day install where the cable has to disappear under a carpet edge.
FAQ
Can I use flat coax outdoors?
Not for long. The jacket is usually PVC and the foil shield is thin, so UV and water ingress destroy it within a season. If the crossing must be outdoors, use a UV rated flat jumper or a proper wall feedthrough with a drip loop.
Does flat coax degrade the picture quality?
Digital television either works or breaks into blocks; there is no gradual softening. The visible effect of extra loss is a higher error rate and then a freeze or a channel that disappears in rain, which is why the signal level should be checked after installation.
Is 75 ohm maintained in a flat cable?
Approximately. The geometry makes exact impedance control difficult, so return loss is worse than RG6 and reflections are more likely at the connectors. On a short jumper the effect is small; on a long one it becomes the dominant loss mechanism.
Next step
Use the jumper only for the crossing, and keep the rest of the run at spec: RG6 cable covers the main run, RG6 vs RG59 explains where the thinner cable is acceptable, and F-connector types covers the termination. Send the application and run length to zongcarrie@zh-zong.com or +86 13543026126 for a loss estimate.
