Why your cable is part of your read range
In a UHF RFID system, the coaxial cable between your reader and antenna is not just “a wire.” The same cable carries your transmit power out to the antenna and the tag’s faint backscatter signal back to the reader. Every decibel (dB) lost in that cable is paid twice — once on the way out, once on the way back. Spec the wrong cable, or run it too long, and you can quietly throw away meters of read range before a single tag is involved.
That’s why, in the RFID world, two cable types come up again and again: LMR-400 and LMR-240.
Quick refresher on dB: every 3 dB of loss halves your power; 10 dB cuts it to a tenth. Small numbers, big effects.
LMR-400 vs LMR-240 at a glance
Both are low-loss coaxial cables from the LMR® family (Times Microwave). The difference is size, flexibility, and how much signal they lose. At the UHF RFID frequency we care about (~900 MHz):
| Property | LMR-400 | LMR-240 |
| Outer diameter | ~10.3 mm (0.405″) | ~6.1 mm (0.240″) |
| Loss @ ~900 MHz | ~0.13 dB/m (~3.9 dB/100 ft) | ~0.22 dB/m (~6.6 dB/100 ft) |
| Flexibility | Stiffer, larger bend radius | More flexible, tighter bends |
| Property | LMR-400 | LMR-240 |
| Velocity of propagation | ~85% | ~84% |
| Best for | Longer runs (≥3–5 m), permanent installs, dock doors | Short jumpers, tight spaces, handhelds/portable rigs |
What that means in practice — a 10 m run:
- LMR-400 ≈ 1.3 dB lost
- LMR-240 ≈ 2.2 dB lost
That ~0.9 dB difference is real read range. As a rule of thumb: use LMR-400 for any permanent run over a few metres, and LMR-240 only for short jumpers or where you physically need the cable to bend.
The golden rules for RFID cabling
- Keep it as short as you can. The cheapest dB you’ll ever save is the metre of cable you don’t install. Mount the reader near the antennas where possible.
- Go thicker for longer runs. Over ~5 m, the lower loss of LMR-400 almost always wins. For very long
runs (15 m+), look at even lower-loss cable (e.g. LMR-600).
- Mind the bend radius. LMR-400 doesn’t like tight corners — kinking the cable damages it and adds loss. Use LMR-240 where you need flexibility.
N. Fewer connectors = fewer problems. Every connector and adapter adds ~0.1–0.2 dB of insertion loss and one more point where water or vibration can get in. Avoid stacking pigtails and adapters.
U. Weatherproof outdoor joints. Outdoors, weatherproof every connection (self-amalgamating tape)
— moisture in a connector is a slow death for read performance.
The connectors: RP-TNC, SMA and N-type
The connector is where cable meets reader or antenna. In RFID you’ll mainly meet three families:
RP-TNC (Reverse-Polarity TNC)
The most common connector on fixed UHF RFID readers — including the Impinj Speedway and R700 readers we deploy. It’s a threaded, vibration-resistant connector. “Reverse polarity” means the center pin’s gender is swapped from a standard TNC (originally a regulatory measure to discourage unapproved antennas). Reliable and the default for most reader-to-antenna links in the FCC/retail world.
SMA (and RP-SMA)
Small, threaded, and good to very high frequencies. You’ll see SMA on smaller or portable antennas, GPS/Wi-Fi gear, and some compact readers. RP-SMA (reverse polarity) is common on Wi-Fi equipment, so double-check polarity before ordering — RP-SMA and SMA do not mate correctly. SMA handles less power and is physically smaller/more delicate than N-type.
N-type
The workhorse for outdoor, industrial and higher-power installs. Larger, rugged, naturally weather-resistant, and excellent for long cable runs and harsh environments. If your antenna lives on a warehouse wall, a yard, or a dock exposed to the elements, N-type is usually the right call.
Watch the gender and the polarity. Connectors come in male/female and, for TNC/SMA, “normal” vs “reverse-polarity.” Mixing them is the single most common ordering mistake. Always confirm what your reader port and your antenna port actually are, then pick the cable (and any adapter) to match.
How it all adds up: the link budget
Your effective output at the antenna is:
Reader power − cable loss − connector losses + antenna gain = EIRP/ERP
In Kenya, UHF RFID runs in the 865–868 MHz ETSI band at a 2 W ERP limit. So your cable and connector choices aren’t just about squeezing out range — they’re part of staying inside the legal power budget.
Lose less in the cable, and you can hit your target field with a more compliant, better-behaved setup.
Talk to us
Specifying readers, cables and antennas for a site is exactly what we do — from a single dock door to a national rollout. Parcy Tech designs, deploys and supports UHF RFID across East Africa. Get in touch and we’ll size the link budget for your site.