2026 · Ashwing RF engineering
TL;DR — When several 5 GHz radios share one tower, a strong transmitter on one sector desenses the receivers on the others — raising the noise floor, dropping modulation rates and cutting range for the whole site. A per-radio 5 GHz cavity band-pass filter rejects the out-of-band and adjacent-sector energy before it reaches the receiver, often recovering several dB of noise floor and restoring capacity. This note explains the mechanism and how to spec the filter.
Why co-located 5 GHz radios interfere
A typical WISP tower stacks multiple access points, backhaul dishes and sector antennas within a few metres of each other, all operating across the crowded 5.1–5.9 GHz U-NII bands. Even with good channel planning, the receivers suffer because:
- Near-far desense. A neighbouring sector transmitting at full power just metres away is enormously stronger than the distant subscriber you are trying to hear. The receiver's front end gets overloaded even when the interferer is on a different channel.
- Transmitter broadband noise. Every PA emits low-level noise far outside its own channel. Multiply that across six radios on one tower and your "quiet" channel is no longer quiet.
- Adjacent-band services. Weather radar (DFS), 5.9 GHz C-V2X/ITS and other users sit at the band edges and leak in.
- Harmonics and intermod from closely spaced high-power radios.
The symptom is a rising noise floor: subscribers drop to lower MCS rates, retransmissions climb, and effective range and capacity fall — often blamed on "RF congestion" when the real cause is on your own tower.
What a cavity band-pass filter does at the tower
A cavity band-pass filter placed on each radio passes only that radio's operating sub-band and sharply rejects everything else:
- Low in-band insertion loss (< 0.5 dB typ.) — it barely touches your own signal or link budget.
- Deep out-of-band rejection (> 50 dB) — the neighbouring sector's carrier and broadband noise are knocked down hard before they reach the receiver front end.
- High power handling (> 50 W) — it can sit on the transmit path as well, cleaning up what each radio emits.
Fit one per radio, matched to that radio's channel plan, and each receiver only "sees" its own band — so the near-far desense that was raising the whole site's noise floor is removed.
Note: A band-pass filter fixes out-of-band and adjacent-sector interference and front-end overload. Two radios on the same channel still need channel/polarity planning and, where helpful, GPS sync.
Where to put it in the chain
Radio ──► [ 5 GHz Band-Pass Cavity Filter ] ──► Sector / Dish Antenna
Install the filter between each radio and its antenna, as close to the radio port as practical, with short quality jumpers and properly torqued connectors. On a transmit-heavy sector it cleans the emitted spectrum; on a sensitive backhaul receiver it protects the front end.
How to spec the filter (checklist)
- Passband — match each radio's channel block. Common WISP splits: 5150–5350, 5200–5500, 5500–5800, 5800–6100 MHz. Narrower passbands reject more.
- Insertion loss — < 0.5–1 dB in-band; every extra dB is range lost.
- Rejection — > 40–50 dB at the specific neighbouring-sector and DFS frequencies you fight.
- Power handling — > 50 W so it can sit on the TX path.
- Environment — IP67-sealed for mast-top weather exposure; -40 to +85 °C.
- Connectors — SMA standard; N-type on request for tower hardware.
Typical measured result
On a congested multi-sector tower, adding matched band-pass filters per radio commonly drops the measured noise floor by several dB. The practical effect: subscribers hold higher MCS rates, retransmissions fall, and both range and site capacity recover — without changing a single antenna. (Every Ashwing filter ships with its own VNA-measured S21/S11 curve; ask us for a before/after test plan for your site.)
Ashwing 5 GHz filters for WISP towers
IP67 cavity band-pass filters, < 0.5 dB insertion loss, > 50 dB rejection, > 50 W, SMA — built for mast-top outdoor use and swept/serialized before shipment:
- AW-BPF-5200-5500-S — 5200–5500 MHz (U-NII-2)
- AW-BPF-5500-5800-S — 5500–5800 MHz (U-NII-2C/3, co-location mitigation)
- AW-BPF-5800-6100-S — 5800–6100 MHz (U-NII-3/4, 5.8 ISM)
Not sure which split fits your channel plan? Send us your tower's radios and channels and our RF engineers will recommend a per-sector filtering plan — or ship samples to test on your own site. See the full cavity filter range.