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RX 75Ω Line Isolator +30 dB CMR (500 kHz – 200 MHz)
RX 75Ω Line Isolator +30 dB CMR (500 kHz – 200 MHz)
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This inline 75 Ω RX choke and surge suppressor integrates two gas discharge tubes (GDTs) and a precision SMD choke on a multilayer PCB inside a rugged aluminum enclosure. It provides wideband common-mode suppression while protecting sensitive SDRs, active antennas, and low-noise receivers from transient surges. The side with the grounded RVS peg must always face away from the antenna or the shack — the choking takes place in front of this grounded section. The opposite connector is DC-isolated via a nylon insert for galvanic separation. Both connectors are 75 Ω F-type for seamless integration into RX-only feedlines.
Baluns in a Nutshell
Why “Common-Mode” Is the Most Abused Term in Ham Radio
How Much Choking Do You Really Need — for RX and TX?
Built for the Long Run
The internal PCB is HV-coated for insulation and moisture protection, while the aluminum enclosure provides mechanical shielding and thermal stability. Only the connector side forms an intentional electrical bond to the housing; all other surfaces are insulated to maintain balance and minimize leakage currents. The dual-GDT network safely diverts surges to the ground peg without altering RF response. Designed for continuous outdoor service, it maintains impedance and CMR even after years of temperature cycling.
Installation Strategy
For optimal suppression, deploy multiple units in a three-stage layout:
- 1) One choke 25 – 50 cm after the antenna feedpoint — isolates the antenna from feedline currents.
- 2) One choke in the middle of the feedline — RVS peg faces the shack to reinforce mid-line rejection and optionally allow small choking capacitance to be added for fine-tuning.
- 3) One choke before the shack entry — keeps stray RF inside the shack and prevents it from coupling into mains or USB lines.
When multiple chokes are installed in series, their impedances add, increasing the total common-mode isolation. Each doubling of total choking impedance provides roughly +6 dB of additional suppression — meaning two 40 dB units in series yield about 46 dB total isolation. As an alternative strategy, you can place a 50 dB+ high-isolation choke at the antenna feedpoint and use this compact version before the shack entry for full feedline suppression.
Maintenance & Corrosion Care
Once a year, apply a small amount of aluminum paste or conductive grease where the RVS peg meets the housing. This prevents galvanic corrosion and preserves the grounding path’s conductivity. Inspect both F-connectors for moisture or oxidation and ensure the nylon-isolated side remains electrically clean.
Measured Common-Mode Performance (75 Ω System)
Conservative, verified results from RF.Guru Lab using the Y-parameter common-mode method.
| Band | Frequency | Choking |Z| (Ω) | CMR (dB @ 75 Ω) |
|---|---|---|---|
| 160 m | 1.9 MHz | ≈ 12 kΩ | 44.1 dB |
| 80 m | 3.5 MHz | ≈ 8 kΩ | 40.6 dB |
| 40 m | 7.0 MHz | ≈ 5 kΩ | 36.6 dB |
| 20 m | 14.2 MHz | ≈ 3 kΩ | 32.3 dB |
| 15 m | 21.2 MHz | ≈ 2 kΩ | 28.8 dB |
| 10 m | 28.4 MHz | ≈ 1.5 kΩ | 26.4 dB |
| All data are conservative, measured in aluminum housing with 75 Ω F-connectors, dual GDT network, and internal SMD choke. | |||
Construction Highlights
- Rugged aluminum housing providing RF shielding and thermal stability
- Internal PCB fully HV-coated for insulation and humidity protection
- Dual GDT surge arrestors with precision SMD choke network
- Grounded RVS peg for direct earthing and mechanical fixation
- Opposite connector DC-isolated via nylon insert
- Two × F-type (75 Ω) connectors, fully shielded
- Optional choking capacitance can be added externally for fine-tuning response
- All impedance and CMR values verified under load
Measurements follow the common-mode Y21 technique using calibrated current probes. This directly measures true common-mode current attenuation, ensuring that the dB figures represent real-world isolation and received noise reduction — not theoretical impedance.
Mini-FAQ
-
Q: Can it handle transmit power?
— No, it’s for RX and active-antenna systems only. -
Q: Is it weatherproof?
— Yes, the enclosure is sealed and the internal PCB is HV-coated. -
Q: What’s the ideal layout?
— One near the antenna, one mid-feedline (peg → shack), one before shack entry. -
Q: Does grounding improve performance?
— Yes, always bond the RVS peg to a local earth or mast connection. -
Q: How to maintain it?
— Once a year, apply aluminum paste where RVS contacts the housing to avoid galvanic corrosion.
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