Cheap BNC-m connector for LMR-400 coax

Created: Feb. 2022

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Introduction

The "Oserjep" branded connector is available from the China-based Shopee retailer, ylkrp.my. The main selling point is a price that is ~1/10 of an equivalent model from established German manufacturer, Telegaertner (RM6.20 vs RM61).

Mechanical aspects

The plating quality is dubious as speckles can be seen against the red grommet (fig. 2). 

As the clamp is different from what I have encountered before, I asked the vendor for an assembly diagram but got no response. The clamp's uniqueness lies in the use of a washer-like ring to lock the braid (fig. 3). However, it is a poor design because the connector body can rotate slightly after assembly (fig. 4).

Electrical performances

Ideally, connectors should be evaluated in conjunction with a very short coax in order to minimize the coax's contribution to the measured performances. However, my intent was to produce a 15m coax and so will need to live with this imperfect evaluation setup.

The coax with the Oserjep connectors was compared to a similar cable fitted with Telegaertner ones. The testing utilized a network analyzer that swept over 10-1000 MHz, although the connectors should ideally be tested up to the specified maximum; i.e. 4 GHz for BNC. 

Both cables have almost similar insertion loss over, but only the Oserjep's loss is shown (fig 5). The return loss trace has ripples that are ~45 MHz apart; these are due to the reflected waves combining either constructively (peak) or destructively (notch) at the test port. 

However the cables' return loss differ significantly (fig. 6); i.e. the Oserjep equipped cable has significantly poorer match at higher frequencies. The Oserjep's poorer match should be tolerable at 435 MHz, but can be worrying above 800 MHz.