RWST Feeds for 23 and 13cm

OK1DFC Septum Feed for Circular Polarisation

This feed was published at EME2002, in Prague, by OK1DFC. It is a design that has been in use by various OK stations, on a number of microwave bands from 23cm to 3cm, for some time.

The design claims better than 1.1dB cicularity, with around 27dB isolation between Tx and Rx ports, without the use of polarizing or matching screws (c/f VE4MA circular and N7ART diagonal waveguide feeds).

I have constructed models, to the dimensions given by Zdenek, OK1DFC, in the presentation. The photos below show various stages in the construction - construction notes for the versions shown can be found here .

RWST Metalwork

Measurement Results

Using 6mm dia monopoles, and 15mm tuning discs, a Return Loss of better than 30dB can be achieved on each port (= SWR<1.07:1)

Summary Measurements obtained:

Tx port RL - -34dB

Rx port RL - -33dB

Isolation: 21dB

Cicularity: V/H +/- 0.1dB

Max cicularity Error: +/- 0.5dB

Transmission Loss: <0.1dB

Initial measurements at Martlesham Microwave Round Table, 2003, gave:

Tx port RL - >-20dB across 2250 - 2450MHz, with a dip at frequency selected by probe resonance

Rx port RL - >-20dB across band as above - also dip at probe resonance

no other measurements were taken on this occasion, but these will be added later.

parts for RWST feed	marking out the side joints of the casing	Rx port and monopole mounted
first assembly stage - rear plate mounted	second assembly stage - laying the septum plate and packing pieces on lower half casing	view into finished assembly
Rx Return Loss display	Measurement set-up for RL, isolation and circularity	Measuring circularity, using WA5VJB PCB log-periodic as reference antenna
G4DDK's RWST feed on 2.3m dish	RWST mounting frame	Feed mounted on my 1.2m prime focus dish for testing
completed 13cm feed, with flare for 0.7f/d dish	A view into the feed	Detail of backplate and support

click here for link to EME2002 Presentation

and you can get the Excel calculation sheet here!

Page created: 16.10.2002 Last updated: 30.11.2003