ON7UN Amateur Radio
 Round Septum Feed
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13cm Septum Feed
Dimentions & construction



Septum transformer in a round waveguide

Walter and I are experimenting with a round septum feed. This should give us the advantages of a good illumination of our reflector, and the easy way of creating circular polarization of the septum feed. Paul, W1GHZ has modelled this setup, with very promising results. Rasto, OM6AA and his team where also modelling a new round septum feed. Walter and I where experimenting with round septum feeds, a lot of copper has been cut and scrapped.

Complete round septum feed for 23 cm, including feed holder, preampbox, good for 15 kgr in the focus.

Happy faces when the first verifications tests where made. ( Left, ON4BCB Right, ON7UN )

Paul, W1GHZ had modelled a new scalar ring for the VE4MA clasic feedhorn, with improved results. We would implement a similar ring on the round septum, but, we also wanted to construct the other choke dimentions to investigate the results in real world.

Round septum feed with super VE4MA choke 23 cm inside the 6,1 m Dish

Rasto, OM6AA and his team where concentrating on a feedhorn for 13 cm. The main design goal was having a 13 cm feed with good circularity and return loss over the entire frequency range, 2304 - 2424 MHz. We constucted 3 samples for testing in an anechoic chamber in Prague. The feeds where tested with very good results, return loss OK over the entire requency range ( better than 25 dB ) and circularity within 1 dB over the entire band. The septum plate was remodelled also, resulting in one extra step, conpared to the OK1DFC design.

Sample feeds for 13 cm, to be measured in an Anechoic chamber in Prague

Inside 13 cm feed with small choke.

Rasto and his team constructed the 23 cm prototype. Axial ratio without choke ring was 0,2 dB , with the large choke ring the axial ratio reduced a bit to 0,6 dB, wich is still outstanding.

Measurements of the 13 cm feed in the Anechoic chamber

RX connector on the 23 cm feed.

TX port on the 23 cm feed

Return loss was OK using N connectors ( 28 dB ), however, later we found out that the return loss could be very much improved. The prototype had female N connectors for measurement. Our feeds needed SMA on the RX side, and 7/16 on the TX side. We constructed SMA connectors using HP ( Agilent ) instrument adaptors sma/N, and installed this inside an N connector, to match the mounting holes. For the TX side we used obsolete lightning arresters from Suhner, using their 7/16 female connector. We constructed the connector very carefull, to make sure we maintained the 50 ohm impedance. This resulted on a return loss way in the 30 dB.

Inside the 23 cm feed

Testing of the 23 cm feed in the Anechoic chamber

Live testing of our feeds gave us very nice results. Measuring sun noise, with the EME calc of VK3UM as a reference, gave us a result where we had to increase efficiency to 65% in the software. Not much that can be improved I guess.

The feed size, due to the larger septum, is large and heavy, therefore the feed was constructed in Aluminium. The weight of the feed alone is around 7 kgr, including the choke, feed holder preamp box etc, the total weight in the focus is now around 15 kgr.

Transport of the feedhorn ...

Antenna with round septum feed



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