Advice on how to build and test the feed for LRO-H2(SCRT)from SARA Member, Don Latham AJ7LL, on SARA RTOP 2/2/2025

Following advice on how to build and test the feed for LRO-H2(SCRT)from SARA Member, Don Latham AJ7LL, on SARA RTOP 2/2/2025

References he recommends I read (1 and 2 below):

1. “W1GHZ Online Microwave Antenna Book” …. https://www.w1ghz.org/antbook/contents.htm
2. “Feeds for Parabolic Dish Antennas” ….  https://www.w1ghz.org/antbook/chap6-3.pdf

Further helpful references:

3. Cantenna design article ….. http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2011/QEX_1_11_PASKVAN.pdf
4. radio telescope for the 21cm neutral-hydrogen line …. https://www.semanticscholar.org/paper/A-compact-radio-telescope-for-the-21cm-line%3B-za-21-Saje-Vidmar/e5baaf7450e0aea72e7926e1bdd75d3175f23424

Ron’s recommendations for how to make cantenna feed:

Cantenna size – anything in 6 inch rough size works well.

• 1/4 wavelength probe.
• 6 inch stove pipe works well.
• Can 12-18 inches long (max 24 inch – longer = narrower frequency cut-off and can be useful to filter strong signals. (Alex Pettit had problem with 750MHz cell phone signal – so he lengthened cantenna and it acted as filter (up to 24 inch) – phase centre will still be just inside front – 2mm.
• Phase centre (focal point of dish needs to go) just inside front lip of can and right in centre – important to fine it – put piece paper across can and use sun to find it.
• Copper-brass-aluminum-steel in order preference – in practice aluminium or buy Victorian stove pipe!
• Coat inside
• Avoid tapered tubes.
• Can coat aluminium can with copper tape – option but not required – if done well improves conductivity of can.

Don’s recommendations for process of tuning the location of the dish feed:

1. Ensure can on focal line of antenna – right angles to antenna plane. Put mirrors on dish and find focal point as often people do not have focal point of dish just inside the lip of can or in centre of can. This should be achieved by:
2. Find centre first. Put mirrors on dish and find focal point as often people do not have focal point of dish just inside the lip of can or in centre of can.
3. Proceed to move cantenna up and down that centre until find ideal place – go out of sunny day, disconnect pre-amp so signal not too high, won’t damage probe, put something across surface (paper), and get that focal point to centre.
4. Must be co-linear with axis of parabola and right on it (i.e. on focal point). If off it, then more ground noise until all you see you see is ground-noise.
5. Paul Wade describes a process of using a string tied to the edges of your dish with the centre of string set to fall at the focal point. I have used this to setup portable microwave dish at different locations.