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, with extra information added from other members:
There have been many expert and knowledgeable comments about cantenna feed design on this group and on the SARA RTOP yesterday by people who truly understand the subject – we are very lucky to be able to get advice from you all – thanks!
Note in particular these recommendations from Alex Pettit on making a cantenna for 1420 MHz:
Click here to download Alex’s recommendations (PDF)
Personally, I am very enthusiastic but, as I am sure you have all noticed, a bit stupid and experience difficulty understanding all the technical jargon – even though supposedly I was taught a lot of it at school – that seems a long time ago now and my brain just isn’t as flexible as it used to be!
So, I have attempted to produce an idiot’s guide on how to make a HYDROGEN LINE CANTENNA from the comments (particularly Don’s yesterday but with additions from Rich yesterday and my attempt to pull out bits form the conversations on the mailing list where I could understand it). This will also be useful for beginners, I hope!
So, please could everyone take a look at my notes below and correct anything that is wrong – the idea is a simple plan to follow to make and setup a cantenna feed for a solar cooker parabolic reflector dish that works.
Also, couple of questions I have:
- 6-inch stove pipe is not usually aluminum but steel – does that matter?
- 6-inch stove pipe in the UK is usually enameled with a black coating – does that matter?
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, with extra information added from other members:
Recommended reading (1 and 2 below):
- “W1GHZ Online Microwave Antenna Book” …. https://www.w1ghz.org/antbook/contents.htm – and in particular read number (2) below out of that book:
- “Feeds for Parabolic Dish Antennas” …. https://www.w1ghz.org/antbook/chap6-3.pdf
Further helpful references:
- Cantenna design article ….. http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2011/QEX_1_11_PASKVAN.pdf
- 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
Cantenna size – anything in 6-inch rough size works well.
- 1/4 wavelength probe. For his waveguide on his 6-inch cantenna made from 6 inch stove pipe designed to detect 1420MHz signals, TE11 mode, Don calculated that the free space wavelength is 0.2113 m or 8.32 inches, and that the waveguide wavelength is 0.362 m or 14.25 inches.
- 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.
- One option is to coat inside of aluminium can with copper tape – option but not required – if done well improves conductivity of can.
Tuning the location of the dish feed during mounting on the dish:
- 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:
- 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.
- 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.
- 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.
- Paul Wade in “W1GHZ Online Microwave Antenna Book” (https://www.w1ghz.org/antbook/contents.htm) 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.