I made a new cantenna - now I'm getting loads of RFI

[Cathal O'Donghaile]

Good morning everyone

It was necessary to replace my cantenna recently and, rather than buy something that happens to come in a 5L can, a friend suggested going to a scrap yard dealer to see if there was any empty ones there. I spotted these metal cases:

The large one happened to measure 10.9cm - exactly the width of the cantenna. So, I tried cutting a new cantenna to shape from that and the DVD player cover sitting next to it:

The steel is 1mm thick. I tried it with and without aluminium lining on the inside. This is the result:

I'm now getting loads of RFI. It's actually worse without the aluminium tape lining. I'm open to suggestions. Can't quite figure out why this doesn't work.

[Alex P]

This is a  1420 MHz Tuned Cylindrical Waveguide built to proper dimensions

 

[b alex pettit jr]

It is more probable that the Signal Decreased rather than that the Noise Increased,

but this can not be determined by the information posted.

[Stephen Arbogast]

It's kinda like  fishin, some  days  you  catch em, some  days  you  don't!    Here  is  my latest catch with  Discovery Dish/Feed,  AirSpy-R2, ezRA,  lots of RFI.

[Cathal O'Donghaile]

Thanks, guys. Alex, that makes sense to me. Looking at the CSV file and my own outputs, the signal is weaker alright. I'll continue to make tweaks to the setup. 

That's a nice looking graph, Stephen, something I aim for is a continuous reading from the Milky Way. Onward and upwards. Hopefully I'll see you guys across Zoom on the 4th.

Cathal

[Stephen Arbogast]

Cathal,

My last graph  is not  typical...    I  am  experimenting   with  post  collection  processing  software.   I  am using  the  Kracken RF   Discovery  Dish  which  is minimum for  H-Line  work...    I have caught nice clean  graphs  when I am not fooling  around. I'm  just saying  fool around and try  different  approaches...   You  never  know what your  gonna get...   Forest  Gump.

[Andrew Thornett]

Hi Cathal,

You must have said before, and I missed it, but what dish are you using with this cantenna and what is its F/D (focal ratio - focal length divided by diameter of dish)?

Where do you live? What is around you? How far away is the closest cell tower?

Apologies for asking you to repeat what I'm sure you have already said.

In addition, you might wish to join us at the Monday evening (19:00 UTC on first Monday each month) hydrogeb line meetings where we have 2 hours dedicated to learning about and supporting each other with hydrogen line observing. Send me email to andrew at thornett dot net if you would like me to send you a link to the next meeting.

Andy

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From: 'Stephen Arbogast' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Saturday, 11 July 2026 07:47:59
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Subject: [SARA] Re: I made a new cantenna - now I'm getting loads of RFI

 

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[Cathal O'Donghaile]

Hi Andy

Thank you for the invite to the meeting and I will take you up on that soon. I'm not using a dish myself, it is a horn antenna to this design:

It's mounted on display board and the lining is from single use oven trays.

I'm currently still tweaking the cantenna element of it hence why it looks a little lob-sided. Last night's reading is looking a bit clearer having done some tweaking yesterday:

I live in Waterford City, Ireland and am surrounded by buildings, trees and foliage. The nearest cell tower is about 2km away. 

Cathal

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[b alex pettit jr]

The acquisition of the extremely low signal strength microwave radiation from

Galactic neutral hydrogen spin flip emission is not trivial. 

Its strength is in the range of being 1/2 to 1/30th the electromagnetic radio frequency

blackbody emission of the Natural environment ( plants / trees / buildings ) and many

times less than that emitted by Digital devices.

If the Analog Receiving & Amplification Hardware portion of the Radio Telescope is not optimized,

it will probably never be fixed in Software...  improved/enhanced/modified but a low Signal to Noise

measurement will never equal good hardware + simple software.

In order to Improve Hardware, Calibrated/ Standardized Software should be used.  

If each person develops their own software without Calibrating it against physical standards,

there can be no useful comparison of results.

Long Story Made Short : 

Evaluate & Optimize your Hardware using SDR# & IFavg     ( since it is widely available & calibration verified ) 

     ( OR Calibrate another software set and Everyone use That ! )

To Demonstrate : This is what I have obtained by Optimizing a simple 3 element Yagi with SDR# & IFavg

>>>  Here's A Suggestion:

The 3 element Yagi is an Extremely Simple / Low Cost Project.

Build It. Get it working  ~ equal to the one shown above. 

The spectral amplitude peaks are ~ 0.9 dB above the cold_sky noise floor

You will then have a Reference System against which you can compare your other designs .

         vs  ???

Alex Pettit

[Andrew Thornett]

I recently built a horn and, given that they are supposed to be RFI resistant, I was surprised at how much RFI it picked up - my talk at Eastern Conference will discuss all that. A deep dish works incredibly well by comparison.

Trees etc certainly make the situation a lot worse.

Can you try your setup somewhere without trees for comparison?

Andy

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Subject: Re: [SARA] Re: I made a new cantenna - now I'm getting loads of RFI

 

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[Cathal O'Donghaile]

Thanks, Alex. That is something I will look at to eliminate background noise. I'm working on my own output graphs based on the the rtl_power output and I've got some of them done. I use a background file based on readings from the LNA attached to a 50 Ohm resistor. But your small Yagi would definitely be more effective. I need to learn more though. My setup is limited to running everything from a 288WH power bank, which isn't enough to run a PC or laptop through the night. Hence I'm restricted to Raspberry Pi and rtl_power or EzCol to collect data and not run out of power.

Andy, currently my antenna looks at one of 2 parts of the sky that are tree, foliage and building free but during the summer it's a challenge finding those spots and I've had to move my antenna few times. Currently, it can see the edges of a tree in the corner. I'll have to wait for it to be a bit cooler before moving it and not collapse from the heat lol.

Cathal

[Andrew Thornett]

When you move tour antenna does the output of the telescope, RFI-wise, change?

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[b alex pettit jr]

Cathal,

You can eval a the system in a matter of minutes .. don't need to do long drift scans.

In Fact, you ideally want to capture data from either of two of the brightest regions.

Dec+20Deg RA0615Hrs  or Dec+40Deg RA2030Hrs.  

You need a consistent signal with which to compare changes to your hardware.

A  calibrated spectrum  is much more useful than a '3D' map for hardware and RFI eval.

Alex

[Stephen Arbogast]

One question....

Is Alex P  and b alex pettit jr the same person?

One  further  comment/explanation...

I   must add in fairness to Ted  Cline  that  I am  using/testing  his ezCon   version  ezCon260214a.py   which he   has  not  approved for  release.

I  ran  ezCol.py   first  with 10  signal  samples  between  reference samples  and then  I  ran  ezCon260214a.py  for  post  processing.

Then  I  removed  all the reference  samples  from  the  raw data and ran  ezCon260214a.py  again

What  I reported was the  ezCon260214a.py. results   from the raw  data with no reference samples.

Having fun with  ezRA, I am still learning,  I think it is great!

Stephen

[Cathal O'Donghaile]

Hi all

After a lot of work I was able to make a new cantenna from a metal casing and big improvement in the RFI:

Admittedly, it's not completely RFI free but it's an improvement from before.