Cantenna feed dimensions & SDR comparison for H-line

[Ayushman Tripathi]

Hi all,

I'm building a cantenna feed for my 2.4m dish for hydrogen line observation. Using the Changpuak calculator with 150mm diameter can at 1420 MHz, I get:

• Cutoff TE11: 1171 MHz 
• Guide wavelength (Lg): 373.5mm
• Probe position from back: 93mm (0.25 × Lg)
• Probe length: 53mm (λ/4)
• Minimum can length: 280mm (0.75 × Lg)

I have two 19cm containers I can join, giving me either:

• 190mm (single container - below minimum)
• 380mm (both joined - well above minimum)

Can anyone please advise should I use the full 380mm length, or would cutting it down closer to 280mm give better performance, as maybe 380mm is a bit more extra? Does 100mm extra length beyond 0.75 × Lg help?

Also I'm using 6mm copper tube for the probe instead of the recommended 2.6mm - is that fine?

Also, I have both HackRF and Nooelec Smartee SDR (RTL-SDR). From my testing, the Nooelec seems to give better sensitivity, but HackRF has a flatter/more linear spectrum (though filled with its own internal spikes). Nooelec picks up more external RFI. After calibration this shouldn't matter, but on my system the RTL calibrated spectrum is still not as linear as HackRF's. Can anyone advise which would be better for H-line work? (I've HackRF One r10, Original,  2024)

Also, I'm observing that even with my LNA and filter chain, the RTL is still able to pick up FM radio. I tried ferrites on the cables - it helped a bit. I tried placing the RTL in a copper box - it was still able to pick up the music. So my conclusion is it's coming through the antenna coax cable from the Discovery Dish feed. This doesn't happen with HackRF - it can't pick up FM music. Though I've already grounded the HackRF, I tried the same with RTL and it didn't help much.

Thanks!


The 2nd much more spiky one is the HackRF (but almost always more linear than the RTL).

[Alex P]

Hello Ayushman,

Is there an FM radio station nearby ?

As an experiment, try over wrapping ~ 2m of the Coax/SDR/USB cable with aluminum foil.

       ========================= FOIL==============================

--------- Coax--------------SDR------------USB_Cable-----------

       ============================================================

( later today/tomorrow I will run some actual tests with a cantenna using 280mm and 380mm tube lengths )

Regards,

Alex P

[Mike Otte]

To block local FM stations , try what is called an "open Stub".  We used on our repeater because the FM station was on the same tower as the repeater.

Open stub is cut to the frequency of the FM station to notch it out.  Lots of video's on how to make one. the components are just a tee in your feed line and a length of coax hanging off of it.  VNA can be used to tune it. 

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/sara-list/6e15128c-64ab-4a96-8da4-960ad2b16f45n%40googlegroups.com.

--

Mike Otte W9YS

[b alex pettit jr]

Mike,

Can you design one and post it as an example ?

Thanks.

              Alex

[Mike Otte]

Google and I am sure AI does all the work now days!  I hope i am not going to be charged for this!

Text was markup and would not copy so i took screen shots (see attached)

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/2129585495.1557541.1765030001932%40mail.yahoo.com.

--

Mike Otte W9YS

[b alex pettit jr]

S21 Spectral Response Tests of the normal 280mm and an extended 390mm tube 

          21cm Tuned Cylindrical Waveguide

==================================================================

 280 mm

280mm In Axis

280mm Out of Axis

=====================================================================================

390mm

390mm in axis

390mm out of axis

=============================================================================

Tx Antenna

CHEERS,

                               Alex P

[Don Latham]

And. . . don't forget to seal the open end of the coax!

Don

---

From: "Mike Otte" <mike....@gmail.com>
To: "sara" <sara...@googlegroups.com>
Sent: Saturday, December 6, 2025 7:28:12 AM
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

To view this discussion visit https://groups.google.com/d/msgid/sara-list/CA%2BG65tT6AeK%2B6a5KuU8kvZp35-%3DUa0-KPxTwCV-pHcrK2JhrkA%40mail.gmail.com.

--

------------
Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304

[Don Latham]

Nice! I clean forgot about using those log periodic antennas as test sources!!!!!!

Don

---

From: "sara" <sara...@googlegroups.com>
To: "sara" <sara...@googlegroups.com>
Sent: Saturday, December 6, 2025 7:58:56 AM
Subject: Re: [SARA] Re: Cantenna feed dimensions & SDR comparison for H-line

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/1698371448.830526.1765033137776%40mail.yahoo.com.

[Marcus D. Leech]

On 2025-12-06 08:53, Mike Otte wrote:

To block local FM stations , try what is called an "open Stub".  We used on our repeater because the FM station was on the same tower as the repeater.

Open stub is cut to the frequency of the FM station to notch it out.  Lots of video's on how to make one. the components are just a tee in your feed line and a length of coax hanging off of it.  VNA can be used to tune it.

I'll note that an open 1/4L stub has a response that repeats on odd multiples of the design frequency.  This may be fine in some applications, and unpleasant in others.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/CA%2BG65tSe5XQWAThajWY0bFWaTLzbAOoB5D6LKLySiad8LiBuWw%40mail.gmail.com.

[Andrew Thornett]

My main advice is cut the monopole probe a little bit too long and then tune it once cantenna made using a NanoVNA and slowly cutting tiny bits off end monopole until maximum VSWR (at least less than 2:1).

Sent from Outlook for Android

---

From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Marcus D. Leech <patchv...@gmail.com>
Sent: Saturday, December 6, 2025 6:22:35 PM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

To view this discussion visit https://groups.google.com/d/msgid/sara-list/a017c2cc-478a-49cb-b43b-59edc57a8ece%40gmail.com.

[Ayushman Tripathi]

Hi all,

Thanks a lot.

Hi Alex,

Yes, I wrapped the full coax cable with aluminum foil just now, and the FM broadcast stopped. As you can see in the image, there is no music now. Thanks a lot. I also placed the RTL in the copper box again. Yes, it seems I do have an FM tower 6.5 miles from my place.

Sure, Mike, Don - I'll try an open stub soon and will report the observations.

Thanks, Alex, for testing both cantennas; both look very similar at 1420 MHz. Do you think 6mm for the probe is fine for the cantenna?

Okay, Andrew, I'll do that. Thanks for the advice, though I don't have a NanoVNA yet. Airspy SDR is on the list next :). I will try to get a NanoVNA after that.

Also, I got my 2.4m dish; I'll set it up by the first week of January with the Cantenna.

[b alex pettit jr]

I wrapped the full coax cable with aluminum foil just now, and the FM broadcast stopped. 

How About That !  

Good News.

6mm is okay, but the 0.95 * WL/4 rule of thumb will be a bit too long .. 

The surface area at the open end will cause a  'capacitance hat' effect of a few mm.

SO, rather than 210mm/4 * 0.95 = 49.8 mm yours may wind up being .. 47 mm 

Alex

==========================================================================

On Saturday, December 6, 2025 at 03:38:54 PM EST, Ayushman Tripathi <ayushmantr...@gmail.com> wrote:

Hi all,

Thanks a lot.

Hi Alex,

Yes, I wrapped the full coax cable with aluminum foil just now, and the FM broadcast stopped. 

As you can see in the image, there is no music now. Thanks a lot. 

I also placed the RTL in the copper box again. Yes, it seems I do have an FM tower 6.5 miles from my place.

[Andrew Thornett]

Hi Don

What do you mean seal the open end of the coax?

Andy

Sent from Outlook for Android

---

From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Don Latham <d...@montana.com>
Sent: Saturday, December 6, 2025 6:18:31 PM
To: sara <sara...@googlegroups.com>

To view this discussion visit https://groups.google.com/d/msgid/sara-list/280444314.11019813.1765041511736.JavaMail.zimbra%40blackfoot.net.

[b alex pettit jr]

A bit more elegant solution than  Al  Foil will be to use Copper Braid

[Don Latham]

just put some silicone seal or other stuff to keep water out. Water loves coax due to capillary action.

---

From: "sara" <sara...@googlegroups.com>
To: "sara" <sara...@googlegroups.com>
Sent: Saturday, December 6, 2025 2:10:12 PM

To view this discussion visit https://groups.google.com/d/msgid/sara-list/FRWP194MB275249358247D22088228767FFA4A%40FRWP194MB2752.EURP194.PROD.OUTLOOK.COM.

[Ayushman Tripathi]

Okay, thanks.

[Alex P]

Please ReRead Marcus' comment :

I'll note that an open 1/4L stub has a response that repeats on odd multiples of the design frequency.  

This may be fine in some applications, and unpleasant in others. 

IF you have removed the RFI with the shield, I'd consider the problem Solved.

[Ayushman Tripathi]

Hi Alex,

Yes, thanks. That's why I'm not trying the Stub method. As for now, the FM is gone. The next task is finding the source of Harmonic RFI.

Will be much better at the remote site with the bigger dish, as there's very little RFI there, only cell towers, FM, no WiFi routers, no PCs, LEDs, etc.

Thanks!

[Ayushman Tripathi]

Hi, I found the source of this harmonic RFI; it was a nearby monitor, and it had a very big effect on the RFI. 

[b alex pettit jr]

YES, they can create a Lot of RFI ..  Typically,  the least radiation is Edgewise to the screen ..

so rotating it 90 degrees may have a dramatic effect !

Alex

[Ayushman Tripathi]

Hi,

Using my Discovery Dish feed (dual QPL9547 LNA, ~36 dB total gain) with RTL-SDR for hydrogen line observations. I noticed that:

• At low RTL gain (0-10 dB): DC spike and a nearby RFI are clearly visible, and the spectrum has good dynamic range.

• At high RTL gain (40+ dB): DC spike disappears completely

My question: Does this disappearance of the DC spike/RFI mean I'm overloading/saturating the ADC?

In other words, should I set the gain to the highest level where the DC spike remains visible?

Attached images show the spectrum at different gain settings.

Is there anyone who made it work with DD feed, and can you please advise what settings you used? I'm trying to find my best settings, using both SDR# and IF AVG (Windows) & also the VIRGO/DSPIRA Spectrometer on Linux.

Soon I'll be setting up a bigger dish with a Nooelec LNA, but still I want to make this setup work, as I have two systems: an HackRF/RTL with a DD Dish Feed and a WiFi Grid.

Also Airspy Mini with 2.4m Dish and Nooelec Sawbird.

I will run both the system at different locations, the bigger dish for SETI, and this one for H-line observations.

Any advice will be appreciated.

Thanks,

[b alex pettit jr]

The mid-band spike is an artifact of that SDR  

Use Max Gain

I use max gain on the AS mini also ..

Here is info on the Air Spy mini that references the problem

Alex

=======================================================

On Tuesday, December 9, 2025 at 06:01:25 PM EST, Ayushman Tripathi <ayushmantr...@gmail.com> wrote:

Hi,

Using my Discovery Dish feed (dual QPL9547 LNA, ~36 dB total gain) with RTL-SDR for hydrogen line observations. I noticed that:

[Stephen Arbogast]

I am using the  Discovery Dish with AirSpy R2,  GNU Radio (not  DSPIRA)  for  data  collection then  I use some developmental ezRA  software  from  Ted  Cline  (still in test)  for  post  processing.  I run  the gain of the  AirSpyR2  at  full (45).  Here is some  very preliminary  results  for a  short 5 hour  drift  scan...  hope this  helps  for  comparison.

[Ayushman Tripathi]

Hi Alex,

Sure, Thanks.

Also, can you please advise best ways to calibrate my system? Currently, I'm using cold sky calibration by pointing the dish towards the cold sky.

But as you can see from the calibrated spectrum, it looks nothing like the linear spectrum in the blog posts using the same setup (apart from the different LNA).
This is where I'm pointed at right now (Perseus Arm), not the galactic centre.

Thanks.

[Ayushman Tripathi]

Hi Stephen,

Thanks a lot! , Good to hear someone has done it with DD Feed!

And I'm not the first one trying. ;)

Thanks, your data helps a lot!

[b alex pettit jr]

THAT plot is reasonable .. and will work fine for a Background Corr.

I purchased the DD feed and will be making tests over the next week or so.

I've fab'd a small 'box' of Conductive Foam as a Background Correction reference ... results TBD..

Alex

========================================================================

On Tuesday, December 9, 2025 at 06:48:14 PM EST, Ayushman Tripathi <ayushmantr...@gmail.com> wrote:

Hi Alex,

Sure, Thanks.

Also, can you please advise best ways to calibrate my system? Currently, I'm using cold sky calibration by pointing the dish towards the cold sky.

But as you can see from the calibrated spectrum, it looks nothing like the linear spectrum in the blog posts using the same setup (apart from the different LNA).
This is where I'm pointed at right now (Perseus Arm), not the galactic centre.

Thanks.

[Ayushman Tripathi]

Okay Alex, thanks. 

[Ayushman Tripathi]

Hi,

I've fixed my system and am now getting much better results. These plots were captured while observing the Cassiopeia/Perseus Arm region.

The key improvement was adding aluminum foil shielding to the back of my dish, fully covering it to block RFI pickup from behind. Within an hour of making this change, I started seeing clear improvements.

My next step is to optimize pointing accuracy. Currently I'm using a phone compass, but it jumps around even after calibration—magnetic compasses aren't stable near metal structures. I'm looking into better solutions for more precise azimuth measurement.

For my larger 2.4 m dish (once it's set up), I plan to use drift scanning rather than daily repointing.

The last plot is from a few hours earlier when I was not pointing at the Cassiopeia region.

Thanks!

[andrew....@googlemail.com]

Drift scanning works well for hydrogen line work, even with small antennas or dishes and I would strongly recommend starting with that. With regards to pointing/compass, make line on the ground before putting your telescope down, and measure the direction of the line to define azimuth and then put telescope down aligned with that. Azimuth then does not change and best to leave the telescope out all the time so no errors due to movement. If you can’t do this then put some markers on the ground so that the telescope can be aligned exactly the same way each time you put it out.

 

For altitude measurements, use a digital inclinometer on the same part of the antenna/dish each time.

 

Both compasses and digital inclinometers are available free of charge as apps on your phone – but they are also so cheap to buy that sometimes easier to get at least the inclinometer as separate device (but not necessary).

 

The effect of metal on compasses can not be removed by buying expensive devices – only by measurement without the metal present, so you need intermediate markers.

 

If you really want to be able to point the telescope in different directions or have projects that require this ability then you need to either use a compass some distance from the metal structure (e.g. on a long boom), or use encoders or other means of identifying angle changes without using a compass.

 

I realise, Ayushman, that you have already done a lot and so moving to these more advanced areas is relevant to you, but for the sake of other readers who might be starting off, I would emphasise the importance of starting simply and with a smaller dish or antenna and building up, rather than jumping in at the deep end – good chance that will lead to failure and frustration, and might even lead newcomers to giving up on radio astronomy completely.

 

………..For all those people who are now raising an eyebrow at the above, thinking of how they advised me to do the same thing, and I ignored them and made a fool of myself, my only response is…..I know, and I was wrong!!!

 

Andy

 

 

From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Ayushman Tripathi
Sent: 12 December 2025 22:55
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

Hi,

I've fixed my system and am now getting much better results. These plots were captured while observing the Cassiopeia/Perseus Arm region.

The key improvement was adding aluminum foil shielding to the back of my dish, fully covering it to block RFI pickup from behind. Within an hour of making this change, I started seeing clear improvements.

My next step is to optimize pointing accuracy. Currently I'm using a phone compass, but it jumps around even after calibration—magnetic compasses aren't stable near metal structures. I'm looking into better solutions for more precise azimuth measurement.

For my larger 2.4 m dish (once it's set up), I plan to use drift scanning rather than daily repointing.

The last plot is from a few hours earlier when I was not pointing at the Cassiopeia region.

Thanks!

On Tuesday, December 9, 2025 at 8:26:42 PM UTC-5 Ayushman Tripathi wrote:

Okay Alex, thanks. 

On Wednesday, December 10, 2025 at 5:29:05 AM UTC+5:30 b alex pettit jr wrote:

THAT plot is reasonable .. and will work fine for a Background Corr.

 

I purchased the DD feed and will be making tests over the next week or so.

 

I've fab'd a small 'box' of Conductive Foam as a Background Correction reference ... results TBD..

 

Alex

 

========================================================================

 

On Tuesday, December 9, 2025 at 06:48:14 PM EST, Ayushman Tripathi <ayushmantr...@gmail.com> wrote:

 

 

Hi Alex,

Sure, Thanks.

Also, can you please advise best ways to calibrate my system? Currently, I'm using cold sky calibration by pointing the dish towards the cold sky.

But as you can see from the calibrated spectrum, it looks nothing like the linear spectrum in the blog posts using the same setup (apart from the different LNA).
This is where I'm pointed at right now (Perseus Arm), not the galactic centre.

Thanks.

 

--

--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/774cfdd9-48f2-4110-9e91-7350fb929133n%40googlegroups.com.

[Ayushman Tripathi]

Hi Andy,

Thanks for the detailed advice!

Sure, I'll try drift scanning for my setup. The marker/line idea for azimuth is clever - much better than fighting with a compass near metal.

I already have a digital inclinometer, so elevation is sorted. It's just azimuth that's been giving me trouble.

And yes, starting simple was key - the foil shielding trick only came after lots of trial and error!

Thanks,
Ayushman

[Andrew Thornett]

Sounds like brilliant idea and fits in with Alex's advice to remove ground noise which is relevant to all of us - so you've demonstrated how effective that is - timely!

Andy

Sent from Outlook for Android

---

From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Ayushman Tripathi <ayushmantr...@gmail.com>
Sent: Friday, December 12, 2025 11:50:30 PM

To view this discussion visit https://groups.google.com/d/msgid/sara-list/eb5c9bc5-e86e-415a-9a0f-d40ae18cd5f2n%40googlegroups.com.

[b alex pettit jr]

Ideally, you want to keep the dish pointed as 'high' in the sky as possible ..

Meaning, point along the Meridian & let the sky drift by

[Steve Berl]

Magnetic compasses have to be corrected for both variation (difference between true north and magnetic north at your location and date), and deviation (error introduced by local magnetic fields from metal objects nearby). It's hard to do that without a true reference direction of some kind.

An accurate way to get an azimuth reference is use the azimuth of the sun, which you can get from any one of several online calculators. One example is https://gml.noaa.gov/grad/solcalc/azel.html

Alternately you can find the time of local apparent noon for your date and location, and at that moment the azimuth of the sun is exactly 180°, due south.

Steve

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/828227956.201160.1765584260516%40mail.yahoo.com.

--

-steve

[b alex pettit jr]

If you pick a fixed location for your setup,

Go out on a clear night, use a star chart pgm to find where :Polaris is WRT True North, and

put two reference stakes in the ground pointing to that position for daytime alignment.  

( Without the planetarium software you'll be not more than  ~ 1/2 Degree from True N )

[andrew....@googlemail.com]

How important is this Steve for small dishes – given the beamwidth of small dishes the exact azimuth pointing direction is perhaps not vital? Or am I wrong? Also, if we are using the same direction with drift scans, then slight errors in pointing can be corrected later in the analysis?

 

…Just thoughts – open for discussion.

 

Andy

 

From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Steve Berl
Sent: 13 December 2025 19:35
To: sara...@googlegroups.com
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

Magnetic compasses have to be corrected for both variation (difference between true north and magnetic north at your location and date), and deviation (error introduced by local magnetic fields from metal objects nearby). It's hard to do that without a true reference direction of some kind.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/CAB4gGneN4nFeG_gmjLZ%3D5WnyU_y1tQQbDs_pdgJnAJQ8OpRm4w%40mail.gmail.com.

[andrew....@googlemail.com]

Of if you want to be really accurate then polar align an astrophotography mount and use that to get an absolute direction – but again how accurate do we need to be for various sizes of dishes, e.g. 100cm, 150cm, 200cm, 300cm dishes?

Andy

 

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 13 December 2025 19:59
To: sara...@googlegroups.com
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

If you pick a fixed location for your setup,

--

--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/1853444121.287245.1765655953637%40mail.yahoo.com.

[b alex pettit jr]

1) with a small dish with beamwidths of 10-15 degrees, not that important ..

2) how would you correct in the analysis if you don't know where it was originally pointed ?

========================================================================

[b alex pettit jr]

I Did Polar align my equatorial mount to True N and have alignment pipes in the ground for the tripod feet to return the mount to the identical location each time,

but that does not mean the Antenna beam matches the axis of the mount..

[Steve Berl]

How close do you need to get to a reference true north? As others have pointed out, it depends.

Depending on where you live, magnetic variation (the location and date dependent difference between magnetic north and true north) can be as big as 30°, so you definitely need to correct for that. 

If you use the included iPhone compass app, there is a setting to show either true north or magnetic north and for astronomy purposes you definitely want true north.

I'm not sure what the default setting is, so check the Settings on your phone.

Visual light astronomers don't typically use the sun, but use Polaris instead, because they typically are out at night and well, it's hard to see the sun then.

But during the day, the easiest thing is going to be to go out at your local apparent noon (from the online calculator, and an accurate time of day clock) and look at the shadow of a stick in the ground. The shadow will point true north. Make sure the stick is as vertical and straight as possible.

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/1549920466.285365.1765656681659%40mail.yahoo.com.

--

-steve

[andrew....@googlemail.com]

You could see how the plots align with what it should look like and adjust the angle so they overlap…..or is that bad practice?

 

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 13 December 2025 20:07
To: andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

1) with a small dish with beamwidths of 10-15 degrees, not that important ..

--

--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/1238281216.284667.1765656446327%40mail.yahoo.com.

[andrew....@googlemail.com]

Good point! I’ve noticed that too on my RTs!

 

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 13 December 2025 20:11
To: andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

I Did Polar align my equatorial mount to True N and have alignment pipes in the ground for the tripod feet to return the mount to the identical location each time,

--

--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/1549920466.285365.1765656681659%40mail.yahoo.com.

[andrew....@googlemail.com]

Not around here – you can hardly ever see the sun during the day even in the summer!!

 

From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Steve Berl
Sent: 13 December 2025 20:55
To: sara...@googlegroups.com
Subject: Re: [SARA] Cantenna feed dimensions & SDR comparison for H-line

 

How close do you need to get to a reference true north? As others have pointed out, it depends.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/CAB4gGndkR%2B5oQfVKeq-UrwhZp22eT3V-4wj3LaMFkh5X7ZU2LQ%40mail.gmail.com.

[Ayushman Tripathi]

Thank you Alex, Steve, and Andrew.

For azimuth, I compared readings across three different phones to rule out magnetometer issues, which helped. I've also been making small adjustments (2-8 degrees) based on trial and error while observing the results.

I've marked the dish position using nearby buildings and towers as reference points for consistent alignment.

I'm using the drift scanning approach rather than frequent repointing - just minor tweaks occasionally. The elevation is adjustable from 20° to 45°, currently fixed at 34°.

I'm using Virgo software running natively on a Raspberry Pi 5. I'm testing this setup on smaller dish before deploying it with my larger 2.4m dish at a remote location - running locally should avoid the latency issues that come with RTL-TCP over network. These observations were captured during 5 and 30 minute integrations. 

I'm also working on a cantenna feed for the larger dish and will share progress soon. 

Here's the plot I recorded today:
Thanks,
Ayushman

[Ayushman Tripathi]

All plots are from the same Cassiopeia/Perseus Arm region.

[b alex pettit jr]

" Cassiopeia "  is in the Dec +60 Deg RA 01:00 Region

Not a particularly bright area  for system evaluation

=================================================================================================

[Ayushman Tripathi]

Hi Alex,

Yes, from my balcony currently only the Perseus arm region is visible due to surrounding buildings - but this won't be an issue with my bigger dish at the remote location where there's a clear horizon.

Based on my plots, is there anything you'd suggest I improve or try differently? Any feedback would be helpful!

Thanks

[b alex pettit jr]

Hello Ayushman,

Honestly, I can not eval the data as I am totally unfamiliar with your software ..

Alex

=====================================

[b alex pettit jr]

With regards to Software :

I have run linearity , calibration, and signal range tests on the AirSpy SDR# Studio with the Kaminski IF_avg plugin software.

They function correctly. 

I have not seen similar tests performed on other software....

Alex

==========================

[Ayushman Tripathi]

Hi Alex,

Sure, no problem! I'll try capturing some data using SDR# with the IF Average plugin and share that tomorrow. I found a version of the plugin that has calibration save/read options, which should help.

Thanks,

Ayushman

[b alex pettit jr]

Hi Ayushman,

The Huge advantage of IF-Avg is its capability of background correcting all data As it is taken ..

Meaning, you can see small details in the Hline spectrum and not have the display overwhelmed with the Giant filter pass-band M Curve !

( this is probably Not the same data, but it illustrates the problem )

And, Yes, if you can capture a clean background file,  that might help.

Regards,

        Alex

========================================================================================