Meteor Scatter Antenna Testing

[IanL]

After beginners luck building my first S@N Yagi / RTL-SDR meteor scatter detector rig, which picks up lots of meteors with good signal strength, I built a second one for my astro-soc's observatory. This one has proved far less sensitive.  A month or so ago, the local Radio Ham club were visiting, so I took the opportunity and had one of them test the antenna using their gear (since the S@N method is to basically build them blind and hope for the best). The results were poor, virtually no resonsance at any frequency across the entire range they could measure, and none at all at the Graves frequency.  They suggested looking for obvious faults like a short or an open circuit, but having tested those (easily with a multimeter) there is no problem apparent.

The radio guys were using a neat little "vector network analyser" (VNA) that basically generates a signal, has a dummy 50 ohm load attached to one port and the antenna to the other. The data is transmitted to an Android App running on a tablet. Looking in to it, the device and software runs to several hundred quid, which is out of the budget for what is supposed to be a cheap project.

I've been looking for alternatives and it seems like I've found something suitable here:

https://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/

It needs some free PC software and an RTL-SDR dongle, which obviously I have as they are basic requirements for the meteor detector setup.  In addition one needs:

A £9 noise generator:  https://www.ebay.co.uk/itm/2018-verison-Noise-Source-Simple-Spectrum-External-Generator-Tracking-Source-SMA/332754389427?hash=item4d79b509b3:g:T1AAAOSwBdRbazs0

An £8 RF SWR reflection bridge: https://www.ebay.co.uk/itm/RF-SWR-Reflection-Bridge-0-1-3000-MHZ-Antenna-Analyzer-VHF-VSWR-Return-Loss/192655050208?hash=item2cdb22c5e0:g:2f0AAOSwqF5bmIoj

A £1 50ohm dummy load: https://www.ebay.co.uk/itm/SMA-Male-RF-Coaxial-Termination-Dummy-Load-2W-2watt-DC-6-0GHz-50-ohm-Pretty/332529587180?hash=item4d6c4ed3ec:g:ZTcAAOSwdd9aZpsP

So it looks like I can measure the antenna performance using an additional £18 worth of parts. It's not as quick and neat as a VNA, especially as I will likely have to run repeated tests as I blunder about tweaking the antenna and feed points, etc. but it is a realistic cost.

Wonder if anyone else has tried something similar and how you got on?

[The Admiral]

27 minutes ago, IanL said:

Wonder if anyone else has tried something similar and how you got on?

I haven't, but it is an interesting project.

However, when you say "built a second one", do you mean to the same spec as the S@N antenna? I can't believe that an antenna to that design wouldn't have a resonant frequency at some point around the intended frequency, given that others have had success with the design, and rather suspect that something else is going on. Personally, I think that rather than trying to test it I would first try replacing the cable and plugs, even as a temporary set up, and see if that improves the situation. The other thing, of course, is are you using a different SDR to your own? Might it not be worth using your own set up, which you know works, with the new antenna, just to try it out? Forgive me if that is what you have already done.

Ian

[Carl Reade]

Hi did they give you any SWR readings on it? 

[IanL]

It's the same design and dongle type, and I was planning on retrieving it and testing it here. Pretty sure it's not the dongle but that will rule it out.

Testing it will require me to remake connections, check cables, connectors, length and placement of antenna elements, etc.  Each iteration would require me to spend (some part of a) day waiting for detections to get an idea of strength and number and whether the antenna was now performing adequately.   Potentially it could run to weeks with no definitive result so I was looking for a way of verifying quickly whether I'd got it working after each change.

This test method could give me a way of base-lining my good antenna against the bad one (or indeed ruling out the antenna as the problem altogether) and then rapidly retesting after each fix.

I don't have any SWR readings of my own as:

a) I don't have an SWR meter.

b) It is a receive only setup, and as I understand it to use an SWR meter you need a transmitter attached at the desired frequency which I don't have access to.

I think the VNA that they were using basically does SWR tests across a wide range of frequencies and what we got was a small amount of antenna resonance at some very high frequencies way above the 143 MHz band needed, and nothing anywhere else. They didn't give me any data to take away, but on screen it was conclusively a dud as the self-test of the test equipment using a 50 ohm dummy load showed it was working to spec.

I agree it doesn't make any sense, given that all the electrical connections check out fine, so wanted to be a bit more scientific as the 'build in hope' method has clearly failed me here.

[robin_astro]

Water in the coax feed ?

Robin

[IanL]

3 minutes ago, robin_astro said:

Water in the coax feed ?

Robin

Pretty sure not - I sealed this one up even better than the first one (which was well sealed and still works fine after many months outdoors) since it is a pain if I have to repair it, and the problem was there from the get-go before it had any chance for water to get in.

[The Admiral]

I usually recommend tuning to the VHF beacon GB3VHF (http://www.gb3vhf.co.uk) which transmits on 144.43MHz to check out viability, as it's not too far from the Graves frequency, which of course can't be received except by meteor scatter. Might save you some time.

Ian

[BiggarDigger]

The homebrew reflectometer described in the RTL-SDR article is a really good application of SDR technology outwith the mainstream radio receiver application.  It's useful for filter setup and can, as they note in the article, be sued to measure VSWR.

However, VSWR isn't so important for receiving antennae.  This is because there is no danger to any transmitter final stage amplifier as a result of poor matching of the feed or the antenna.  Nonetheless, a very bad VSWR will yield poor results for your meteor detector, so ideally you want a decent impedance match between the antenna, feed and RF input stage of the SDR.

A reasonable approximation can be obtained as The Admiral notes by tuning for decent performance on the GB3VHF beacon.  It's some way off the GRAVES frequency, so if your antenna is narrowband, as most Yagi arrays are, you could be a little way off optimum though.

I'm outside the groundwave range of GB3VHF, at least with a low-gain, low-height antenna anyway, so  that route wasn't available to me.  I used the wideband noise emanating from the Sun to tune the antenna.  It doesn't matter that the Sun is high in the sky: your'e not looking at absolute noise levels, but tuning for the maximum noise achievable.  Solar radiation at 144MHz is quite noticeable and can usually be discounted from other sources by the directional nature of the antenna.  Yes, it took a bit of rotating the antenna to be sure of noise source and a bit more back and forth to the SDR and software, but I was able to adjust the tuning capacitor to maximise the wideband solar noise and hence centre the tuning of the antenna on GRAVES fairly easily.

I use a so-called HB9CV antenna which is a two element active phased array, similar in performance to a 3 element parasitic array such as a Yagi.

[The Admiral]

15 hours ago, IanL said:

The results were poor, virtually no resonsance at any frequency across the entire range they could measure, and none at all at the Graves frequency.

Prompted by BiggarDigger's comment about the Yagi being narrow band, I thought I'd see just what the SWR should look like. So, using my very rusty knowledge of EZNEC I managed to get this plot. This is for the S@N Yagi, 5m above typical ground. There is a definite resonance, as you'd expect, but it isn't desperately narrow. The plot goes from 120MHz to 180MHz in 1MHz steps.

So it should be fine for the GB3VHF beacon, and certainly for comparative purposes.

Ian

[The Admiral]

You might also be interested in some plots I made 3 years ago. The antenna here is at 6m elevation.

Here the antenna is vertical, as recommended. The first plot is in the vertical plane, and the second the horizontal, azimuthal, plane.

 

Here the antenna is horizontal (as opposed to the S@N recommendation to use it vertically). The first plot is in the vertical plane, and the second in the horizontal, azimuthal, plane.

I think that for this role, there isn't really a significant difference between the two arrangements. In fact I think it could be argued that the polar response for the vertical antenna is perhaps unnecessarily wide. Your call!

Ian

 

[BiggarDigger]

That's right Ian, the VSWR of the yagi antenna should be fairly broadband (as the feed to the dipole would show).  However, being a parasitic array, the forward gain achievable should be fairly narrowband; At least for a reasonable sized yagi array anyway.  I'm not certain how many elements the Sky at Night array had, but is it were only 3 or 5, then the forward gain might be reasonably broadband.

Some years ago I used multiple stacked and bayed 17 element antennas at 144Mhz and similarly, multiple 21 element antennas at 432MHz for EME (moonbounce) work and can be sure that the bandwidth of the forward gain of those monsters was pretty tight.

Anyway, that's straying off topic: I think the idea of using an SDR with a directional couple and noise source is an excellent little project to tune the antenna, but as noted, there are equally good methods to get "close enough" for meteor logging.

Cheers,

Richard

[The Admiral]

48 minutes ago, BiggarDigger said:

That's right Ian, the VSWR of the yagi antenna should be fairly broadband (as the feed to the dipole would show).  However, being a parasitic array, the forward gain achievable should be fairly narrowband; At least for a reasonable sized yagi array anyway.  I'm not certain how many elements the Sky at Night array had, but is it were only 3 or 5, then the forward gain might be reasonably broadband.

Some years ago I used multiple stacked and bayed 17 element antennas at 144Mhz and similarly, multiple 21 element antennas at 432MHz for EME (moonbounce) work and can be sure that the bandwidth of the forward gain of those monsters was pretty tight.

Anyway, that's straying off topic: I think the idea of using an SDR with a directional couple and noise source is an excellent little project to tune the antenna, but as noted, there are equally good methods to get "close enough" for meteor logging.

Cheers,

Richard

Ah OK, that's interesting. So, correct me if I'm wrong, does that mean that the sharp forward lobes of such multi-element arrays rapidly broaden and shrink at frequencies only a little away from the centre frequency, even though the VSWR is still low?

The S@N Yagi is only 3 element, and I've just looked at what peak forward gain that EZNEC predicts. It's about 12.5dB over a remarkably wide frequdncy range, if I'm understanding things correctly. But at least, the VSWR does peak, unlike what was found in IanL's 2nd edition.

Ian

[IanL]

Thanks for all the suggestions:

- I have SpectrumLab configured with a button to retune to GB3VHF for testing already. On the good antenna I get a reasonably strong signal, but on the second one I also get a signal. It's not quite as strong but it seemed acceptable when testing immediately after the build, but I might have been fooling myself as I didn't record the absolute -dB figures for each setup. I guess I could try adjusting the antenna whilst monitoring it but I doubt I'd be confident of any possible improvements until I get better/stronger meteor detections too.

- I did my own plots using EZNEC and came to the same conclusions - the pattern is fairly broad so azimuth pointing direction isn't critical (but both are lined up on Graves accurately anyway). Elevation pointing also not critical. The good antenna is pointed up at about 10 degrees, the poor one was horizontal but over time has drooped to maybe 10 degrees down as the mounting needs a bit of fettling. The difference on the test beacon and on meteor detection rates/strength did not change noticeably between the planned horizontal and unplanned 10 degrees down elevation which confirms the modelling somewhat. The main source of improvement is supposed to be height above ground. The first is at about 2M above ground on a wooden pole, the second is 3M above ground on a satellite dish mounting pole (bolted to a stud and ply wall). Getting up to 6M would probably improve matters on both antennae but isn't an option for the second one in any event.

- The first antenna is vertically polarised as it was easier to attach to the pole that way, the second horizontal as more convenient at the other site. Modelling suggests it should not matter much either way.

- The plot the radio guys did for me looked nothing like the modelled one above - as I said, no response across most of the spectrum, except a small area at the top of their frequency measurement range, suggesting something is badly amiss. Given I have limited opportunities to get over to the site where the second one is installed, and that it is a pain to mount and demount it, I just want to make this a one shot job if possible otherwise I'm not going to get it sorted this side of Xmas. It's probably somewhere in the connections, cabling or adaptors used to connect to the SDR dongle.

Will let you know how I get on in a few weeks once I have had a chance to work on it.

[The Admiral]

2 hours ago, IanL said:

I have SpectrumLab configured with a button to retune to GB3VHF for testing already. On the good antenna I get a reasonably strong signal, but on the second one I also get a signal. It's not quite as strong but it seemed acceptable when testing immediately after the build, but I might have been fooling myself as I didn't record the absolute -dB figures for each setup. I guess I could try adjusting the antenna whilst monitoring it but I doubt I'd be confident of any possible improvements until I get better/stronger meteor detections too.

- I did my own plots using EZNEC and came to the same conclusions -

If I understand you correctly, the tests of the existing and new antennae with GB3VHF were done at different times? My experience had been that signal strength is quite variable depending on when I test, though may be you have a much better signal being that much closer.

I'm not sure how much effect the height of the antenna has intrinsically, I'm assuming it has more to do with how clear the view is to the horizon.

Gratifying that our EZNEC plots are consistent :<).

For information, my antenna is in my loft space, but the signal is good enough I feel. I originally built a Yagi (non-S@N) but now I use one of these.

Good luck with your investigations.

Ian

[IanL]

I've checked the signals a few times and they are fairly consistent between sites as we are not that far distant from the source here. The horizon isn't that relevant as radio at this frequency can propagate from beyond the horizon quite readily and, as I understand it, a clear view isn't needed and even small buildings in front will not make much difference. The height issue is related to constructive / destructive interference between waves direct from the source and those reflected from the ground relatively near to the front of the antenna. The local type of ground also has a bearing, as you will get different results from (say) a body of water vs. dry sandy soil vs. heavy wet clay, etc.

[The Admiral]

10 minutes ago, IanL said:

The horizon isn't that relevant as radio at this frequency can propagate from beyond the horizon quite readily and, as I understand it, a clear view isn't needed and even small buildings in front will not make much difference. The height issue is related to constructive / destructive interference between waves direct from the source and those reflected from the ground relatively near to the front of the antenna.

My understanding, rightly or wrongly, is that whilst signals can propagate ionospherically under certain conditions, it is the ground wave we are interested in. After all, we don't want to receive Graves signal directly.

I understand that the creation of the sensitivity lobes is due to interference between the direct wave and that reflected from the ground, I found that my EZNEC plots for 2m and 6m weren't much different. In general the lowest peak lobe was at about 5° above the horizon. Even line of sight at this elevation allows one to see events at ~100km altitude well over France. My thoughts were that if there were significant obstructions at this sort of elevation, like from houses or hills, then this would reduce sensitivity. I suppose it is possible for a ground wave to creep over a hill? Anyhow, you are much better placed than myself for detecting meteor reflections.

Ian

[Stub Mandrel]

Interesting. I built mine to the design on page 5 of this guide:

https://www.britastro.org/radio/projects/Antennas_for_meteor_radar.pdf

 

Except I used 6mm anodised aluminium tube for the element (the one more than a meter long has a 8mm tube connector in the middle).

Be aware that anodising is an excellent electrical insulator. I made sure the connector had excellent connection to the outer parts and that my wires were well connected.

I used 3D prints to hold everything together onto a central fibreglass tube and the connections were covered with hot melt.

I get truckloads of meteors, and can also pick up the GB3VHF  beacon and Radio 4 really well. As it's only a 3-element design it shoudl be fairly broadband and not too directional at the expense of gain.

One thing I found helped was fitting a heatsink to my aluminium-cased USB SDR, this reduced noise quite a lot. The received is as close to the aerial as possible while being inside, then I have a 4m USB lead to the computer, with a clip on ferrite fitted.

 

- just to add, my aerial is nearly 6M up has a pretty much unobstructed view and points south-south east (ish)

[IanL]

Ok had a chance to get over there today unexpectedly. The radio hams were also there by sheer chance so we retested using their gear. Problem found and fixed.

I had used an N-type connector due to the diameter of the coax, but also needed an N to SMA adaptor to match the SDR dongle. The pin in the N type was set slightly too far back to make contact with the adaptor to it was a wonder we were getting any signal. Remade the connection and the test kit showed a pretty good response around the 143 mark, bit long but good enough.

Test beacon much stronger and detected four small meteors in 10 mins so looks good to go.

[The Admiral]

Great news Ian, I bet you're a happy chappie! Who'd have thought that could be the culprit, well done for tracking it down.

Ian

[IanL]

Yep very happy. I continuity tested everything but without the adaptor in place so didn't spot the problem. Lesson learned.

[BiggarDigger]

Easy mistake to make Ian!  Glad you found the culprit!

To pick up on a couple of earlier points, Yes, the gain bandwidth of a Yagi antenna tends to be narrow and generally gets more so, the more parasitic elements you add to the array.  Eventually the array becomes so sensitive to external effects as to be unstable.  Fewer elements usually mean broader usable bandwidth, less environmental sensitivity and broader frontal lobes.

I'd be a little sceptical of the 12.5db gain figure calculated by the software for a 3 element array.  A more realistic value might be in the range 5-7dBd.  Even allowing for gain over an isotropic radiator adding an extra couple of dB over a dipole would make 7 to 9dBi.  In any case however, since we are not interested in weak signal propogation and want reasonable broad horizontal forward lobes, a gain of around 7 to 9dB is perfectly acceptable.

GB3VHF is an excellent source to check functionality, but directly comparing two antennae against each other using it would a bit hit and miss.  Environmental effects and propogation effects will make much more difference than tuning or antenna gain unless (assuming like for like antennae).  This makes a good case for the refelctometer described in the original post since, assuming the noise source is stable and antenna feeds are calibrated, means the absolute tuning points can be compared.

Radio signals at 144Mhz can and do propogate over the radio horizon tropospherically and ionospherically.  Indeed, the meteor refelections we detect are ionospheric propogation and the ground wave, beyond radio horizon, is tropospheric.  Depending on local weather conditions tropospheric propogation can increase or decrease the received signal dramatically.  It would not surprise me if over the coming couple of months, I am able to hear the GB3VHF beacon here in Southern Scotland even with a modest antenna when super-refraction occurs in the troposphere.  Ionospheric propogation at 144Mhz comes in many forms: but for our purposes, we are interested in the plasma effect in the ionosphere of incoming meteors.

One other item that I've been pondering for a little while is the path of signals from my receiver to meteor plasma trail and onwards to GRAVES.  I'll start a new thread for that so as not to hijack this one!

Richard

[The Admiral]

Thanks for that Richard. I must admit I thought a 12dB gain unrealistic too, but that's what EZNEC came up with. I am not sure why that should be so, as I'm not sufficiently well versed in these matters.

Ian

[pawelp]

hi,

how 'directional' is your Yagi antenna for meteors ?  I'm looking for a broader %% of the sky coverage. more antennas would be needed ? 

thx

p.

[IanL]

1 hour ago, pawelp said:

hi,

how 'directional' is your Yagi antenna for meteors ?  I'm looking for a broader %% of the sky coverage. more antennas would be needed ? 

thx

p.

Take a look at the original article, first link on this page:

https://www.britastro.org/radio/downloads.html

It has a pretty wide angle in azimuth, probably 130 to 140 degrees of reasonable gain. In elevation there are significant lobes and nulls, but that seems fairly common with Yagis and is more a function of height above ground that anything else. The design is a simple half wave dipole with a single reflector and single director.

It is optimised for the GRAVES radar transmitter on 143.05 MHz so depending on what you're using as your radio source you may need something different. No idea how you'd combine multiple antennae to improve coverage; most of the stuff I've read by Radio Hams is aimed at the opposite, i.e. making the antenna more directional not less.