ADS-B Filter

freddoair

New Member
A while back I read a question about a particular ADS-B filter that was on Ebay, so I took a look https://www.ebay.com/itm/132406298935?ul_noapp=true

Given that the filter was from Mini Circuits, a long established and well respected US manufacturer of microwave components, and the price was quite reasonable - even with freight to Australia, and a not so favourable exchange rate - I took a punt and bought one. This is a bandpass filter for ADS-B, and insertion loss is respectable at about 2.0 - 2.5dB. It's a lot more convenient from a mounting perspective than the coaxial cavity filter I was building!

My RPi and DTV dongle have lived in a weatherproof enclosure directly under the antenna for about 18 months, and the antenna directly fed the dongle input via 600mm of RG214 (double screened RG213) coax (and a short adaptor cable). Apart from a not-so-nearby lightning strike killing the original dongle (it took three weeks to finally die) after nearly a year in service, the installation has worked well.

Today, I relocated the RPi and dongle to the shed, and mounted the MiniCircuits filter and a LNA4ALL pre-amp in the weatherproof box under the antenna. I have about 6m of RG213 coax between the mast head unit and the RPi/dongle, it's loss being of no consequence given the gain and noise figure of the pre-amp. As is said, the proof is in the eating, so ......

A few hours of uptime has returned around 60% increase in the number of planes observed, due to a significant improvement in system noise figure. I normally send about 2,000,000 packets to Plane Finder each day. After just three hours today I've sent 1,600,000 and I'm keen to see what the total is tomorrow. Aircraft on approach to a nearby (45km away) airport used to disappear at about 900-1100 ft. Today I see them down to about 600ft. Now I can't wait to get the antenna from 4m above ground, to about 7.5m to increase path clearance for the S to NW sector. Sorry about the mixed units, I tend to work in both.

Even though it's early days (hours, even) I consider the MiniCircuits filter/LNA4ALL pre-amp combination a great asset to my ADS-B installation.

Ray
 

Stealth

New Member
Impressive results on yours, as a fairly low cost simple LC filter it seems to be doing a great job.

I've found that signal overload of a dongle SDR can be very sudden and pronounced. I live in a good elevated location in terms of take off in most directions, but at the same time because of the attraction of the location to commercial operators I have saturation levels of radar and cell-phone signals.

Initial operation was with modest antennas, akin to the 1/4 wave GP and some reasonable signals were received. Creation of a co-linear antenna (not coax) followed by testing on an antenna analyser looked promising, but when connected up I got practically no signals. Assuming that there was a problem, I obtained a known to work commercial antenna, but the same thing happened, only slightly better than sod-all signals.

A SAW filter had been on order for a while and arrived during this puzzling period. Put in line and bingo, both antennas brought in good signals.

The next step was to swap the SDR for a Mode-S Beast. I retained the SAW filter and it was OK, but not exceptional. Without the SAW the signals flooded in, not only greater number of reports, but also greater consistency in reports and range over extended periods. I've tried filter in and out on a number of occasions, but performance is definitely better without it, even though measurement shows it clearly reduces out of band signals and the insertion loss is minimal.

So it looks like I have to get some more height and signal! So much entertainment from one signal source.
 

wiedehopf

New Member
The Mode-S Beast has 2 SAW filters and LNAs internally.
(https://wiki.jetvision.de/wiki/Mode-S_Beast:System_Design)

The LNA used has enough dynamic range so it's not oversaturated by cell phone or broadcast signals.

The rtl-sdr LNA has a similar arrangement to the Mode-S Beast: two LNAs followed by a SAW filter each.
Main difference is probably a high pass filter to filter out FM or TV stations and maybe cell signals a little, i'm not sure how good the high pass filter rolls off. (LNAs aren't quite as high dynamic range probably so it needs the high pass filter to ensure that it's not going into saturation)
(https://rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/)

Of course the ModeS Beast has a better receiver compared to a rtl-sdr dongle.
(The airspy with an external LNA can keep up with the ModeS Beast though)

You could probably use an external LNA like the rtl-sdr blog LNA with the ModeS Beast if your cable run is long.
But then you need a minimum amount of attenuation not to oversaturate the Beast with signal. (Probably around 20 dB)
 

freddoair

New Member
Thanks guys,

Having worked in the two-way radio industry for 40 years, and being a licenced amateur operator for 50 years has provided me with plenty hands-on awareness of the need for adequate filtering for receivers (and transmitters, too, to keep them clean). Those, well-elevated, busy radio sites, or near them, is no place for a receiver without adequate filtering! I have sympathy for anyone who lives near such installations. I live in a quiet, rural village where the nearest communications site is about 20km away, so I don't have to worry too much about strong RF from various sources causing me grief. The exception is in the next paragraph.

I considered the possibility of a nearby cellphone site, 297 metres away with transmitters at 763MHz, 950MHz and 1.8GHz, causing the dongle grief but the filter, a LNA with a high-level third order intermod point, and an antenna with a shorted matching stub have obviously kept the gremlins at bay. If I was being picky, the 2dB loss in the cable between the LNA and the receiver also helps ... a tad! I was prepared to finish construction of the cavity filter, if necessary, but the MiniCircuits BPF is working for me, in my circumstances. I considered another MiniCircuits BPF following the LNA to further protect the dongle, but it's not been necessary, for me. The specification of the filter may be seen here https://ww3.minicircuits.com/pdfs/VBFZ-1065+.pdf

I had to fit a HPF ahead of the TV mast head amp to keep the 763MHz cellphone transmitter out of the TV system, but that's a worse scenario due to a resonant TV antenna looking almost directly at the cellsite, and that's another story to be shared elsewhere.

I didn't comment on the antenna I use because the before and after results weren't antenna dependent, again, in my case. But for the record it's a homebrew collinear using five, halfwave elements in phase. It is not made from coaxial cable. The tuning stub was adjusted using an antenna analyser, an essential piece of test equipment for tuning such antennas while still retaining as much hair as possible!! Why five halfwaves? Well, the antenna started life as a commercial collinear for 470MHz, and five radiating elements was all I could squeeze into the original radome. I estimate the gain at 4.5 to 5 dBd.

I don't know anything about the ModeS Beast but given that it's purpose built, one should expect its performance to exceed that of a cheap dongle, and by a significant amount. However, with a LNA at the antenna - the correct place for it - system noise figure, which relates to sensitivity, is very much improved to make the dongle somewhat competitive, if you don't run into the overload situation that Stealth mentioned. The last paragraph from weidehopf pretty much sums up the situation when the receiver already has a lot of front-end gain and good performance. With such a receiver, and provided the coax run from the antenna isn't an unreasonable length, use the best coax you can get your hands on and forget the outboard LNA. To pick up on Stealth's second last paragraph, any loss ahead of a receiver or outboard amplifier will worsen the noise figure and reduce sensitivity. That loss can be the coaxial feeder, or it can be the insertion loss of filters. That's why it's best to place the LNA and filter at the antenna. Sure, a filter ahead of the LNA will contribute to a worsening of the noise figure of the LNA but it will be a minor worsening for any half respectable filter. Once the LNA has amplified the signal, the losses between it and the receiver are a LOT less significant, and it becomes possible to fit a second filter or an attenuator (length of lossy coax, e.g.) to shed excess signal that would cause receiver blocking (overload), without worsening overall performance. The exercise is not to get the highest signal strength reading, it's to extract the weakest signal from the noise.

The white box in the image housed the RPi and dongle until yesterday. It now houses the LNA and filter.
 

Attachments

Stealth

New Member
I have mulled over the idea of a mast mounted pre-amp, but if the claimed figures are correct, the 20m of LF400 cable only has a total loss of around 2.5db. My main signal limiter is actually the trees around the block that exceed any mast height that I can realistically attain (currently 14m), as confirmed by range in clear directions approaching 300nm. So far I haven't found a suitable variable gain narrow LNA, but open to suggestions,
 

wiedehopf

New Member
So far I haven't found a suitable variable gain narrow LNA
While the rtl-sdr LNA linked in my previous post isn't variable gain you could always use some attenuation with it.
For example you could put in that SAW filter for some attenuation but maybe you have another attenuator?

With the ModeS Beast that would be a record of 5 SAW filters in the signal chain ;)

Not sure such a setup would be an improvement over your current one though.
 

Stealth

New Member
While the rtl-sdr LNA linked in my previous post isn't variable gain you could always use some attenuation with it.
For example you could put in that SAW filter for some attenuation but maybe you have another attenuator?

With the ModeS Beast that would be a record of 5 SAW filters in the signal chain ;)

Not sure such a setup would be an improvement over your current one though.
Ummm, put up a LNA and then attenuate it by replacing the feeder with RG58 maybe :D
Seriously though, variable gain would be the ideal, then just sit there watching and twiddling until optimum signal level is acheived.
 

freddoair

New Member
Hey ab cd, some good advice. The Australian licencing authority - ACMA - has an online tool into which you load your Lat/Long, search distance from that location, and a frequency range. After a lot of number crunching it spits out a list of all transmitter and receiver licences that meet the search criteria, so I tend to use it as a first step, without having to wait for transmitters to transmit<grin>. From individual licences I can obtain the Lat and Long of each site, antenna type, and transmit power. Armed with information from such a search I did use a DTV dongle as a spectrum analyser to confirm TV interference from the local 763MHz cellphone site.

Moving to cable loss et al, it would be an each way bet for me if I had a cable run with a loss of 2.5dB. To amplify on not. My decision would be based on the quality of the receiver. For the ModeS Beast, with good performance, probably not, you can get an ulcer trying to gain an extra 1 dB that you'll never notice in practice, but for a DTV dongle with a lousy noise figure (and modest performance, at best) I'd probably opt for a mast head LNA. I don't know of a LNA with variable gain but inserting some loss after an amplifier is easily accomplished. 30 ft of RG58 will provide about 6dB loss. For users of RG6 style 75 ohm cable there are cheap (about $2) attenuators with F connectors for the TV industry that are rated to around 850MHz (maybe 2GHz for the satellite TV IF range). Their performance will be adequate in a situation where we want to get rid of excess signal and are not fussy about the method. When you have too much signal (coming from a LNA) you can afford to be less fussy!

I wish I could erect the antenna at 14m. The tallest mast I can erect without involving expensive town planning is 8m, and the top of the antenna is to be no more than 10m. My location is only about 9m ASL but take off to the east is excellent, with the Pacific Ocean being about 15km away. Sadly, the west involves a tall house behind me that I have to live with, and mountains from 3,000 ft to 5,000 ft just 75km away that impact on what I see from that direction. In that direction every extra foot of antenna height is a bonus.
 

Stealth

New Member
I'm over the other side, similarly placed but from the Indian Ocean, and about 250m higher. The crazy thing is that coverage predictions show near unlimited over the ocean and I can can see airport ground traffic at the bottom of the hill about 6kms away. However actual cover over the ocean is relatively poor. So if the predictions are correct and terrain isn't the issue, it then shows what good attenuators trees are at UHF and above. I suppose the upside is that the trees make the antenna mast very hard to see.;)
 

freddoair

New Member
Placing an attenuator after the LNA might appear strange, until one realises that the purpose of the LNA is NOT to make signals stronger, it's to enable "lifting" weaker signals out of the noise created by the receiver's front-end. A LNA with variable gain would be quite useful, I agree. I did read about the gain of the LNA4ALL being reduced by dropping the supply voltage, but doing that trashes the noise figure and the intermod performance. You can't win, you just have to decide by how much you loose!!

Moving to coverage, you can't get unlimited range, not even over water because the curvature of the earth gets in the way. There are two figures that come into play; the line of sight (optical) distance, and the longer distance obtainable due to characteristics of the ionosphere that causes the wave to "bend", the radio distance. For radio distance, the earth is assumed to be flatter than it really is, with the earth's diameter being considered as 4/3 (1.333 times) it's actual diameter. The value is predominantly determined by the amount of water vapor in the air for a given path, being greater over water, and smaller over dry, arid regions. There is a third condition that shows up in a more or less unpredictable way that's related to prevailing weather patterns, and in particular a temperature inversion in the atmosphere. The most commonly used name is a tropospheric ducting, or just tropo, because the radio wave is "trapped" in an invisible curved duct that can cause it to travel unusually long distances. Looking over the Pacific Ocean, as I do, I occasionally see planes out to in excess of 600km. Others living close to a similar ocean path might also experience this. The mechanism that creates tropospheric ducting is very different to that which permits long distance short wave transmission/reception.

The horizon calculator at the following site might be of use. It mentions radar, but radar signals are just another radio wave. http://members.home.nl/7seas/radcalc.htm
 

Stealth

New Member
Yes, by unlimited I mean unlimited by obstructions. I was a avid VHF/UHF fan using Es, tropo and MS until I came to WA almost thirty years ago and discovered a almost total lack of activity bar the odd beacon and the FM repeaters. Damned if I was going to build an EME station just to work VHF/UHF DX.
 

freddoair

New Member
We might know or know of each other, I'm VK2TV. Was previously VK2BVO and originally VK2ZSX. All you need now to work V/U DX is a dual-band radio and a hand-held yagi. There's quite a few LEOs running FM voice repeaters 2m up and 70cm down. I'm gating the APRS satellites.
 

Stealth

New Member
Don't see you in my computer log (2011 on). VK6CR since 1991. I've been operating almost exclusively on HF digital modes since 2012. Dabbled with the sats in the UK, both Oscar 10 and the RS series. Although always regarded them as one step from a voice repeater. EME was always the plan, even had the PSU and the two 4CX250s in the cupboard, but hadn't the room in the backyard for the array. 1296 would have fitted, but it was exotic territory then, more akin to plumbing than pcb componentry.
 

Feek

New Member
A few hours of uptime has returned around 60% increase in the number of planes observed, due to a significant improvement in system noise figure. I normally send about 2,000,000 packets to Plane Finder each day. After just three hours today I've sent 1,600,000 and I'm keen to see what the total is tomorrow.
Out of interest, how do you know how many packets you're uploading to PlaneFinder on a daily basis? Is this figure in the stats somewhere?
 

Stealth

New Member
Out of interest, how do you know how many packets you're uploading to PlaneFinder on a daily basis? Is this figure in the stats somewhere?
If you open the Planefinder client there is a tab 'Stats'. Its a very quiet day here today and even a busy day only gets 300 or so aircraft.
pf_client.PNG
 

Feek

New Member
Gotcha - Thanks. I wasn't sure if that was the place as it's been a while since I looked at my stats!

2824


The filter posted by @freddoair does look interesting, I'm just about to take the receiver down that's giving me these figures which is a FlightAware dongle with a FlightAware light blue filter and replace it with an Airspy and RTL-SDR LNA. I'm not sure whether to install the filter or not but my logic is that it can't hurt to try.
 

freddoair

New Member
Don't see you in my computer log (2011 on). VK6CR since 1991. I've been operating almost exclusively on HF digital modes since 2012. Dabbled with the sats in the UK, both Oscar 10 and the RS series. Although always regarded them as one step from a voice repeater. EME was always the plan, even had the PSU and the two 4CX250s in the cupboard, but hadn't the room in the backyard for the array. 1296 would have fitted, but it was exotic territory then, more akin to plumbing than pcb componentry.
It's unlikely that I'd be in anybody's log! I started running a BBS in 1996 and I've hardly touched a microphone since! The BBS grew to 1x70cm, 2x2m, 1x20m, 1x30m and 1x40m ports. I was on 25 acres so room for antennas was readily available. When Packet started to die I moved to an APRS Igate and satgate. Then a gap in activities until I started to get the satgate running at this new location. That, ADS-B and compiling Linux programs and kernels keep the mind active. When I think EME I think of the late VK3ATN who achieved 2m moonbounce in the 60s despite being told it would never work. Ray had a huge, steerable stacked Rhombic antenna. I've heard a few FM signals trough the LEOs but voice doesn't interest me these days.
 
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