SDR testing 433 MHz antennas

friis

Senior Member
Hi,
I bought the RTL-SDR from Amazon. I followed the instructions for installation (Windows 10) as shown
on the manufacturer's home page and it went without a hitch. I also bought "The hobbyist's guide
to the RTL-SDR" (Amazon/Kindle). With that I could easily handle the subset of the system's
comprehensive set of parameters, that I needed.

I have used the SDR-RTL on a system consisting of a central system with a Pixaxe 08M2
and a Picaxe 28X2 chips and 4 decentralised systems each containing 14M2 chips (in its final
configuration). The distance between a decentralised system and the central system is in the worst
case about 10 m with a house in between. The TX- and RX-transmissions are handled by Dorji DRA
887/886 modules at 433.92 MHz.

The first screen of the RTL-SDR is shown in the second picture below. The most important parameters
are:
- on the left side of the screen (the only thing I had to choose) is AM (radio) - the rest is
default
- on the right top side:
- at the top I have selected the frequency - 433.92 MHz
- the upper half of the screen shows:
- the frequency chosen marked by a red line
- the frequencies surrounding 433.92 MHz
- the noise level
- a spike shows up at the corresponding frequency and the height of the spike shows the
strength of the signal with the scale on the left
- a vertical green line shows up under the curser with the frequency indicated. If you move
the curser so the green linen covers the red one, the green line disappears and
you are shown the frequency, the strength of any spike on the frequency, the noise floor
and the SNR.

The second screen of the RTL-SDR is shown in the first picture below. It shows three
transmissions:
- at the bottom is shown the transmission from the Picaxe 08M2 - a short burst.
- a bit higher up ( a bit later) is shown the reply from the decentral system's Picaxe 14M2 to the
Picaxe 08M2's request for data - also a short burst, but sent from location about 1.5 m from
the RTL-SDR's antenna.
- at the top is shown the central system's Picace 28X2's confirmation of receipt. The transmission
is prolonged for testing purposes.

All three signatures will move down and out of the bottom of the screen as a set. Apparently the
three Picaxes dont quite agree on the frequency to transmit on.

Result:
The four antennas used are shown in the picture in the next subject. The monopole antenna showed good strength
and rather poor SNR compared to the dipoles, and when I used it in the system some time ago it gave
a lot of transmission errors. Its a bit of a mystery that the helical ones work at all:

Below are the figures for a distance of about 10 and 20 m and with a house with one outer and one
inner (light) wall plus a shed in between (strength/SNR):

Antenna 10 m:
dipole (173 mm): -6/44
dipole (168 mm): -8/40
helical (long): -21/32
helical (short): -20/32
Monopole of plastic covered copper wire/173 mm: -15/37
A commercially available monopole produced: -12/38

Antenna 20 m:
dipole (173 mm): -12/36
dipole (168 mm): -9/40
helical (long): -24/31
helical (short): -26/28
Monopole of plastic covered copper wire/173 mm: -17/35
A commercially available monopole produced -18/34

Feed line is 50 Ohm coax, about 10 cm. A small car and a metal mailbox at about 3 m distance.
Quite a bit of uncertainty is associated with the figures.

best regards
torben
 

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PhilHornby

Senior Member
My "RTL-SDR" arrived today - can't wait to have a play.

I'm particularly interested to see what numbers I get from this :-
PCB Trace.jpg

Surprisingly, I couldn't find a standard design (for 433MHz), so I made my own up :D
It works and is not susceptible to being repositioned into a non-optimal position, like my previous wire coils were.

UPDATE I got it working, but I don't really know what I'm looking at. There appears to be a constant signal @ 433.958MHz ... and absolutely no sign of anything of mine :confused:.

SDR1.jpg
 
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manuka

Senior Member
Surely some other 433 MHz signal nearby- yours or neighbours ! Take your set up elsewhere & retry?
 

srnet

Senior Member
Looks like a strong signal that's very close.

Can you provide some more details of your setup, transmission frequency, radio module etc, so that we don't have to guess ?

Is the signal on the SDR present when your project is powered off ?
 

PhilHornby

Senior Member
It's worse than that, it's dead Jim ;-)

Surely some other 433 MHz signal nearby- yours or neighbours ! Take your set up elsewhere & retry?
I reckon a next door neighbour is sitting on their Car Key Fob :)

Is the signal on the SDR present when your project is powered off ?
Yes - I it was the first thing I saw when I got the software fired up.


I pressed STOP on the SDRSharp software - went to the pub - and on my return it claims it "can't open RTL device" :( ... I'm currently going through the rigmarole of falling back to the System Restore Point, I cunningly created before embarking on this.

I'll have a go at installing it on a laptop instead, so I can have a wander around the environment with it.
 

friis

Senior Member
Hi PhilHornby,
What is "this"?. I think I see room for a Picaxe 14M2. Is the framed thing really an antenna?
torben
 

techElder

Well-known member
Phil, just to keep you thinking, a quick search reveals a "wireless headphone" with these specs:
"Takstar UHF-958 Wireless Headphones"
"Carrier Frequency: 432.6MHz-434.1MHz"
 

PhilHornby

Senior Member
Well I successfuly repaired my PC ... and installed this software on a laptop instead. The strange constant signal is still there, but shows as 433.702Mhz now. I probably need to familiarise myself with what this software is actually telling me ... and figure out what all the options do.

Hi PhilHornby,
What is "this"?. I think I see room for a Picaxe 14M2. Is the framed thing really an antenna?
torben
It's an aerial , in English :p. Well sort of anyway :)

I copied the design from a commercial board, but I think I must have transposed the inside and outside measurements (or something), because it looks nothing like the original!

The board does indeed have a Picaxe on it :-



It's a Room Temperature Controller/Logger. The HC-12 is command-and-control ... and is using 458MHz. The 433MHz ASK transmitter communicates with Energenie Remote Control sockets to power a heater On/Off.
 

hippy

Technical Support
Staff member
The strange constant signal is still there, but shows as 433.702Mhz now.
It could be something outside, drifting with temperature. With the weather how it currently is it could possibly be doing that inside.
 

techElder

Well-known member
Phil, the trick is to find a local broadcast station and compare the frequency from your SDR to that known frequency. There's an adjust setting on your SDR software and, in English, "Bob's Your Uncle!" :D
 

friis

Senior Member
Hi,
Could it be that a lot of people are transmtting at that frequency? It looks like a composite peak.
torben
 

PhilHornby

Senior Member
Could it be that a lot of people are transmtting at that frequency? It looks like a composite peak.
Ah OK, that makes sense. I've been for a walk around the street with a (huge) laptop and the signal looks the same wherever I go. I thought it must be a really strong signal, from far away. Your suggestion sounds more plausible.

Phil, the trick is to find a local broadcast station and compare the frequency from your SDR to that known frequency.
I'd wondered how accurate the frequency readings were and had noticed the 'adjustment controls' - but it hadn't occurred to me how to use them.

I think I need to buy a USB-C 'OTG' cable, so I can try something like this on my phone: http://sdrtouch.com/ https://play.google.com/store/apps/details?id=com.mantz_it.rfanalyzer&hl=en_GB
 
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friis

Senior Member
Hi,
The adjustment has to do with the quality of the crystal in RTL-SDR (not so good) and you find it under the sign like a wheel in the upper left corner of the screen. Is that it? It should only be used once.
torben
 

pxgator

Senior Member
The 'wandering signal' could also be a product of strong harmonics from a powerful
lower frequency RF source. Good luck Phil....at least you're having some fun figuring
things out.

Cheers to All
 

manuka

Senior Member
PhilHornby: Bulky laptops - bah- this is 2018! An OTG (On The Go) cable typically costs just a few $ & allows nimble Android based spectrum monitoring. Get organised & go for a decent wander away from urban RF clutter. It'd help if you specify your location- urban areas globally can be awash with 433 MHz signals these days.

Failing a rural walkabout perhaps rustle up a well shielded environment, or make your own simple Faraday Cage with mesh or bird netting etc. Even a basement, metal walled garden shed or shipping container can do.

EXTRA: Ah- best you FIRST ensure that the deviant signal is not associated with something you're wearing/pocketing- car/garage remote, smart watch, hearing aid or heart pacemaker etc! Stan.
 

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PhilHornby

Senior Member
Good luck Phil....at least you're having some fun figuring things out.
I started with two mini-projects...one is my (Picaxe-related) PCB aerial - the other is to try and diagnose a fault with a Ford Tyre Pressure Monitoring System (since the Ford dealer can't). I didn't realise I was going to start by dealing with the Cosmic Background Radiation, or whatever else it is, that has shown up in my house :D

An OTG (On The Go) cable typically costs just a few $
I know, I know ... it's just all this USB-C stuff, on my new phone, took me by surprise. A suitable cable should be winging its way to me, as we speak :rolleyes:

I spend most of my time in the great (as in big!) Manchester metropolis - but once a month I escape to rural Cornwall. I bet this alien signal follows me there :)
 

manuka

Senior Member
I bet this alien signal follows me there
It may be if you drive there! Direct TPMS (Tyre Pressure Monitoring Sytems) often work at 433 MHz ...
Perhaps you're carrying a flakey TPMS transmitter in your pocket?!

Re harmonics - the 3rd overtone of a signal on ~144.64 MHz comes out at ~433.92MHz. Radio hams on the 2m VHF band (144-146 or 148 MHz) may hence have 3rd harmonics at this frequency from their powerfully legal VHF transmitters. It's fair to say some hams grieve the sharing of a slice of their 70cm UHF spectrum, & may even deliberately run a -ah- "prolonged transmission test" on 433.92MHz.

However initially suspect more mundane sources, such as those mentioned in this posting from Sydney (Australia) -
I've been working for a manufacturer of garage door and commercial door openers for awhile now and we have found some interesting issues with wireless devices.

This all stems from people who say they have issues with their garage door not working. Our Field Service Technicians will attend site and check for RF issues on the same 433.92 Mhz frequency as the garage door. More often than not we find its actually the home owner has caused the interference themselves (or a neighbour).

The things we find that are most common to cause issues are:
1) baby monitors. Same frequency with a decent range blocks out the signal from a remote control.
2) remote control ceiling fans. Same frequency and when faulty constantly sends out a signal which will stop all 433.92 transmissions including some cars. This is an evil device as the range can extend a couple of houses. Easiest to identify when people can't lock or unlock their car.
3) Wireless doorbells. Same frequency and will block out a signal from a remote. When the transmitter battery starts to die, the receiver starts transmitting a high powered signal looking for the transmitter.
4) Wireless weather stations: same as above only for a greater range.

Now for the big one.
5) AV senders. These are the devices people buy to send an AV signal from the source unit to another TV wirelessly over the 2.4Ghz or 5.8Ghz range. These devices also send the remote control signals, but on the 433.92 Mhz range. The big problem with these units are the range and the signal strength. We have seen these units block out a 5 house radius. No one can operate their garage door, unlock their car, doorbells stop working, baby monitors stop working.

I've set a few units up (i don't want to name brands in case I defame someone) and we have tested the signal strength. for some of the units, we can't reduce the sensitivity of our meters enough to acurately track the signal under 100m from the source of ISM band interference.
 
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PhilHornby

Senior Member
It may be if you drive -direct TPMS (Tyre Pressure Monitoring Sytems) often work at 433 MHz ...
I think Ford TPMS uses FSK @ 433MHz ... at least according to the code on this web page: https://github.com/merbanan/rtl_433/blob/master/src/devices/tpms_ford.c

There's seem to be a lot of different implementations of FSK, so in the first instance I'll be happy if I see similar signals, from all five wheels. (I'm pretty sure that Ford forgot to sync the system with the optional full-size spare and one of the other sensor's battery may have died).

It should keep my neighbours amused, if nothing else :)
 

PhilHornby

Senior Member
I'm particularly interested to see what numbers I get from this :-
PCB Trace.jpg

Surprisingly, I couldn't find a standard design (for 433MHz), so I made my own up :D
It works and is not susceptible to being repositioned into a non-optimal position, like my previous wire coils were.

UPDATE I got it working, but I don't really know what I'm looking at. There appears to be a constant signal @ 433.958MHz ... and absolutely no sign of anything of mine :confused:.

SDR1.jpg
I think this strange signal is just a short-coming of this software...
...I swapped to using HDSDR and there's no sign of it. HDSDR has the advantage that it seems to stay working, unlike SDR# (It failed on my laptop, in exactly the same way as it failed on my desktop ..."Unable to access RTL Device").

I got some encouraging results for my PCB Aerial, using HDSDR. I get a good strong signal from about 7m away (which is far more than I need) and the only thing which seemed to affect it, was having a battery pack behind it (in front was fine?!?).

I compared it against a bone-fide Remote Controller, who's functions it emulates in this application. The PCB aerial generated two measurable signals, with one being much smaller in amplitude and about 333KHz lower in frequency than the main one. In contrast, the commercial unit (which uses a small coil of wire), produced at least 8 additional signals above and below its main, centre signal. They were spaced about 115KHz apart; with the even numbered ones being much larger in amplitude.

I have no idea what that signifies, but it's interesting ;)

I also had a spy on the 458MHz band, where my HC-12's operate. Nobody there but me :cool:
 
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manuka

Senior Member
PhilHornby: OK-"resolved"! Mmm- perhaps you'd an ECSS (Exalted Carrier Selectable Sideband) option on? ECSS injects a carrier on the freq. of interest to aid in poor AM signal resolving. This may have hence appeared at 433.920 MHz on your screen? Please check -it's high summer "away from the workbench" here in NZ & I don't have a RTL-SDR setup handy.

EXTRA: As I'm guilty of suggesting this SDR (Software Defined Radio) Forum thread, perhaps further input is in order - especially since requests asking "which USB SDR dongle is best" have emerged.

As basic USB SDR modules tend very reasonably priced (US$10-20 range) choice may relate to features, size, freqs. of interest, extras, antenna, packaging, thermal stability, hackability, display cleanliness, warranty etc. Choice really depends on ones specific needs!

Although an early SDR "TV dongle" enthusiast, my changing e-interests mean I've lamentably rather fallen behind the current state of play. I'm rather hence out of touch with preferred SDR software & hardware,both of which are experiencing enormous developments - it transpires some 6 hardware generations have evolved!

With USB dongle SDR offerings & reviews dating E X T R E M E L Y rapidly it's difficult hence to make rational recommendations, but see links such as this and this for insights. Stan.
 

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PhilHornby

Senior Member
As basic USB SDR modules tend very reasonably priced (US$10-20 range) choice may relate to features, size, freqs. of interest, extras, antenna, packaging, thermal stability, hackability, display cleanliness, warranty etc. Choice really depends on ones specific needs! ... With USB dongle SDR offerings & reviews dating E X T R E M E L Y rapidly it's difficult hence to make rational recommendations, but see links such as this and this for insights. Stan.
From those review sites, mine would seem to be "3rd generation" "2nd generation!". It cost £7.43 delivered (from China). For the some reason, eBay have just removed most of this seller's item, so I can't give a link :(

 
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PhilHornby

Senior Member
Testing 433MHz aerials

I did some tests...

I set up two 'normal' FS1000A's on a small breadboard, using different types of aerial. I connected my PCB aerial to the breadboard too, with flying leads. The PCB was fitted with its intended 433MHz ASK transmitter. I chose this device purely because of its Pinouts, but it is noticeably different in the amount of current it draws. Therefore there is a difference in more than just the aerials - but it is pertinent to what I am actually building.

I fed each of the transmitters (in turn) with the 1KHz signal from my 'scope. This seemed more realistic than just connecting Data to Vcc, but the signal from the scope is quite low (~1.5V), so may not have been driving the transmitters fully on. (So many pitfalls!)

Using HDSDR with my cheap 2nd generation RTL dongle and its supplied magnetic stick aerial, I took readings at a distance of ~2.4Metres.

Configurations :-

  • (A) - Centre of picture, is a simple coil, mounted vertically. I have tended to use this arrangement on my transmitting modules and found it to work satisfactorily. However, it is very susceptible to being knocked out of position (in relation to the battery pack, I think - and I wanted to address this shortcoming). The design dimensions came from here: https://github.com/OpenHR20/OpenHR20/wiki/2.1)--433-MHz-and-868-MHz--Antenna-Design-Examples
  • (B) - Nearest the camera, is a 'loaded coil' aerial, described here: http://www.instructables.com/id/433-MHz-Coil-loaded-antenna/ (this has definitely been posted on the forum before). I have only used these as receiving aerials, because of their size.
  • (C) - The PCB is my eye-of-toad, leg-of-newt contraption ... I have no design data for this :D.

IMG_20180212_225241.jpg

The results I got (from HDSDR), are as follows :-

ConfigurationSignal @ RBW (dB)Signal/500Hz (dB)Frequency (MHz)
A (Simple Coil)-72.1-58.6433.915
B (Loaded Coil)-66.3-52.7433.917
C (PCB)-63.9-50.3433.999
Car key fob!-57.1-43.5434.405

and here are the screenshots (though I forgot to grab one for the Car key fob)

ThreeAerials.jpg
The last result - the Car Key Fob - was just thrown in for comparison. What do VW/Audi know, that I don't :confused:

Probably quite a lot, given that I didn't know what RBW stood for until I googled...and what has 500Hz, got to do with anything?!?
 

manuka

Senior Member
Great data but surely you want to use 433 MHz modules at more than a mere 2.4 metres!? Even 2.4GHz Bluetooth betters that distance !

Most 433MHz setups are over 10s-100s of metres,with the likes of LoRa good for several km. Best you explain your setup more fully, as at very close in distances Near Field issues arise.
 

PhilHornby

Senior Member
Great data but surely you want to use 433 MHz modules at more than a mere 2.4 metres!?
In this particular project, long distance is not really a requirement - the 433MHz ASK module is used by a temperature sensing module to control a commercial RF switch, to which, a heater is attached. I just need some separation between the temperature sensing and the heater that is being controlled. I was (vaguely!) aware of the issues of having them too close together, but reckoned that wasn't the case with the setup I lashed together.

The main thing this experiment showed, is that a cheap SDR dongle can at least give some insight, as to what is actually happening with the RF portions of my projects - those signals being way outside the measuring range of my oscilloscope.
 

PhilHornby

Senior Member
Car key fob internals

The last result - the Car Key Fob - was just thrown in for comparison. What do VW/Audi know, that I don't :confused:
I found a picture of the internals of a key like mine. Is that some sort of a 'loop' aerial - containing a tuning capacitor arrangement? :-

 

manuka

Senior Member
The main thing this experiment showed, is that a cheap SDR dongle can at least give some insight, as to what is actually happening with the RF portions of my projects - those signals being way outside the measuring range of my oscilloscope.
Well said sir, & the reason why I am enthusiastic about SDR dongles for all manner of cost effective RF work.
----------------------------------------------------------------------------------------------------
I'm still chuckling over Phil's 433 MHz "handspan" use,as even IR (a PICAXE strength) may have done! Ranges are usually wished orders of magnitude greater of course. Typical was from an outback Australian wanting a 433 MHz slow data link to 20km(!) to monitor stock watering ponds. Initial thoughts related to LoRa & elevated antenna,but his terrain was dead flat & a $$$($) tower would have been needed...

FWIW: My 50+ years of antenna wrangling shows boring practical aspects may often be as important as wavelength/impedance/gain/ bandwidth calculations. This especially emerges when salt spray corrosion, snow or large bird loading, insects (spiders/ants),wind, neighbourhood appeal (!) or -as here in very clear sky NZ- UV deterioration of fittings may arise. Naturally budgets are an issue too...

In contrast to past large & "difficult to work with" VHF antenna, the global move to UHF digital TV means compact antenna now abound. Here in NZ numerous "FreeView" programs occupy just 562-602 MHz, readily received by Yagi style antenna such as the rugged wideband one pictured. It's folded dipole driven element even looks to suit 433 MHz use, although the directors (normally slightly less than a half wavelength) are a tad short. It may well suit SDR exploration ! Anyone ? Stan.
 

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marks

Senior Member
Sorry if this is off topic but This is one of those backshed moments one can not resist lol.
you can't beat the yagi style element, the one manuka has shown is perhaps a good allrounder probaly ideal for 433
I would be tempted to choose a slightly smaller element if just targeting the freeview range ,the smaller directors pulin the higher frequencys

I use to need the largest antenna (3m span(i liked my footy clear) for analogue reception
but not needed here for digital.The birds use to make too much noise i was glad to get rid of it.
hell even the backshed needs one (as shown) so I dont miss those sporting and news moments lol.

mine tuned for the 170 to 230 range and found the ideal element loop to be from memory about 430mm long
(with 40mm inside width which you vary slightly to fine tune) the balun is about 550mm coaxial rolled under the coffe lid)
it works good for radio too ,eventually will make a seperate one with the same element using a longer balun which will chop out those tv signals.
I have two for the house as well inside the roof the tv shows 100% add a second telly tho it still works but drops to 75%(so 1 for each socket lol)
there is one chanel here WTV at 557.5 you can still get shows 58% with signal quality still 100% . i only watch the main channels anyway.
 

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AllyCat

Senior Member
Hi,

It's folded dipole driven element even looks to suit 433 MHz use...
Yes that's very probably the case because "classic" yagis and dipoles generally have quite a gentle fall-off in response at lowering frequencies but a "cliff edge" in the response at higher frequencies. To my simplistic mind it's when the electrons discover that they can't rush up and down the antenna elements (resonate between "mismatches" at the end and maybe centre) faster than the speed of light. ;)

My antenna theory is rather rusty and I've always considered their design very much a "black art", but the 360mm (width?) dimension of the "folded dipole" seems too large for 862 MHz, is it because of its "bow tie" shape ? My antenna theory told me that the purpose of the folded dipole, i.e. with a "short-circuit" (mismatch) at the centre of the fold raises its impedance (back to 75 ohms?) to compensate for the lowered impedance caused by the proximity of the reflector and all the directors.

Conversely, the directors seem remarkably short (if they are each a total of 130 mm long and spaced 90 mm apart along the length of the boom). But then my "theory" always considered dipoles and yagis to be relatively narrow-band devices so times appear to have changed. ;)

On the topic of key fobs, they have traditionally always used (magnetic) tuned loop antennas, for their smaller size and better resistance to "proximity effects". You can't get a much worse proximity effect than holding it directly in your hand!

Cheers, Alan.
 

manuka

Senior Member
PhilHornby: I've run across similar rooftop characters in the lower Himalayas,but NZ is thankfully spared such -ah- monkey business. However our vexacious "Kea" alpine parrots are infamous for their mountainous mischief,with vehicle antenna,windscreen wiper baldes & rubber window trim their favourites - see pix. You can imagine what they'd get up to with light duty coax feedline or flimsy antenna elements...

Check the ruggedness of the pictured commercial VHF antenna (pictured), built to handle such environmental woes. It saw decades of exposed service -note the pre late 1960s Mc/s instead of modern MHz.

Allan- "Alleycat": Antenna design may indeed be a black art, but it's the performance that matters. With tradeoffs in electrical/mechanical/budget monitoring via SDR can now be hence very useful. Stan.
 

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