Antenna question

In grad school I played (with someone else's)* marginal oscillator, trying to see an NQR signal. It was painful, part of the problem was we weren't sure if the crystal (in the tank circuit) was any good. I wonder if a super regen would have been better? (NQR super-regenerative gives me plenty of hits.)

George H.

*I knew little about RF at the time, well I still only know a little. :^)

I just want to make sure I haven't missed something. The SIM card would have to be an active SIM, with a phone number associated with it, which means an active account that someone is paying for. Is that right, or is there some way around that?

I raise this issue because for me the whole point of doing the project is to do it as simply and inexpensively as possible. I mean, otherwise we could just hire someone to sit in the lobby for an hour and let people in. Jeff mentioned FRS devices, which don't require cell accounts, but those are considerably more expensive than the $5.15 I have invested so far. Of course I have several old cell phones, but the only active SIM I have is the one in my current phone, and it might be getting calls from a variety of sources during the meeting. Plus, you know, I want to be able to use my phone during the meeting.

Well, I know nothing about GPRS/SMS relays and such, so will read up on them today. Thanks for the suggestion. But I just wanted to be sure that all of those devices would need an active SIM.

On a sunny day (Thu, 05 Oct 2017 12:13:43 -0500) it happened Peabody wrote in :

No, that is right. Prepaid works, and 10 Euro over here (say 8 $??) is OK for a year.

Ebay has those things, maybe something like this (have not tried it):

I think I have seen some with more than one relay output too.

It is more expensive than your solution, but can be used later to switch on you heating or whatever from your car, what not. It is more secure than the 430 MHz sets, I have hacked those:

If you have more than one cellphone, like I do, just use the card of one of those.

It requires an active SIM but you can use a PAYG account and just have to arrange that it makes (at least) one chargeable call every 6 months.

In the UK at least you don't get charged for receiving texts (and on many cheap contracts charged for sending them either). I guess cheap is

have to make a chargeable call at least every 6 months to stay active.

If cost is an issue why not use Wifi or bluetooth to do the comms? Or even those dreadful hobby RF relay switches for lightbulbs. (and slightly more secure versions)

--
Regards, 
Martin Brown

Well the Ebay remote control arrived today, and things are not off to a good start. The device is listed as 315 MHz, but the SAW thing on the transmitter board is marked R433, from which I conclude that the transmitter at least is

433 MHz. But I don't see a way to tell how the receiver is tuned. That board is marked YK04, but apparently that can be supplied in either frequency.

Is it a fair conclusion that if it works at, say, 50 ft., the receiver is also 433 MHz? So at least they are the same, if not what I ordered.

Ebay really sucks sometimes.

And it turns out the receiver does have a trace antenna. I'll try to get a picture of it.

transmitter

I set up the receiver with a 5V supply and tested the connection. The transmitter blinks its LED when I press any button, so it appears to be working, but I get no reponse of any kind from the receiver. All outputs remain low.

So I'm dead in the water on this one. Oh well.

1) Line-of-sight between receiver and transmitter 2) Large RF Output power 3) Sensitive receiver(s) 4) Directive gain antenna(s) 5) Quiet RF operating environment (a quiet channel with good propagation characteristics)

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Just FYI: Cheap Chinese-type LED landscape lighting (and similar) often radiate a lot of RF noise. This noise could interfere with your setup. If you have LED lights on nearby, try turning them off and re-test.

I've made some good money tracking these kinds of interference sources down over the years. LED light noise doesn't seem to be as prevelant as it was a few years back. Guess the manufacturers got wise... Anyway, still wort h a try if it applies to your situation. Good luck.

Sorry to hear about your bad luck with ebay. There's one more test that you can perform (if you're of a mind to). You can use an oscilloscope to measure the fob's RF. The SAW can on my 434 MHz fob is labeled BCT RT0433B. The fob PC uses a curved "bow" trace as an antenna. (In my home grown parlance a fob's "bow" is the surface that protrudes slightly beyond the tip of your thumb as you hold the fob in your hand.) When a fob button is pressed with my 2465B connected between the negative battery terminal and the "bow" antenna the 2465B displays 11 full cycles in 25 nS.

25 nS / 11 = 2.3 nS 1 / 2.3 nS = 434.7 MHz

This thread's great. It inspires me to take a closer look at super-regenerative receivers, which apparently are timeless.

Thank you,

--
Don Kuenz, KB7RPU

On a sunny day (Fri, 06 Oct 2017 19:24:16 -0500) it happened Peabody wrote in :

Maybe open collector outputs?

No, I have a schematic showing them driving an NPN base to turn on a relay. So they source 5V when on.

Anyway, as Jeff suggested a while back, I found a pair of FRS walkie talkies, with charging base, supposedly working, for $4.99 on Ebay with free shipping.

That should eliminate any question of having enough range. I'm not sure what model these are, but I think these radios start at 1/2 Watt, and go up to as much as 2 watts.

But I have to say, the super-regenerative receiver is really something. Another invention of Edwin Armstrong, who also gave us superheterodyne receivers, and FM radio.

Which will add interference and "falsing" to the puzzle. Choose your channel and CTCSS tone frequencies wisely. You'll need to add some kind of tone activated relay to open the door. FRS/GMRS radios have a "call" button, which belches an obnoxious irritating tone over the channel to get everyone's attention. It can probably be used to activate a tone operated relay. Fire up a PC based audio spectrum analyzer: and determine what frequencies are being belched. Set your decoder frequency accordingly. Otherwise, build a tone encoder.

They're FRS/GMRS radios. Uniden GMR-2240-2CK. Bothe FRS and GMRS can now be used with up to 2 watts.

Yep. I'm rather embarrassed to admit that I didn't recognize a super-regen receiver from the schematic. That's because nobody used them in land mobile, marine, and consumer receivers, which is my background.

Super degenerative drivel: I was in 1st year college (1966?) when I was asked to design a helmet receiver for the local police department. Please do not ask how this project landed in my incompetent lap. For a receiver, I built a

45.5MHz wide-band FM super-regenerative slope detecting receiver version of a similar 50MHz ham radio receiver that I had built: Everything from a single 2N1409, which on paper, should not have worked. The 45.5MHz version was much nicer looking and was given to the PD for testing. I never saw the prototype again but was told that they liked it, whatever that meant.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Thanks and sorry for the delayed reply. I've been rather busy and didn't have time to fabricate a suitable excuse. (Firewood, customer crisis, and helping friend after surgery).

So, that's how quenching works for AM data receivers. The super-regen receivers that I've played with were rather unstable, probably due to the lack of an RF stage to isolate the antenna from the frequency determining network. Since they were also FM (slope detection) demodulators, frequency stability was critical. That's not the case with AM (OOK or on-off-keying) receivers, which can drift around quite a bit and still function properly.

Also, my background is land mobile, marine, and consumer radios, where the only super-regen receivers I've seen were in really awful pager receivers. I also failed to recognize a super-regen from the schematics of the data receivers and assumed that they were TRF (Tuned RF) type. My apologies for the misinformation.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

The manual suggests I can select a channel and one of 121 "sub codes", which I assume is CTCSS, and transmissions from any other source will be squelched. It also describes five selectable call tones, but doesn't say whether the sub code selection also applies to the call tone. Logically, it should.

Assuming all that works reasonably well to limit what audio is ouput by the receiving radio, I may just detect any strong audio to trigger the door unlock - rectify the audio signal, etc.. I'd rather avoid adding intelligence if I can.

Yes, I think it's pretty much just garage door openers these days. And of course RF remote controls. :-)

Anyway, thanks for the FRS/GMRS suggestion.

Nope. There are 38 analog CTCSS (continuous tone coded sub-audible squelch) tones available. In addition, there are 83 digital DCS codes available. 38+83=121. DCS (digital coded squelch) is somewhat faster to respond and is more immune to noise and voice falsing (false activation).

However, setting the privacy code on some FRS/GMRS radios is an ugly mess. For those radios that just count from 1 to 121, CTCSS 1-38 matches up to 1-38 and DCS 1-83 matches up to 39-121. If such a radio is set to code 55, that would be DCS 17. As long as you're using all one brand of radio, you're probably ok. However, if you mix brands, be careful with the privacy code settings.

These are suppose to be 5 or 10 different "tones" to alert 5 or 10 different people using the same channel and privacy codes. As I recall, they are a rather dirty sawtooth wave.

I was using FRS radios to triggered an animal trap. Triggering and timing was critical as a misfire might injure the critter if the trap closed early or late. I went for two tones (as in telephone touch tones) and no CTCSS or DCS decoding. Triggering was within +/-2 msec (2 cycles of 1KHz tone) at any RF signal level, which was good enough. Another benefit to using 2 tones was that it wasn't triggered by chatter on the channel. Your requirements are less critical, so except for the falsing requirement, you can use something less elaborate.

Looks like the Uniden has a COS (carrier operated squelch) output available: It probably won't be in the same location on your radio, but probing the pins for COS activity should find the correct pin.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Jeff Liebermann wrote on 10/3/2017 8:59 PM:

If not a straight wire, this is what I see on most devices. It is hard to imagine using enough PCB trace to get a long enough conductor for 433 MHz.

I am working on a project with very similar components. The thing the OP has said that is confusing me is he thinks the receiver and transmitter are not on the same frequency. The listing says 315 MHz but he thinks the transmitter is actually 433 MHz because of the markings. Possible they mixed that up. That is a good reason to get a refund at least, but it is likely the receiver is also 433 MHz.

He talks about the signal going out a window, done the building and back in another window, etc. Is the building solid metal? 433 Mhz will go through a wall otherwise. If the straight line distance is less than 100 feet it should work, or maybe more like 50 feet... He said the listing claimed 100 meters, but it says, "50m -100m" which says 50 meters to me.

At least, this unit came with a battery installed. Most don't. So this is a "quality" ebay vendors. lol

I've ordered similar devices without the decoder as it will be attached to an MCU and I can program it to work how I want.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

On the receiver of my Ebay "315" MHz remote control (the one that doesn't work), there is indeed a small trace antenna. Here's a back-lit picture showing it pretty well:

To get a sense of the size, note that on the left of the picture is an 18-pin dip. So the trace antenna extends about .4 inch, but because of the back and forth, it is effectively much longer than that. But still, nowhere near

7 inches. The question is whether the board is tuned for the impedance of the short trace antenna, or for an added monopole plus the trace. Also, how does the presence of the trace antenna, and its related LC stuff, affect the optimal length of the monopole?

I should add that there is significant gound plane on the bottom of the board, taking up probably 2/3 of the board area.

Here's something I ran across that might be useful for 433 MHz antennas:

OP here. The transmitter has a small metal can that looks an awful lot like a SAW resonator, labeled Y1, with the marking R433 on it. To me, that's pretty convincing evidence that the transmitter is 433. I agree that the receiver is probably also 433, but there is nothing on the board that tells me that, and I have no way to test it. In any case, it doesn't work. The transmitter appears to work

- the LED lights up when I press a button, but there's no output at the receiver. I think it's possible there is a frequence mismatch since the listing calls it 315 MHz, but of course the problem could be something completely different. Either way, it's going back.

It's an old brick building, but there are probably half a dozen walls (construction unknown), and two floors, that a direct signal would have to get through. I think a more likely path is reflections off cars parked in front or across the street to get back into the lobby.

Indeed, If only the rest was good.

The thing I liked about the remote control was that with the built-in authentication chips, for my purposes the need for arduinos was eliminated. Just like setting dip switches on the old garage door openers (8^3 choices), you get pretty good authentication, and those chips take care of the timing and protocol of transmitting and receiving the auth code. So the output of the receiver is a simple high when the button is pressed on the transmitter.

I would be fun to plug the dimensions into 4NEC2, which includes a helical antenna model generator. I'll play with it and see what appears. My sense of smell tells me such a tightly wound spring is not a good antenna.

I have a small pile of similar transmitters and receivers. The SAW filter cans all say "433.92". I think it would say 433.925 but there wasn't enough room for the "5".

From the photo, at least 2 brick walls and a right angle turn into the entranceway. Did you see my posting on calculating the end to end losses? Change the numbers to suit your reality (i.e. drop the -5.5dB brick losses, and change the gains to the 433MHz version) and see what you get.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I just wanted to report back on using walkie talkies to remotely trigger the door opener in case anyone else ever needs to do something like that.

I received the radios from Ebay - Uniden GMR2240-2CK - and they work fine. They have five different call tones, #3 being the best for this purpose. They also have a headset jack (2.5mm stereo) that provides audio and silences the speaker. Audio is provided on the middle ring of the stereo jack.

I still don't know what device will need to be triggered to open the door, but I've breadboarded and tested a circuit to convert the incoming audio from the received call tone into a momentary relay closure. Here's the schematic:

I used what I had in my junque box, and basically it's a CMOS dual op amp in an 8-pin dip, a relay, an various other stuff. The first amp stage is an inverting amp with a gain of 100. The + input is tied to ground, which is also the bottom power rail. So this stage just amplifies the positive going portion of the audio wave.

The output then goes through a diode to a filter and delay section, which helps prevent any noise from triggering the device. The resulting signal goes into the second amp stage, which is rigged as a comparator. The output of that stage drives a transistor, which in turn drives the relay coil. The whole thing takes about .5 second to turn the relay on, but about 5 seconds after the call tone ends to turn off.

I tested all this from various points up to 1/2 mile away from my house, and everything works fine, so I suspect the radios will work well at the brick office building.

Thanks for everyone's help on this.