Antenna question

This evening I tested it from my driveway at home. Not the same as dealing with the brick office building, but interesting just the same.

Working with the new transmitter and receiver, 315 MHz, you'll recall that yesterday I got about 50 feet with no receiver antenna installed. Tonight, using the telescoping antenna on the transmitter, and with a 23.8 cm straight wire sticking up from (perpendicular to) the receiver board, both antennas in the vertical orientation, I got perfect performance as far as I could go and still have open air, straight line transmission with no obstructions, which was

420 feet. After that I had to turn a corner, and went behind other houses, and it no longer worked well at all. So it clearly doesn't like obstructions. Duh.

Then I tried a coil loaded antenna, but there wasn't any detectable improvement. I know that a lot of people using the plain boards with no encoder or decoder chip found that the coil loaded antenna worked much better than a straight

1/4 lambda wire, but my experience with the encoded boards was just the opposite. Maybe there's a big difference in ground plane. Mine has a pretty good one. Maybe the others don't, and the coil makes up for that.

Anyway, I'mn pretty pleased with this performance. I think it's still unlikely it will work at the meeting building, but I wouldn't be shocked if it did. There are always cars parked in front and across the street, and maybe a reflected signal would get back into the lobby that way. We'll know Wednesday evening.

Actually, while clearly not suitable for every use, I think these encoded versions are pretty freaking awesome for the money. And you even get a battery.

I find your ideas on obstructions to be interesting. You suppose the brick building won't pass enough signal through the walls to work, but expect the scattered reflections from a few parked cars to work. Your test above doesn't really show much since you can't say how much further it would have transmitted had you not gone around the corner. Do you have a closer corner you can go around? I guess we'll see what happens when you try it in the actual building you need to use it in. I'd like to see where in the building you can get it to work and how many walls you can go through.

I don't follow your logic. You say there was no detectable improvement with the coil loaded antenna, since you didn't say it was worse, I assume they were about the same. But then you say your results were "opposite" which implies the coil loaded antenna worked more poorly. Can you clarify?

Lol!

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Rick C 

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

I don't expect the reflections to work. I just think it's possible they would work.

No, but I can put the receiver in an interior bathroom in my house and see what difference that makes.

They were about the same. What I shuld have said is that a lot of people using the non-encoded modules reported that even with a 1/4 lambda wire antenna, they only got two or three meters of range, whereas that worked quite well on my receiver - 128 meters and maybe more. I don't know why that would be the case, but it seems clear that these modules are not all the same. Even the decoder receivers are not all the same since the first one I received had that little trace antenna, but the new one does not. Well, based on my anecdote, the decoder receiver marked R02A appears to be a good one so long as you add the antenna.

I suppose even if the circuit is the same and the same passives are used, the really low cost units might be using crap transistors. I know the encoder/decoder chips are made by at least two companies. One seems to be legit with an data sheet you can understand. The other seems to have copied parts of the other company's data sheet, but not enough to make sense. lol Clearly one is a very low cost chip maker and the other is even lower cost.

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Rick C 

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

Do you think it would be reasonable to test operating the receiver on a 9V supply instead of 5V? Both the decoder chip and the LM358 are rated higher than that, but the two transistors are unknowns - marked R25. And none of the passives are marked in silkscreen, so I can't tell for sure that there are no zeners there.

I'd hate to find a receiver that works and then destroy it. But it would be convenient to use a 9V battery, and it kinda looks like the higher voltage would be tolerated. What do you think?

It looks like your board has the Princeton PT2272 chip which is rated for up to 15 volts. I can't see the part number on the other chip. I can't find an application circuit for the receiver. I know I've seen one that used two transistors in the ASK receiver. But I also have seen schematics that punt the receiver off and just show a chip which I think is your 8 pin chip.

Since your image has two transistors *and* an 8 pin chip, I can't say what would happen if you tried a 9 volt battery. In any case, I wouldn't expect the receiver to work any better really.

I'm curious about the chip now. Here is a data sheet for the MICRF007 which is a very popular although obsolete chip on the Chinese modules.

Page 10 has a schematic. Can you match this up with the chip on your board?

Here is the receiver I've ordered.

Nothing on the back side and one ground plane. At least one of the chip data sheets I've looked at shows separate RF and digital planes.

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Rick C 

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

On Wednesday, October 18, 2017 at 7:29:42 AM UTC-7, Peabody wrote: ..

It's schematic is probably similar to fig 6 here:

It has an input RF amplifier followed by a super-regenerative detector.

One of the stages in the LM358 opamp amplifies the signal from the detector with the second opamp being used as a data slicer to give the data to the PT2272 decoder.

kevin

No, the small chip on my receiver is just an LM358. A dual opamp.

Looks pretty fancy. What is the 6.7458 thing?

The remote device didn't work at the office building. The best I got was an occasional flash from the LED. I only had time to test from the conference room to the lobby. I guess it's possible it would have worked from a nearer point.

I still haven't played with replacing the telescoping antenna on the transmitter with a 1/4 lambda one. That might improve things somewhat.

However, the walkie talkies worked like a charm. So I think we're going to use them.

The other part of the project was finding a way to trigger the door unlocking. Our group leader, who offices there, did that part. He used an arduino driving a servo to which was attached a pipe cleaner. He mounted it in a shower basket with suction cups that mounts to the lobby window right above the motion sensor.

Once I read Kevin's post I realized that was it. What Kevin calls a "data slicer" is just a comparator. So the first opamp has a gain of about 300 as an IF amp maybe, and the second is the comparator that does the same thing as a diode for all practical purposes, demodulating the ASK to a pulse train. If you look at the schematic you will see the opamp on pins 1, 2 and

3 is biased on the inverting pin to Vcc/2 and a 200K resistor sets the bias point of the non-inverting input to the same voltage. The output of the first stage, pins 5, 6 and 7, is AC coupled to the comparitor. A feedback resistor from the output provides some hysteresis.

I'm not an RF guy, but it looks like Q1 is an RF amp and Q2 is an oscillator and likely a mixer. I don't think the opamp can handle 300 MHz signals. So they have to be down converting the UHF signal. I'd like to understand how the RF circuit works if anyone can explain it.

Either a crystal or more likely a ceramic resonator. There seem to be a lot of sources for the 8 pin chips on these boards. Here is one.

Change the frequency by changing the crystal/resonator.

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Rick C 

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

rickman wrote on 10/18/2017 11:54 PM:

Here is the info on the actual chip used on the receiver I ordered. They actually tell you! LR680

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Rick C 

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

On Wednesday, October 18, 2017 at 8:54:56 PM UTC-7, rickman wrote: ...

tor

So

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...

Q2 is a self-quenching super-regnerative detector.

It is an oscillator arranged to cyclicly go in and out of oscillation.

In the non-oscillating state the gain gradually rises due to the choice of biasing components at some point oscillation will start form either circuit noise or a received signal. The larger the signal the sooner oscillation starts. This affects the average supply current of the stage that is then filtered and amplified by the LM358.

The technique was invented about 100 years ago by Edwin Armstrong

It can give extremely high gains in a sin gle stage with low gain devices. It has largely been outdated now.

kevin

Thanks for the explanation. I should have figured that out as the superregen receiver has been mentioned in this thread already I believe and I think I've seen it in some of the eBay listings.

"Outdated" just means something that works better has been found which I assume would mean the superhet type. What is superior about the superhet specifically? Maybe the advantages of the superhet don't matter so much in this case. Certainly the super-regen has an advantage in cost or it wouldn't be used when the superhet is available in a single chip requiring nothing but a few passives to match the antenna impedance and a ceramic resonator. Two transistors and a very common opamp are pretty cheap.

Trying to get my head around the operation of the superregen makes me wonder how someone came up with it. I wonder how much serendipity was involved.

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Rick C 

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

You are probably correct. A bias-starved oscillator will often quench and restart spontaneously. It's not hard to imagine someone doing this by accident.

Jeroen Belleman

"Optimum Design of Short Coil-Loaded High-Frequency Mobile Antennas" (Brown, W6TWW) appears in _The ARRL Antenna Compendium Volume 1_. It answers many, if not all, of my questions about how to make an loaded coil 315MHz antenna similar to the antenna show at this website:

If you click on the link you can see an image of the 433MHz antenna. Note how the ratio of the coil's diameter to its length is half or less. Meanwhile, the ARRL article suggests that the exact opposite works better. My reading of the following excerpt leaves me with the impression that the coil's diameter ought to be twice as large as its length:

High Q can be obtained by going to large-diameter coils having a diameter-to-length ratio of two, by using larger diameter wire, by using more spacing between turns, and by using low-loss polystyrene supporting and enclosure materials.

An anecdotal survey of the coils used to tune my short whip for ham bands reveals a larger diameter, which works its way up to twice as large for the lowest bands. For a given Q, is the coil's diameter inversely proportional to the frequency? In other words, for a given Q, does the coil's diameter monotonically decrease with frequency regardless its length?

Thank you in advance.

73,

--
Don Kuenz, KB7RPU 

Genius is nothing but a greater aptitude for patience. - de Buffon

What modules are you using. Some cheap one don't really work. Try a HC-12 module. Then you don't need a good antenna. For any frequency you need a coil which has as much inductive reactance than the short antennas capasitive reactance. Perhaps small coils do not radiate so much as larger coils. If some one cares. But what I know short coils with large diameter have lower lossess (they don't get hot. ahem )