Extending the range of this cordless intercom - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: August 17, 2015, 06:27:58 pm
content: I have an OLD, crappy cordless intercom based on Motorola MC13111 which works at 40.668MHz FM.It's a prototype model with some limitations, it serves both as interphone and gate opener. It works very bad, the voice communication is quite instable and unusable (it works good only at a few meters from the control unit, while the gate opener works correctly at +50 meters).Since I live just a little bit out of range from the control unit, I'd like to extend the range of the gate opener, and since this model is used for the same gate by my neighbors, I can't simply replace my intercom. All the neighbors have more or less the same problem with voice communications but their gate opener works fine, since they are closer to the gate.By now to open the gate I must put the intercom outside my window. So I thought I can extend the range of this device just by installing a better antenna tuned to 40.668MHz.Once opened the device, I discover that other than the extendable antenna, there is also a PCB antenna surrounded by a bunch of RF circutry.I'm not an expert, expecially in RF field, so I'm a little bit confused JUST to find 2 antennas.Using a near field probe, I discovered that the PCB antenna is the one (mostly?) used to transmit the signal when I press the gate opener button, so I guess the stylus is (apparently) used for other, i.e. to receive the confirmation of the command sent by the gate control unit (and for crappy voice answers to the door rings).The 40.668MHz signal that is applied to and amplified by the BFQ17 and then goes to the pcb antenna, comes from pin 8 of the MC13111 wich is strangely marked as input (??) on his datasheet.Code:PIN 8, Input, Tx-VCO: Transmit divide counter input which is driven by an ac–coupled external transmit loop VCO. The minimum signal level is 200 mVpp @ 60.0 MHz. This pin also functions as the test mode input for the counter tests.What do you suggest to improve the range of the gate opener PCB antenna? My doubts are about the path the signal takes AFTER that antenna and what kind of effect can have adding capacitance/inductance to the circuits by adding an external antenna; I guess I can't simply take the output of the BFQ17 breaking the path of the signal...

author: mcinque
author_rank: Unknown
timestamp: August 23, 2015, 06:43:39 pm
content: Hmmm... maybe I have not provided enough details?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 23, 2015, 07:13:09 pm
content: Those spiral patterns on the PC board are more likely RF inductors and not intended to be the antenna.No, I would not disturb the circuit on the PC board. It is likely part of the tuned RF circuit.I didn't see any details about using the external antenna?Is there a connector for the antenna?Is there a built-in antenna? An extendable "whip" antenna? How long?What is the possibility of connecting a proper antenna tuned to 40MHz?

author: Flump
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:26:06 pm
content: That looks like a center fed dipole to me

author: LaserSteve
author_rank: Super Contributor
timestamp: August 23, 2015, 07:28:46 pm
content: Consider that you need about 46 inches of antenna length with a base loaded (inductor at base of antenna) antenna for that band.... Its range is designed to be minimal. It would take a lot of work to extend the range. It is designed to be a low power, short range device.Steve

author: mcinque
author_rank: Unknown
timestamp: August 23, 2015, 08:00:30 pm
content: QuoteThe external antenna installed doesn't seem to affect the range of transmission.Doing some tests, I found first the distance range-limit where the central unit can receive the signalwith the antenna retracted.Then I moved just a couple of meters away and retried with the antenna fully extended, but the control unit doesn't receive the command anymore.QuoteNo, as you see in the picture, there is only a screw that secure the whip antenna to the board.QuoteThere is an extendable, 49cm long whip antenna.QuoteI tried to connect a DIY 1/4 wave "solid wire" antenna (no inductor at base) cutted for 40Mhz, (so replacing the whip), with just a slight improvement but the issue is that this improvement is not constant: one time the signal get received, one time not. This could be dued to the "wire" nature (so not fixed in one stable position) of the DIY antenna?Due to the frequency, it's not practical to have a full wave length antenna on a portable deviceQuoteTo me too. Otherwise, why most of the RF signal radiates from that area?QuoteIndeed.Do you think a center loaded dipole wouldn't be enough as improvement?

author: qno
author_rank: Frequent Contributor
timestamp: August 24, 2015, 10:09:58 am
content: You could try to increase the power supply voltage of the rf output stage.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 24, 2015, 04:47:59 pm
content: You will see a big electric field from the two PCB spiral inductors. However they do not actually radiate much at all. They are not the antenna, they are a high Q factor output stage or bandpass filter. Did you ever consider if that is the case, then the output amp transistor is probably dead?Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: August 24, 2015, 04:55:25 pm
content: See attached for what does what....Steve

author: mcinque
author_rank: Unknown
timestamp: August 24, 2015, 06:34:55 pm
content: Thank you Steve! I appreciate the time you've spent to make that explaining picture!QuoteHmm... I've checked if the transistor is working correctly, and the signal is amplified, but following the path I found a big attenuation in the signal just at the output antenna; as I said I'm not an expert expecially in the RF field, so is this to be considered normal? or there is something in the path that is shorted/broken that cause this?

author: LaserSteve
author_rank: Super Contributor
timestamp: August 24, 2015, 08:08:47 pm
content: How did you check the signal? Because applying just about any kind of probe detunes the phone's output stage somewhat. Well, that can range from "somewhat" to "adsorb all the output", depending on your instrument.Steve

author: mcinque
author_rank: Unknown
timestamp: August 24, 2015, 08:29:33 pm
content: With a 100MHz DSO (1MOhm input impedence) and a 10x passive probe; is this the wrong way to probe the signal in this case?

author: mcinque
author_rank: Unknown
timestamp: August 27, 2015, 12:28:23 pm
content: What do you suggest to use to probe the signal without affecting the circuits? 1M impedence should be enough.This are the attenuation values. It is safe to desolder that inductor and try without it?

author: LaserSteve
author_rank: Super Contributor
timestamp: August 29, 2015, 05:20:11 pm
content: Quote from: mcinque on August 27, 2015, 12:28:23 pmThat inductor is there to match the electrically short antenna to the RF output stage. The small Inductor is there as a DC return and is needed by the circuit, it is not to limit the signal. My bet is it is optimized for best matching and thus best range. Remember if you take it off the receive impedance matching and the duplexer go to hell in a handbasket at such long wavelengths.The signal power SHOULD drop there. The tank circuit is highly resonant. You only want to take off part of the energy in the tank circuit and feed it to the antenna.I'd suggest you read up on how Class A and Class C RF amplifiers function before you remove components.Use one of these with a 40 inch antenna on it or a 6" diameter loop antenna on it as a probe:http://www.techlib.com/electronics/detect.htm.Dr Wenzel was kind enough to test those diode detectors for me long ago. Long ago when I could not afford a proper, calibrated, RF power meter.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: August 29, 2015, 05:28:30 pm
content: Short of a high level of reverse engineering, the only way you can increase the range on this system is the traditionally best way. That is improving the antenna to a directional antenna or a higher gain antenna.All the RF power increase in the world is basically useless if you are radiating with an electrically short antenna.TV and the Movies have given people the idea that increased transmitted power equals increased range. This is not always the case. While increasing the power will slightly increase the phone's range, increasing the antenna's ability to couple the signal to free space (known as "Antenna Gain") will give you far more range.You have a system that was designed to be range limited so that the frequency space could be reused by other phones nearby. It is deliberately designed to be a license free, un-coordinated, short range system. It is designed to comply with a law that limits the transmitted field strength.Place a high gain antenna designed for 40 Mhz service on the receiver at whatever the phone controls and be happy.A quarter wavelength at this frequency is a bit less then two meters. Add a two meter random wire antenna to the receiver and see what happens. Or add wire to the existing receive antenna till it is a bit less then two meters.Find a scanned copy of the ARRL Antenna Handbook on line and read it.Steve

author: mcinque
author_rank: Unknown
timestamp: August 29, 2015, 05:48:30 pm
content: Thank you very much Steve, you've been very clear and instructive. I really appreciate your timeI'm going to read that webpage and about RF amplifiers, other than download the ARRL Antenna Handbook as you suggested.Thank you again for your advices, you're really a kind person.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 29, 2015, 05:54:49 pm
content: IMHO, using any kind of RF amplifiers is impractical for this situation. Remember that you have a transceiver here, that is both a TRANSMITTER and a RECEIVER. It takes very different kinds of RF ampllifers for transmitters vs. receivers. So you must make TWO amplifiers, one for transmit and one for receive. And you will have to switch between transmit and receive mode. None of this is the kind of project one would expect to be successful without significant previous RF circuit experience.@LaserSteve's suggestion to use PROPER antennas is far and away the most practical suggestion, and the most likely to result in significant improvement.

author: mcinque
author_rank: Unknown
timestamp: August 29, 2015, 06:04:11 pm
content: Yes Richard, you're absolutely right about the two way communication. I will try with a proper antenna asap. Thank you too!