Please help me extend the range of this PCB loop antenna

Hello, I would like to modify the antenna on this circuit board to extend the broadcasting range of the antenna. I have little experience in antennas and am seeking the wisdom of this board. There are no restrictions on design, my first thought is to add a whip antenna, but I'm unsure if this is best or if so how to connect it? Or should I create a bigger wire loop? How big? How should I do this, should I just solder on a new loop or disconnect the current loop at the red arrows I added to the graphic? Thanks for any help.

BTW the PCB is a gate opener, the current range I get is about 10', I know modifying the receiving antenna would be a much simpler solution but this is not possible. Also why are there 2 antennas, are they both used at the same time?

UPDATE: Full circuit diagram and schematics posted below, along with RF section explanation, and all FCC details.

FULL RESOLUTION IMAGE HERE https://i45.tinypic.com/191a3d.jpg

It rather looks like two "printed" inductors (for the respective one-transistor oscillators) than "antennas". Because we
don't know the purpose of the board, we hardly can guess, why there are two of them.

If it's a transmitter, it obviously hasn't been designed to achieve a range. You surely can't change the loop size, and most
likely not add a whip antenna without threatening the intended operation. You have to modify the oscillator circuit or add
output stages.

An obvious reason to limit the transmitter range is compliance with legal regulations. 300 or 310 MHz aren't allowed
"on the air".

FvM said:

It rather looks like two "printed" inductors (for the respective one-transistor oscillators) than "antennas". Because we
don't know the purpose of the board, we hardly can guess, why there are two of them.

If it's a transmitter, it obviously hasn't been designed to achieve a range. You surely can't change the loop size, and most
likely not add a whip antenna without threatening the intended operation. You have to modify the oscillator circuit or add
output stages.

An obvious reason to limit the transmitter range is compliance with legal regulations. 300 or 310 MHz aren't allowed
"on the air".

Click to expand...

I tell you go for monopole antenna
Length of antenna wire is λ/4 of your transmitting frequency


Regards
Ganesh
EmbSpark Technologies

A λ/4 monopole for 300MHz is 25cm...which is approximately 5 times bigger than the actual PCB.

The radiation resistance of the small-loop antenna (circumference < λ/2) is extremely small, and typically the antenna may be able to radiate only a few percent of the power that comes from the transmitter.

The radiation resistance of a small-loop can be increased increasing the number of turns, or inserting within its circumference a ferrite core with high permeability.

vfone said:

A λ/4 monopole for 300MHz is 25cm...which is approximately 5 times bigger than the actual PCB.

Click to expand...

Thank you very much for the help vfone as I said there are no design limitations, I could add a 25cm antenna if needed. What I don't understand is how I would add it, where do I connect it? Just solder it too the loop? Should I disconnect the loop then? Use one antenna for each loop?

vfone said:

The radiation resistance of a small-loop can be increased increasing the number of turns, or inserting within its circumference a ferrite core with high permeability.

Click to expand...

So you're saying another modification I could do is to create multiple windings of the same loop diameter with a ferrite core? The more windings the better I would assume or how many? Do I do this to both loops or only one? Trying a monopole sounds like it would be easier.

Another idea I had was to simply over power the entire circuit, currently it runs on 9V, do you think it would damage anything to run it at 12V? Would it increase the transmitting power any? The main thing to worry about I would think would be the EPROM, I found its datasheet and it looks like it can handle 2-6V (currently with 9V the chip runs at 5V) and can probably handle slightly more than that.

Another option would be to amplify the signal at the antenna loop, how hard would this be, do they sell specific amps or pre-made circuits to do this?

I have done some digging and found the schematics and several FCC documents relating to the device.

CIRCUIT DIAGRAM IMAGE - Please see PDF for full vector diagram and circuit board legend.

Also found this in the information

RF circuitry
TriCode transmitters have two fixed antenna outputs – 300 MHz & 310 MHz.
C1, C4, C7 & C9 are the primary components of the 300 MHz antenna circuit. R4, R1, C8, Q1, R5, &
C11 are the means by which a coded message from the microprocessor, U1, may be transmitted on the
300 MHz antenna.
C2, C3, C5, & C10 are the primary components of the 310 MHz antenna circuit. R2, R3, C6, Q2, R6,
& C12 are the means by which a coded message from the microprocessor, U1, may be transmitted on
the 310 MHz antenna.

Click to expand...

It looks like Q1 and Q2 are oscillators. If so, then the loop antenna is an important part of the frequency determining circuit, and can not be easily changed.

You could try loading the loops slightly with a metal ground plane, which might increase the radiation efficiency of the loops.

You could replace the loops with inductor chips (ie redesign the oscillators), and add additional transistors to form an RF amplifier. Then, with the RF amplifier buffering the oscillators, you could easily try different antenna configurations.

What does the receiver antenna look like, and what is its orientation with respect to this transmitter board? You need the polarizations to match up.

Thanks for your input biff. I don't know what the receiver antenna looks like. How would I create and connect a metal ground plane, just put a metal plate under the PCB and connect it to ground?

Indeed they are two microstrip oscillators using transistors MMBTH10 (SMD E3), where the resonators are actually small loop antennas.

Changing the dimensions (or the number of turns) of this loop antenna will change the oscillation frequency.

The easiest way is to let as it is and add another stage amplifier which feeds a λ/4 monopole.

Indeed they are two microstrip oscillators using transistors MMBTH10 (SMD E3), where the resonators are actually small loop antennas.

Click to expand...

Yes, this could be seen without a schematic.

The easiest way is to let as it is and add another stage amplifier.

Click to expand...

Yes, but do you think, the original poster knows how to?

I took some EE classes in college and have built small circuit boards before, I have a breadboard and wiring, resistors, etc. I have no doubt I could build it, designing it on the other hand that information has long since leaked out of my brain...

If someone could do me a huge favor of drawing a schematic, or giving me an example of what I should be looking at that would be great.

Also what about the idea of simply increasing the voltage from 9V to 12V?

This is just an example of what you can do.
Input of the buffer has relative high impedance to be able getting a low coupling with your oscillator. Good point for coupling is the emitter of the transistor oscillator.
The antenna matching components L* and C* must be tuned for best radiated power depending by the antenna used.

Also what about the idea of simply increasing the voltage from 9V to 12V?

Click to expand...

The oscillators are running from stabilized 5V, furthermore the operating point (and output "power") is defined by control voltage and the emitter resistor.

vfone, wow, thanks so much, this is awesome, hopefully it works Can I use a simple straight piece of 25cm wire for the antenna? If so what would L* and C* be? Do you have any guess on how much better this circuit and antenna will be over the current design (2x, 10x, 100x)?

BTW the PCB is a gate opener, the current range I get is about 10', I know modifying the receiving antenna would be a much simpler solution but this is not possible. Also why are there 2 antennas, are they both used at the same time?
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1. Why you can not modify the receiver side.
2. What range do you want to control?

The LC matching network values depends by the impedance transformation needed, mainly by the antenna impedance which rarely is a perfect resistive 50 ohms.

Ideally have to measure the impedance of the antenna using a VNA and also the output impedance of the final stage, and find the matching values using a Smith chart software (e.g. WinSmith).

For this frequency L* could be in a range of 5nH to 50nH, and C* from 1pF to 30pF, but as I said depends by the antenna impedance.

You can use a tuned λ/4 monopole wire (cut the length for proper VSWR) but in this case the antenna needs a good RF ground, which can be the metal case. Anyway a simple open-air λ/4 wire it will work better than any printed small-loop antenna.

BTW, if you just want "a little" more transmit power, you could lower the values of R4 and R2, which will provide more current to the oscillator transistors. Similarly, you could raise the values of R1 and R3, or possibly even eliminate them. You can only do this a little bit, because you need the transitor to still stay inside of its "active region" for oscillations to start up.

Assuming the antenna is a perfect 50 ohms, what would the values of L* and C* be? I am just trying to figure out what to order for parts to start with. The VSWR <2.0 typ. at center. Also I assume I will need to build 2 amps, 1 for each oscillator, or can I tie them both into the same amp?

Here are the antennas I am looking at:
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