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> Antenna Circuit, increasing transmission range
deth502
Posted: October 12, 2016 10:00 pm
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i have this project where i repurposed an old rc car for the transmitter and receiver, but i have to have the transmitter within 12-16" of the receiver antenna to work, which is far from acceptable.

being a complete idiot afa circuit design goes, i think im picking up just a little. now, i *think* i have an idea, but im checking here first. heres what i have now. the signal comes out of a cap into an inductor, then both sides of the inducor have a cap to ground. as such:

user posted image

user posted image

now, would i be correct in assuming that either one or both c7 and c8 are there just to bleed signal off to ground and make the transmitter weaker? and removing one or both would increase the power of the transmitter? or am i completely barking up the wrong tree here?
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kellys_eye
Posted: October 12, 2016 10:35 pm
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That circuit is what is known as a 'pi filter' and is used to filter harmonics from the desired output signal and thereby prevent you breaking all sorts of laws pertaining to interference. It also 'matches' the output of the circuit to the antenna.

What you need to do is make an antenna that matches the frequency of operation and the impedance of the output. This can be as simple as getting the LENGTH of the antenna wire correct.

Then, if you really wanted/needed to, you could design a directional antenna that gives a 'boost' to the signal (by concentrating the signal in a given direction).

Same rules can also be applied to the receiving end antenna.

Mind you, if the transmit signal is THAT weak then there's probably something wrong with the circuit - or maybe the receiver circuit has become 'detuned' and requires trimming?


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deth502
Posted: October 12, 2016 10:43 pm
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it is somewhere in the 24mhz region, so by calculations that puts a proper antenna min around 10', which is quite unweildly and impractical attached to a 2 sq in transmitter. i do have a 9' 9 3/4" (iirc, the 1/4 wave value i calculated) antenna on the receiver. as i said, i need to get the small telecscoping (maybe 14"?) antenna on the transmitter within about a foot of the receiver antenna to work reliably. i can ga it to work intermittently to about 10', i need about a 25' range.
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deth502
Posted: October 12, 2016 10:57 pm
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and lol, as i was typing i figured the "breaking fcc regs" thing would come up. yes, i know "technically" and all, but this is being used at our site garage which is about min 1/4 mile from any signs of civilization, and im fairly certain that theres not ANYTHING i can do to this to make it transmit that far, so i wasnt really worried about interfering with others electronics.

so they really dont bleed off any power from the signal, they just narrow the bandwidth of whats coming out and trim of all of the garbage being added to the signal? i was fairly certain this was taking something out. i thought it was reducing the power (said fcc regs), but they are more for reducing any emissions that are not within the exact target frequency then?

follow up, this transmitter runs on 3 aaa batteries, roughly 4.5v. assuming that the components (the electrolytic, the ic, and the transistor) are rated for higher voltages, would increasing the power help with the transmission power?
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deth502
Posted: October 12, 2016 11:03 pm
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another follow up question, in an instance like this, what if i took of the antenna wire, and hooked that into a transistor or op amp to amplify that signal, then put the output of the op amp/ tranny to the antenna, would that do what im thinking?

more importantly, instead of me asking stupid questions and being a pain in the ass, are there any good books/references out there on antennas i could read? ive never really done anything with radio wave propagation before, but i recently got my ham technicians licence, so im starting to get into it.
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Sch3mat1c
Posted: October 13, 2016 12:04 am
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Two possibilities:
1. Transmitter is way under Part 15 limits (hence the shitty range), or
2. Receiver is shite, or detuned or something, and can't hear anything.

If it's #1, you probably have a lot of room to go before causing problems for others. But it would be nice to know. Say if you had a cute lil' EMF sniffer thingy -- an antenna, crystal diode and voltmeter. Enough to get a crude idea if it's working or not.

So be careful with which one it is.

As for L's and C's, their presence may well be enhancing performance, not hindering it. As KE said, that's typical of filtering. A filter can also provide matching, which means more power into the antenna.

You aren't going to get any more power than the output transistor is capable of, which may be limited by many things. Its input power is probably fixed by earlier stages (say, whatever's coming out of that IC), and it's only going to have so much gain. Its output power depends on impedance matching, which may be intentionally poor (i.e., running the transistor with a light load, since that's all that's needed for the application, and to not exceed limits).

So, fine, maybe you could change the impedance, but it's not as simple as turning a potentiometer. Remember how filters can match? Yeah, the ratio of L/C gives impedance, while the product L*C gives frequency. You have to tweak both at once, to change impedance while keeping frequency the same.

It's not at all difficult to match and tune an RF amplifier -- but it is quite involved, making sure every single component is set right. And generally speaking, all components in that filter will have to change, in order to change the impedance. So it's a tricky thing to try and attack, without practical knowledge or theory guiding you.

Tim


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kellys_eye
Posted: October 13, 2016 12:34 pm
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There are many tomes on radio frequency stuff and the ARRL handbook could be a worse place to start - easily obtained and aimed at beginners to advanced alike.

Rather than using an op-amp you can build simple PA stages using just one transistor and a few coupling/matching/biasing components - outside the scope of this thread - but covered in aforementioned book.

But those basic remote control units are built DOWN to a price and performance is always going to suffer as a result. If you need to do anything you need to establish how well the circuit is working 'now' and what adjustments can be made to improve it i.e. does the receiver side have any adjustable (tuning) components?

From what I can tell the circuit operates at 27.145MHz (as stamped on the crystal) - basic stuff and commonly used - but equally it's well documented stuff too so try Googling for some basic tech knowledge on 27MHz radio controlled stuff.



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deth502
Posted: October 18, 2016 11:56 pm
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this might just push me over the edge. i had been thinking about joining the aarl, but got kind of ticked off before, so i put it off. i might join now and have a look at what publications they offer. right after i got my radio licence, they sent me a letter, join now and get a discount, only $xxx. i forget the exact amount. so i went to the website, they had a certain price up which was like $5 more than my "coupon code", so i typed my code in, and when doing so, the price WENT UP $10!! thats just poor web site set up, but i digress.....

inductors, to me, are a black art. the entire circuit is there in the pic, the inductor has some sort of wax potting over it, i dont know if thats to hold it together, or because it is adjustable and that holds the adjustment, other than that, no variable components on it.

@tim, im fairly certain its #1, but have no way of telling. in going down this road learning about broadcasting radio waves, googling "a cute lil emf sniffer thingy" might be a very good and useful project at this point. but from the rest of the responses, im thinking theres a lot more to this than i think there is and should probably shelf this project for a while (or start over with better products to hack)
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kellys_eye
Posted: October 19, 2016 10:20 am
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The wax does indeed prevent frequency changes due to physical movement of the coil.

A simple 'replace bits until it works' approach might help in this particular instance as, for most designs like this, the output drive transistor is the usual culprit of poor transmission range so changing the part without even bothering to make tests may sort the problem.

If you can't find an exact replacement then practically any transistor of the right polarity (NPN or PNP whichever is being used) will work provided the basic parameters such as Vce/Ic/Ptot and Hfe match - or near enough match.

What part number IS the drive transistor perchance???

As for signal measurement - you could attach a simple schottky diode (or point contact germanium) to the output, bypass the diodes output to ground using a small capacitor (say 1nF) and measure the change in reading (on a DC voltmeter connected at the diode) as you key the transmitter.

Basically you'd be rectifying and 'smoothing' the RF to something a meter can detect given they're normally useless last a few 100Hz on AC/DC unless specifically designed for the purpose.


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