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Am receiver - Is this correct?

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scubadog

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Can someone please help me with this.

I have built this circuit https://www.electro-tech-online.com/custompdfs/2012/11/3063-1.pdf

it uses the Mk484. i am trying to tune it to 457kHz.

It has a coil and ferrite bar. The coil is made from 80 turns of 3 strand litz wire. I have calculted this to be 32uF.

There is a tuning capacitor of 160pf that i have had to wire in parallele to enough capacitors to get me to 3.8nf.

Now my calculation is that this should be tuned to some where near the frequency i am after.

I have a bug transmitting on 457kHz and I can hear its pulsed output on the speaker. the trouble is that it is not very loud and if i move the bug any further than 5 inches away from the coil the signal is lost.

Can someone please tell me what I am doing wrong? I just can not seem to move past this stage

Thanks
 
The design uses a tuning capacitance that is lower than 160pF. The coil must be designed to provide the necessary inductance in order for resonance to occur. I understand that you added capacitance to get to 3.8nF because this is the value necessary to resonate with 32uH. However, the literature including the referenced website and the datasheet for the MK484 ( http://www.datasheetcatalog.org/datasheet/RECTRON/MK484.pdf ) all say that the inductor, when wound correctly, will resonate with capacitance of less than 160 pF. This implies that the inductance should be around 530 uH. I think that you are calculating the inductance of your inductor incorrectly. To construct the inductor, you wind the litz wire around the ferrite bar. The ferrite bar is the core of the inductor. When you wind wire on a ferrite bar, the ferrite magnifies the inductance a great deal, so you cannot use a formula for an air core coil to calculate the value of this one. Also, ferrite bars are different depending on its size and its composition, so the chances are good that you don't know the exact amount by which the ferrite will magnify the inductance. For this reason, I recommend that you figure out how to measure the inductance of your coil. These are some methods:

- find someone with an inductance meter and measure your coil with it
or
- wire your inductance in parallel with a ceramic capacitor of known value, for example a 150 pF ceramic cap and then couple an AC voltage into it. Vary the AC voltage to find the point where the tank voltage is at a peak
(if you can borrow a "grid dip meter" or "gate dip meter" (a useful instrument used by Hams to measure resonance of tank circuits) use that to measure the coil resonant frequency). this can be tricky for someone with no tools or instruments but it can be done. Do you have any instruments such as a function generator or an oscilloscope?
or
build a simple inductance meter yourself. its not complicated unless you have no meters or instruments of any kind.
or
wire your inductor in parallel with a variable capacitor. Place the inductor ferrite bar very close to the antenna of an AM receiver and tune the AM receiver to a weak station. Then adjust your variable capacitor to see if, while you are adjusting the capacitor you find any spot where the sound from your AM receiver gets a lot louder. If this happens, your ferrite plus variable cap is resonating at the receiver frequency.


Another approach is to assume that your inductor is fairly close, but not right on. Change your capacitance to only have the variable capacitor of less than 200 or 300 pF and leave out the rest of the capacitors. Then tune the capacitor to see if there is a point where you hear your bug very strongly.

edit: I found a couple of AM radio antennas here in my junk box and measured the inductance. I get 700 uH for one and 745uH for another. The one with 700uH is typical of an AM antenna inside a portable AM radio, in that the ferrite bar is 55mm long and it has about 80 to 100 turns of wire on it. In addition, I also have the windings from an AM bar antenna that have slipped off their bar, essentially just an air core coil. So I measured that air core coil and get 70uH. Then I slipped that coil onto a ferrite bar. The inductance increased to 745 uH. This coil is approximately 100 turns of 3 cond litz wire, with diameter of roughly 11mm. This is roughly like yours, I think. So as you can see, the ferrite magnifies the inductance by a lot, about 10 times in my case. Perhaps you could assume that your inductor is around 550 uH. Such a coil would resonate at 457 KHz with a 220pF ceramic capacitor, or with your 160pF variable (set about halfway) in parallel with another 150pF fixed ceramic cap.
 
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Thanks for that Ron!

i will grab some more caps.

I have a function generator and a scope but do not have an inductor meter. Maybe I need to build one!
 
Instead of building an inductance meter, it is better to put it into your circuit then try different capacitor values in parallel until you get it to resonate. Start by assuming your coil is 500 uH and put a variable capacitor across it, adjust the variable capacitor and listen to see if the sound from the bug gets louder. Put the bug further away if it is too loud. If the sound is loudest when the variable capacitor is at one end of its adjustment range, then you have to add more capacitance or add more turns of wire to your coil. Then try it again.

Of course, this assumes that the rest of your circuit is working ok. Is it?

If everything is working the way it should, there will be a very narrow range of capacitor adjustment where the sound gets louder, so don't turn the capacitor adjustment too quickly.

I tried to build a really simple inductance meter on my bench just now, to see if you could do it. I measured a value of 400uH for my AM bar antenna. Then when I measure using accurate instruments that I trust, I get 730 uH, so the simple meter I built isn't working very well. For this reason, I think it is a waste of time for you to build this simple meter idea. For your info, the idea is to simply connect a resistor in series with your inductor. I chose a 100 ohm resistor. Then I attach a function generator output across both parts (ie. the gnd of the function gen is at the free end of the resistor and the signal of the function gen is attached to the free end of the inductor). The center connection, where the inductor joins the resistor has no function generator connection. Now, in theory, at some frequency coming out of the function generator (sine wave), the voltage across the resistor will be exactly half the voltage across both the inductor and resistor. At this point the reactance of the inductor equals the resistor value, and you can calculate the inductance knowing its reactance and the frequency. Here is a reference explaining this and the other ideas:
https://daycounter.com/Articles/How-To-Measure-Inductance.phtml

The problems that I ran into trying to do this include that the AM antenna is not just an inductor, it has some parasitic capacitance in it too, so as the function generator frequency goes up, the impedance doesn't necessarily just keep going up. For this reason it is best to keep the test frequency quite low, like less than 50KHz. In turn, this means that you should stick to fairly low value of resistor.

edit: I also tried the method of coupling my coil to an AM radio and finding the frequency that it resonates at with a known capacitor value. This method is tricky, and the result I got was 920uH, which is wrong. I suspect that this method is touchy because there is some mutual coupling to the antenna in the radio which will appear to add inductance to our test coil, and the amount of coupling depends on distance, so, like I said, it would be tricky to maximize the distance to minimize this coupling. I moved the coil further from the radio and used an accurate frequency source as my test signal, and the result was 763uH, so this method can get close if you avoid the problems.
 
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wow.

I realy appreciate your help.

I have ordered another variable cap. That should be here by the weekend.
Tommorow i am at university so wont have time to play again untill the weekend any way.

But once again i realy appreciate your help.
 
At the risk of stating the obvious thing, which you may have done already...

First of all just build the radio as standard and check that it works correctly as a medium wave broadcast receiver.

Compare what you receive on the kit radio with what you hear on a bought receiver.
Compare the tuning ranges of both recevers and see how low in frequency the kit built receiver will go.

Then when you have established to lower limit of the tuning range, some simple calculations and inspired guesswork will indicate how much capacitance to add and/or how many turns to add to the coil.

JimB
 
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