The radio and the antenna matcher

 

This is the radio to be built.  Note the 20pf trimmer. If the antenna is long enough it could be all the trimming needed.

 The MFJ-16010 would be ideal for the job with a shorter antenna.

 

Note a quick search will find the manual with connecting and tuning instructions.

The hope is we build the unit as presented and it works fine. With our short antenna it probably will not. Adding a MFJ-16010 would be the next step before changing the circuit from the design.

Next thing to consider is the ground. Poor ground can cause you to lose half your signal. MFJ has an artificial ground to consider.

 

It worked so well for Dave. Why does it not work for me?

 This is what we would prefer. Need an acre to build it on!

 The actual antenna is a single wire. It looks like a capacitor  up to 1/4 wavelength.

So Daves antenna is about 1/4 wavelength. (little reactance)

Our antenna is very short and presents a lot of reactance.

 Assuming C2 is the antenna reactance we lose a lot of signal. The radio may be top notch but is deaf because it ears are not in tune. So, what to do?

 By adding L1 C1 we get our signal delivered to the radio input.

So 2 2 word answers - loading coil  - antenna tuner.

 The well loaded - tuned - neutralized input soars.

Next post the radio and possible additional circuit.

 

 

 

Is it a regen?

 The question is can you build a regen around an opamp?

 Things to consider:

How do we modulate and demodulate an AM signal.

What is the number one requirement for mod / demod?

I produced some screen shot that help answer the question.

I have some wheres to be shortly so am just posting the screen shots now 

and will comment further later. 

The shots are in reverse order. Reading from the bottom up makes more since. 

Using one input create an audio output.

Adding a filter separates the signals.

Driving with an AM signal you can see the distortion created. 

Here I tie the inputs together and we see distortion (ripple). This is the non linear action required to demodulate.

 The plus and minus inputs are inverting and non-inverting. Here we have the same phase out as in. The shot above inverts.

Add low Z output

 Note R10.

 

3 stage JFET amp

 Plenty of voltage gain just needs an output stage or transformer.

 It could drive a high Z phone but I want to drive an earbud.

 

Why is it motorboating?

It is motor boating but stops when the meter ground lead is connected to the chassis?

Could it be lead dressing or interstage coupling through the power supply (battery)? 

 

 

2SK2539 SPICE MODEL

 .MODEL 2SK2539 NJF(VTO=-3.1621 BETA=5.01519m BETATCE=-0.5 LAMBDA=2.40112E-2 RD=1.48485 RS=1.48485 CGS=1.05000E-11 CGD=1.40000E-11 PB=5.68050E-1 IS=9.11151E-16 XTI=3 AF=1 FC=0.5 N=1 NR=2 MFG=PHILIPS)

MOSFET Slider review

 If you build this one using a variable resistor for R2 you can use the MOSFET you have on hand and adjust the bias to suit the circuit. R8 and R9 represent a high or low Z load. The screen shots display the output with different MOSFET as R2 is adjusted from 10K - 20K.

 

9 volt version of AF amp

 When I built this circuit I had a 6 volt lantern battery that was needing using. I tried a 9 volt battery to compare results. My earbud is rated at 1ma and it puts out 1ma with 100uv signal. Looks like this one is a keeper?

 

 

Let's take a look at the Z transformation. Insert a resistor in series with the input and load. When the voltage drop across the resistor is one half the input the resistor equals the input Z.

 So Zin is 500 ohms and Zout is 64 ohms.

If you need an output transformer for your next build it may be worth a try.

 You could use a 2N3906. I use  the 2N5401 because I have a drawer full of them. 

 What would happen if we change the base resistor?

 

 

One stage headphone Amp

Z matching vs neutralizing

3 stage amp with filter and feedback

This little 3 stage amp works really well in sim. R4 and R5 form a voltage divider for the degenerative feedback. They could be replaced with a pot or rheostat for volume control. C2 could feed  a low Z speaker or headset. The input PI network is a low pass filter which should keep RF out of the amp. 

* I built this one and it passes the finger buzz test.