Question Re: 2 FET follower use

Are those phasing dots right?

The 5.2V reading could be influenced by the voltmeter the OP used?

piglet

Reply to
piglet
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** Really ?

So the total input capacitance is under 0.5 pF ?

.... Phil

Reply to
Phil Allison
300 kohm is a ridiculously large impedance on a RF frequency.

Mike, how much capacitance corresponds to 300 kohm at 1 MHz?

It is practically impossible to get less than around 10 pF or more of stray capacitances.

--
-TV 


On 21.5.20 06:18, amdx wrote: 
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Reply to
Tauno Voipio

The antenna is a voltage source with a 330 ohm internal impedance, if I match that with a 330? to 50? transformer and connect it to a 50 ohm radio, I loss 1/2 of my voltage across the internal impedance, vs feeding the antenna into the high input impedance amp. Second, a 330? to 100? impedance transformer is a voltage step down of 1.8 to 1. The amp does the impedance conversion without any voltage step down. The amp gain is relative to the transformer step down.

Mikek

Reply to
amdx

About 0.5pf.

I don't know what that havoc conspires to do. Resonance, but...

Mikek

Reply to
amdx

I recall calculating the meter as a 10M? resistor in the circuit. But please, I posted that schematic with those numbers when I was trouble shooting the amp. I should have removed those voltages before I posted it. Mikek

Reply to
amdx

Ok, and I suspect it is mostly a capacitive load. My feeble attempt at measuring the input resistance was to insert a series resistor with the input, when that resistor was 330k? the output voltage dropped by 1/2. That's where I got my 330k? from.

Mikek

Reply to
amdx

The usual practice is to match an antenna to the impedance of the radio, in my case I'm matching the antenna to the 100? feed line and then I match the feed line to the radio with a 100? to 50? transformer. The amp does the impedance transformation without a voltage step down. Half of this project is to see how this amp works. The other half is getting an antenna up. I lost all my antennas in the hurricane 1-1/2 years ago.

S/N is why I put down the Beverage on the Ground, (BOG). I don't know the noise figure, I do trust that the designer is well aware that the first amp sets the noise figure and his design was with that in mind. I built 6 amps, if someone is interested in doing measurements, I would send them one.

I found a commercial version that is based on the Dallas Lankfords design. Has schematics, voltage readings and IP measurement method and specs.

Mikek

Reply to
amdx

Phil,

I found a commercial version that is based on Dallas Lankfords design. It has schematics, voltage readings and IP measurement method and specs.

Mikek

Reply to
amdx

I'm aware of dozens of antenna preamps I can build. I'm working with this one for now. I went to a lot of trouble building it into a relay switchable configuration, just so I can easily comparison testing. Mikek

Reply to
amdx

I'll be trying it with and without the resistor.

Mikek

Reply to
amdx

It spoils your impedance level by a factor of at least a decade.

--

-TV
Reply to
Tauno Voipio

How dis you determined that the antenna was 330 ohms ? Was it really the impedance or just the resistive part of the impedance ?

What is your antenna like ? Is it a resonant antenna with pure resistive Z=330 ohms (R=330 ohm, X=0). For instance a full size 1/2 wave folded dipole would produce 330 ohms (with slightly tweaking tube diameters) ? A transformer should work OK.

Or is it some electrically small highly reactive antenna with Z=330 (e.g. R=5, X=-329) ohms ? For instance a short whip on HF

Even worse, has the antenna resistance 330 ohms and the capacitive reactance tens of kilo-ohms and the complex impedance also in tens of kilo-ohms.

These kinds of short antennas are usually interfaced with a very high impedance but low capacitance preamplifier.

To make sense of your questions, one has to first know the antenna impedance (both resistance and reactance).

That suggests that the antenna is small relative to the wavelength and hence highly capacitive.

If the difference is only 6 dB this would indicate a power mach vs. voltage match situation.

??

If the antenna has a highly capacitively reactive impedance, any stray capacitance to ground will form a capacitive voltage divider, attenuating the signal. A step up transformer will have some extra stray capacitance, so the gain obtained by the turns ratio might be lost due to the stray capacitance.

If you really need more gain, put a voltage amplifier stage _after_ the impedance matching source follower. Do not mess up with the input side.

,
Reply to
upsidedown

That is a valid method at low frequencies, did you measure this at 1MHz or somewhere lower?

A garden variety 0.25W through hole resistor will have approx 0.5pF end to end capacitance without even thinking about wiring strays so that could have confounded your measurement. Hi-Z and RF do not mix easily.

piglet

Reply to
piglet

Perhaps there is no need since one pot will set the Id for both FETs?

piglet

Reply to
piglet

Yes, measured at 1 MHz.

Reply to
amdx

Here's the measurement I made, from 200kHz to 4MHz.

No, see graph.

It is a 260ft wire on the ground with a 340 ohm termination resistor. The Value of the termination resistor was chosen because it gave the flatest impedance curve as posted above. I'm not sure that's the optimum termination, there my be other important characteristics that may be affected by the termination resistance, like pattern, F/B.

Part of the project is to see how this amp works. I'm aware there are dozens of antenna preamp designs on line. I'm working with this one, until I'm not.

Mikek

Reply to
amdx

and a

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the purpose of this amplifier and the transformers you may or may not use?

sformer.

ty adequate unless there is a problem of noise pickup in the feed. The bat tle is typically S/N ratio rather than voltage. The first stage of amplifi cation needs to add as little noise as possible which means a design optimi zed for that, not gain. Even just 10dB of gain means noise added by any su bsequent stages will be minute in comparison.

some problem with the antenna system?

So why do you think you need to worry about the voltage from the impedance match? Have you tried it and found it lacking? As I mentioned, typically the enemy is S/N, not voltage.

Why not try a simple matching transformer and see what it does. Then if th e reception is not good you can return and consider low noise amps.

I just don't understand the reason for trying to fix a problem you don't kn ow you have. It was some time ago I realized the battle at the antenna is not as much about signal strength as it is about S/N.

--
  Rick C. 

  -- Get 1,000 miles of free Supercharging 
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Reply to
Ricketty C

Right. I have a matching transformer. The part sequence is 330? to

100? transformer and then a 100? to 50? transformer, with a 50 ohm termination resistor. I scanned 600kHz to 1600kHz. At 600kHz, R=360 ohms, X= +165 ohms, at 1600kHz, R=430 ohms X= +23 ohms. Could use some adjustment, but, I'm not sure how to correct the reactance. Do I need more turns with just a slightly adjustment to the turns ratio?

Now the surprise to me. I have the 330? to 6,250? ohm transformer driving the input to the FET amp. At 600kHz R=105 ohms X=-380 ohms, at 1600kHz R=14 ohms and X=-122ohms.

Now with a 6,250 ohm resistor across the amp input making the transformer match. At 600kHz R=201 X=-235 ohms at 1600kHz R=36 ohms X= -133 ohms.

What do I do to tame the input to the the amp?

Mikek

Reply to
amdx

As you like. But don't marry circuits because you designed them or built them.

John Larkin is right sometimes good designs appropriate for the job shouldn't fight you too hard. If they do it may be indicative one needs to come at the problem another way.

Reply to
bitrex

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