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Transform active whip antenna for 9.6v

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neazoi

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Hi I would like to transform this preamplifier https://dl1dbc.net/SAQ/Mwhip/Article_pa0rdt-Mini-Whip_English.pdf on page #14
In order to operate from 9.6-12v (8x1.2v rechargable or 8x1.5v alcaline).
Can you please help me what components do I need to change?

Also I am looking for an smd version or equivalent of the 2n5109. Which one should I try in this circuit?
 

biff44

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i would drop the value of the two 3k3 resistors to the point that the same DC voltage shows up at the transistor bases with battery supply as when the 12V supply was used. Probably to around 2.2K
 
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betwixt

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The 2N5109 is used in emitter follower configuration so it has no voltage gain and as it runs at relatively small current, little or no power gain. You should be able to use almost any small NPN transistor in that position.

Brian.
 
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neazoi

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i would drop the value of the two 3k3 resistors to the point that the same DC voltage shows up at the transistor bases with battery supply as when the 12V supply was used. Probably to around 2.2K

I am looking at the circuit at page 14 of the pdf !!!
This is the circuit I want to transform.

- - - Updated - - -

The 2N5109 is used in emitter follower configuration so it has no voltage gain and as it runs at relatively small current, little or no power gain. You should be able to use almost any small NPN transistor in that position.

Brian.

Even a 2n2222 (smd version)
I have seen that kind of amplifier using the 2n2222 instead. See this page https://www.chirio.com/mini_whip_e.htm
However all high dynamic range amplifiers I have seen insist on the 2n5109. Why is that?
 

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The only special characteristic of the 2N5109 is it's maximum continuous collector current rating of 400mA while still being good at high frequencies. The 2N3866 (which is available in SM packages) is very similar.

I have reservations about both designs, for one thing they have no static protection at the antenna. It would make more sense to place a pair of 'head to tail' diodes from the antenna input to ground and then capacitively couple to the FET. Performance would be unchanged but it would be far less prone to damage.

The 2N2222 should work fine, bear in mind the output impedance of the emitter follower in relation to the input impedance of the receiver. The 2N5109 design is better at driving low impedances such as 50 Ohms at the receiver antenna socket, but only because it is run at higher current. If you substituted the resistor values from the first design into the second one it would work better, even with a 2N2222 fitted.

Brian.
 
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neazoi

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The 2N2222 should work fine, bear in mind the output impedance of the emitter follower in relation to the input impedance of the receiver. The 2N5109 design is better at driving low impedances such as 50 Ohms at the receiver antenna socket, but only because it is run at higher current. If you substituted the resistor values from the first design into the second one it would work better, even with a 2N2222 fitted.

Brian.

Ok, I think I am goint to use the 2N3866 then as you suggested.
About the resistor values, you mean keeping all the resistor values from the pdf (first design) and just replace the 2n5109, or the opposite?
 

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I mean to use the 2n2222 or 2N3866 but make it pass more current than in the second design. The problem in the second design is the output impedance is relatively high so it will give a poor match to the low input impedance of most receivers and connecting cables. Ideally, the output impedance would match the receiver but it isn't practical in such a simple circuit. You can get nearer to a match by using a lower value emitter resistor but inevitably that means more current through the transistor.

Try using a 2N3866/2N2222 in the second stage using the circuit and values in the first design. You can experiment with the first stage to see which works best.

Brian.
 
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neazoi

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Try using a 2N3866/2N2222 in the second stage using the circuit and values in the first design. You can experiment with the first stage to see which works best.

Brian.

Ok I got it.
Do you believe the 2n4401 for the second stage would be ok as well? (mmtb4401lt1s).
I am asking only because I have at hand.

Also if I use anti-parallel diodes for the gate protection and a series gate capacitor, what value of capacitor should I use for 100KHz to 30MHz? !00nF?220nf?570nf?...

Finally, for 9.6v of operation is it enough to calculate the second stage base resistors to have the same boltage at the base, as if it operated on 12v?
 

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MMTB4401 should be fine.

100nF should be OK, the value isn't critical. It forms a potential divider with the input impedance of the JFET and gate resistors in parallel as the 'bottom' resistor and Xc as the 'top' resistor. Given that the JFET side has very high impedance and at the lowest frequency of 100KHz, Xc of a 100nF capacitor is less than 0.02 Ohms, it will be almost a short circuit to the RF.

The current is not critical, you can if you wish recalculate the values but for such a small change in supply you can probably just substitute the next lower standard value for the bias feed.

Brian.
 
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SunnySkyguy

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The problem with active FET input is ESD prone to failure and low capacitance diode caps add noise. This why RF transformers were used on front ends for wide band reception over 2 decades. Loop antenna shunts ESD risks but picks up stray leakage currents in band from triacs or FL ballasts or other H field noise.

That being said I used FET whip for VLF in 1975 in the arctic and it worked for a while then stopped. It was mounted on spring for Polar bear mechanical resistance but not lightning protected. :( oh well live and learn
 

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