I'm trying to build a Mini Whip active antenna that receives 10KHz to 20MHz. But I'm having trouble finding the right components especially the transistors.

It calls for a J310 and a 2N5109, which are both hard to find.

When designing an antenna circuit what exactly is it you need to look for in a transistor? High frequency response? How does one go about choosing a suitable replacement?

I've tried using www.alltransistors.com to find similar transistors, but I'm not too sure what to compare. Obviously: polarity, max power dissipation, max voltages, maybe transition frequency? maybe the noise figure?

Other people have substituted these for a MPF102 and 2N3904 or BF245C and 2N2219A

I've also found other people using these as replacement, but I'm having trouble sourcing these too: BF245A, BF245B, BF245C, BF246 A/B/C, MPF102

I guess that these transistors are all very similar but subtly different, the problem for somebody out of their depth like me is knowing if it matters or not, if I use the wrong one will I end up with a terrible antenna?

On a similar note what being an RF circuit do you think its necceasary to have tighter tolerances on the resistors too? I was thinking about choosing +/-1% resistors over +/- 5% resistors?

Many thanks!

Scroll down the pdf for circuit diagram: http://myweb.tiscali.co.uk/wgtaylor/PA0RDT-Mini-Whip.pdf


N channel Jfets are all similar ,substitution should be easy for your non critical 20MHz job.You dont have to worry about smithcharts or anything horrible.You may want to change the source resistor value because drain current varies a lot between different part numbers and within a batch of the same part number anyway . This variability issue is why JFETs are seen more in hobby stuff and less in professional stuff . On your frequencies of interest atmospheric noise is high so some exotic low noise device wont show any discernable improvement . So just get a local equivalent .I got 100 MPF102 from a local component supplier a few months ago for 35cents each .


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