On Wed, Mar 12, 2025 at 05:28 AM, hisami dejima wrote:
Hi All,
?
I found an interesting site, there a archive of the defunct "ham radio" magazine.
?
I have been reading this article intensively for the past few days.
"active antenna preamplifiers by R.W.Burhans, ham radio May 1986" pp.47-54
?
There are things I can't understand.
Please someone tell me what they are?
About "inductive feedback and input capacitance"
that says "A circuit that can reduce the input capacitance and improve the linearity for MOSFETs involves a noiseless feedback method" and "with the FET input capacitance reduced to pratical levels comparable to ..."
Does this mean that the inductance and capacitance cancel each other out?
Or does the NFB current or voltage physically reduce capacitance?
?
I try to explain it in few words. Yes, inductance and capacitance are canceling each out, this is the fact how the resonant circuits works. In old times, when the tubes have a lots of internal capacitance and feedback, the neutralisation was used and it was based on this principle too. How this work in active circuit is a bit complicated, you have to simulate the circuit and see what happens there. The negative feedback decreases the gain, but? can improve input impedance as well.
?
?
About "active notch"?
I have tried to the BC band notch between the Probe and the Amplifier or between the Small Loop and the Amplifier, but it does not work well.
It seems to be difficult with High-Z or Low-Z line.
Putting it in the ?NFB line looks like a good idea.
Have you tried it? Did you get good results?
?
The graph in the magazine answers clearly to that. Without the feedback, the reflective behavior of the BC bandstop is presenting high impedance on the input, which leads to the peak in gain and sometimes even to instability. The feedback can prevent such problems.
?
?
Actually, the MiniWhip I made at the end of last year looks exactly like fig.2 circuit and works well for VLF.
it running at?
?
and I hope to improve it.
I have no electrical or physics education (other than Ham radio), so please forgive my naive question.
?
73, Hisami 7L4IOU
?
73s, OK1VAW, Vojtech.
?
?
