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A few ideas from K.P.S. Kang's blog of 2017

Brian L. Davis
 

Jerry Gaffke
 

That Kang T/R switch is interesting, gets around the need for a PIN diode.

As in the stock rig, pin 2 of T7 drives the LPF and the antenna via two 0.1uF caps in parallel, or 0.2uF.
At 7mhz, the impedance of the two 0.1uF caps is  1/(2*pi*freq*farads) = 1/(2*3.14*7e6*0.2e-6) = 0.1 ohms,
so 500 times less impedance than the 50 ohm impedance of the transmit LPF, and thus of no consequence..

During receive, he drives the base of Q1 through 47+4.7 pF of capacitance, or about 50pF,
At 7mhz, an impedance of  1/(2*pi*7e6*50e-12) = 455 ohms.
That 50pF is still there during transmit, so about 10% of the transmit energy is getting shorted to ground through
that 455 ohms and the two back-to-back diodes (the diodes prevent Q1 from blowing out on transmit).
Also, the 455 ohms means that the receiver gets a little bit less signal than on a stock rig
So extremely weak signals might be harder to hear, but will have less distortion receiving very strong signals.
So the 47+4.7pF caps are chosen to be a good compromise between transmit and receive performance at 7mhz,
I'm OK with losing 10% of my transmit power..

On the uBitx we want this to work from 3.5 to 30mhz.
Perhaps could do this by mechanically switching in different values of capacitance to replace that 47+4.7pF
depending on the operating frequency.  Or go with one of the 1n4007 PIN diode T/R switch designs.

Jerry, KE7ER

Sandeep Lohia
 


Diode switching was my idea to incorporate ;)

more we discuss here :

Jerry Gaffke
 

Perhaps we'd get 3.5 to 30mhz operation by replacing Kang's 47+4.7pF capacitors 
with a 1000pf cap and a 470 ohm resistor in series?
Anybody see a problem with that?
Also, not obvious to me why he bothered to put 4.7pF in parallel with the 47pF cap. 
 


On Tue, Aug 6, 2019 at 10:50 AM, Jerry Gaffke wrote:
On the uBitx we want this to work from 3.5 to 30mhz.
Perhaps could do this by mechanically switching in different values of capacitance to replace that 47+4.7pF
depending on the operating frequency.  Or go with one of the 1n4007 PIN diode T/R switch designs.

K. P. S. Kang
 

Dear OM Jerry,
10uH and 52pf form a series resonant circuit that offers minimum impedance on 7MHz during recieve.

Jerry Gaffke
 

Thanks for responding!
Very clever, during transmit the path to the diodes is not resonant.
For some reason, I didn't notice the inductor.

Jerry, KE7ER


On Tue, Aug 6, 2019 at 09:53 PM, K. P. S. Kang wrote:
Dear OM Jerry,
10uH and 52pf form a series resonant circuit that offers minimum impedance on 7MHz during recieve.