[ARC5] Receiver input impedance...

[J Mcvey via ARC5]

Ok, I took a look at the circuit and I see that at resonance , it a direct shot into the grid of the RF amp in parallel with whatever the max impedance of the L-C happens to be. So, yeah, it's going to be a lot higher than 12 ohms!

Another person was talking about transformed impedance ( can't find the darn post!!) and I wanted point out a misconception. 

If the antenna has the SERIES model of say, 12-j50, the Z is going to be about 52 ohms capacitive. While the PARALLEL equivalent resistor does indeed increase to 220 and the cap stays close to the original value at 53,
the impedance is STILL 52 ohms capacitive either way.  That's what they call it and "equivalent circuit".
This is another can of worms which really doesn't  have too much to do with the receiver matching itself, but I didn't want to let it confuse things further.

The parallel rf grid circuit , consisting of the Grid resistance in parallel with the L-C tank at resonance boils down to 


1/Z= 1/Xl  + 1/Xc + 1/Rg
If you know the value of the cap reactance  at f0, the Xl value is the same. Plug them in the formula and that is what the impedance is looking into the the antenna terminal. 




On Monday, October 20, 2014 3:38 PM, Glen Zook via ARC5 <arc5 at mailman.qth.net> wrote:
 


I have found that matching the 50-ohm coaxial cable to receivers, that have the old 3-terminal antenna/ground connections, definitely helps the sensitivity.  The Collins 75A-1 is one receiver that matching the impedance definitely makes a difference in apparent sensitivity.

I am going to try a TV balun on a BC-454 and BC-455 later today.  Since I do have several service monitors, with calibrated attenuators, I can see just how much difference is made in the LDS / MDS.

Glen, K9STH 

Website:  http://k9sth.net


On Monday, October 20, 2014 2:30 PM, "Fuqua, Bill L" <wlfuqu00 at uky.edu> wrote:



  Generally, impedance matching is not very useful at below 30 MHz, especially below 10 MHz due to the atmospheric noise. Any signal you can receive must be above the atmospheric noise.
That is why ferrite antennas are just fine for AM broadcast receivers, but useless for transmission of signals. Reciprocity still applies it is just that you don't need much sensitivity at low frequencies.
73
Bill wa4lav

________________________________________
From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Kenneth G. Gordon [kgordon2006 at frontier.com]
Sent: Monday, October 20, 2014 11:49 AM
To: ARC5
Subject: Re: [ARC5] Receiver input impedance...

On 20 Oct 2014 at 8:08, J Mcvey wrote:

> The ARC-5 system was designed for low-Z (12 ohm?), short capacitive
> antennas. So, in this case, the low Z side would be at the receiver terminals?

Yes. Those are correct, yet the receiver input-impedance IS about 4K ohms.

Kinda like a VTVM with an 11 megohm input impedance being used to
measure a battery's voltage.

Probably this was done in this case so that one antenna could be connected
in parallel to mulitple receivers which all tune to different frequencies (which
was done in the original setup) without causing any interaction or signal loss.

Possibly, the input circuitry was done the way it was so that it inherently is
already a type of impedance transformer.

As per the discussion here, I am coming to the conclusion that matching that
impedance is not really necessary, nor particularly useful or helpful, and it
appears, at this point, that doing so is hardly worth the effort.

To satisfy my curiousity, I MAY build an 80:1 Un-Un and take some
measurements....just to see...but at this point, I would not expect there to be
a noticeable improvement.

Ken W7EKB
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html