[ARC5] Receiver input impedance...

Tue Oct 21 00:39:11 EDT 2014

The effective resistance of the tank at resonance will be
(Q * wL) or  (Q /wC). So with a 150pF tuning capacitor at
half-mesh, and a Q of 25 or so, you are looking at something
like 12.5k ohm. There is an 11pF capacitor between the
antenna terminal and the tuned circuit. Don't forget to add
(1/x + 1/y) this into the series capacitance of the antenna
circuit before doing the series-parallel transformation...

In practice I imagine that the idea was to provide a relatively
low-loss input connection without loading any of the antennas
excessively.

73, ian K3IMW

On Mon, Oct 20, 2014 at 8:22 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net>
wrote:

> 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
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