[ARC5] Receiver input impedance...

[Mike Hanz]

Ken, the "practical" Bible on antenna matching issues for receivers is 
the Radiotron Designers Handbook Fourth Edition, pages 912-922.  I say 
practical because the author not only includes theoretical aspects but 
also plenty of pragmatic advice.  One thing to remember is that you are 
complicating an already complex design in that reference by adding 
*another* impedance matching device to the one that is already in the 
receiver.  Add the variability of the antenna across its frequency range 
and you would need a Cray supercomputer to simply define the 
characteristics of every added circuit element across the bandwidth of 
the receiver.  It's more doable for a narrow bandwidth, but not so easy 
when you're trying to provide a match for a band switching receiver.

Since you are apparently trying to match a 50 ohm line (which will not 
be a constant 50 ohms across the entire frequency band of interest 
unless the antenna is dynamically conditioned to that impedance as you 
tune around, BTW), I might suggest trying a low gain FET preselector as 
an impedance matching device.  There are a number of good designs lying 
around - the RSGB Handbooks have excellent examples.

Having said that, I suspect any increase in S/R ratio due simply to 
impedance matching is going to be disappointing...too many other 
variables in the soup at your particular QTH.

73,
Mike

On 10/19/2014 12:26 PM, Kenneth G. Gordon wrote:
> I have wondered about how much of an improvement in receiver operation
> could be achieved by adding an Un-Un to the antenna input.
>
> I've always felt that "matching" impedances is the correct thing to do in every
> case.
>
> Doing some simple calculations, it appears that with the 12SK7 RF amp, the
> input impedance is about 4K ohms, based on the grid-to-cathode
> impedance.
>
> Thus, it seems to me that an 80:1 Un-Un might improve matters somewhat,
> perhaps by improving the S/N ratio, especially in a noisy environment....like
> mine.
>
> Has anyone here tried this? If so, how did you build an effective 80:1 Un-Un?
> What core did you use? A toroid? Which one? How many turns primary and
> secondary? How did you calculate these? Is there an "on-line" calculator
> which could help? Do you think it would be worth the effort?
>
> Etc?
>
> Enquiring minds need to know.
>
> I am thinking that this might be applicable to my other Boat Anchor
> receivers...
>
> Ken W7EKB
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