[ARC5] Receiver input impedance...

[Dennis Monticelli]

Ken,

If by noisey environment you mean the annoying stuff we urban dwellers have
to put up with, improving the power transfer from antenna to receiver won't
matter as long as the noise coming off the antenna is dominant over the
receiver noise (the usual case below 20M).  You would need to address the
local noise via a separate receiving antenna such as a small shielded loop
or other aeriel type that rejects the vertically polarized local fields.

If you do wish to improve the power transfer to the receiver anyway (for
best instrinsic S/N) then why not just use a simple L network?  A low Q L
network has low loss.  If you're dealing with a single band there is no
need to go to the effort of making a broadband transformer.  Should you
elect make a broadband transformer you should be able to achieve <1 db loss
with a proper design.  Just as with your transmission line, every dB of
input network loss increases the S/N ratio oft the receiver but does
not degrade the overall S/N ratio if antenna noise is dominant.

BTW, I don't bother to shoot for a match to my command sets because my
local noise dominates.  I do have a well shielded mag loop that I
will employ from time to time on 80M or 160M, especially with the main
transceiver in the shack.  It helps but doesn't stop my cursing the urban
QRN.

 Dennis AE6C
On Sun, Oct 19, 2014 at 9:26 AM, Kenneth G. Gordon <kgordon2006 at frontier.com
> 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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