'corse, from this "troublemaker", on VHF, (and granted, the noise figure
of a reciever , or preamp is less compromised by swr, THE FURTHER from
the SOURCE(antenna), because of the loss's in that line (great dummy
load for 432: 200 foot of rg-58u, don't even terminate it! If a Bird
reads ANY swr,meter has a problem (because of the loss)! But, then look
at the loss's from the standpoint of a (Scatter, Moonbounce, Long
Distance VHF (ect)) Operator, trying for the "HOLY GRAIL" of a
BI-DIRECTIONAL 20 + dB gain, noise figure of less than , say 1.3 dB (sky
noise), and a KW ,+ Minimum FEED LOSS'S, on 2 meters to hear your own
echos. When at THAT point, and keep in mind:
1) that when stacking antennas, the MAXIMUM (maybe you know different)
GAIN accomplished on a bay, is 3 dB , for 2 antennas, 6dB, for 4
antennas,ect., 2) that The Reciever front end, Maximized for BEST NOISE
FIGURE, is adjusted to the that point, by intentionally MISADJUSTING the
front end impedence, to obtain THAT optimum point,at 50 OHMS!
and 3) that anything that is misadjusted, to add ANY LOSS's to the
system means the difference (because bad stacking distances, mis- fed
coax(out of phase), change in the front end impedence of the LNA, ect.)
means the difference between sucess,or failure!! Perhaps was wrong on
initial assumption that swr was bi-directional, but doesn't negate the
original premise that the swr has no effect on recieve-- and, btw, will
the stacking actually provide THAT 3dB?? (before, or after the added 3:1
mismatch)?? Yours for comment?? Jim NN7K


Richard Clark wrote:
On Sun, 15 Aug 2004 23:57:02 GMT, "Jerry Martes"
wrote:


It is my understanding that the transmission line
loss isnt increased excessively when the line loss is under about 2 DB and
the VSWR is as high as 3:1, and that S-meter readings arent measureably
degraded when the receiver sees these signals thats not coming in with a 50
ohm internal impedance.


Hi Jerry,

It seems your question isn't going to be answered except to three
decimal places.

You are right, no one will notice much difference to mismatches such
as you describe. I know that your interest is in satellite plots of
weather conditions. You may experience some drop out - snow in the
picture. However this would be for marginal signals, and I am sure
that the uncorrelated noise would only slightly degrade the contrast
or detail.

I've played with WEFAX over HF to worse conditions and those pictures
came out quite readable.

73's
Richard Clark, KB7QHC

On Mon, 16 Aug 2004 02:09:49 GMT, Jim - NN7K
wrote:

Perhaps was wrong on
initial assumption that swr was bi-directional, but doesn't negate the
original premise that the swr has no effect on recieve-- and, btw, will
the stacking actually provide THAT 3dB?? (before, or after the added 3:1
mismatch)?? Yours for comment?? Jim NN7K


Hi Jim,

My experience in the very short wavelengths is confined to RADAR. I
have not pursued satellite nor EME. RADAR comes with its own
compensations in that if you have one, you can afford to do it right
the first time (I pine for the day when the FCC allows Amateur RADAR
operation).

As for Transmit/Receive, they are so intimately wed, that it is
sometimes difficult to separate them and judge their needs on their
own merits. A Receiver doesn't need to have an input Z of 50 Ohms,
but given that the Receiver of a Transceiver shares the same path ways
of the transmitter, it is foolish to go a different direction. Why
would you put a 300 Ohm first RF stage after a filter designed for 50
Ohms? A 6:1 SWR from the get-go is simply stupid when you can do it
right with so little effort.

I've seen some discussion that it doesn't matter because front ends
only take voltage and need no current. This is a 0Hz analysis and at
10MHz is thoroughly dead in the water. Stray capacitance negates any
claims to an input being Hi-Z and the whole point of low Z inputs is
to swamp nature's capacity to send your signal straight to ground
before it sees that amplifier.

For the mild SWRs such as described by Jerry, most receivers have a
lot of head room (capacity) to amplify what makes its way in. The
only down-side is degrading S+N/N ratio for very small signals where
this capacity fails to make up for information loss.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
I've seen some discussion that it doesn't matter because front ends
only take voltage and need no current. This is a 0Hz analysis and at
10MHz is thoroughly dead in the water.

Once again, what I said has been thoroughly misquoted.

Stray capacitance negates any claims to an input being Hi-Z and the
whole point of low Z inputs is to swamp nature's capacity to send your
signal straight to ground before it sees that amplifier.

Now that analysis really *is* dead in the water!

My simplification to "the amplifier takes what it needs from a 50-ohm
source" is just that - a simplification. But it is based on actually
knowing something about the subject. If you wish to discuss input
network design for FET RF stages in terms of Smith-chart circles of
constant gain and noise figure, and the device manufacturer's quoted
data for gamma-opt, then I'm willing and able.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Mon, 16 Aug 2004 23:06:43 +0100, "Ian White, G3SEK"
wrote:

Richard Clark wrote:
I've seen some discussion that it doesn't matter because front ends
only take voltage and need no current. This is a 0Hz analysis and at
10MHz is thoroughly dead in the water.

Once again, what I said has been thoroughly misquoted.

Hi Ian,

If it was you that said it, otherwise you are misquoting me.

My simplification to "the amplifier takes what it needs from a 50-ohm
source" is just that - a simplification.

Ah yes, you are misquoting me.

73's
Richard Clark, KB7QHC