"Dr. Slick" wrote in message
om...
Hello again,

Well, surprise, surprise...the thread deviates again! Hehe.

If i may add to the discussion:

By "reciprocity", a transmit antenna can also be used as
a receive antenna, assuming you want the same polar pattern.

A mismatch on the receive side will adversely affect the signal
to noise ratio, or would increase the noise factor of the system (which
is why you always want your low noise amplifiers as close to the
receive antenna as possible, usually mast-mounted, to avoid the
losses of a long coax).

However, at the very least, the mismatch on the receive side
will not result in catastrophic destruction of your output transistors,
which is what a mismatch on the output of a transmitter can result in.

So one mismatch is a bit more serious than the other.


Slick

Slick

I'd submit that, in a practical situation, the mismatch of an antenna for
receiving can be as high as 2:1 or even 3:1 without degrading the strength
of the received signal when the transmission line losses are low. I
thought that, if the receiver is tuneable, the actual impedance the
transmission line presents to the receiver can be 'accounted for. I
thought that, for a given antenna and transmission line, the effects of VSWR
are less important that for delivering power by a transmitter.
Am I wrong when I consider VSWR to be less important for receivers than
for transmitters?

Jerry

Jerry Martes wrote:
Am I wrong when I consider VSWR to be less important for receivers than
for transmitters?

Depends upon the source of the noise. My 40m vertical couldn't
copy stations that gave me an RST of 559.



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Jerry Martes wrote:

I'd submit that, in a practical situation, the mismatch of an antenna for
receiving can be as high as 2:1 or even 3:1 without degrading the strength
of the received signal when the transmission line losses are low. I
thought that, if the receiver is tuneable, the actual impedance the
transmission line presents to the receiver can be 'accounted for. I
thought that, for a given antenna and transmission line, the effects of VSWR
are less important that for delivering power by a transmitter.
Am I wrong when I consider VSWR to be less important for receivers than
for transmitters?

Jerry

It sounds like you (and some other posters) might be confusing the
transmission line SWR with the impedance seen by the
transmitter/receiver, which is often indicated with an SWR meter. The
two aren't the same.

Consider, for example, a 50 ohm antenna and 50 ohm tx/rx, with a 300 ohm
half wavelength transmission line connecting the two. The antenna sees a
perfect match (50 ohms) when receiving, and the transmitter sees a
perfect match (50 ohms) when transmitting. The transmission line SWR is
6:1 when transmitting and receiving. On the other hand, if the antenna
and transmission line are both 300 ohms (+ j0), the line SWR will be 1:1
when transmitting, 6:1 when receiving. And so forth.

The effects of impedance mismatch seen by the transmitter when
transmitting, the impedance mismatch seen by the antenna when receiving,
and the transmission line SWR are three separate issues. Each has its
own effect on system performance, and each needs to be treated
separately. The importance of one or the other depends on the individual
situation.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Jerry Martes wrote:

I'd submit that, in a practical situation, the mismatch of an antenna
for
receiving can be as high as 2:1 or even 3:1 without degrading the
strength
of the received signal when the transmission line losses are low. I
thought that, if the receiver is tuneable, the actual impedance the
transmission line presents to the receiver can be 'accounted for. I
thought that, for a given antenna and transmission line, the effects of
VSWR
are less important that for delivering power by a transmitter.
Am I wrong when I consider VSWR to be less important for receivers
than
for transmitters?

Jerry

It sounds like you (and some other posters) might be confusing the
transmission line SWR with the impedance seen by the
transmitter/receiver, which is often indicated with an SWR meter. The
two aren't the same.

Consider, for example, a 50 ohm antenna and 50 ohm tx/rx, with a 300 ohm
half wavelength transmission line connecting the two. The antenna sees a
perfect match (50 ohms) when receiving, and the transmitter sees a
perfect match (50 ohms) when transmitting. The transmission line SWR is
6:1 when transmitting and receiving. On the other hand, if the antenna
and transmission line are both 300 ohms (+ j0), the line SWR will be 1:1
when transmitting, 6:1 when receiving. And so forth.

The effects of impedance mismatch seen by the transmitter when
transmitting, the impedance mismatch seen by the antenna when receiving,
and the transmission line SWR are three separate issues. Each has its
own effect on system performance, and each needs to be treated
separately. The importance of one or the other depends on the individual
situation.

Roy Lewallen, W7EL

Roy

It might be that I'm wrong with my conclusion that when receiving, the
loss of received signal wont be appreciably worsened by VSWRs as high as,
say, 3:1, when the line atenuation is under a couple DB. But I dont think
I've been confused about the Line to Load Mismatch and VSWR.

I recognize that the lowest VSWR and lowest line loss and the lowest noise
figure are all important.

Since I'm always working with systems that arent perfect, I dont get
concerned with low VSWR when working with receivers except when there is
need for phasing arrays. 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.

It occurred to me that the original poster of this "Phasing Harness" might
have some sophisticated need for phase or low mismatch. If the two antennas
being harnessed are only to maximize radiation toward the horizon, I'd
consider the question a simple one to answer. ie Feed them in phase with
any convenient equal lengths of line. If bandwidth is a factor, expect
sidelobes to vary with frequency.

But, my knowledge and experience is very limited, so any 'redirection' of
my thinking is appreciated.

Jerry

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

'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

Jerry Martes wrote:

It might be that I'm wrong with my conclusion that when receiving, the
loss of received signal wont be appreciably worsened by VSWRs as high as,
say, 3:1, when the line atenuation is under a couple DB. But I dont think
I've been confused about the Line to Load Mismatch and VSWR.

As long as we make sure the readers understand we're talking about the
actual SWR on the transmission line, that's true for both transmitting
and receiving.

I recognize that the lowest VSWR and lowest line loss and the lowest noise
figure are all important.

If the VSWR and (matched) line loss are roughly within the bounds you
mentioned, the transmission line VSWR isn't really important, since the
extra loss it causes is generally negligible. However, as I somewhat
indirectly mentioned earlier, the match between the antenna and its load
determine the amount of signal that gets to the receiver. And as Ian has
mentioned, the receiver noise figure might be best when the receiver is
mismatched to the source impedance it sees. Further qualifying the
matter is the fact that at HF, losing signal when receiving due to
mismatch, line loss or SWR doesn't impact the signal/noise ratio unless
the attenuation is very great, since the dominant atmospheric noise is
attenuated by the same factor. I do realize, though, that the original
posting was directed toward VHF or above, where those things do matter.

Since I'm always working with systems that arent perfect, I dont get
concerned with low VSWR when working with receivers except when there is
need for phasing arrays. 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.

It occurred to me that the original poster of this "Phasing Harness" might
have some sophisticated need for phase or low mismatch. If the two antennas
being harnessed are only to maximize radiation toward the horizon, I'd
consider the question a simple one to answer. ie Feed them in phase with
any convenient equal lengths of line. If bandwidth is a factor, expect
sidelobes to vary with frequency.

But, my knowledge and experience is very limited, so any 'redirection' of
my thinking is appreciated.

Your thinking looks fine to me.

Roy Lewallen, W7EL

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