"Roy Lewallen" wrote
Oops. I meant, WHEN RECEIVING, fiddling with the antenna or the
antenna/feedline match won't have any effect at all on the line SWR. It
will, of course, change the line SWR when transmitting.
_____________

However that doesn't mean that it's unimportant to match the feedpoint
impedance of a rx antenna to the feedline connected there.

A rx antenna with a mismatched feedline will not deliver to the rx end of
the feedline the maximum possible energy available from the fields in which
that antenna is immersed. Whatever received power that is reflected by a
mismatch at the rx antenna feedpoint is re-radiated (less losses).

RF

Richard Fry wrote:
"Roy Lewallen" wrote
Oops. I meant, WHEN RECEIVING, fiddling with the antenna or the
antenna/feedline match won't have any effect at all on the line SWR. It
will, of course, change the line SWR when transmitting.
_____________

However that doesn't mean that it's unimportant to match the feedpoint
impedance of a rx antenna to the feedline connected there.

A rx antenna with a mismatched feedline will not deliver to the rx end of
the feedline the maximum possible energy available from the fields in which
that antenna is immersed. Whatever received power that is reflected by a
mismatch at the rx antenna feedpoint is re-radiated (less losses).

What both Roy and Richard say is correct in principle, but may be
missing the point about what a receiver needs.

What's usually important is to present the RX input with the *source*
impedance it was designed for. (Most often this is 50 ohms, and let's
also assume 50-ohm line for the rest of this discussion.)

Likewise the transmitter needs to be presented with a 50-ohm load
impedance, so those two requirements coincide.

In order to achieve a 50-ohm load impedance for the transmitter, and a
50-ohm source impedance for the receiver, the antenna itself must be
matched to 50 ohms - so that's your design aim.

Now when Richard says:
Whatever received power that is reflected by a
mismatch at the rx antenna feedpoint is re-radiated (less losses),

that is true in principle, but more important is that if any energy is
reflected from the receiver input, that is perfectly OK - that energy
was "not wanted" by the receiver. The receiver *does* want a 50-ohm
source, but it only takes what it needs from that source. For example, a
simple tuned-gate FET amplifier only needs a voltage swing at the input
- it doesn't need current as well, so most of the incident power is
reflected. That type of situation is very common in receiver design, and
completely OK.

It is a myth that a receiver input is not optimized unless it presents a
50-ohm load. What it does need is a 50-ohm source impedance. The design
details about input reflection coefficient are much more complex, but
the underlying principle is simply "The RX input takes whatever it needs
from a 50-ohm source, and reflects the rest."

RX inputs *can* be designed to present a 50-ohm load impedance, even
with FETs, but this requires special design techniques that generally
involve feedback. It is usually done when some other device has to be
inserted between the feedline and the RX input, eg a filter which
requires a 50-ohm load impedance. But that device probably requires a
50-ohm source impedance too, so you still have the same requirement for
the antenna to be matched to the feedline.


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

"Ian White, G3SEK" wrote
... the underlying principle is simply "The RX input takes
whatever it needs from a 50-ohm source, and reflects the rest."
______

The above statement might be read as though there is a threshold voltage
limit above which a receiver will not deliver improved performance.

A receiver amplifies and detects whatever voltage is present at its input
terminals, if just thermal noise. Even if no current flows in the input
stage device itself, the wanted signal voltage present there should be as
high as possible above the thermal noise voltage in order to maximise SNR.

RF

Richard Fry wrote:
"Ian White, G3SEK" wrote
... the underlying principle is simply "The RX input takes
whatever it needs from a 50-ohm source, and reflects the rest."
______

The above statement might be read as though there is a threshold voltage
limit above which a receiver will not deliver improved performance.


If you truly believe that danger exists, then please don't make it worse
by quoting my statement out of the context in which it was made.


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

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

"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