vswr on receive-only antenna
Daniel Becker
heard of doing vswr tests on a receive-only antenna? Seems to me in theory,
a receive-only antenna should have as good an impedence
match as a transmit or duplex antenna, and a low
vswr would indicate that. but I dont recall ever seeing anyone do a vswr test
on a receive-only antenna as a means of testing its performance. Depending on
the
antenna construction, and depending on the amount of forward power applied to
the
antenna, you could burn it out. but is this really practical or useful, and what
gotchas are waiting?
--
Dan Becker
dcbe...@lucent.com
W6RCecilA
Most receivers have a large enough dynamic range to handle losses in an HF
receive antenna system. What usually matters on receive is the S/N ratio
and losses don't appreciably affect that ratio. Bottom line: Most people
don't worry about losses in an HF receiving antenna.
--
73, Cecil, W6RCA http://www.bigfoot.com/~w6rca
Richard Harrison
Seems to me in theory, a receive-only antenna should have as good an
indicate that---
You are correct. Radiation resistance is a transmitter load impedance
and it is a receiver source imprdance. You need a matched impedance for
maximum power transfer between the antenna and the receiver, just as you
would between a transmitter and an antenna.
Only when the receiver matches the receiving antenna can you get as much
as 50% of the power the antenna intercepts. This is the very best you
can do. In the case of a perfect match, 50% of the received energy is
absorbed by the source resistance (the radiation resistance), i.e. it is
re-radiated.
In the case of a mis-matched antenna and receiver, more than 50% of the
received energy is bounced by the receiver input, to be re-radiated or
absorbed by losses.
Not much concern is spent on receiver matching at h-f because signal to
external noise ratio usually determines what the receiver can detect.
Various losses often take the same toll on noise as they do on signal,
and a receiver with low internal noise isn`t bothered by a somewhat
weaker signal input. The external noise is that much weaker too. There
is usually ample receiver gain.
Best regards, Richard Harrison, KB5WZI
Gray Frierson Haertig
W6RCecilA wrote:
>
> Most receivers have a large enough dynamic range to handle losses in an HF
> receive antenna system. What usually matters on receive is the S/N ratio
> and losses don't appreciably affect that ratio. Bottom line: Most people
> don't worry about losses in an HF receiving antenna.
> --
> 73, Cecil, W6RCA http://www.bigfoot.com/~w6rca
Gonna have to disagree with you there, Cecil. A lot of people worry
about receive antenna impedance matching. All the antenna tuners and
matchers in the Grove, Crane, Universal Radio etc catalogs is proof
enough of that.
Of course worrying about it is foolish, but that never stopped them. (c:
You should hang out over in rec.radio.shortwave for a while, to see
where the masses stand on matching HF receive antennas. Even the
sainted Yuri took exception to my pointing the foolishness of this out
to a fellow.
Gray
--
Telecommunications Engineering
Gray Frierson Haertig & Assoc.
820 North River Street, Suite 100
Portland, Oregon 97227
503-282-2989
503-282-3181 FAX
g...@haertig.com
Robert Lay (W9DMK)
<dcbe...@lucent.com> wrote:
>Not sure if this falls under silly question category or not. has anyone ever
>heard of doing vswr tests on a receive-only antenna? Seems to me in theory,
>a receive-only antenna should have as good an impedence
>match as a transmit or duplex antenna, and a low
>vswr would indicate that. but I dont recall ever seeing anyone do a vswr test
>on a receive-only antenna as a means of testing its performance. Depending on
>the
>antenna construction, and depending on the amount of forward power applied to
>the
>antenna, you could burn it out. but is this really practical or useful, and what
>gotchas are waiting?
The problem here is that VSWR is meaningless in regard to an antenna.
The term VSWR is only meaningful on the transmission line. If the
impedance matching between the antenna and receiver is optimal and if
the impedance matching between the antenna and the transmission line
is optimal, then I would think that the S/N ratio may be slightly
improved.
If your operating environment requires the best possible S/N ratio in
terms of KTB noise, then perhaps all of this would be important. If
you are worried about S/N in a typical QRN environment, as is typical
in HF work, then I doubt that you can make any general statements
about any advantages of matching, since the antenna's relative
sensitivities to signal and QRN is going to be more related to such
things as polarization and pattern. Does that make sense?
73 de Bob, W9DMK, King George, VA
Change "nobody" to "w9dmk" for e-mail
http://www.qsl.net/w9dmk
http://www.fortunecity.com/meltingpot/albania/784
py...@my-deja.com
> Gonna have to disagree with you there, Cecil. A lot of people worry
> about receive antenna impedance matching. All the antenna tuners and
> matchers in the Grove, Crane, Universal Radio etc catalogs is proof
> enough of that.
>
> Of course worrying about it is foolish, but that never stopped them.
(c:
>
> You should hang out over in rec.radio.shortwave for a while, to see
> where the masses stand on matching HF receive antennas. Even the
> sainted Yuri took exception to my pointing the foolishness of this out
> to a fellow.
>
> Gray
Well, we are dealing with antenna X line match and NOT with line X radio
match. It is obvious that a coupler will perform the latter job but not
the former one.
Well, as Cecil wrote, HF antenna X line coupling is not important
because the S/N ratio is practically independent of that match (the main
noise on HF, specially at lower frequencies, is atmosferic, NOT the
receiver front-end one).
But on VHF/UHF and above it is not true. A 3 dB on the S/N can be the
difference between copy and do not copy a weak signal. So, it is a good
idea to have antennae and cables well matched for those bands.
>
> --
> Telecommunications Engineering
> Gray Frierson Haertig & Assoc.
> 820 North River Street, Suite 100
> Portland, Oregon 97227
> 503-282-2989
> 503-282-3181 FAX
> g...@haertig.com
>
--
Greetings,
Luiz - PY1LL
Sent via Deja.com http://www.deja.com/
Before you buy.
Daniel Becker
say I have an unattended repeater, and the system is no longer receiving. I want
to do some unattended diagnostics. (assume i still have dialup or some other
means of accessing controllers on the site.) would it make sense to do a vswr
test on
the receive antenna to see if it was damaged, or if something damaged the
transmission line from the receive antenna? If I could arrange to transmit up to
the site with a known
and consistent transmit power, and measure received signal strength, that still
doesnt
tell me if it is the antenna or the receive amplifiers.
--
Dan Becker
dcbe...@lucent.com
domin...@my-deja.com
TX the match is important. Matched loads improve power transfer. We match the
coax to dump power into the antenna, or vice versa in RX.
Mind you, the VSWR on an antenna is high, in many cases. The power is
radiated into the sky :-), and we model that as radiation resistance. Think
of the antenna as a matching network from the transmission line to the sky.
To prove this make a (poorly tuned for 7Mhz) random wire antenna, and listen
to a weak AM station. Use a receiver without auto tune. Note the strength,
which is weak. Tune the antenna with a noise bridge or a VSWR analyzer. Then
listen to the same station, it will be much better received. It worked for me
last night SWL on 40m wit a TS-520, and I am sure it will for you.
In article <381ed124...@news.crosslink.net>,
--
+++
atz
W6RCecilA
> To prove this make a (poorly tuned for 7Mhz) random wire antenna, and listen
> to a weak AM station. Use a receiver without auto tune. Note the strength,
> which is weak. Tune the antenna with a noise bridge or a VSWR analyzer. Then
> listen to the same station, it will be much better received. It worked for me
> last night SWL on 40m wit a TS-520, and I am sure it will for you.
But did the tuner appreciably improve the signal to noise ratio existing
at the antenna?
domin...@my-deja.com
HF band has 30db S/N and you have the dynamic range to get to that noise
floor, no improvement will be noticed. There's a common misconception with
many people about signal to noise and noise floors. The noise floor is the
key here, and matching will give you more signal (and of course it's
associated noise) to chew on.
If the receiver noise floor is higher than the noise signal received, then
you will get an improvement. If the noise at an antenna is -80dB, and you can
only read signals down to -60dB, more signal (max of +20dB) makes your
receiver read weak signals better.
In receivers like my trusty dusty 520, the noise floor is way too high and
matching has a dramatic improvement. In others I would postulate that
improving the power transfer would help in marginal reception conditions.
Besides, having those extra tuner dials gives you something else to tweak!
:-)
In article <381EFA92...@IEEE.org>,
--
Richard Harrison
I want to do some unattended diagnostics.
Replace the receiver with a transmitter and measure the field strength
to determine your receive antenna system condition.
Gray Frierson Haertig
Daniel Becker wrote:
>
> I really appreciate the thoughtful answers. they raise another related question.
> say I have an unattended repeater, and the system is no longer receiving. I want
> to do some unattended diagnostics. (assume i still have dialup or some other
> means of accessing controllers on the site.) would it make sense to do a vswr
> test on
> the receive antenna to see if it was damaged, or if something damaged the
> transmission line from the receive antenna?
> --
> Dan Becker
> dcbe...@lucent.com
That DOES make sense, though I would consider using some coax relays to
switch the receive antenna to the output of a low powered transmitter
(or swap receive and XMT antennas on the main repeater, if the RCV
antenna can handle the power) and note the SWR. Doesn't matter if it's
1:1 or not, just that it hasn't appreciably changed since installation.
This would test both the RCV antenna and XMSN line.
Good luck!
Gray
Wes Stewart
>Not sure if this falls under silly question category or not. has anyone ever
>heard of doing vswr tests on a receive-only antenna? Seems to me in theory,
>a receive-only antenna should have as good an impedence
>match as a transmit or duplex antenna, and a low
>vswr would indicate that. but I dont recall ever seeing anyone do a vswr test
>on a receive-only antenna as a means of testing its performance. Depending on
>the
>antenna construction, and depending on the amount of forward power applied to
>the
>antenna, you could burn it out. but is this really practical or useful, and what
>gotchas are waiting?
>
>--
>Dan Becker
>dcbe...@lucent.com
Excuse the pun, but it depends on a point of view.
If you are concerned with the match from the receiver end of the line looking
back toward the antenna, then this is no different from the transmit case.
Maximum efficiency (lowest loss) occurs with the line and the antenna "matched."
Given the above, the case looking into the receiver can be (usually is)
different. In this case, the antenna/line combination becomes a generator with
a particular source impedance that is loaded by the receiver input Z. Any
mismatch here causes a reflection back to the antenna, where the signal is
reradiated and lost.
In the HF case, most of the time this is immaterial. The SNR is set by external
noise and is unaffected by additional mismatch/loss. Where it can have an
effect however, is when some type of additional filter is used, for instance to
reject BC interference, etc. The filter performance will be negatively affected
by deviation from the design impedance.
In the VHF and above case, the effect of receiver input mismatch can be very
detrimental. This is especially so with very low noise amplifiers(LNA) that
typically have very large input mismatches. LNAs are often tuned using a noise
generator that is a very good 50 ohm source.
The low noise devices used in LNAs have an optimum source impedance that
delivers lowest noise figure. This is almost never 50 ohm. Consequently,
during tuneup, the input matching is adjusted to make the 50 ohm noise generator
"look like" the optimum source Z.
When the LNA is connected to the antenna/transmission line, the source Z is
almost always different and the "measured" NF is no longer realized. In my
opinion, the "optimum" method of adjustment is by radiating the noise signal and
receiving it with the antenna/line/LNA combination.
I realize I've probably strayed far from your question but perhaps this will be
useful.
Wes N7WS
Ed
<I dont recall ever seeing anyone do a vswr test
<on a receive-only antenna as a means of testing its performance.
antennas regardless of its intended use.
There is a point I haven't seen mentioned in this string. A 'good' VSWR on an
antenna is an indication of how well it is tuned to the desired frequency(s) .
Hence, how well it can transform an EM field into a current on the rx line, or
visa versa as is how VSWR is checked.
Regards Ed.
Reg Edwards
a resistive load of 50 ohms for the transmitter, and the receiver
input resistance is nominally 50 ohms (most modern receivers are) then
a CONJUGATE MATCH exists in the receiving mode of a transceiver. The
antenna then delivers maximum available power to the receiver.
I'm too lazy to look for references but I think a conjugate receiving
match is also the condition which minimises internal receiver noise
and therefore maximises signal to noise ratio when receiver front-end
noise is the dominant factor.
So although a conjugate match for the transmitter may not matter very
much, it often pays to tune up for minimum SWR on the assumption that
it does.
--
Reg.
William E. Sabin
Reg Edwards wrote:
> I'm too lazy to look for references but I think a conjugate receiving
> match is also the condition which minimises internal receiver noise
> and therefore maximises signal to noise ratio when receiver front-end
> noise is the dominant factor.
Maybe not. Sometimes a deliberate mismatch at the receiver provides a
small improvement in noise figure. This is common and important at
microwaves (called noise match) and less common at HF, but it happens.
Bill W0IYH
Robert Lay (W9DMK)
That isn't true.
An antenna VSWR is for all practical purposes irrelevant - it's the
VSWR on a transmission line that is of interest. The antenna being
"tuned to the desired frequencies" is also not terribly relevant -
it's the question of whether or not you have an efficient antenna -
not what frequency it is tuned to.
Generally speaking, the interest focuses on VSWR on the transmission
line - with two perspectives - one is the loss in that line and the
other is whether or not the transmitter can transfer power to it.
Richard Harrison
A good VSWR on an antenna is an indication of how well it is tuned to
the desired frequency(s).
A good VSWR is an indication of a match between the antenna and
transmission line.
The surge impedance of a lossless transmission line is a resistance.
This does not mean the line has loss. It means the current in the line
is in phase with the voltage across the line. It also means the voltage
to current ratio must conform closely to the stated surge impedance (a
resistance) of the line.
If a resistor matching the surge impedance of the line is connected as a
load, the resistor will convert all the energy from the line into heat.
An antenna, when resonant, that matches the line`s characteristic
(surge) impedance, will accept all the energy from the line, but
hopefully convert most of it to radiation rather than heat. Some energy
will be converted to heat because the antenna wire has some resistance,
and the antenna will be close enough to some lossy surroundings to
dissipate some energy as heat.
But, if the antenna resistance total, loss and radiation resistances,
equals the feedline surge impedance, and all of the reactance is tuned
out of the antenna, no energy will be reflected from the antenna. Thus,
there will be no wave interference pattern established. Maxima and
minima will be the same and their ratio will be one.
If the antenna is not a terminated traveling wave antenna, it will have
reactance when it is not resonant. Reactance causes reflection and SWR,
as mismatched resistances between feedline and antenna would.
So, if you have a resonant antenna, a good VSWR may very well be an
indication of how well an antenna is tuned to the desired frequency.
This condition only holds when the antenna and line are a natural (or
contrived) match at resonance. You really need to neutralize antenna
reactance not so much to keep SWR down as to get a lot of current into
the antenna. The reactance can be more than ten times the radiation
resistance, and if uncorrected could reduce loading and radiation
accordingly.