Who can measure VSWR of transponder antenna?
Eric Greenwell
operating properly; nonetheless, I'd like to measure the VSWR of the
antenna and cable installation. The avionics shop here can't do it.
So, does anyone know of a place/person that can do it somewhere along
Interstate I-5 between Portland, OR, and San Diego, Ca? I might be in
the Reno/Minden area also, so anyone there?
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Eric Greenwell
Richland, WA (USA)
Armand A. Medeiros
BTW, it isn't like measuring the SWR on a CB radio because the duty cycle is
so low. I would think that if you put a transponder on continuous output,
the output section would fry after getting very hot.
The manufacturers have special equipment that can take a snapshot of the
transmit event at the output section and compare to another snapshot from
the antenna end. This is very tricky stuff (I worked in microwave for years,
not easy to even make test equipment to do what you want).
I was informed by a friend that someone out there made a peek
detecting/sample & hold type meter lately that could be used for what you
want... have no idea what it is or who makes it.
In any case, following the manufacturers instructions for cable/connector
type and length should be satisfactory.
Armand
"Eric Greenwell" <REMOVEeg...@prodigy.net> wrote in message
news:MPG.17ccade87...@flashnews.prodigy.net...
Larry Goddard
Cut the wire going to the antenna, solder in a Radio Shack CB SWR meter and turn
that sucker on!!!
:-)
Larry
Janus2k
frequency) in an Avioncs shop(20 years in field). This is pulse output with low
duty cycle, not continous RF output like a Com. Frequency is up at 1Ghz, not
going to happen. Transponder is check and certified, it should show on test
equipment the proper power out. Verify the transponder outputs the proper power
on the bench. If not need to check the antenna or coax in aircraft.
Mark Mahan
2K
Robin Birch
people who do amateur work in the microwave area, they may have some kit
to do it. However, if the set was designed and installed right, correct
connectors and so on, then you should be ok.
Robin
In message <20020821110438...@mb-mu.aol.com>, Janus2k
<jan...@aol.com> writes
Eric Greenwell
jan...@aol.com says...
The avionics tech that did the check on it (handheld test box about
3' from the glider fuselage, which was pulled out of the trailer and
sitting on the trailer ramp) said it appeared to putting out the
proper power, but the situation seemed so uncontrolled (and different
from the usual airplane check) I'm skeptical any valid estimate could
be made.
Eric Greenwell
zar...@sbcglobal.net says...
> In any case, following the manufacturers instructions for cable/connector
> type and length should be satisfactory.
Ay, there's the rub. The Becker installation manual calls for a
minimum 40 cm x 40 cm metal ground plane, which I don't have: The
antenna is mounted on the belly of my ASH 26 E and uses the carbon
fiber for the ground plane, which is the way the Schleicher factory
mounts them.
So, I'm slightly skeptical about the glider factory method even though
it seems to work. As a former EE with residual geek tendencies,
getting the VSWR would put my mind at ease.
Armand A. Medeiros
out, no prob.
Armand
"Eric Greenwell" <REMOVEeg...@prodigy.net> wrote in message
news:MPG.17cd9270e...@flashnews.prodigy.net...
John Giddy
power. It doesn't have to be the transponder. In this case
one can use a low power continuous source and a suitable SWR
meter. The SWR isn't sensitive to power level to the
accuracy normally used.
Cheers, John G.
"Robin Birch" <rob...@ruffnready.co.uk> wrote in message
news:pxRwheAr...@falstaf.demon.co.uk...
Janus2k
accurate. Also will check the receiver sensitivity of the transponder.
Carbon fiber normally works as ground plane in home built aircraft.
Good luck.
Mark Mahan
2K
tango4
> power. It doesn't have to be the transponder.
Connect it to your microwave!
:-)
Ian
John Giddy
This is too far from the transponder frequency to be useful.
Good try however !!!
Cheers, John G.
"tango4" <i...@tango4.demon.co.uk> wrote in message
news:1029992749.12435....@news.demon.co.uk...
Finbar
then: you just want to know if the antenna has the proper impedance at
the operating frequency. You can measure that with low power - which
just leaves the problem of the measurement equipment! In my microwave
days I'd have borrowed the company network analyzer for a quick
Saturday measurement...
Is this a genuine concern, or more of a curiosity issue?
Eric Greenwell <REMOVEeg...@prodigy.net> wrote in message news:<MPG.17cd8e787...@flashnews.prodigy.net>...
Willy VINKEN
<REMOVEeg...@prodigy.net> wrote:
>In article <l%L89.2831$Ex1.17...@newssvr14.news.prodigy.com>,
>zar...@sbcglobal.net says...
>
>> In any case, following the manufacturers instructions for cable/connector
>> type and length should be satisfactory.
>
>Ay, there's the rub. The Becker installation manual calls for a
>minimum 40 cm x 40 cm metal ground plane, which I don't have: The
>antenna is mounted on the belly of my ASH 26 E and uses the carbon
>fiber for the ground plane, which is the way the Schleicher factory
>mounts them.
>
>So, I'm slightly skeptical about the glider factory method even though
>it seems to work. As a former EE with residual geek tendencies,
>getting the VSWR would put my mind at ease.
I would not worry about this. If you have a suitable antenna, good
connectors and the right coax cable, the shape of the groundplane
could indeed shift your SWR somewhat. But this would not be relevant.
What happens with a 'bad' SWR, is that the impedance mismatch
reflects part of the RF power back to the transmitter, then back to
the antenna, aso. After every cycle, part of the RF energy is
transmitted and part of it is again reflected. The only power that is
not radiated by the antenna is that part that heats the coax cable
due to dielectric losses. OK, these losses may be substantial at
1GHz, but then your coax cable is probably not that long.
Enjoy flying and forget it.
Willy VINKEN - ON5WV
Armand A. Medeiros
Armand
"Willy VINKEN" <on...@RadioLink.net> wrote in message
news:fgm9mugam0n7cd74g...@4ax.com...
Eric Greenwell
zar...@sbcglobal.net says...
> Eric, just go up and see what approach says...if they have you a long ways
> out, no prob.
That's the ultimate test. The one flight I've had with it, approach
had a good signal at 20 nm, but I didn't try it further away, or at
another facility.
An IFR instructor suggested I try VFR flight following, and as long as
they don't report any difficulty, it's probably working
satisfactorily.
Eric Greenwell
on...@RadioLink.net says...
>
> I would not worry about this. If you have a suitable antenna, good
> connectors and the right coax cable, the shape of the groundplane
> could indeed shift your SWR somewhat. But this would not be relevant.
> What happens with a 'bad' SWR, is that the impedance mismatch
> reflects part of the RF power back to the transmitter, then back to
> the antenna, aso. After every cycle, part of the RF energy is
> transmitted and part of it is again reflected. The only power that is
> not radiated by the antenna is that part that heats the coax cable
> due to dielectric losses.
The power can also be absorbed by the transmitter itself, causing it
to heat up. And it is a mistake to think most of the power gets
radiated anyway. The effectiveness of the antenna (transmit and
receive) drops off sharply as VSWR rises above 3, which is why it is
an important characteristic of antennas.
The fact the power might be rattling back and forth doesn't mean most
of it gets out eventually. After all, that is basically what the VSWR
is measuring: ALL the power going out (including re-reflected power)
vs ALL the power coming back.
Eric Greenwell
finbar...@yahoo.com says...
> The power output & pulse length from your transponder don't matter,
> then: you just want to know if the antenna has the proper impedance at
> the operating frequency. You can measure that with low power - which
> just leaves the problem of the measurement equipment! In my microwave
> days I'd have borrowed the company network analyzer for a quick
> Saturday measurement...
>
> Is this a genuine concern, or more of a curiosity issue?
Partly curiosity, partly just tying up what seems like a potential
loose end. The one flight I've had with it, approach had a good signal
at 20 nm, but I didn't try it further away, or at another facility.
--
John Giddy
If the output impedance of the transmitter accurately
matches the cable impedance, there will not be any signal
reflected at the transmitter end, so you can also lose power
by absorption in the transmitter output stage. A very bad
SWR of the antenna will reflect so much of the output energy
that the transmitter will be damaged.
However I agree that if the cable is correctly connected to
a properly designed antenna, there should be very little
problem. I also believe that carbon fibre makes a
sufficiently effective ground plane.
To make doubly sure, a sheet of thin soft copper or
aluminium could be glued to the inside of the fuselage with
the antenna mounting making good electrical contact to it
Cheers, John G.
"Willy VINKEN" <on...@RadioLink.net> wrote in message
news:fgm9mugam0n7cd74g...@4ax.com...
Eric Greenwell
says...
> However I agree that if the cable is correctly connected to
> a properly designed antenna, there should be very little
> problem. I also believe that carbon fibre makes a
> sufficiently effective ground plane.
> To make doubly sure, a sheet of thin soft copper or
> aluminium could be glued to the inside of the fuselage with
> the antenna mounting making good electrical contact to it
A bulkhead about 2" aft, and the landing gear opening about 1"
forward, make this difficult to do. Left and right would be easy, but
the strip would only be 2.5" wide and 8" long. Before I tried measures
like this, I wanted to see if it needed it, and have a way of
measuring changes to see if they actually improved the situation.
Adding the material to the outside of the fuselage would be a lot
easier.
I suspect the 16" x 16" plate mentioned in the installation
instructions is far bigger that needed, but I haven't seen any figures
on how small is adequate.
Interestingly, the Becker is smart enough to know when no cable is
attached to it, but doesn't complain when a 13 foot cable with no
antenna is attached to it. The manual doesn't mention how much
mismatch it will tolerate before it displays an error code.
Finbar
http://www.lancair.com/flash/faq/avionicsfaq.html
2. How large of a ground plane do I need for my antennas? Is carbon
fiber a good ground plane?
* A ground plane is a large surface area at the base of the antenna
constructed from a conducting material. The Ground plane serves two
primary purposes: A) To provide an electrical match to the antenna for
proper tuning of the antenna. B) To provide shielding and noise
immunity from other electrical noise in the airplane. The ground plane
should generally have a diameter about twice the length of the antenna
itself. It will typically be mounted on the inside of the fuselage as
a piece of aluminum sheet. There has been some success using a copper
mesh on the outer skin of the fuselage. This copper mesh is glued into
place with Flox or Micro, then blended into the fuselage. Another
option is to use the Carbon fiber itself as a groundplane. To do this,
you must sand down into the weave of the carbon to get a good
electrical bond with the ground base of the antenna. Carbon Fiber will
work decently at Comm and Transponder frequencies. It is not a
suitable ground plane at lower frequencies such as what the ADF, and
Strikefinder / Stormscope use. For these applications, a metal or mesh
ground plane is still the best.
George Emsden
except that;-
a) they use a lot of juice (even when switched off) and
b) cost a lot here in the UK
but what is VSWR please?
George Emsden
Moderator - Kestrel Newsgroup on Yahoo
<http://groups.yahoo.com/group/kestrel401>
"Eric Greenwell" <REMOVEeg...@prodigy.net> wrote in message
news:MPG.17ccade87...@flashnews.prodigy.net...
news.optonline.net
"efficiency" of an antenna. A VSWR of 1.0 is perfect; VSWR up to about 2.0
is generally acceptable, and anything much over that is NFG.
"George Emsden" <yq...@dial.pipex.com> wrote in message
news:3d68a4d1$0$8514$cc9e...@news.dial.pipex.com...
Dave
Radio.
"news.optonline.net" <dha...@pipeline.com> wrote in message
news:%wca9.207912$EJ4.5...@news4.srv.hcvlny.cv.net...
Stephen Cook
news:%wca9.207912$EJ4.5...@news4.srv.hcvlny.cv.net...
> VSWR=Voltage Standing Wave Ratio. It's basically a measure of the
> "efficiency" of an antenna. A VSWR of 1.0 is perfect; VSWR up to about
2.0
> is generally acceptable, and anything much over that is NFG.
Absolutely, but allow me to bore you with the details.
VSWR is actually a measure of how well matched the antenna and cable are.
When you are transmitting a radio signal you want all the signal to go along
the cable and out of the antenna. If there is a mis-match between the the
transmitter and cable, or cable and antenna, then part of the signal will be
reflected at the mis-match. The reflection combines with the original
signal and creates a standing wave. This is bad news and in the worst case
you can damage your transmitter with a poor match. This is why its very
stupid to operate a transmitter without its antenna connected. The same
principles apply when you are receiving but the consequence of a mismatch is
just poor reception rather than damage.
The property of the cable or antenna that must be matched is the
"characteristic impedance". Glider VHF systems are 50 ohm. Terrestrial TV
receivers use 75 ohm. The T shaped bit of wire you get with a domestic VHF
receiver is 300 ohm. Video cables are 75 ohm.
Its important when installing a radio frequency (RF) system that the cable,
connectors and antenna are all of the correct impedance and, to mimise the
potential for mismatch, its best to keep the number of connectors to a
minimum. If you are forced to make a join then you must do it with the
proper connectors. Splices with insulating tape, TV antenna connectors
(I've seen both in gliders) etc. are asking for trouble. RF signals don't
work like 12V power suppies and you can't take liberties with them.
Now, that's got that off my chest.
Stephen
Eric Greenwell
@dial.pipex.com says...
>Please forgive the dumb question but I do not know much about transponders
>except that;-
>a) they use a lot of juice (even when switched off) and
Actually, they don't use any juice when shut off, except I hear the
Microair does use 0.010 amps (definitely not "a lot", though it will
run your battery down in a few weeks).
The newer units, like the Becker and Microair, use much less when
running than older units, or even the usual units used in airplanes.
My Becker uses 230 (idle) to 430 ma (525 interrogations per second;
normally I see about 1 a second).
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