Com Standing Wave Ratio?

Can someone explain a com radio's Standing Wave Ratio?

During our trip to Key West, our #1 KX155 got damp. I removed it, dried it
out, and just to be safe, swapped places with our #2 KX155. After several
hours of flying and just after take off from Champaign IL, we noticed that
when in altitude hold mode our Stec 60-2 would enter a slight dive when
transmitting on the #2 com (now in the top slot). This happened when in
either Nav or HDG mode and when coupled to either the VOR or the GPS. I
found the following on Stec's web site and am looking for a little
education.

Q: Where do I start when the aircraft either climbs or dives when the mike
button is pressed to transmit?
A: Over the years it has become customary to place the autopilot at the
bottom of the radio stack. Check the standing wave ratio on the comm.
radios, a high S.W.R will affect the altitude transducer. Make sure the coax
cables are routed away from autopilot wiring, and check for proper antenna
bonding.

Thanks as always,

Jim

Jim B wrote:
Can someone explain a com radio's Standing Wave Ratio?

During our trip to Key West, our #1 KX155 got damp. I removed it, dried it
out, and just to be safe, swapped places with our #2 KX155. After several
hours of flying and just after take off from Champaign IL, we noticed that
when in altitude hold mode our Stec 60-2 would enter a slight dive when
transmitting on the #2 com (now in the top slot). This happened when in
either Nav or HDG mode and when coupled to either the VOR or the GPS. I
found the following on Stec's web site and am looking for a little
education.

Q: Where do I start when the aircraft either climbs or dives when the mike
button is pressed to transmit?
A: Over the years it has become customary to place the autopilot at the
bottom of the radio stack. Check the standing wave ratio on the comm.
radios, a high S.W.R will affect the altitude transducer. Make sure the coax
cables are routed away from autopilot wiring, and check for proper antenna
bonding.

Thanks as always,

Jim


The ratio of voltage and current of a RF signal in a transmission
line (called characteristic impedance) is fixed and determined by
the dimensions and materials of the transmission line.

For the coaxial cables used in aircraft equipment the value is
usually 50 ohms.

If the transmission line has a load that is equivalent to its
characteristic impedance (he 50 ohms), the signal enters
completely the load. If the load is not equivalent to the
characteristic impedance, part of the signal gets reflected
back to the source. The effect is the same what can be demonstrated
by tying a rope to some fixed point and whipping a wave into the
rope. When the wave meets the tiepoint (which looks different than
the rope for the wave), the wave reflects back.

The ratio of the load impedance and characteristic impedance
is defined as the Voltage Standing Wave Ratio (VSWR or SWR).
It is always calculated so that the larger number is divided
by the smaller one, so the SWR ranges from 1.0 to infinity.

The antennas (the load) and the radios (the source) are designed
to give a reasonably small SWR when all is in working order.

My guess is that the instructions above are using the SWR as an
inidicator that the antenna and its feedline are OK. The most
common causes are broken or shorted connections at the antenna
base, bad grounding of the antenna base to the aircraft skin
and a broken antenna (an element of incorrect length does not
give good match anymore).

If the antenna feed system is bad, the RF signal (10 - 20 W of
VHF) starts to wander around in the other wiring, easily wreaking
havoc in sensitive electronics, like the altitude encoder.

HTH (Hope This Helps)

--

Tauno Voipio, Avionics Engineer, also CPL(A)
tauno voipio (at) iki fi

Tauno Voipio wrote:
The ratio of voltage and current of a RF signal in a transmission
line (called characteristic impedance) is fixed and determined by
the dimensions and materials of the transmission line.

Considering how much abuse the typical piece of coax absorbs over GOK how
many years of being scrunched into the rats nest of wiring behind most GA
panels, I suspect the characteristic impedance is anything but fixed.

Not to mention that a lot of radio guys barely know which end of a
soldering iron to pick up. SWR Meter? That's the box which tells you how
****ty a job you did of putting the connectors on before you button up the
panel and write out the customer's bill.

Roy Smith wrote:
Tauno Voipio wrote:

The ratio of voltage and current of a RF signal in a transmission
line (called characteristic impedance) is fixed and determined by
the dimensions and materials of the transmission line.


Considering how much abuse the typical piece of coax absorbs over GOK how
many years of being scrunched into the rats nest of wiring behind most GA
panels, I suspect the characteristic impedance is anything but fixed.

As long as the scrunch is short compared to the wavelength
(in the cable, for coax about 2/3 of free-space wavelength),
the signal could not care less, and, for practical purposes,
the characteristic impedance is constant. If we have to take
the attenuation into account, the impedance is not even real,
but complex, containing capacitive or inductive components.

Not to mention that a lot of radio guys barely know which end of a
soldering iron to pick up. SWR Meter? That's the box which tells you how
****ty a job you did of putting the connectors on before you button up the
panel and write out the customer's bill.

You can also crash a new car before delivering it to
the customer ...

--

Tauno Voipio
tauno voipio (at) iki fi

Standing wave ratio is a measure of how efficiently power from the
radio is being coupled into the antenna. A high standing wave ratio
is a result of impedence mismatch between any or all of the radio,
the coax and/or the antenna. Any power that is reflected back (not
coupled into the antenna) will travel back up the coax to the radio,
and, since it is out of phase with the forward power, will produce
a standing wave on the line, even on the outside of the coax.

Needless to say this is unsatisfactory and these standing waves can
radiate into other nearby equipment. Wet coax and bad connections
are the usual cause especially in a setup that was previously working.
One culprit is the coax balun... and/or the antenna connections
themselves. If your coax and connections are old you are better off
just replacing them all.

Thanks both Tauno and ktbr.

Second question. After we returned home, I swapped the KX155's returning
them to the original slots. #1 KX155 (the wet one) didn't give us any
pitching problem with the auto pilot, however, thinking of coax, conection,
or antenna problems, we had a local tower operator complain about very
staticy transmisions. Reception was fine, Nav functions were fine.

#2 transmitted much better however only #1 is coupled so we couldn't check
the #2 radio/tray/coax/antenna against the autopilot.

Next step to swap coax antenna leads?

Thanks again,
Jim

As I stated, if the coax is old (I'm talking like 10.. 15 or more)
years... just replace it. I don;t think I'd waste a lot of time trying
to troubleshoot old coax. Don't forget about the balun to the NAV
antenna, it can get lossy when old and degrade NAV reception.

A high VSWR can ultimately damage the transmitter as well, since that
reflected power if coming back into the RF power stage where it will
increase the operating temperature and ultimately cause failure.

Jim B wrote:
Thanks both Tauno and ktbr.

Second question. After we returned home, I swapped the KX155's returning
them to the original slots. #1 KX155 (the wet one) didn't give us any
pitching problem with the auto pilot, however, thinking of coax, conection,
or antenna problems, we had a local tower operator complain about very
staticy transmisions. Reception was fine, Nav functions were fine.

#2 transmitted much better however only #1 is coupled so we couldn't check
the #2 radio/tray/coax/antenna against the autopilot.

Next step to swap coax antenna leads?

Thanks again,
Jim

Replacing the coax will have to wait until annual time when we can pull the
interior and the headliner. We're planning on changing the headliner
anyway, so the ceiling will be open at that time. Meanwhile we'll keep a
record of what occurs, swap antennas, and try to follow the problem.
Thanks
Jim

Also check that the radio is going ALL the way in. The lip on the rack
should tocuch the faceplate. Sometimes the faceplate is hitting the
mounting rails and not going ALL the in and the atenna connector is
make a poor connection.



On Feb 26, 10:52�am, "Jim B" wrote:
Replacing the coax will have to wait until annual time when we can pull the
interior and the headliner. *We're planning on changing the headliner
anyway, so the ceiling will be open at that time. *Meanwhile we'll keep a
record of what occurs, swap antennas, and try to follow the problem.
Thanks
Jim

Jim B wrote:
Thanks both Tauno and ktbr.

Second question. After we returned home, I swapped the KX155's returning
them to the original slots. #1 KX155 (the wet one) didn't give us any
pitching problem with the auto pilot, however, thinking of coax, conection,
or antenna problems, we had a local tower operator complain about very
staticy transmisions. Reception was fine, Nav functions were fine.

#2 transmitted much better however only #1 is coupled so we couldn't check
the #2 radio/tray/coax/antenna against the autopilot.

Next step to swap coax antenna leads?

Thanks again,
Jim

You should take the wet (and now hopefully dry again) unit
to a shop for a checkout. Swimming can create weird effects
in electronics.

I once repaired a handheld COM, which had been diving, and
all the glass-tube diodes were broken, as electrolytic corrosion
ate the wire at the more positive end, and water got into
the glass encapsulation.

--

Tauno Voipio (PA28RT-201T, OH-PYM)
tauno voipio (at) iki fi