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