Effects of Wave Propagation on KGOU/KROU Reception

by David White, KGOU Chief Engineer 1983 - 2006

I'm sure many of you have experienced difficulties in picking up the KGOU or KROU FM broadcast signal at one time or another.  Of course, obvious equipment failure in the broadcast transmission chain for KGOU or KROU will always create listening difficulties.

However, there are more subtle things that can create occasional reception problems for the listener.  FM broadcasting is normally a line-of-sight operation.  Unfortunately, there are events in the world of atmospheric physics that can temporarily change this, making the signal nearly or totally unlistenable.  In these cases, it is not your radio or the KGOU=KROU equipment.

First, let me describe something that can cause reception problems primarily in the Spring and Fall for our listeners.  This is probably the most common scenario causing an occasional problem with receiving KGOU or KROU - especially in the morning hours.

Technically, this effect is called tropospheric ducting.  This primarily occurs due to temperature inversion where the temperature of the atmosphere actually rises with altitude.  Effects of temperature inversion, on occasion, permit other FM broadcast stations located beyond line-of-site to interfere with local stations on the same frequency.  In this instance, the interfering station can sometimes be strong enough to over ride local signals such as KGOU and KROU.  This most commonly will occur in the early mornings for us and effects will generally disappear by noon.

Next, something called sporadic E is another possible cause of reception problems for KGOU and KROU listeners.  In this instance, the E Layer of the ionosphere, under certain conditions, can reflect the signals of other FM broadcast stations located beyond line-of-site.  Once again, signals from distant FM stations can override local FM signals on the same frequency.

Finally, I would like to mention something about our STL systems.  The STL systems are the Studio-to-Transmitter Links consisting of microwave transmitters and receivers to deliver the program audio from the studio to the transmitter sites of KGOU and KROU.  If something should disrupt the STL systems, then the listener's reception is also disrupted.  The same processes of tropospheric ducting and sporadic E can also effect the STL microwave signal.

Since the STL operates on a much higher frequency than the main transmitters of KGOU and KROU, effects are generally limited to an up and down fading of STL signal strength.  When the received STL signal at the main transmitter site fades below a certain level, the program audio is lost and the main transmitter will automatically shut down.  In this case, nothing can be done until the STL signal strength recovers to a near normal level.

There are many other variables and factors that can affect broadcast reception of KGOU and KROU, but these are probably the most subtle and interesting.  Well, interesting at least from an engineer’s point of view but not from the Manager’s.  Of course, Karen thinks the leaves on the trees affect reception too.