[AMRadio] Propagation question

Thu Aug 19 09:53:27 EDT 2010

I'll add a little more to that John.  And I know very little about it.  The ionosphere is moving up and down with respect to the earth surface causing a Doppler shift in the sky wave or one of the sky wave paths (there maybe multiples).  This Doppler frequency shift causes a low freq beat to appear in the receiver as it mixes with the other waves from other paths, (ie ground waves).  If the beat is rapid we call it rapid QSB otherwise is slow fading or QSB.  I'm not sure why it is so selective but I know that the more ionized the reflective layer is the stronger it reflects, and it is frequency sensitive, hence propagation differences from band to band.  My understanding is that as the Solar activity changes, especially sunset and rise. The density and height of the layers changes. Certain densities reflect certain frequency more than other densities.  During this transitional period it can be very selective, removing one sideband more that the carrier or just the carrier.  When the carrier is reduced then the received signal level drops and it sounds over modulated because the sidebands are stronger than the carrier.  If the carrier is increased you can receive a very strong signal but may sound under modulated or lacking in high frequency sounds.  I have watched the effects as it sweeps across the 75 meter band starting at one end creating havoc and ending at the other end.  Then perhaps 10 to 30 min later it all gets right again.  This seems to be more slightly after sun set in the fall.

A very interesting topic.
John Coleman, WA5BXO

BTW Brian, W5AMI, sorry we couldn't get together last weekend, another time perhaps.

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of johndtate at post.com
Sent: Wednesday, August 18, 2010 3:24 PM
To: kf5ccn at arrl.net; amradio at mailman.qth.net
Subject: Re: [AMRadio] Propagation question

The culprit is selective fading....  You have a constant carrier on AM. 
  When it arrives at your antenna from more than one path (for instance 
ground wave and skywave) the singals arriving at different angles are 
likely out of phase causing attenuation or they may be in phase causing 
a brief gain.  It happens on all modes but because of the carrier, it's 
more pronounced in our ears on AM.  Popular methods to combat this is 
to use synchronous detection or diversity reception.

This is really a primitive explanation and I'm sure others can add to 
the subject.

73 John KX5JT

-----Original Message-----
From: Brian - KF5CCN <bzwiener at sbcglobal.net>
To: AMRadio at mailman.qth.net
Sent: Wed, Aug 18, 2010 12:23 pm
Subject: [AMRadio] Propagation question

This has been bugging me. Why do signals QSB up and down worse at times 
on AM
than SSB? Even on or near the same frequency. I was talking with a 
friend on
3890 KHz AM and we switched to 3897 KHz LSB and there was little or no 
fading.
Things that make me go, hmmmm.

73
Brian/KF5CCN

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