[AMRadio] Interesting FM Bcast Propagation

screwdriver he20c at swbell.net
Sun Jul 16 03:18:20 EDT 2006

Take a look here may give some insight as to what conditions were occurring.


73 SD

----- Original Message ----- 
From: <doxemf at aol.com>
To: <amradio at mailman.qth.net>
Sent: Saturday, July 15, 2006 8:06 PM
Subject: [AMRadio] Interesting FM Bcast Propagation 

> Hello,
> I was listening to my usual station 93.9 North Hampton MA yesterday, ( 
> Friday), which is about 50 miles or so east over the mountain
> when it faded out and was replaced by 93.7 from Albany NY about 40 
> miles west over another mountain.
> This reception was swinging between the two stations in a similar 
> fashion to AM QSB but without the phasing for several hours in the 
> afternoon. Tuning in between the frequencies yielded the usual 
> distortion and if I tuned to the 93.7 station it was receiving the same 
> way with the cross fade between the two stations perfect.
> Now the signals from both stations are normally just at or below the 
> stereo detection point using an SAE 8000 analog tuner. But without the 
> shifting. Antenna is a normal 300 ohm inside FM balanced dipole 
> broadside E/W.
> I did fire up a wide band receiver to look at the IF spectrum but the 
> signals were so close to the noise floor that any measurements were not 
> possible, although enough of the levels were visible to see the see-saw 
> strength between the two stations.
> There is a 1000 ft+ N/S mountain ridge immediate west and another one 
> to the east about 5 miles.
> This behavior has been heard several times especially when there are 
> significant temperature gradients low in the atmosphere.
> I would suspect that however the reflective or ducting effect is 
> producing this could be visualized as similar to the waves
> on the surface of a pond reflecting light at a slow rate. IE, 
> consistent, slow ripples over a thermal reflective layer of the 
> ionosphere.
> I am just surprised at the short distance and would think that there 
> would be technique to take measurements of the distance involved and 
> determine the actual height of the reflective layer.
> Just an interesting observation of propagation i had not observed 
> before moving here to MA.
> Has anyone here in the NE ever noticed similar propagation with 6 or 2 
> mtrs?
> Bill KB3DKS/1
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