AM radio propagation #antennas


Rich - KN6JLF
 

Requesting perspectives on propagation.

Foresthill Fire District has an emergency radio operating on 1020 AM under TIS
rules.  The usual distance/signal falloff has been noted in contour mapping.
Note, no spectrum analyzer on hand, just receivers with relative
dBu/v readings.  Power is 4 Watts CW, but with 90% mod when messages are played;
PEP 7-8 watts estimated (simple power meter) Inital setup of the transmitter field density
is 2mv/.93 miles. Antenna is at 3200 MSL, in downtown Foresthill.

However, it was noted that the signal was higher, and obtained receiver quieting
while crossing the Foresthill Bridge at about 15 miles distance.  From that point towards
Foresthill, the usual PGE, growlights and cable system noises interfere as expected while running at
this low power level.  Why does it sound so good there?   Is it the bridge structure and groundwave?

Take a crack at this one Extra Class!

Rich
KN6JLF


Jef - N5JEF
 

Interesting question.  Think I'll try to check it out.

- Jef


On Sun, May 2, 2021 at 7:42 AM Rich - KN6JLF <dustywood98@...> wrote:
Requesting perspectives on propagation.

Foresthill Fire District has an emergency radio operating on 1020 AM under TIS
rules.  The usual distance/signal falloff has been noted in contour mapping.
Note, no spectrum analyzer on hand, just receivers with relative
dBu/v readings.  Power is 4 Watts CW, but with 90% mod when messages are played;
PEP 7-8 watts estimated (simple power meter) Inital setup of the transmitter field density
is 2mv/.93 miles. Antenna is at 3200 MSL, in downtown Foresthill.

However, it was noted that the signal was higher, and obtained receiver quieting
while crossing the Foresthill Bridge at about 15 miles distance.  From that point towards
Foresthill, the usual PGE, growlights and cable system noises interfere as expected while running at
this low power level.  Why does it sound so good there?   Is it the bridge structure and groundwave?

Take a crack at this one Extra Class!

Rich
KN6JLF


Jef - N5JEF
 

Just a thought.  Could the bridge be acting as a kind of travelling wave antenna, like a Beverage, and re-radiating to the vehicle driving across...?

On Sun, May 2, 2021 at 9:10 AM Jef Allbright <jef@...> wrote:
Interesting question.  Think I'll try to check it out.

- Jef

On Sun, May 2, 2021 at 7:42 AM Rich - KN6JLF <dustywood98@...> wrote:
Requesting perspectives on propagation.

Foresthill Fire District has an emergency radio operating on 1020 AM under TIS
rules.  The usual distance/signal falloff has been noted in contour mapping.
Note, no spectrum analyzer on hand, just receivers with relative
dBu/v readings.  Power is 4 Watts CW, but with 90% mod when messages are played;
PEP 7-8 watts estimated (simple power meter) Inital setup of the transmitter field density
is 2mv/.93 miles. Antenna is at 3200 MSL, in downtown Foresthill.

However, it was noted that the signal was higher, and obtained receiver quieting
while crossing the Foresthill Bridge at about 15 miles distance.  From that point towards
Foresthill, the usual PGE, growlights and cable system noises interfere as expected while running at
this low power level.  Why does it sound so good there?   Is it the bridge structure and groundwave?

Take a crack at this one Extra Class!

Rich
KN6JLF


Greg - KO6TH
 

Possible, but kind of unlikely that it's related to that station.  I suspect all AM stations come in a bit louder over the bridge, and that might make for an interesting test.  Could just be a difference in what appears to be ground to the car's electrically very short antenna.

I have a vague recollection of a similar effect from the deep past, but can't quite place it.  Now it will keep my up all night trying to figure it out.

Greg  KO6TH


Jef - N5JEF wrote:

Just a thought.  Could the bridge be acting as a kind of travelling wave antenna, like a Beverage, and re-radiating to the vehicle driving across...?

On Sun, May 2, 2021 at 9:10 AM Jef Allbright <jef@...> wrote:
Interesting question.  Think I'll try to check it out.

- Jef

On Sun, May 2, 2021 at 7:42 AM Rich - KN6JLF <dustywood98@...> wrote:
Requesting perspectives on propagation.

Foresthill Fire District has an emergency radio operating on 1020 AM under TIS
rules.  The usual distance/signal falloff has been noted in contour mapping.
Note, no spectrum analyzer on hand, just receivers with relative
dBu/v readings.  Power is 4 Watts CW, but with 90% mod when messages are played;
PEP 7-8 watts estimated (simple power meter) Inital setup of the transmitter field density
is 2mv/.93 miles. Antenna is at 3200 MSL, in downtown Foresthill.

However, it was noted that the signal was higher, and obtained receiver quieting
while crossing the Foresthill Bridge at about 15 miles distance.  From that point towards
Foresthill, the usual PGE, growlights and cable system noises interfere as expected while running at
this low power level.  Why does it sound so good there?   Is it the bridge structure and groundwave?

Take a crack at this one Extra Class!

Rich
KN6JLF


Skip - K6DGW
 

The bridge simulates having your [likely] ferrite loop antenna on a 750 foot tower, which at 1020 kHz is about 0.8 wavelengths. All the rest of the way to Forest hill [and anywhere else], your ferrite loop antenna is essentially lying on the ground <K6DGW>