Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Get involved. We want your input!
Apply for Membership and join the conversations about everything related to broadcasting.
After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post. If you use a disposable or false email address, your registration will be rejected.
I've heard various theories on this and just want to get an authoritative word on this question.
Why do AM signals BOOM out over water? (For stations whose tower is near a large lake or ocean).
Also, does the FCC allow for this? In other words, does the FCC care how far an AM's signal will go over the ocean?
The FCC licenses commercial AM broadcast stations for specific transmitter powers and radiation patterns, which may vary at different times of the day, and different seasons of the year.
The groundwave field strength that such radiated power may produce at a given distance/direction/location depends on the h-plane power radiated in that direction at that time and date, and the aggregate earth conductivity along that path.
Sea water has very high conductivity, which means that groundwaves propagated over such paths may have less loss, and generate higher fields for a given radiated power and distance than those radiated over terrestrial paths (including all/part of such paths over fresh water lakes).
Your signal radiates as well from your ground system as it does from your tower.
Explanation for this is like two towers combined and working together. Antenna and Antenna Ground.
No ground. No signal.
Add more towers to the radiator and the signal changes (good or bad). Add water and conductivity to your ground system and it is the same principle. RF guy can expound.
Am stations with sprinklers on the tower field? connecting to a creek or stream is good too.
A station had a temporary ground. Copper with insulator. One wire. (Not evenly spaced underground wires)
Sometimes the transmitter would go off for no reason. Wondered and Wondered.
One day was called. Station off. On arrival saw the lawnmower man unrolling the ground wire.
"I have to make sure not to grab a part with no cover cause it shocks me."
A buried radial ground system doesn't produce any useful radiation from its wires. The r-f current in an opposing pair of radials flows in opposite directions and the net, far-field radiation from them is zero. Only the vertical monopole produces useful radiation.
But the ground system of a monopole radiator is in series with the r-f current that will flow along the monopole. If ground resistance is high then less current can flow for a given applied power, and received field strength will be reduced.
//
Even in professional circles, there is still no agreement to how ground radial current flow equates into
"effective radiation" from the monopole. I like Chief Engineer's analogy and description as well as any text I've read. The story about removing the ground current connection physically silencing the station is clear enough.
Every word he says is worth gold in mv strength. Got sprinklers? Pass the salt.
I do believe that stations on east side of Lake Michigan seem to do really well here on the Illinois side, despite
the 90 miles of water. WGTO 910 in Cassopolis Mich, compared to WLBK, De Kalb, Il are (were before iBOC)
an example of this. There are others. I think it has to do with a good connection to the water table,
as the Cheif notes.
If I recall correctly, one of the stations I once worked for had (and possibly still have) some radials connecting to one of the local rivers, which is right behind the tower by just several yards. As you walked north along the river, the signal would remain strong, but if you get up north of town, a good 4 miles or so away from the tower, and move away from the river, the signal would diminish. I recall once listening while driving on a road about 5 miles north of town over the river and noticing the signal getting stronger as I drove toward the bridge and weaker as I drove away from it.
AM signals and water certainly do some interesting things together!
I do believe that stations on east side of Lake Michigan seem to do really well here on the Illinois side, despite the 90 miles of water. WGTO 910 in Cassopolis Mich, compared to WLBK, De Kalb, IL are (were before iBOC) an example of this.
Both stations use 1 kW daytime. WGTO is directional, and radiates about 2 kW ERP toward Chicago, while WLBK is omnidirectional. Also WLBK is on 1360 kHz, so propagation losses are higher than for WGTO on 910 kHz, other things equal. Finally, Lake Michigan is only about 55 miles wide for the 270 degree radial from Cassopolis, and it has about the same conductivity as much of the dry land in the region: 8 mS/m.
Tom Wells said:
Even in professional circles, there is still no agreement to how ground radial current flow equates into "effective radiation" from the monopole.
Some may disagree about it, but that doesn't mean that the radiation physics of monopoles with radial ground systems is unknown. In fact it was carefully measured by RCA in 1937, and that work is the basis for the FCC's AM rules in use to the present day.
The 1937 RCA work showed that the groundwave field measured 3/10 of a mile from a monopole of 90 degree height (and somewhat less) was within a few percent of the inverse distance field when used with a system of 113 buried radials each 0.41 wavelengths long. The inverse distance field is the field generated by a perfect monopole over a perfect ground plane. This experiment was done in the sandy soil of New Jersey, about 4 mS/m.
Below is a ], or about 24%, which value is dissipated by heating the earth around the monopole, as the r-f earth currents have longer paths through the resistive earth to reach the radials.
Once the signal is launched it becomes subject to the field loss resulting from the terrain over which it propagates.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
