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Re: [IRCA] QSL request
Hi Craig,
Thanks for the good technical info.
Steve
----- Original Message -----
From: "Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>
To: "Mailing list for the International Radio Club of America"
<irca@xxxxxxxxxxxxxxxx>
Sent: Thursday, October 30, 2008 7:29 PM
Subject: Re: [IRCA] QSL request
As an AM TX and general domestic AM dummy, what does all this mean, about
not being able to put a 1kW signal into some type of tower? What does the
power and or tower have to do with not being able to feed full power into
the antenna?
Different classes of stations are required to have a minimum efficiency from
their tower. Put a kilowatt in and the signal strength at a kilometer away
should be a specific number or more. Clear channel stations have the
highest minimum. Regionals are lower. Locals like 1340 are the least.
When a station is licensed at a specific power and tower height (efficiency)
it's given an effective radiated power. When a station puts a kilowatt into
a 5/8 wave tower it is about the same as putting 3+ kilowatts into a 1/4
wave tower. So, many times a station using a taller tower has to reduce
transmitter output to keep the coverage the same. One of my clients on 1110
was originally a kilowatt non-directional into a 1/4 wave tower. They put
in a much taller tower to add an FM. So, they had to drop transmitter power
to around 680 watts. Same coverage on groundwave.
5/8 wave is the best height for a local station that isn't looking for
coverage beyond 40 miles or so. There is a secondary high-angle lobe that
comes back down from skywave and causes interference about 70 miles out.
That's why the sweet spot for tower height for the big guns is a bit over
half wave, 195°.
Low band stations can reach the minimum with a shorter tower. For example,
local WDDZ-550 has 1/6 wave towers but still meets the groundwave efficiency
requirements. There are other problems with short towers, but that's a
subject for another chat.
WNBH is lucky in that they are grandfathered in for a full kilowatt into a
tower that's effectively around a half wave. For a local channel station,
they do cover well. And the fact that they are not far from the coastline
gives some pretty interesting DX reports now and then. As I mentioned, this
is the third report from Scandinavia in the past couple of years. The
previous two were quite easily identified. Probably have the .mp3 of those
around somewhere.
Yeah, I probably ought to schedule some sort of DX test out of that station.
Let me see what I can do...
Again, thanks to all for the help. My ears aren't anywhere as good as they
used to be...
Craig Healy
Providence, RI
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