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Author Topic: NVIS at AM broadcast band frequencies?  (Read 5076 times)

2X2L

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NVIS at AM broadcast band frequencies?
« on: November 23, 2005, 08:25:29 AM »

Hello All,

On the Community Radio message board, the subject of NVIS in connection with emergency AM broadcasting antennas (to replace destroyed AM towers) came up after Hurricane Katrina struck the New Orleans area.

One AM station engineer (formerly from KVON 1440 in Napa, California) posted that in 1986 when KVON's transmitter site was flooded, he lashed up an offcenter-fed "Tee" wire antenna and connected it to an old backup AM transmitter that could only put out 500 watts.  He used two 4-foot ground rods for grounding.  The signal range and signal strength were considerably better than he had expected, and he attributed it to NVIS.

What AM broadcast band frequencies would work best for NVIS propagation?  Also, what kind of antenna would work best for NVIS as these low frequencies?  (Dipoles are pretty big at these frequencies, especially toward the bottom of the band!)

Many thanks in advance to anyone who can help.  --  J. Jason Wentworth
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KE4SKY

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NVIS at AM broadcast band frequencies?
« Reply #1 on: November 23, 2005, 08:43:21 AM »

MUF related to critical frequency (fo) by takeoff angle:   MUF = fo /Sin(takeoff angle)
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2X2L

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NVIS at AM broadcast band frequencies?
« Reply #2 on: November 23, 2005, 11:08:25 PM »

That sounds like low frequencies are better, down to a point.

The higher the frequency, the more shallow the angle of incidence on the ionosphere must be to reflect the waves or they will simply punch through it.  (X-rays will only reflect off surfaces at very shallow angles, unlike visible light.)  --  J. Jason Wentworth  
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WB6BYU

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NVIS at AM broadcast band frequencies?
« Reply #3 on: November 28, 2005, 02:13:24 PM »

NVIS really works best with horizontal polarization.
I suspect in the case you cited that the "off-center" fed
T antenna acted like a Windom (which was originally fed
with a single wire) the top of the T was actually radiating
in dipole mode.

While the broadcast band does have the advantage that it
is below the Critical Frequency almost always, the other
problem for propagation is path losses, especially in
the "D" layer of the ionosphere.  So the frequency window
where NVIS can be used is limited at the top end by the
Critical Frequency, and at the bottom end by the signal
absorbtion.  (This is why 160m, and often 80m, is quiet
during the day, but has lots of signals at night.)  Both
of these factors vary with time of day, season, sunspot
activity, etc., so the "optimum" frequency is constantly
changing.

For an antenna, if the existing tower can't be salvaged
somehow (perhaps by rescuing one or two sections and
adding a wire top hat to make up for the missing length)
the I'd consider a dipole or full wave loop.  If the
station had room for the radials for its tower, it will
have enough room for either.

There are, though, a number of practical issues that
will probably be the determining factors.  First, the
optimum frequency for a station is that stations's
assigned frequency.  They probably don't have any other
equipment available.  And most AM BC stations don't
have a convenient PL-259 connector that you can hook
your antenna to:  they probably use a large, rigid coax
to the antenna (especially if they are running over 5kW.)

Tuning and matching the antenna will be another issue -
you don't just hook and old Johnson Matchbox to the
output of a commercial transmitter.  The station engineer
might look out in the Tuning House and see how the
circuit might be reconfigured to suit a different
antenna, but it usually isn't like readjusting the
ordinary antenna tuner.  Might be easier to replace the
tower with a kite-supported long wire.

But, IF there was a low-power backup transmitter
available, it could be pressed into service just like
any station for 160m.  The ordinary antenna dimensions
still apply.  The problem is likely to be finding
enough antenna supports that survived the initial
disaster - I suppose you could disassemble the original
tower sections and them individually for antenna supports
if you have the equipment to put them up.

2X2L

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NVIS at AM broadcast band frequencies?
« Reply #4 on: November 29, 2005, 12:07:57 AM »

That's true about the offcenter-fed "Tee" antenna--had the top hat wire been symmetrical, there would have been little if any horizontally-polarized radiation.

I imagine a low "Inverted L," dipole, or loop antenna would be better for NVIS at AM band frequencies.  Some 60 meter tropical band Short Wave broadcast stations use low-mounted loops for local NVIS coverage.

Many AM radio stations do have low-power back-up transmitters, and those that are required to reduce power at Sunset have small 30 watt or 60 watt night-time transmitters that could be pressed into service with NVIS emergency antennas if needed after a disaster.  --  J. Jason Wentworth

 
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