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AM Near-Vertical Incidence Skywave question

J

JasonW

Guest
Hello All,

The previous thread about emergency broadcasting (I will post no more on it in deference to Mr. Tarr, the moderator) raises another interesting question.

There are occasions when AM radio stations lose their towers to tornadoes or hurricanes, but their primary transmitters are undamaged. Ham radio operators often use a regional (good out to 400 miles or so) form of propagation called Near-Vertical Incidence Skywave (NVIS). They use very low-mounted horizontal dipole or loop antennas to reflect signals off the ionosphere directly overhead. It works well for the lower ham frequencies, including the 1800 kHz - 2000 kHz (160 meter) band that is just above the AM broadcast band. (In equatorial countries that use the 60 meter "tropical band" for local area Short Wave broadcasting, stations commonly use low-mounted NVIS dipoles and loops.)

Have any AM stations ever tried using NVIS transmission after losing their towers? At the higher AM band frequencies (1600 kHz - 1700 kHz in particular), a low NVIS dipole or loop fed 1 kW or more could provide a listenable if not great signal in the station's normal coverage area and beyond. A portable "crank-up" tower would, of course, be better, but for stations without this option, an NVIS dipole or loop would be a "quick and dirty" way to get back on the air. -- J. Jason Wentworth
 
> Hello All,
>
> The previous thread about emergency broadcasting (I will
> post no more on it in deference to Mr. Tarr, the moderator)
> raises another interesting question.
>
> There are occasions when AM radio stations lose their towers
> to tornadoes or hurricanes, but their primary transmitters
> are undamaged. Ham radio operators often use a regional
> (good out to 400 miles or so) form of propagation called
> Near-Vertical Incidence Skywave (NVIS). They use very
> low-mounted horizontal dipole or loop antennas to reflect
> signals off the ionosphere directly overhead. It works well
> for the lower ham frequencies, including the 1800 kHz - 2000
> kHz (160 meter) band that is just above the AM broadcast
> band. (In equatorial countries that use the 60 meter
> "tropical band" for local area Short Wave broadcasting,
> stations commonly use low-mounted NVIS dipoles and loops.)
>
> Have any AM stations ever tried using NVIS transmission
> after losing their towers? At the higher AM band
> frequencies (1600 kHz - 1700 kHz in particular), a low NVIS
> dipole or loop fed 1 kW or more could provide a listenable
> if not great signal in the station's normal coverage area
> and beyond. A portable "crank-up" tower would, of course,
> be better, but for stations without this option, an NVIS
> dipole or loop would be a "quick and dirty" way to get back
> on the air. -- J. Jason Wentworth
>

Actually, posting there is OK, though my head hurts from all the arguing!

Anyway, excellent question. Typically when an AM loses a tower, engineers go with some sort of long-wire antenna (which often has those properties). I've seen a folded unipole over an existing structure, as well as a slant-wire antenna. Because of interference concerns, the folks that set these up are looking for more groundwave than skywave.

Anyone else?
<P ID="signature">______________
</P>
 
That reminds me...

Back in 1986, when the KVON 1440 site in Napa, CA went underwater (!!!)
I decided to try and get it on the air somehow, as KVON was THE community station, and quite a bit of town was flooded. I remembered that we had given an old Collins 20T 1kw rig to the local junior college electronics program.
Hmmm.... It turns out it was set up and operational in a classroom, still on our frequency, wired to a test load. Cool! They had replaced a bad plate transformer with another that would only allow about 500 watts or so out, but what the heck. It was better than a soggy, muddy rig with no RF out at all.
They had a crank-up ham tower there, so I strung a long wire between it and a foot peg on a telephone pole a hundred feet away or so, then dropped a line down from the horizontal section. Came out to be an offcenter fed T. I built a tuning/matching network on a chunk of plywood, using scrounged up parts from my hamshack, and a few odds and ends from the station. I drove a couple of 4 foot ground rods from the local radio shack in, did a bit of tweaking, and pretty soon we had a decent load for the transmitter, and we were on the air. The boss was pretty pleased! I drove it with a stereo FM tuner, simulcasting the sister FM station. We pretty much ran news and emergency information 24/7 at that point anyway.
The signal was better than I had expected. I had never heard of NVIS back then, but I'm sure that's what it was.
I certainly remember getting my butt chewed out later on, when a bay area FCC agent found out what we had done, without getting permission first. That never even crossed my mind! I just wanted the station on the air.

Dave Fortenberry, CE Salem Communications Sacramento


> > Hello All,
> >
> > The previous thread about emergency broadcasting (I will
> > post no more on it in deference to Mr. Tarr, the
> moderator)
> > raises another interesting question.
> >
> > There are occasions when AM radio stations lose their
> towers
> > to tornadoes or hurricanes, but their primary transmitters
>
> > are undamaged. Ham radio operators often use a regional
> > (good out to 400 miles or so) form of propagation called
> > Near-Vertical Incidence Skywave (NVIS). They use very
> > low-mounted horizontal dipole or loop antennas to reflect
> > signals off the ionosphere directly overhead. It works
> well
> > for the lower ham frequencies, including the 1800 kHz -
> 2000
> > kHz (160 meter) band that is just above the AM broadcast
> > band. (In equatorial countries that use the 60 meter
> > "tropical band" for local area Short Wave broadcasting,
> > stations commonly use low-mounted NVIS dipoles and loops.)
>
> >
> > Have any AM stations ever tried using NVIS transmission
> > after losing their towers? At the higher AM band
> > frequencies (1600 kHz - 1700 kHz in particular), a low
> NVIS
> > dipole or loop fed 1 kW or more could provide a listenable
>
> > if not great signal in the station's normal coverage area
> > and beyond. A portable "crank-up" tower would, of course,
>
> > be better, but for stations without this option, an NVIS
> > dipole or loop would be a "quick and dirty" way to get
> back
> > on the air. -- J. Jason Wentworth
> >
>
> Actually, posting there is OK, though my head hurts from all
> the arguing!
>
> Anyway, excellent question. Typically when an AM loses a
> tower, engineers go with some sort of long-wire antenna
> (which often has those properties). I've seen a folded
> unipole over an existing structure, as well as a slant-wire
> antenna. Because of interference concerns, the folks that
> set these up are looking for more groundwave than skywave.
>
> Anyone else?
>
 
Oh yes, Dave--I'm sure they asked "And where is *your* STA?" :) If you'd cut the horizontal wire to a specific length, you'd have had an old-style, classic multi-band Windom antenna with single-wire feed--they look exactly like what you described. (Come to think of it, a Windom would make a good emergency antenna for a tower site where multiple stations' transmitters share a single tower, if the transmitters' frequencies are harmonically related.)

Surprisingly, NVIS usually gives pretty good signal quality even with low power (3 - 5 watts on HF). Successful operations are even sometimes conducted with antennas lying on the ground! (During World War II the Soviet and German Armies and Soviet & German spies used these ground-mounted "spreading" antennas [as the Russians call them] between 1 MHz and 50 MHz).

NVIS antennas mounted at least 5' - 10' high are much more efficient, though, and reflector wires can be laid out on the ground under them to boost efficiency even more. The reflector makes an NVIS dipole (or loop) a two-element yagi (or quad loop) that's pointed straight up. -- Jason

> That reminds me...
>
> Back in 1986, when the KVON 1440 site in Napa, CA went
> underwater (!!!)
> I decided to try and get it on the air somehow, as KVON was
> THE community station, and quite a bit of town was flooded.
> I remembered that we had given an old Collins 20T 1kw rig to
> the local junior college electronics program.
> Hmmm.... It turns out it was set up and operational in a
> classroom, still on our frequency, wired to a test load.
> Cool! They had replaced a bad plate transformer with another
> that would only allow about 500 watts or so out, but what
> the heck. It was better than a soggy, muddy rig with no RF
> out at all.
> They had a crank-up ham tower there, so I strung a long wire
> between it and a foot peg on a telephone pole a hundred feet
> away or so, then dropped a line down from the horizontal
> section. Came out to be an offcenter fed T. I built a
> tuning/matching network on a chunk of plywood, using
> scrounged up parts from my hamshack, and a few odds and ends
> from the station. I drove a couple of 4 foot ground rods
> from the local radio shack in, did a bit of tweaking, and
> pretty soon we had a decent load for the transmitter, and we
> were on the air. The boss was pretty pleased! I drove it
> with a stereo FM tuner, simulcasting the sister FM station.
> We pretty much ran news and emergency information 24/7 at
> that point anyway.
> The signal was better than I had expected. I had never heard
> of NVIS back then, but I'm sure that's what it was.
> I certainly remember getting my butt chewed out later on,
> when a bay area FCC agent found out what we had done,
> without getting permission first. That never even crossed my
> mind! I just wanted the station on the air.
>
> Dave Fortenberry, CE Salem Communications Sacramento
 
Actually you do not want this type skywave on a MW signal. But for HF it can work well. I tested this with my 40 meter dipole lying FLAT on the ground. I got good results at 90 miles, but folks in Atlanta 200 miles away could NOT hear a trace of me. 100 watts


Powell W4OPW<P ID="signature">______________
NNNN</P>
 
> Actually you do not want this type skywave on a MW signal.
> But for HF it can work well. I tested this with my 40 meter
> dipole lying FLAT on the ground. I got good results at 90
> miles, but folks in Atlanta 200 miles away could NOT hear a
> trace of me. 100 watts
>
>
> Powell W4OPW

Did you have to use an ATU to match its impedance to your transceiver? Also, what power were you running into it? Lying on the ground, I'm sure it had a pretty narrow lobe pointing straight up. Raising it just a few feet (even just chest-high) will broaden the lobe considerably.

I agree, vertical polarization is best for Medium Wave (plus, NVIS might not work at all below 1 MHz), but for higher-frequency AM stations low NVIS antennas could serve in a pinch if no other antenna was available. -- JasonW
 
> > Actually you do not want this type skywave on a MW signal.
>
> > But for HF it can work well. I tested this with my 40
> meter
> > dipole lying FLAT on the ground. I got good results at 90
> > miles, but folks in Atlanta 200 miles away could NOT hear
> a
> > trace of me. 100 watts
> >
> >
> > Powell W4OPW
>
> Did you have to use an ATU to match its impedance to your
> transceiver?

Absolutely I did. The Ten Tec was NOT happy until I did.


> Also, what power were you running into it?


100 watts PEP

> Lying on the ground, I'm sure it had a pretty narrow lobe
> pointing straight up. Raising it just a few feet (even just
> chest-high) will broaden the lobe considerably.


That is true.


> I agree, vertical polarization is best for Medium Wave
> (plus, NVIS might not work at all below 1 MHz), but for
> higher-frequency AM stations low NVIS antennas could serve
> in a pinch if no other antenna was available. -- JasonW


Indeed. Now the DOG (Dipole On Ground) heard all the way out but not on transmit.


Powell<P ID="signature">______________
NNNN</P>
 
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