P15 AM skip

Mr. Crime, I am one person standing alone in my belief that my .1mW signal will show up many miles away for the benefit of keen DXers probing the sky for weak traces of life.

But the nay sayers are everywhere claiming things like something I read, which I can't seem to forget...

"To reach the ionosphere with a strength sufficient to make the return journey on a skip wave, transmission power on the medium wave band would need to be at least 20 Watts."

If that's true we can realize that in drafting the Part 15 rules the FCC wanted to make absolute sure that we'd never enjoy skipping by setting our power so low that we'd never have reason to hope.

I think we should be allowed 19 Watts, just below skip power, then our lives would amount to something and we wouldn't need to conceal our small radio activities.

I'll wake up in a few minutes and have no memory of saying these things.

Quite afew people have operated morse code

(carrier on and off) beacons between 525 and 1705 kHz.

Some people have done digital modes (WOLF? PSK31?)

and others.  These can and have been heard on Part 15

AM BCB for several hundred miles (just barely.)  So to be

clear -amplitude modulation won't work - it's too weak but

the RF energy will skip and can be recovered via digital modes.

I know a couple people who have done it.

I think the best Part 15 AM distance I have heard of

is 13 miles????  And with a fantistic receiving set-up.

Best wishes

Bruce, CBJB

Completely

Broken

Junk

Broadcasting

Where did the notion that it takes 20 watts to bend a signal in the ionosphere come from?

Any signal in the AM band can be bent but hearing it is another matter. To extract the information on a signal at the receiver a certain minimum signal to noise ratio is required. An example of this is what happens to Part15 AM signals at night. Local range is reduced not because the signal is getting weaker but because the signals from distant stations appear as noise lowering the signal to noise ratio.

The path loss of the signal is a major factor in whether the signal will be heard and .1 Watts minus the skip path loss will almost always result in an unreadable signal to noise ratio.

In theory, the 100 mW signal travels infinite distance but the farther it goes the lower the S/N ratio becomes.

Long range communications, such as from space probes, lower the noise by reducing the bandwidth (using, for example, 10 Baud signaling) thereby lowering the noise and improving the S/N ratio at the receiver. Morse code is a low bandwidth signal and greatly improves the distance over which the signal can be "read" because of this.

Neil

I was hesitant to quote the 20 Watt notion regarding a minimum power to produce a sky-wave, but I know I "read it somewhere once."

If I ever run across it again I'll remember to sign on here and name the source.

If the 20 Watt idea has any validity, perhaps it was meant to suggest the power level necessary to produce a receivable signal above the noise floor in an average situation.

Neil's presentation is much to my liking because I want to believe that our little signals have infinite capability, we are on the air whether anyone can detect it or not.

Since this subject came up, way back when I used to constantly surf the dial at night seeing what I could pick up(in Toronto) and found that the stations all came from the south, southeast, or southwest.....never from the north. I always listened to stations from NY city, Boston, Detroit, Charlotte North Carolina, Chicago, St Louis, Fort Wayne, just to name a few but why couldn't I get Ottawa, Montreal, Winnipeg, Kingston, North Bay, Sudbury, etc.

Was always aware of this but never thought to find out why. Maybe someone here knows...?

Mark

The "Critical Hours" are the two hours before sunrise and later in the day the two hours from before and after sundown.

Long distance reception starts happening in changing patterns during the critical hours and becomes more stable during the darkness hours.

Perhaps the morning critical hours will bring stations from a different direction than the nightime critical hours.

This is a guess.

Don't spend your life guessing, join the ALPB.

Here is a rough idea of the greatest nighttime skywave field strength a Part 15 AM system could produce at a distant receiving location.

Setup:

• 3-meter monopole with base at the surface of the earth, using a set of eight horizontal radials each 20 feet in length, buried in soil of average conductivity (5 mS/m, d.c. 13)
• Transmitter output power at input of loading coil = 85 milliwatts
• Operating Frequency = 1650 kHz
• Loading coil resistance at system resonance = 15 ohms

For these condtions the greatest field directed toward the ionosphere occurs at an elevation angle of about 26 degrees.  Its value at a distance of 100 feet from the antenna is about 5 millivolts/meter.

If the useful reflecting layer of the ionosphere is located about 60 miles above the surface of the earth, then the total path length for a space wave to and from the ionosphere is on the order of 300 miles (flat earth).

If the ionosphere reflected 100% of the signal reaching it back to the earth, then the reflected skywave field intensity arriving at the earth downrange would be reduced by a factor of 0.019/300 = 0.000063.

So the original 5 millivolt/meter field at 100 feet would drop to about 0.32 microvolts/meter at the receiving location.

Unfortunately, a field intensity of 0.32 microvolts/meter is far below the level of atmospheric noise and co-channel interference in the medium wave band, let alone the sensitivity specification of typical and even expensive AM broadcast receivers.

Ham radio operators often can communicate with other hams over 1000 miles away on the 160-meter band -- which is just above the top of the MW broadcast band.  But they are using much more power, and much more efficient antennas than are permitted for unlicensed operators covered by Part 15.

As an experiment one time, well maybe 3 times, I transmitted an unmodulated 100 mW input power with a Rangemaster. Then I used a Kenwood TS-50 attached to a Texas BugCatcher that was tuned to 3900 KHZ.  Not an optimum match for 1610 KHZ.  The TS-50 was in the CW mode.  I could hear my tiny signal about 3.5 miles.  So with a better match on the RX antenna, probably more.

I had meant to say that before.

Some related comments to follow.

Bruce, CBJB

Is a longwave Part 15 experimenter

doing something similar.

For those who are not familiar - the rules

between that band 160 to 190 kHz are I watt

DC into the final with a roughly 50 foot antenna.

The experimenter uses some kind of receiving loop

on the far end with a portable LW receiver of some kind.

He is several miles from his beacon transmitter and you can

hear the cw signal on the video.  It's not very strong - but it's

there.

I remember he could not put up a 50 foot antenna - I think he only

was able to do 30 feet.  It still worked though.

It's a cool video.

Bruce, CBJB

Bruce I think this is it... everything you said fits this video...

The first spring-like day of the year and everyone is walking a dog, or rushing out to buy a dog. But not me.

Instead, I went over to Save-So-Much for snacks and a public appearance with all the shoppers.

I noticed that other men about my age were shuffling around with glum expressions, a dead giveaway that they don't have personal radio stations.

In my head I realized that if these people knew I was Carl Blare, group owner of part 15 radio stations, they might insist on picking up the tab at checkout. Instead, I just smiled and greeting everybody as the crossed paths.

At times I think my brain receives some thoughts by direct path, and other thoughts come in as sky-waves, giving me that dizzy look.

It's never too late to join the ALPB.

I had lost this post, didn't see it on the page when I came back, now I found it, sorry guys.

Thanks for the ideas on Skip, and the breakdown on why it's almost impossible. My idea was that a signal could suffer from some strange propagation that would "beam" it to a certain area, even a tiny signal. That happens on FM a lot, and a small station will skip in from hundreds of miles away for a while with certain weather conditions.

I wonder what the record for distance on a groundwave signal would be? Not that someone would listen, but thinking of experimenters who do it with wifi signals between mountain tops, and other ham type things.

I noted a funnelling effect when operating on Bowen Island on FM.  Now, it should be recognized that the allowed field strength up here in Canada is 4 times that of the U.S., so all things being equal, we'll get 4 times the range (which worked out, line of sight, to between 1/2 - 1 km with a very sensitive car radio at the receiving end).

The transmitter was located at the top of a hill, overlooking my desired coverage area (so it really was line of sight).  However, off to one direction, there was a valley with 2 steep mountains on either side (Bowen is basically some very large rocks sticking out of the ocean) and the transmitter was at the start of that valley (opposite to the desired coverage area).

I noted a very interesting effect.  The signal went a lot further up the valley - in fact, I could sometimes hear it where I lived, at the opposite end of the valley and again, line of sight - and I lived (line of sight) over a mile away.  It was not a great signal, almost at the noise level, but it was there.  The weather played an important factor as well - I noted best range when the weather was sunny, but cool and crisp, usually early in the day.

I never did that experiment with AM, but I would imagine that the much higher wavelength signal would get absorbed easier in the surrounding terrain.