Realistic Part 15 AM Range Estimates

Yes, at lower frequencies the coil stray effects are less important. The stray capacitance effects and the skin effect on resistance are reduced.

I don't have any references or links handy but you can probably find some good info. by doing a search using "160 meter antenna" or similar keywords. I found a nice technical writeup by a ham who designed a short 160 meter mobile antenna.

If you want to go with a classic dipole, the ARRL Antenna Handbook is a good place to start. A horizontal dipole is affected by the height above the ground and this book has charts describing this.

Neil

Efficiencies can be improved by taking measures as simple as impedence matching, or actually CUTTING the length from 3 meters to a length better resonant with 600 kHz and mitigate that electronically (loading, tuning, trimming, whatever). Have you got numbers for some antenna length other than 3 meters that would be resonant with 600 kHz?

I did some quick rough number crunching and find that 3 meters isn't even close to being resonant with 600 kHz, and we're cut down to 1/64th wave getting down to 12 feet and change. Going to 1/128th wave takes me down to 6 feet and change. More precisely 6 feet, 1 inch and 1/8th.

The numbers I posted were based on resonance on each frequency. The loading coil adds the inductive reactance needed to cancel the capacitive reactance of the electrically short radiator needed for Part 15 AM, per 15.219.

Shortening the antenna from 3 meters would not make the antenna radiate better. The antenna system would radiate worse, at a given frequency, because its radiation resistance would go down. And the radiating structure can't be made longer without violating 15.219.

An antenna does not need to be related to the wavelength used in order to radiate efficiently. It will radiate virtually all of the power that can be made to flow into it, regardless of its electrical length. Making the antenna resonant makes it possible for the maximum amount of power available from the tx to flow into the antenna, after considering other system losses in the coil and r-f ground.

Also, relating to another of your posts... horizontal polarization is OK on MW for "yardcasting," but its groundwave propagation losses are much higher than if vertical polarization is used. That's why all broadcast stations use vertical towers.
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Well, thanks for your input; I'm not certain that I buy the entirety of that, but it certainly was informative.

Thanks again.

I know that I am re-visiting an old post but for the newbies here this is good info. What I have discovered is that many times distance is measured with an automobile odometer. For example, I got into a peeing contest a couple of years ago about the distance received from my station. The listener told me that he could hear me loud over a mile. I reality it was about a mile driving distance for him but about 1/4 mile "as the crow flies from my station."

I recommend for those who want accurate measurements to use the distance tool in Google Earth or Circle Plot generator from the FCC. You may be surprised.

I generally agree with the conclusions here and they certainly match my real world observations as well.

But a few caveats (you knew there had to be some).

There is a very, very wide range of sensitivities and selectivities in both indoor and car radios, which will greatly affect range, both to the positive and negative.  It would be more helpful if some assumptions were made vis a vis those values, and the results re-calculated.

As just one example, I've been in cars where it was difficult to hear my (legal) AM signal at half a mile.  And in a car with a particularly sensitive car radio, I was able to hear my signal over 2 miles out (it was very noisy but still could be heard).

Selectivity is one facet of radios that is often ignored.  And yet, poor selectivity is often the reason an indoor radio can't hear you - strong signals bleed over into adjacent frequencies and interfere with a puny Part 15 station.

And finally, what is an acceptable signal?  It probably varies from person to person.  I'd like to see some definition there as well (i've made attempts in the past to do just that).

The basic problem I've seen in discussing range is the lack of rigor (not mortis) in the description.  Just because someone says they get 2 miles range doesn't necessarily mean they're illegal.  They could be using a long antenna, they could have a particularly good receiver or they might just be barely hearing the signal in amongst the noise.

On the other hand, if someone says that they get a crystal clear, noise-free signal 1 1/2 miles away on ordinary indoor radios, I think I'd be suspicious and take the claim with a grain of salt.

Good idea, John WDCX, to bring this post out of the moth balls.

None of my experience with AM Part 15 transmitters differs in any way with the estimations calculated by Rich.

Artisan makes a very good point about the widely variant AM receivers, which can account for a compliant operator reporting 2-miles or so.

The odemeter (straight line) issue mentioned by John comes into play when blocks and streets are involved, the ones around here curve and have wildly different lengths, so I rarely know where I am in terms of knowing a direct route to the transmitter.

I was reflecting just yesterday about how appropriate 100mw/3-meters is for the Part 15 AM allowance, for these reasons:

I don't think anyone would disagree that the purpose behind Part 15 AM intentional radiation is PERSONAL use; permitting sole individuals to transmit to their own radios around a home environment. For that the rules are intelligently measured.

At 100mW with a 3-meter antenna one of my radios within 6-feet of the transmitter overloads and distorts, showing that any more RF power would overload radios farther away, in other rooms.

To enable use of the sensitive radio I roll back the power of the AMT5000 to its minimum, 36 milliwatts, and not only is the reception cured, but the signal is still perfectly robust everywhere in the house and most of the yard.

The fact that rich's calculations extend to a mile, albeit weak and noisy, there is the open door for experimentation and the "sport" of trying to do slightly better within the confines of the rules.

The case exists for a medium power AM service, in the lower Wattage range, but that's a different subject from Part 15.

Let me look around about factory car radios.  Also, the antennas in cars as well.  My bride drives a Honda Pilot. Weak transmitters both FM and AM suck in this vehicle.  I have a "friend" who runs 7 watts on 87.9.  He is solid on my Supertuner with 31 inch Ford antenna all over town. Reception in the Honda is terrible.  Maybe not the radio but more the Window antenna in the driver side rear windo.

I think we need more comprehensive data

from more Part 15 stations? 

My best data: AMT-3000 ground

mounted, 3 meter stick, 16 radials,

big loading coil-

Lobes and nulls  changed a lot (every day.) - it was

nowhere anything like a circle.  Nulls

accured at street intersections.  (Think

about that one.)

2 miles very noisy on car radio in JUST

ONE PLACE, and mostly the signal was

NOT there.  (Why?)

In a really good car radio - various sections

downtown - 1/2 to 1 mile away - fading up

and down all over the place.  (Not the same

every day.)  But good enough for my friend

to listen all over down town - but on a fantastic

car radio...  Following a

powerline for about a mile in the opposite

direction, but NOT receivable when more than

500 feet from the powerline.  (Why?)

On my street - best known inside house receptions -

600 feet away - almost no noise day or night -

but this was on a GE Superadio - a really good

radio.  The fact that it was a good signal at

night, though - means something?  This

was 1700 kHz at the time.  And at that time -

1700 was almost empty day/night.  Except

for a station about 800 miles south of me.

On an "quality unknown, brand unknown"

boombox - in a house - 800 feet away -

noisy - signal 60 or 70 percent - noise

30 or 40 percent during the day.  No

data for night.  Don't know which way

the radio was pointing - i.e., the loopstick

antenna inside.  (This was the 10th house

away - our lots are very narrow.)  No data

for "really bad clock radios."

I think this is realistic for a fairly

good 15.219 set-up.  (?)

I have given this data before

(sorry to those who have seen

it so much) - it might be useful

for newcomers. 

By the way - looking back on this

thread - I miss SCWIS.  He had

fantastic info.  I hope he is

well wherever he may be.  I am facinated

by this thread for whatever reason that

may be. 

Bruce

Re 'encouraging' posts...

It's nice to be encouraging. It's also nice to have your eyes & mind open. If a Part 15 station gets significantly more range than the estimates posted here, then questions should be asked. There may well be a valid reason (I've given several) but there also may not.

It's not being negative. It's being realistic and understanding the laws of physics.

In Reply #12 of this thread, SCWIS wrote: The only hope I see of getting folks to operate legally is to give beginners and other experimenters some possible hope of getting some kind of useful range. I agree wholeheartedly that we must be realistic (if I didn't certain posts would not be here, I am admin, after all) but the tone of realism needs to be positive and encouraging.

What puzzled me then in the quote above, and still puzzles me today is the "tone of realism" that SCWIS was suggesting when others commented on systems clearly non-compliant with Part 15.

The definition of "useful range" lies mostly in the expectations/hopes of unlicensed AM/FM broadcasters.

That "useful range" is not a GIVEN permitted to such operators complying with Part 15.

Information proving such should not be dismissed as negative and/or discouraging.

I see it as a POSITIVE that both theory and practice demonstrate that Part 15 AM ranges of a mile and perhaps a bit more with a listenable signal are certainly possible.

Methinks this might be a place to drop in the link for the article I wrote about AM Part 15 coverage.  Geared more toward a beginner than an engineer, but may offer some useful tips.

www.ironrangecountry.com/coveragearea.html

Tim in Bovey

Indeed, Rich.  My article does mention using a radio with any sort of a signal strength meter on it if you can. You won't get actual, accurate readings of signal in the air, but you will indeed get a reading that you can use to compare with other readings around the signal area and again later to look for changes in signal strength.  And it can also be used to see changes in signal strength with changes in the transmitter site and configuration.  I have one of the little portable radios you mention and it works just fine, so do a wide array of communicatiions and portable short wave receivers that include medium wave (AM) band and an "S" meter.

Tim in Bovey

Along the lines Tim suggested:

http://transition.fcc.gov/mb/audio/bickel/circleplot.html