Part 15 in the 160-190 kHz Band

Interesting analysis. The antenna diameter used was 4 inches. Would using a wire (~0.1 inches) make the bandwidth narrower?

Neil

Would using a wire (~0.1 inches) make the bandwidth narrower?

Yes, it would.  If the conductor was 0.1" OD, then its capacitance drops  to ~100 pF and its Xc rises to ~9100 ohms -- which would need a loading coil of about  ~8300 µH for resonance on 175 kHz.

That inductance is 1.78 X greater than when using a 4" OD radiator, so the r-f loss in the coil would be considerable higher, which would reduce radiated power.

If the coil loss went up to 50 ohms, then the 3 dB r-f system bandwidth would be about 1.9 kHz, or less than 1 kHz audio bandwidth (at -3 dB) at the output of an AM receiver.

Such small r-f bandwidths make the system very difficult to tune to resonance, and to keep resonated during changing environmental conditions.

I appreciate this subject opening up because a LW project is happening in slow motion and when I finally move to the next stage I will post on this thread rather than the older thread on the subject.

What we have on paper is a draft for a LW transmitter built around the shortwave transmitter previously built by a committee of forum members. The LW project has the project name "Deep Voice" and as soon as I can I'll update the schematic draft.

Here is the project link, but it is not up to date

http://kdxradio.com/deepvoice.html

I will add Rich's data from above

After circling back to absorb more carefully the findings of the analysis, two questions explain my now raised hand:

1.)  "resistive loss of the ground connection would probably be greater (than a comparable MW system)" ----  What accounts for this greater loss? Why wouldn't the loss be equal in either instance?

2.) With an audio bandwidth under 2kHz, where is this audio bandwidth centered? If it includes only the very bottom of the audio band it would sound like the .6 bass channel in a 5.6 channel audio system, which would be somewhat pointless.

1) The difference in r-f ground loss is due the practical length likely for buried radials for Part 15 systems on 175 kHz relative to 1600 kHz or so.

If both systems used buried radials with the same free-space wavelength at their frequency, then for the same number of radials buried in the same earth, their r-f ground connection loss would be the same.

The r-f currents flowing in the earth within 1/2 wavelength from the monopole need to be collected by the ground system and returned to the "ground" terminal of the antenna system

2) The lower limit of the audio output of the receiver could extend down to 20 Hz or less, if the transmitter and program supported that (as long as the r-f bandwidth of the signal was least 40 Hz),

If the r-f bandwidth of the antenna system is 4 kHz, the upper frequency limit for the receiver output audio will be 2 kHz.

For DSB AM signals the upper frequency limit of the audio at the receiver output always is 1/2 the r-f bandwidth.

I actually understand the expalanations of both 1) and 2), even though it was initially not registering. The best question is an answered question.

I want to find out how that will sound, so I plan to set the bandpass in the StereoTool Plugin for the 2kHz bandwidth and hear it on an MW radio.

Question # 3:

Will an audio pre-emphasis help a little?

People with a Tecsun PL310 can select 1, 2 , 3, 4, or 6 kHz audio when listening to a local AM station.

3)  Yes, a moderate amount of audio pre-emphasis can help a little.  But if it is too much the transmitter can overmodulate at those frequencies, and/or the average modulation level might need to be reduced to keep the HF content at/below 100% modulation.

Based on this input by Rich it would be my recommendation that the Deep Voice LW transmitter now under design, include the addition of a pre-emphasis circuit.

It remains to plot the curve of that circuit within the confines of the limited bandwidth.

It should probably start to roll-up right at the point where the antenna bandwidth starts to roll off.

What else can be said about such a slight high-end boost?

By good fortune I am among people with a TECSUN PL-310 Radio (eternal gratitude to Rich for the recommendation) and have done the AM listening tests at 6, 4, 3, 2 and 1 kHz audio bandwidth.

Using speech for the test here is what we found:

6kHz bandwidth sounds very bright and clear, and is probably typical of most AM radios;

4kHz bandwidth is still pleasant, the ear adapts to it, and sounds like the treble control is reduced somewhat;

3kHz starts to sound like maybe you're listening from under a sheet, but is at the limit of acceptable audio quality;

2kHz puts your head in an overloaded laundry basket with the ears surrounded by overcoats;

1kHz is what you'd hear from a cat buried in a 5-foot hole in the ground.

What we have learned today is that forcing decent AM audio quality out of a LW transmitter is going to be among the many challenges of building to Part 15 limitations.

You get used to that 1 kHz selectivity

setting after a while.  WTIC on 1080

in Hartford - the gigantic 50,000 watt

flame thrower has IBOC hash on 1090

and 1070, but I can mess around with 

the PL-380 and get WBAL in Baltimore

on 1090.  Is it easy?  No - but it can 

be done with that radio.  Oh, by the 

way, the PL-380 I have is not sensitive

on the longwave 160 -190 kHz part 15

band.  (For whatever it's worth.)

It is not easy to find a REALLY GOOD

longwave radio.  And if you are going

to test a Part 15 LW set-up, you have

to have one.  You have to be able to

hear a lot of the NAV beacons on LW.

And if you are on the east coast, you

have to be able to hear LW broadcast 

stations from Europe and Asia.  Then

you know you are getting somewhere

with the receiver set-up.  But I know you

guys know this.  In fact, based on your

above discussions, you guys know a lot

more than I do.  

My favorite logwave BC station

is Iceland on 189 kHz.  Great music!

I can't understand what they are 

saying, though.  

Bruce, 

Dogs and Radios, and... Furniture

How does the Iceland LW bandwidth sound on their music?

Of course they have proper antenna/ground, so it is probably much better than we will get at Part 15.

Hi Carl!  How does the station from

Iceland sound on 189 kHz?  Well, the

radio I use is an old marine direction

finder that is really really sensitive on

longwave.  I do not know it's selectivity.

So all I can say is that the Iceland

broadcaster sounds like a regular AM

radio station - or maybe a shortwave

broadcast station.  These statons hardly

fade at all - you can listen to them all

night long - well - until THEIR local sunrise.

Some of the stations in this

band (about 153 to 279 kHz)

are running a million watts. 

I think there is one that is

running 2 million. 

The BBC European domestic service

is on 198 kHz.  Their ground wave

covers a very large part of Europe,

and they still have a great many listeners.

And a lot of European car radios have

the longwave band. 

I have heard the BBC here too, but they are

almost always covered up by a NAV beacon.

How do they do it - with the audio and

everything else?  Gosh, I have no idea.

It really is quite a feat.

By the way, in order to receive

these stations, I have to go

outside in the back yard with

the radio.  EMI from in the house

is too great.  I have almost never

heard a LW broadcaster indoors. 

The marine direction finder has a

loop antenna that works great.

If I want to listen indoors, I just

put the receiver out in the back

yard and send the signal back in

the house with my C. Crane transmitter.

Best Wishes,

Bruce, Radios and Dogs

During the first half of infinity I have never thought of taking a LW radio outdoors!

Wonder what would happen if a super wealthy and very eccentric person, one of us for example, applied to the FCC for a LW license at high power.

If Alex Jones and other dooms-dayers are correct, we will all end up living outdoors, and a big LW radio station might start being important.

I had always assumed Longwave had the same bandwidth as medium wave, arent all modes of AM in North America 10khz? (Excluding the air band)