What's the deal with SSB frequencies? - SailNet Community
 
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post #1 of 7 Old 06-06-2009 Thread Starter
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What's the deal with SSB frequencies?

Can someone refresh my memory in layman's terms? What is the difference between transmitting on 4.105 vs. 8.104 vs. 6.221? I know that when I receive a lot of interference on the 4MHz frequencies I usually also receive similar interference on 8MHz, but I'm often OK on the 6MHz. This was explained to me once, and I'm trying to remember how it all works... a pointer to a web tutorial in plain english would suffice too... Thanks!

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post #2 of 7 Old 06-07-2009
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There is a lot of good information on Pactor-II/III Radio Modem sales/support, FCC License filing, Marine SSB & HAM Radio Net schedules/frequencies.

Channel list
SSB Simplex Channels

East Cost Nets
East Coast Cruising Nets

They publish a sailor's guide to SSB
The Quick-Start Guide to SSB Installation and Operation Table of Contents

Different frequencies have different propagation characteristics. This is because the signals interact differently with the various layers in of the atmosphere. For example the 10 meter band (28 mhz) requires a strong E-layer which is created from solar activity so it's best during the middle of the day in the summer. You have to work around these issues plus deal with the skip zone, so you need to know the approximate range of the target station. The skip zone is the distance before the wave is reflected back to earth. Take a look at this document for more details...
www.alconaradio.org/images/NVIS_W8CX.pdf

You have very limited access to frequencies with the a basic marine Restricted Radiotelephone Operators permit. You really need a Ham General Class license to get decent access to the spectrum. I just got mine a few weeks ago.
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post #3 of 7 Old 06-07-2009 Thread Starter
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Thanks Steve - I should clarify. I'm looking for info on the science behind radio frequencies - what the waveform differentiation is between 4MHz and 6MHz. Specifically, why I can receive on the 6MHz frequency in marinas but not a multiple of 4MHz. I'm very familiar with the actual use of the SSB and the various nets out there. I appreciate it!

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post #4 of 7 Old 06-07-2009
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I think you will have better luck looking for your answer on QRZ.COM There's more people with pocket protectors and slide rules over there.

Good luck. Please let me know if you get to the bottom of it.

Last edited by SteveInMD; 06-07-2009 at 11:52 AM.
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The difference between, e.g., 4mHz, 6mHz, and 8mHz waveforms is primarily their LENGTH. Also, as Steve pointed out, their BEHAVIOR is different because they penetrate and are reflected by different layers in the ionosphere.

A 6.227 mHz wave is 936/6.227 = 150 feet long. A 8.152 mHz wave is 936/8.152 = 114.8 feet long.

Shorter waves tend to be more appropriate for communication over longer distances, particularly in daytime. If, for example, you were in the Bahamas and wished to contact a station in Europe during daytime, you'd be better off trying on 12mHz or 16mHz frequencies than lower ones. At night, however, it could very well be that 6mHz or even 4mHz would do the trick.

That said, the reason you are seeing differences in reception in a marina may have VERY LITTLE to do with wavelength and propagation. Rather, it's likely to be associated strongly with RFI...that generated on your own boat, on nearby boats, and by equipment belonging to the marina itself.

Bill
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post #6 of 7 Old 06-07-2009
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I don't get very good TX or RX while in my marina either. I agree this is likely due to RFI and the mast, shrouds, etc. in the area. I have not noticed that 50 meters is better than 75 meters or 37 meters. I would think that the shorter the wavelength the less local interference you would experience.
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Quote:
Originally Posted by labatt View Post
Can someone refresh my memory in layman's terms? What is the difference between transmitting on 4.105 vs. 8.104 vs. 6.221? I know that when I receive a lot of interference on the 4MHz frequencies I usually also receive similar interference on 8MHz, but I'm often OK on the 6MHz. This was explained to me once, and I'm trying to remember how it all works... a pointer to a web tutorial in plain english would suffice too... Thanks!
In a word: Harmonics. The first hamonic of 4MHz is 8MHz. (There'll be another at 12MHz, 16MHz, ad infinitum. The energy decreases at each harmonic. [Don't recall the forumula anymore.]) So if there's a strong broad-band noise source at 4MHz, you'll get its harmonics at 8MHz.

Marinas will be particularly bad for noise. Reason is all the metal-on-metal points. Any metal-on-metal contact that is less than perfect can turn into a form of diode. (Diodes rectify AC--which is what RF is.) Us radio guys call the effect "rusty bolt rectification." The result is the original RF signal poorly/partially rectified and re-emitted on a range of different frequencies and their harmonics. Because the re-emitted signal is partially rectified, it will have sharp edges. Square waves are rich in harmonics. (In fact: A square wave can be thought of as the integral of a primary frequency and all of its harmonics--out to infinity.)

Then there's all the other noisy electrical stuff running in a marina, in close proximity. And all that noise will be seen on their harmonics.

Jim
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