HFU HF Underground
General Category => General Radio Discussion => Topic started by: ka1iic on January 15, 2016, 1152 UTC
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I like the AM mode best, SSB the least but why have I not heard NBFM being used in the 6900-6990 band... ?
Now I'm not in anyway encouraging anyone to do anything illegal but still. I remember a number of years ago Amateur Radio Operators using that mode on the 75 meter band and it worked out great for the times they used it
Most modern radio gear does have NBFM detectors and even the older radios that don't can still hear the transmissions via 'slope detection'. Slope detection is nothing more than tuning a receiver in the AM mode slightly above or below the center carrier frequency.
Am I not thinking clearly because it is so early in the morning... most likely... <sigh> Never mind... ::)
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FM requires a pretty decent signal strength. There has been some FM on the 43 meter band (I think WBNY ran some years ago?) but I don't recall spectacular results.
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http://www.hfunderground.com/board/index.php/topic,14771.0.html
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man!!!... not the bunny... good grief!
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Radio Bob of RBCN used to try it now and then. Not great results, as I recall, but he could be heard.
man!!!... not the bunny... good grief!
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Well then how about DSB reduced carrier... You can get good fidelity with that... better that USB or LSB but not as good as AM.
DSB reduced carrier has one good point that it doesn't need a high power modulator and I have heard some great signals from such a rig.
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Radio Bob of RBCN used to try it now and then. Not great results, as I recall, but he could be heard.
I heard a rumor he was going to be back on the air soon.
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DSB can work well, no need for steep filters, so you can run as much bandwidth as you want. It's a good idea to leave some carrier leak as a tuning aid for people, otherwise it can be very difficult to get the tuning right.
DSB is also quite easy to generate, even at low levels, and is conducive to class D/E amplification.
+-RH
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Yep DSB equipment can be built very easily... I know because I built a unit from the W1FB QRP handbook (crystal controlled) it is a small exciter and it works amazingly well considering the amount of parts and minimum complexity... wait... I mean simple but it works great... even I was surprised!
I did add on a (near) hi-fi preamp with a 741 op amp as an audio driver to the original design and I get excellent audio... BTW I always use a simple crystal detector to do an 'air check' and run the audio to my M-Audio studio monitors. The audio was better than many local broadcast band stations audio. The audio source was my el-cheapo $8 mp3 player connected to the 741 op amp to the exciter.
Add a little carrier you say... not a problem all that needs to be done is to un-tune, slightly, the variable resistor that is used to balance out the carrier... there you go the small amount of carrier to help those folks tune in the station. done... un-tune it totally and you have an AM transmitter with low level audio modulation, that's how I tested the audio quality with my crystal detector.
I built the DSB exciter in the 'Manhattan style' construction and in modules; 1) the oscillator, 2) the balance modulator stage, 3) the 741 audio modulator stage. I always build things in modules so I can exchange newer designs as they might come along. Makes life more fun and saves in the long run... part wise that is ;-)
Now all it needs is an power RF stage and it is 'on the air'... simple
Anyway... that's the way I done did doood it <heh>
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I like the AM mode best, SSB the least but why have I not heard NBFM being used in the 6900-6990 band... ?
Now I'm not in anyway encouraging anyone to do anything illegal but still. I remember a number of years ago Amateur Radio Operators using that mode on the 75 meter band and it worked out great for the times they used it
Most modern radio gear does have NBFM detectors and even the older radios that don't can still hear the transmissions via 'slope detection'. Slope detection is nothing more than tuning a receiver in the AM mode slightly above or below the center carrier frequency.
Am I not thinking clearly because it is so early in the morning... most likely... <sigh> Never mind... ::)
FM could work easily if the operator modifies or builds a modulator that is capable of a frequency response 20Hz-20Khz, its quite easy to do.
most transceivers (not home brew) have audio circuits with "curved", limited audio response and modulation, with a diagram and knowledge, you can easily "bypass" those circuits.
the problem lies in two areas, first, limiting the correct amount of each audio frequency to a specified deviation for FM, second, some listener receivers including SDR receivers, have a low frequency cutoff filter at 70Hz or higher, the high frequency response is dependent on the receive bandwidth setting of the user or radio.
ive tested FM at a deviation of 10Khz with excellent audio response up to 10Khz with very good results using various radio receivers and SDR receivers.
to get the best AF response from an SDR receiver requires disabling any audio filters present besides the bandwidth filter (SDR sharp has this ability).
while disabling the audio filter on an SDR receiver can produce excellent AF response results, there is a couple of caveats, on AM, the DC from the carrier will be present in the audio and any slight distortion from over modulation in the broadcast will be emphasized (especially AM).
the key is to strictly limit all audio frequencies by using a 12 band audio compressor/limiter/clipper and a HF AF bandwidth filter when transmitting.
while the FM modulation may be a bit low, it will have the same fidelity as 10Khz AM with less static (on a good signal) and QRM.
the additional benefit is that you can operate full carrier and not worry about carrier saturation on AM transmit.
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The problem with FM is the amount of power required for good reception. I did some tests a few years ago with an HP broadband generator and concluded that the bandwidth needed was just too wide for our little chunk of spectrum. For good fidelity, I needed close to 40KHz of bandwidth or so, and that would probably upset some folks. 2KW or so would probably give nice coverage coast to coast...
+-RH
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The problem with FM is the amount of power required for good reception. I did some tests a few years ago with an HP broadband generator and concluded that the bandwidth needed was just too wide for our little chunk of spectrum. For good fidelity, I needed close to 40KHz of bandwidth or so, and that would probably upset some folks. 2KW or so would probably give nice coverage coast to coast...
+-RH
and obviously opening up the receiver bandwidth to 40Khz drops sensitivity but increases fidelity (and adjacent interference).
it is possible to "cram" the audio spectrum up to 10Khz though, you obviously wont get hi-fi sound but it will be similar and comparable to commercial SW broadcasts that utilize 10Khz (5Khz each side band), it just requires lots of "fine tuning".
in general, it not really worth it for purposes of broadcasting unless the general audience listening will be using specialized SDR receivers capable of allowing the full AF spectrum being only limited to the bandwidth filter itself.
this same AF spectrum issue exists for receivers using USB/LSB but not for DSB and not for AM though DC is filtered on AM, USB and LSB, DC is not filtered on DSB.
i totally agree with you that you would require more RF power to achieve the same results though.
as it goes for anything, the more bandwidth you use, the more power you need.
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It doesn't really lower sensitivity, but wider bandwidths also contain higher average noise power, hence the apparent loss of sensitivity. Carson's rule for significant sidebands are related to maximum audio modulating frequency and peak deviation. I was testing with 7.5KHz audio and 5 KHz deviation, and had significant sideband energy out to around +/-25KHz, which is a little on the 'hoggy' side. If it were practical, I'd have done it by now.
You can always close down the receive bandwidth, but the distortion rises very quickly as sideband truncation begins to occur. AM for all its faults is one of the most spectrum efficient modes.
+-RH