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Hi Fi AM

Nov 13th 2013, 03:14

kc2ifr

Joined: Nov 13th 2013, 02:57
Total Topics: 0
Total Posts: 0
I am an AM operator and was wondering if other AM stations are sick of these new Class E stations taking up at least 20kc of the am window. I am NOT criticizing class E equipment.....its great stuff. But the way it is designed (home brew) it will pass dc to light audio.
There are a few stations on 3875 at night here in the north east that really think they are an FM station. One in particular is at times 16KC wide........................WHY? If someone complains to these stations about being "wide".....they will tell u "sos your ass". I have heard these guys say on the air........."sos your wife's ass". Its unfortunate that this kind of CB crap has invaded the AM window. BTW...at least 2 of these class E stations are running over 2KW carrier on am.
Many of the usual am'ers will not get on because of these am bullies.
I can say more.......but lets see how this post goes!

Bill
KC2IFR
Nov 13th 2013, 23:49

KF0XO

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I don't know how you determine what the power of station is running unless you are close enough to take a measurement with calibrated equipment. I know that people say that there is an "AM window" but in reality anyone with a General class license can work any frequency from 3800 to 4000 kcs. I don't particularly like the attitude of many of the people running "wideband AM"but I will move to another part of the band if I need to. I also find that many people on the ham bands have seemed to set their radios on a frequency and cut off the tunning shaft. I will operate AM where ever I find an open space. I don't restrict myself to a "band plan" that a group of people have arbitrarialy set years ago.I like using the lower end of the 75 meter band 3.700 to 3.725 kcs. and on 40 I like 7.125 to 7.150 that way I don't have to contend with SWBC at the top of the band.
73
Norb
KF0XO
Nov 14th 2013, 18:37

WB1GCM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Bill,

There is no limit on bandwidth. One must use good engineering practices. That means, if the band is crowded, it's not exactly good engineering practices to take up a large portion of the radio dial. If it's the middle of the day and the (75M) band is quiet, then it can be fun to experiment a bit. When the band is crowded, it makes more sense to roll off the higher frequencies and concentrate the bandwidth for more effective communication.

The best way to "see" the transmitted bandwidth is with a spectrum analyzer. It's a bit harder to determine the transmitted bandwidth by ear using a receiver only..

As far as the name calling goes, that's not good operating practice and it gives the wrong image of Amateur Radio in the eyes of non-Amateurs. It's fodder for commercial interests that we misuse the airwaves and our frequencies should go to the highest bidder.

Bob
Nov 26th 2013, 17:42

K4KYV

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Quote by WB1GCM
There is no limit on bandwidth. One must use good engineering practices. That means, if the band is crowded, it's not exactly good engineering practices to take up a large portion of the radio dial. If it's the middle of the day and the (75M) band is quiet, then it can be fun to experiment a bit. When the band is crowded, it makes more sense to roll off the higher frequencies and concentrate the bandwidth for more effective communication.


The FCC intended it that way. The Part 97 limits on bandwidth were left deliberately vague, defined in terms of "good engineering and amateur practice", in order to give amateurs the maximum flexibility for experimentation and self-instruction in the radio art. A few years ago, Riley reiterated that at the FCC forum at Dayton when someone posed a question regarding so-called ESSB, although for whatever reason, he went on to raise the issue of whether ESSB has any place in amateur radio at all.

Just as variable selectivity is a desirable feature in a receiver, some variability in audio bandwidth is equally desirable in a transmitter. There are many ways of accomplishing this; in the audio chain that drives my transmitters I have incorporated switchable, passive, low-pass audio filters, something I picked up decades ago at a surplus store and at a hamfest. One filter has a very sharp brick-wall cutoff at 3400 Hz, while the other has a more gradual cutoff that begins just above 5 kHz with everything gone at 7.5 kHz. The third option is no filter at all, with the highs limited only by the frequency response of the audio transformers. Normally, the 3400~ filter is used under congested band condition or when there is a nearby adjacent QSO, and the 5000~ filter is used when the band is less congested. The no-filter option is rarely used except for testing purposes.

Passive filters like mine are hard to find these days, and usually the ones that show up are expensive, but current technology allows one to easily build effective filters at very low cost using active circuitry. I have seen circuits published on websites, using nothing more than a handful of components, usually a couple of transistors and IC chips, plus a few resistors and capacitors. More sophisticated filter circuits can be found using digital technology, for those so inclined.

There is little use in transmitting audio frequency response that would produce 20 kHz wide signals, since very few amateur receivers would be set to wide enough selectivity to receive such a signal in its entirety, and it would be poor engineering and amateur practice to deliberately transmit such a broad signal merely to keep the adjacent channels clear. But, OTOH, "wide" signals that cause harmful interference to adjacent channels are more often the result of spurious distortion products than the frequency response of the audio used to modulate.

I would recommend first and foremost, a "clean" transmitter that is not pushed beyond its modulation capability, and then an appropriate low-pass audio filter in line to limit bandwidth as needed. Merely lopping off the higher frequencies beyond a certain point doesn't cut it; the overall audio response curve needs to be adjusted for balance to produce pleasant sounding audio that is still readable under adverse conditions.

I often get reports that my audio is "broadcast quality" when using the 3400~ filter. A "presence rise" of some 9 dB in the upper mid-range seems to allow the articulation of consonant sounds to still pass through, and compensate for the limited high frequency response. Nevertheless, when running an A/B comparison under less congested band conditions, most reports tell me that the signal sounds better with the 5000~ filter. But they usually tell me there is little or no difference in audio quality between the 5000~ filter and no filter at all.

An interesting tutorial on AM and audio frequency response can be found at http://www.amwindow.org/tech/pdf/eam.pdf

Don k4kyv

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