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Author Topic: CW Signal Width, Beginner Question  (Read 10012 times)
K6JCR
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Posts: 12




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« on: February 22, 2011, 02:17:00 PM »

Got a question regarding signal width.  My License manual shows the CW signal's width is 500Hz.  But how come I can hear stations across 3kHz of spectrum?  Albeit the tone pitch is much higher or lower, but I can still hear their CW signal.  Doesn't that mean their signal is wider than 500Hz?

I don't have a filter, and I assume this is what it's for.  I just want to make sure I stay within the band edge, and avoid QRMing others.

Thanks!

73's

John - K6JCR
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WB2WIK
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« Reply #1 on: February 22, 2011, 03:06:28 PM »

Where does it say a CW signal's bandwidth is 500 Hz?

That would be incredibly wide for a CW signal!

A continuous carrier without any keying at all is close to "zero" bandwidth.  Keying it on and off (Morse) widens the signal, but 500 Hz is ridiculous.  Most CW signals likely have a bandwidth of less than 100 Hz, including all factors attributed to keying.

You hear signals across 3 kHz due not to the width of those signals, but the width of your receiver.  If you had a very narrow CW filter in your receiver, you'd see the same signal looks much narrower, and would disappear completely when you tune just a few hundred Hz away from zero beat.

Modern transceivers display the carrier frequency, usually with better than 1 kHz accuracy.  Some with much better accuracy than that.  Stay 1 kHz away from the band edge based on the frequency display, and on CW you'll be fine.  If you can verify display accuracy (confirmed with WWV or some similar standard), you can get closer than that.
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DJ1YFK
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« Reply #2 on: February 22, 2011, 03:11:31 PM »

Hi John,

Quote
But how come I can hear stations across 3kHz of spectrum?  Albeit the tone pitch is much higher or lower, but I can still hear their CW signal.  Doesn't that mean their signal is wider than 500Hz?

no. The tone you hear is the product of a mixing process between the received signal and the local oscillator of your receiver. As such, it can have any value, whatever you tuned your receiver to, e.g. 3 kHz. But that doesn't mean the bandwidth of the signal is 3 kHz. The bandwidth is simply the difference between the highest and the lowest frequency in the signal spectrum. And no matter if the signal appears at 500 Hz or 3 kHz in your receiver, it's just the same signal (with the same bandwidth) shifted up or down in the frequency spectrum.

For a CW signal, the bandwidth (with a properly adjusted transmitter) is approximately "wpm * 4", e.g. for 20 wpm it would be about 80 Hz only.

73
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K6JCR
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Posts: 12




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« Reply #3 on: February 22, 2011, 03:33:12 PM »

Thanks for the replies.  That makes sense.  A filter is definitely in my future then.  I was bummed because the 20M band seemed so crowded to me, I hear CW signals all throughout that portion of the band.  I hardly ever transmitted because of it, on top of being intimidated because I'm learning.

To answer the question:
Quote
Where does it say a CW signal's bandwidth is 500 Hz?

On the ARRL Extra Class License Manual, Table 3-3:
A CW transmitted carrier frequency at 14,040.00 kHz, the transmitted signal occupies 14,039.75 to 14,040.25 kHz assuming a 500 Hz wide signal.

I guess they are being overly cautious?

Anyways, thanks for opening up a HUGE amount of radio spectrum for me!  Much appreciated.

73's

John - K6JCR
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KE4JOY
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« Reply #4 on: February 22, 2011, 03:45:17 PM »

I'm kind of supprised that any rig does not have some type of filtering in it for ssb vs cw.

Look for a mode select there somewhere  Wink

In otherwords put the rig in CW even if your just listening.

One trick I will use is to listen with the ssb filter in to find a signal then switch to cw filter and fine tune it.
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NI0C
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« Reply #5 on: February 22, 2011, 03:53:36 PM »

This is a fascinating topic, because there are no easy answers to the question of how wide is a CW signal.  Because CW is a form of amplitude modulation, its bandwidth is determined by the sidebands created by the modulating envelope, which in turn, depends on the shape and timing of the transmitter keying.  W8JI has some good information on his website.  

Just yesterday, Scott Ellington posted (to the Elecraft Reflector) a link to his measurements of his "modified" (presumably for reducing keyclicks) FT1000D keyed at 40 wpm:
http://www.mail-archive.com/elecraft@mailman.qth.net/msg111883.html

Note that the signal is down about 35 dB at the band edges of his 400 Hz filter.  Other transceivers may be wider! In practice, Morse code can be copied in very narrow bandwidths, and the formula provided by DJ1FJK sounds intuitively correct.  

Probably too many people think CW communications require a 500 Hz bandwidth, just because that is a "standard" receiver filter bandwidth.  

73,
Chuck  NI0C

 
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K6JCR
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« Reply #6 on: February 22, 2011, 04:16:25 PM »

I'm kind of supprised that any rig does not have some type of filtering in it for ssb vs cw.

I'm using a Yaesu 817nd(with amp).  Looking at the filter section of the manual now and it is optional.  And it's $164 option!  Holy smokes.  I'm not too keen on that, but it looks like I don't have a choice.

Any recommendations on my two CW options:  500Hz/2.0kHz or 300Hz/2.0kHz ?

Or sell this darn thing and get a real rig.
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NI0C
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« Reply #7 on: February 22, 2011, 04:48:45 PM »

Quote
Any recommendations on my two CW options:  500Hz/2.0kHz or 300Hz/2.0kHz ?

I'd advise the 300 Hz option.  It is a good bandwidth for enhancing s/n ratio and reducing QRM, while being manageable for tuning across the band.  I generally tune the CW bands with a bandwidth between 350 and 250 Hz, when I'm searching for DX.  I use a 500 Hz bandwidth for calling CQ on Field Day, where people are sometimes pretty sloppy about zero beating.

73,
Chuck  NI0C
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VA7CPC
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« Reply #8 on: February 23, 2011, 12:30:01 AM »

The FT-817 _is_ a "real rig" !!!

The receiver is decent, it's got a pretty stable master oscillator, a built-in keyer -- its only "flaw" is that it only puts out 5 watts.  If you have a good antenna (full-size dipole or better), that is often enough.

I have the Yeasu 500 Hz filter.  I think, if I were doing everything again, I'd get the 300 Hz filter instead.  The 300 Hz filter might be a little narrow for RTTY, but it's better for CW.

W4RT has a less expensive filter option than Yaesu:
 
www.w4rt.com

Last weekend was unusually busy -- the very important ARRL DX CW contest was running.  But even in "normal" conditions, a narrow CW filter will be useful.

If you want a ton of FT-817 expertise, join the Yahoo group for it:
 
http://groups.yahoo.com/group/FT817/

                   
                   Charles
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KJ4FUU
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« Reply #9 on: February 23, 2011, 07:03:15 AM »


I'm using a Yaesu 817nd(with amp).  Looking at the filter section of the manual now and it is optional.  And it's $164 option!  Holy smokes.  I'm not too keen on that, but it looks like I don't have a choice.

Any recommendations on my two CW options:  500Hz/2.0kHz or 300Hz/2.0kHz ?

Or sell this darn thing and get a real rig.

AHEM...I have an FT817ND and I've reached Serbia on 5w SSB (still waiting for the card, though). It IS a real rig!

I have the TCXO and 300hz filter options. No, it's not cheap, but you will notice quite a difference when it kicks in. I wanted the 500hz, but would have had to wait, since they were out of stock, but they did have 300hz filters, so I went with it. No regrets.

I will be starting to do some PSK31 soon. No amp for me.

-- Tom
« Last Edit: February 23, 2011, 07:05:14 AM by KJ4FUU » Logged
KC9TNH
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Posts: 304




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« Reply #10 on: February 23, 2011, 07:31:02 AM »

Good question, good answers, and interesting info (preferences) on the 300 vs. 500Hz CW filters. Gent at w4rt.com answered my query and recommended the 500 for general use in my 817nd (have zero interest in RTTY, other than the Army girlfriend I had 30 yrs ago who ran a RATT rig....).  Haven't actually purchased the filter yet.  Have heard some regard 300 as a bit too narrow and, spinning around to find places to practice my copying, wondering... why? Sounds like a subjective preference thing to me, but am curious for a few more opinions on 300 vs 500.

Regards,
Smiley
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
AE4RV
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« Reply #11 on: February 23, 2011, 07:47:11 AM »

I would pick the 300 over 500 if I could only have one, but the 500 is more than adequate most of the time. My Icom 703 arrived from ebay with a 500 Hz filter and I wanted to sell it in favor of a 350, but I didn't want to wait so I went ahead installed it. It's fine, but for big contests like last weekends ARRL DX CW, I enjoyed using many different filter widths with my 746Pro, even down to 100 Hzs occasionally.

The 500 Hz filter is probably a little more pleasant on the ears and good for general, daily use. Big contests don't happen every weekend. I usually do casual CW with a wide BW, say 1200 Hz and only turn on a filter when needed. Very nice to have when someone starts tuning near you or does a brief QRL then starts CQing right next to the signal you're tying to copy...

73 and enjoy your new filter, you'll be glad you got it.

Geoff
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KC9TNH
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« Reply #12 on: February 23, 2011, 09:05:27 AM »

AE4RV many thanks. Not a contester; 500 sounding like a normal duty solution.

I can understand your preference, though:
"...someone starts tuning near you or does a brief QRL then starts CQing right next to the signal you're tying to copy..."

In the short duration since licensed I don't have sufficient limbs & digits to count those occurrences. Smiley
Big ripples in water; filter will be my wing dam.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
KE4JOY
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Posts: 1335




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« Reply #13 on: February 23, 2011, 09:13:23 AM »

Well no wonder he cant find a clear spot .. no CW filter.  Tongue

I think its kind of ridiculous to sell a HAM rig that is CW capable without a CW filter as 'standard' equipment.

Just another way to wring out a little more money as I see it.

That being said go ahead and get a filter you will be amazed at the difference adjacent signals just disappear, the signal to noise ratio of the desired station increases (even though the 'total' signal level may drop a bit). It's a win win for all involved.

Besides it will increase the re-sale value of your rig.
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AE4RV
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« Reply #14 on: February 23, 2011, 09:20:53 AM »

Glad to help and I agree with KE4JOY including the part about adding to resale value. You're not throwing this money away - you'll likely get most of it back someday.

Like I said, most of the time maybe 60% I don't use any filter - the bands are usually not that crowded where the casual slower speed code lives (<20 WPM). The 500 filter will sound better but will occasionally not be enough. I don't think you can go very wrong with either of them. It sounds like you're settling on the 500 and I think you'll love it.
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