{long} RE: [NLRS] Question on emission type

[Baker, Donn B]

Hi Mike,
I'm not sure what the FCC definition of "pulse emission" is, but...  pulse is generally either fully ON or fully OFF.  In many respects, CW is "pulse" but not classified as such.  However, if you ran CW at (guessing !) a several thousands of words per minute you'd come close to real pulse emission.  (Think of a series of morse "dits."  Each is 1 element long, with a space of one element between them.  Send them fast enough that each element was only a (very) few microseconds long, and you'd have it.)

There are advantages with pulse: you can have very efficient amplifiers (non-linear); by changing the duty cycle (ON vs OFF time), you can have very high output power ("peak power output") without exceeding the "average power" limitations of, say, your power supplies, etc.  In the old days, when ham's maximum power limits were 1000W DC input, pulse would let you have 2,000 or more watts peak output power.

There are also some down sides to pulse, mostly having to do with bandwidth.  The pulse shape itself, along with the very sharp rise/fall times of the pulse, cause LOTS of harmonics and a "spreading" of the signal.  From my experience, signals in the UHF band were several MHz wide.  In a word, interference.

Pulse, per se, doesn't convey a lot information in our terms.  Just a pulse train is OK for RADAR, although typically there's a lot of modulation on RADAR signals.  For communication purposes, two types of modulation are very common: pulse-width, and pulse-position modulation.

You might be familiar with pulse-width (PW):  the idea is used to control the speed of DC motors and model airplanes.  With the motors, the higher the duty cycle, the faster the motor runs.  That is, if the ON time is short compared to the OFF time, the AVERAGE DC value of the pulse train is low, and the motor runs slowly.  If the pulse is ON more than its OFF (high duty cycle), the DC value is high, and the motor runs faster.

For model airplanes, a given pulse width, (1ms ?) is "zero." If the transmitted pulse is shorter, the servo moves left; if the pulse is longer, the servo moves right.  Voice can be put on PW by making longer widths correspond to high amplitudes, and shorter widths equal low amplitudes.  This is how the Ramsey laser communicators work.  The basic pulse is around 40KHz.

Pulse position (PP) has a couple of variations: the easiest actually uses two pulses: the reference, and the one carrying information.  How soon (or late) after the reference pulse the second occurs is the information.

RADAR systems can have many variations of PW, PP as well as frequency modulation (called "chirp") on any give pulse or pulse train.  Typically, long pulses, with an FM chirp on them are used for long range detection of targets.  A long, chirpy pulse puts more effective RF on the target, which makes a better return, which means that smaller targets may be detected at longer distances.  (This requires signal processing... you don't just display it on the B-scope like you see in the movies).  Conversely, a short pulse is used to determine the distance and/or radial velocity of a target VERY accurately.  Most tactical-type RADARs will use a combination of short, long, and chirpy, pulses in every cycle (called the pulse repetition interval).  I don't know about the ARS-x stuff (Aircraft surveillance radars) as I've never been around them.  Likewise, I don't know about the modern NWS doppler radars (WSR-88D). They may use a chirp, but I just don't know.

As Jerry, K0CQ pointed out, hams lost pulse on 23cm most likely due to the use of TACAN, etc., in the 1000 to 1100 MHz band.  The splatter of poor equipment 23 cm would have wreaked havoc on the unsophisticated airborne receivers (longer range, and LOS to boot!) in use (and STILL used today!).  There used to be some airborne IFF gear in the 8-9GHz range, and there were early precision tactical radars up there, too.  Today, a lot of aircraft WX radars are in that band (as are ship radars).

Pulse is permitted in the 6cm band... maybe we should give WiFi some competition ?  1 microsecond wide, 1500W pulses (max peak power under todays rules) should really improve the range of a WAP !!

73 Donn
WA2VOI/0


> -----Original Message-----
> From: nlrs-bounces at mailman.qth.net
> [mailto:nlrs-bounces at mailman.qth.net]On Behalf Of Mike
> Sent: Thursday, August 12, 2004 1:40 PM
> To: nlrs
> Subject: [NLRS] Question on emission type
> 
> 
> Hi all, I had a good time in the contest last week after I found the 
> problem I was having.  Thanks.
> I have a question though, I was reading the new ARRL petition for 
> regulating by bandwidth not specific mode and there was something I 
> don't understand.  It said that "(c) Pulse emissions are permitted on 
> all bands authorized to the control operator above 902 MHz 
> except in the 
> 23 cm and 3 cm bands."
> First off what are pulse emissions and why wouldn't they be 
> allowed on 
> 23 and 3 cm?  Would this have something to do with radar???
> 
> Thanks for your thoughts
> 
> Curious Mike
> 
> Kb0ozn
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