[Milsurplus] 10 meters w/BC684,924 xmtrs

[WA5CAB at cs.com]

Ben,

I didn't say that you couldn't operate FM below 29.0 MC.  I said that you 
couldn't legally (and I should perhaps have added "using an amateur license 
issued by the US FCC") operate a BC-684 below 29.0 MC.  47 CFR 97.305(c) shows that 
all phone sub-bands below 29 MC are subject to 97.307(f)(1).  97.307(f)(1) 
says 

"No angle-modulated emission may have a modulation index greater than 1 at 
the highest modulation frequency."

Modulation index is peak deviation divided by modulating (base-band) 
frequency.  Thus if you take 3 KC as being the highest modulation frequency (and other 
rules prevent you fron using 10 KC or 20 KC base-band just to get around the 
rule), then below 29 MC, peak deviation is limited to 3 KC.

You can, of course, significantly decrease the peak deviation in the 
transmitter.  However, if you do the receivers will be deaf.  If you try to retune a 
BC-603 or BC-683 receiver for narrow band usage, you will find that (a) the 
tuned circuits aren't sharp enough and (b) the HFO stability is too poor for it 
to work.  Plus any noise in the transmitter audio circuits is effectively 
multiplied by five.  I guess you could get around that by using a fixed attenuator 
right at the end of the audio chain.  But I doubt that the frequency stability 
of the transmitters is up to it, either.

One further comment to forstall getting jumped on by someone who has read the 
BC-1000 alignment instructions - the effective bandwidth of an FM voice 
signal is wider than twice the peak deviation.  So a transmitter running a peak 
deviation of 15 KC with around a 3 KC base-band has a typical bandwidth in the 40 
to 50 KC range.

Finally, it would probably be better to describe the WW-II and Korean War 
vintage FM sets as medium band rather than wide band FM sets.  The Vietnam War 
vintage sets when set to operate with the KY-8/TSEC are really wide band.

In a message dated 8/29/2006 1:18:08 PM Central Daylight Time, 
Military1944 at aol.com writes: 
> In a message dated 29/08/2006 19:10:11 GMT Daylight Time, WA5CAB at cs.com  
> writes:
> 
> In the  past, I've tuned up both radios for MV owners.  But never tried them 
>  
> on 10.  Don't forget that the ITU (i.e., the French) changed the  rules 
> since 
> the War.  You can't legally operate them below 29.0  MC.
> 
> 
> Really, why not ? I can't recall my license stating no FM below 29. I  think 
> 
> I can run FM on 160 if I want. It does state bandwidths should be  kept to 
> the 
> minimum required to achieve two way communication but does anyone  really 
> expect there to be hundreds of WW2 vintage sets pumping away at any one  
> time. 
> 
> Frankly, if others can not put up with the odd wideband FM signal for the  
> odd afternoon well, maybe we should all give up now and switch over to Fox. 
> 

Robert & Susan Downs - Houston
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MVPA 9480
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