[Mobile-Portable] IC706

[K0DAN]

----- Original Message ----- 
From: "K0DAN" <k0dan at comcast.net>
To: "Mobile-Portable Reflector" <mobile-portable at mailman.qth.net>
Sent: Wednesday, January 17, 2007 9:44 PM
Subject: Re: [Mobile-Portable] IC706


> The fact that the loud ones on the adjacent channel muted the desired 
> signals says
> a) the front end filtering is not up to protecting on-center from 15 khz 
> away
> b) FM capture effect permist the strongest signal (off-freq in this case) 
> to quiet the receiver
>
> This might have been an overload problem, but chances are that this mode 
> is just too wide to handle a lot of dynamic range with close spacing. If 
> you were operating a non-linear mode such as CW, SSB, you might not have 
> had such problems. Such are the trade-offs of modes such as FM.
>
> 73
> Dan
> K0DAN
>
>
> ----- Original Message ----- 
> From: <lcomden at comcast.net>
> To: "Mobile-Portable Reflector" <mobile-portable at mailman.qth.net>
> Sent: Wednesday, January 17, 2007 1:59 PM
> Subject: Re: [Mobile-Portable] IC706
>
>
>> Paul,
>>
>> Some of the signals were 20 over S9  on my mobile 1/4wave antenna. Others 
>> which were S3 were perfectly readable  when the loud ones in the adjacent 
>> channel shut up. Signals 2 channels (30KHz) away didn't seem to be 
>> annoying.
>>
>> If it is not a front end overload or mixer problem maybe this means the 
>> shape factor of the IF passband is poor.
>> Didn't have an attenuator handy to rule out the overload problem. Wasn't 
>> using the preamp, of course. And was ignorant of the how to turn down the 
>> RF gain (combo of RF gain/squelch is selectable and I didn't)
>>
>> Other mobiles (really rovers)in this exercise had yagis and different 
>> xcvrs.
>>
>> 73
>>
>> Larry K3VX
>> -------------- Original message ----------------------
>> From: Paul Goble <goblefam at swbell.net>
>>> To whom it may concern,
>>>
>>> Putting my toungue securely into my cheek, the following actual facts 
>>> apply:
>>>
>>> The situation you describe is why God invented squech.
>>>
>>> Additionally, if you look at your Bessel functions, the energy out at 
>>> 16KHz is
>>> so low as to be laughable.
>>>
>>> On top of that, look at the FM quieting curve of any FM receiver and you 
>>> will
>>> see that the weakness of the FM "sidebands" that far out means, clearly, 
>>> that
>>> the FM capture effect will easily override any adjacent channel "stuff".
>>>
>>> FM is, by definition, a strong-signal mode - who worries about 
>>> weaksginal
>>> characteristics of a strong signal mode?  Enquiring minds are curious - 
>>> HIHI!
>>>
>>> um, "FM" satnds for "forbidden mode", doesn't it?
>>>
>>> Another "gem":
>>>
>>> "REAL mobile antennas are omnidirectional!" (You ought to see the fire 
>>> in some
>>> folks eyes when I say this UNTIL they realize I'm kidding! - HIHI!).
>>>
>>> Enjoy!
>>>
>>> Paul ND2X/5
>>> http://www.nd2x.net/pix/new.jpg
>>>
>>>
>>> Gary Pearce KN4AQ <kn4aq at arrl.net> wrote:
>>> This is straying off-topic from purely mobile-portable, but I'd like to
>>> note that EVERY radio will be unable to completely reject strong signals
>>> from a 15 kHz adjacent channel.  That's because the actual spectrum used 
>>> by
>>> a properly operating narrow-band FM transmitter is 16 kHz, so there will 
>>> be
>>> some RF within the receiver pass band of even the tightest receive 
>>> filters.
>>>
>>> 15 kHz channel steps were a "necessary evil" when ham repeaters filled 
>>> up
>>> all the available 30 kHz channels in metro areas back in the
>>> 70's.  Repeater councils in the eastern half of the country decided to
>>> "split" the channels to 15 kHz, and use about 50 miles of physical
>>> separation between repeaters, which theoretically makes the adjacent
>>> channel signal weak, to reduce the overlap problem.  Repeater councils 
>>> in
>>> many western states (plus Michigan and Alabama in the east) bit the 
>>> bullet
>>> and went to 20 kHz steps, requiring many repeaters to shift to a new
>>> frequency.  In the repeater band from 444.5-145.5 MHz, which was 
>>> developed
>>> after many 15 kHz "split" repeaters were on the air, the entire country
>>> decided to go with 20 kHz channels.
>>>
>>> 15 kHz channel steps for simplex are a bad idea, since there's no way to
>>> provide physical separation.  3 miles between 50 watt base stations with
>>> good outdoor gain antennas is "nothin'."  Repeater owners debate to this
>>> day which plan, 15 or 20 kHz, works best for repeaters.
>>>
>>> Some receivers have narrower IF filters than others, so some get hit 
>>> worse
>>> than others.  I'm not sure about the Icom 706.  You can see the specs in
>>> ARRL equipment reviews.
>>>
>>> Interference from paging and other out-of-band signals is a separate
>>> problem.  Adjacent channel rejection is provided by a filter in the IF
>>> chain.  "Out of band" signals are rejected by filters in the RF or
>>> "front-end" area of the receiver.  "Out of band" is a bit of a misnomer 
>>> in
>>> most radios, as they are designed to receive signals across a wide
>>> spectrum.  They rely on tunable filters to keep from having the "barn 
>>> door"
>>> front-ends some hams complain about.  Some tunable filters are better 
>>> than
>>> others, and out of band rejection is quiet variable between makes and
>>> models (Kenwood 733: bad.  Yaesu 8900: good).  Good, fixed band-pass
>>> filters are better than tunable filters and will stop all but the 
>>> strongest
>>> out of band signals cold.  But they also prevent you from listening to 
>>> NOAA
>>> weather, the local police and fire departments, etc.
>>>
>>> If you're bothered specifically by paging transmitters, PAR sells notch
>>> (and band pass) filters that will take out several common VHF and UHF
>>> paging channels while allowing the rest of the public-safety and 
>>> business
>>> spectrum to pass through.  They go in your antenna line and can handle 
>>> 50
>>> watts of transmit power, so you don't need switching.
>>>
>>> You can buy an "on channel" crystal filter for limiting the RF stage of
>>> your receiver to a single narrow channel.  It's a solution for some
>>> repeater problems, but not your home station.  And it won't keep out the
>>> on-channel RF from an adjacent 15 kHz signal.
>>>
>>> 73,
>>> Gary KN4AQ
>>>
>>> At 11:30 PM 1/14/2007, you wrote:
>>> >----- Original Message -----
>>> >From: "Larry Comden"
>>> >Sent: Sunday, January 14, 2007 4:44 PM
>>> >Subject: [Mobile-Portable] IC706
>>> >
>>> >
>>> >We just ran our club 2M simplex contest with 50 to 100 stations in the 
>>> >local
>>> >area.. I was mobile with my IC706 MkIIG. Strong signals on adjacent 
>>> >(15KHz)
>>> >FM channels were bleeding thru so badly that weak signals (the life 
>>> >blood of
>>> >a contest like this) were swamped. We were definitely not operating 
>>> >side by
>>> >side. My competitors were 3 or more miles away - some mobiling some 
>>> >fixed.
>>> >All stations limited to 50W.
>>> >
>>> >I've also noticed this effect in other locations near paging, radio and 
>>> >TV
>>> >transmitters. Does anyone on the list know what front end VHF/UHF 
>>> >filtering
>>> >is done in this rig?
>>> >
>>> >At home on HF I can use the RF gain to reduce the effect.
>>>
>>>
>>>
>>>
>>>
>>> ARVN: Amateur Radio//Video News
>>> Gary Pearce KN4AQ
>>> 508 Spencer Crest Ct.
>>> Cary, NC 27513
>>> kn4aq at arvidnews.com
>>> 919-380-9944
>>> www.ARVidNews.com
>>>
>>>
>>> ----
>>> Your Moderator: Dick Flanagan K7VC, 
>>> mobile-portable-owner at mailman.qth.net
>>>
>>> ----
>>> Your Moderator: Dick Flanagan K7VC, 
>>> mobile-portable-owner at mailman.qth.net
>>
>> ----
>> Your Moderator: Dick Flanagan K7VC, mobile-portable-owner at mailman.qth.net
>>
>
>