[Mobile-Portable] IC706

[Paul Goble]

Larry:

As you probably already know, the bandwidth of a rig is determined primarily in the IF - the front end on most rigs without preselectors is pretty wide.  The IIP3 of the 706 is somewhere between -12 and -18 dBm (I can't remember the exact figure) which is on a par with most of today's rigs (there are some exceptions, like the Elecraft stuff).  This is, of course, lousy.

If the problem is intermod in your rig, guaranteed it's in the front end of your receiver.  The 706 has an attenuator which can be switched in and out, but since  I don't have mine anyomore, I can't remember how to switch it in and out.

GL!  Keep us posted!

Best regards,

Paul

lcomden at comcast.net wrote: 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 
> 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  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