[Mobile-Portable] IC706

[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 <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