wrote in
om:

I'm glad that someone wrote in response. The purpose of the post was to
pique the interests of those that also have been plagued by intermod.

As to signal strengths and feedline loss, the signals are actually
greater now then when using the "rubber duck" antenna. I was unable to
use the "Service Searches" that are part of the Uniden 785D system. There
simply was no signals available with the indoor antenna, save for the
blasted and incessant intermod. Nothing has changed in the local cell
tower, since my Yaesu VX-5R is virtually destroyed in this location, as
are the other handhelds that I have if I use them un-attenuated. I have
still to try the experiment of using the VX-5R coupled to the outdoor
antenna to see if it is still plagued by intermod.

Brand new, RG6, Radio Shack supplied and about 85 feet. The antenna has a
300 to 75 ohm matching transformer. At first I was concerned that the RG6
should have been RG58, since the antenna is older (I came across the
Radio Shack info for another scanner antenna that they offered, and it
recommended RG-58). I notice that the newer scanner antennas (such as my
model) recommend RG6. Eventually I came across this link
http://www.barovelli.com/radio/scan/scanant.htm as well as other links,
and came to the conclusion that for scanner work, the difference between
50 ohms and 75 ohms could be ignored.

Anyway, if it wasn't for the greatly augmented "Service Search"
capabilities with the outdoor antenna, everything ranging from CB,
Aircraft, Marine, Railroad, through FRS, I would also have thought that
attenuation would have explained it. Someone suggested a bit of
directionality might account for the increased resistance to intermod,
but with the antenna involved, it would be hard to believe that was the
case.

Regards.


Soliloquy wrote:
Any insight into why the scanner antenna would permit normal
operation of the 785D without the problems of intermod being a real
nuisance? My outdoor antenna is identical to the Antenna Craft ST-2
seen (minimally) here http://www.durhamradio.com/scanacc.htm

Very strange. Obviously your scanner can't handle the strong local
signals picked up by your small "rubber duck" antenna. So obviously
the outdoor antenna is delivering LESS signal to the scanner. You may
be hearing more signals, because the scanner is no longer being
de-sensitized by strong signals.

I suspect you're getting a lot of loss in the feedline.

What type of coax are you using? What length of coax? And how old is
the coax?
If it's a long run of old weather beaten RG-58, that would explain it.

As the saying goes, "Scanners don't need pre-amps, they need
attenuators."

Art N2AH

"Soliloquy" wrote:

As to signal strengths and feedline loss, the signals are actually
greater now then when using the "rubber duck" antenna.

How are you determining what the signal strengths are? (I'm talking about
actual rf signal levels at the input to the scanner, NOT what you're seeing
on an "S" meter or hearing from the speaker.)

If your outdoor antenna were delivering more signal strenghth to your
scanner, your intermod would get worse, not better.

As I tried to explain, when the scanner is subjected to STRONG local
(unwanted) signals, the scanner will be de-sensitized. This will make it
difficult to hear weaker signals. If attenuation is placed between the
antenna and scanner, this "de-sensitization" effect will be reduced, and
weak signals will actually appear stronger. In other words, less signal
strength may actually improve reception in your case.

Often folks try using pre-amps to improve reception, only to find that the
preamp degrades reception (especially with handhelds). This is similar to
what I described above. Sometimes, less is more!

my Yaesu VX-5R is virtually destroyed in this location, as
are the other handhelds that I have if I use them un-attenuated.

There you go. Adding attenuation improved your reception. I'm speculating
that the attenuation (loss) in your feedline is having the same effect.

Brand new, RG6, Radio Shack supplied and about 85 feet. The antenna has a
300 to 75 ohm matching transformer. At first I was concerned that the RG6
should have been RG58

RG-6 is much better than RG-58. However, 85 feet is a pretty long run. All
coax cables have higher loss as you go higher in frequency. If your
overload/intermod problems are indeed caused by nearby cell sites, those 800
MHz signals will be attenuated more than signals at lower frequencies. And
your outdoor antenna is probably less effective at 800 Mhz than at lower
frequencies. So your antenna and feedline are acting like a "low-pass"
filter in that they attenuate the higher frequencies. All of that is
consistent with what you are hearing.


Art N2AH

"Harris" wrote in
. net:

Thanks very much for the lengthy response. I see that I have much to
learn about scanners and antennas.

Regards.

How are you determining what the signal strengths are? (I'm talking
about actual rf signal levels at the input to the scanner, NOT what
you're seeing on an "S" meter or hearing from the speaker.)

If your outdoor antenna were delivering more signal strenghth to your
scanner, your intermod would get worse, not better.

As I tried to explain, when the scanner is subjected to STRONG local
(unwanted) signals, the scanner will be de-sensitized. This will make
it difficult to hear weaker signals. If attenuation is placed between
the antenna and scanner, this "de-sensitization" effect will be
reduced, and weak signals will actually appear stronger. In other
words, less signal strength may actually improve reception in your
case.

Often folks try using pre-amps to improve reception, only to find that
the preamp degrades reception (especially with handhelds). This is
similar to what I described above. Sometimes, less is more!

There you go. Adding attenuation improved your reception. I'm
speculating that the attenuation (loss) in your feedline is having the
same effect.

RG-6 is much better than RG-58. However, 85 feet is a pretty long run.
All coax cables have higher loss as you go higher in frequency. If
your overload/intermod problems are indeed caused by nearby cell
sites, those 800 MHz signals will be attenuated more than signals at
lower frequencies. And your outdoor antenna is probably less effective
at 800 Mhz than at lower frequencies. So your antenna and feedline are
acting like a "low-pass" filter in that they attenuate the higher
frequencies. All of that is consistent with what you are hearing.


Art N2AH

Soliloquy wrote:
: wrote in
: om:

: I'm glad that someone wrote in response. The purpose of the post was to
: pique the interests of those that also have been plagued by intermod.

: As to signal strengths and feedline loss, the signals are actually
: greater now then when using the "rubber duck" antenna. I was unable to
: use the "Service Searches" that are part of the Uniden 785D system. There
: simply was no signals available with the indoor antenna, save for the
: blasted and incessant intermod. Nothing has changed in the local cell
: tower,..."
(snip)

Curious .. maybe the short hand held antennas are more resonant to the
cell towers signals than the outside antenna. Thus using the outside
antenna will decrease the ratio of 'bad' signal to 'good' signal enough to
let the radio work as it is supposed to....but that's just a guess.

Richard in Bsoton, MA, USA