Outdoor Antenna and lack of intermod
 

Once upon a time, there was a semi-crazy man that posts mainly to the
shortwave NG. This crazy man had bought a Uniden 785D multiband scanner
but soon discovered that the scanner was virtually destroyed by intermod,
even with the simple telescoping antenna that came with the radio.
Unfortunately this man lived very close to a cell tower and not so far
from some television transmitter antennas. Woe was this man, being
reduced to using a 1 ½ inch portable antenna for the Uniden, lest the
intermod monster drive the man even more insane. The man lived with
tolerance of his 785D, realizing that it could be much much more, but
having to content himself with the abundance of local police, fire, and
paramedics. One day, as the man was cleaning his basement, he discovered
his outdoor scanner antenna from days of yore. He thought, "for old times
sake, I'll hook this antenna to my 785D to see how bad the intermod is!"
To his great surprise, when connected in a causal manner, (the antenna
standing on one of the elements, with another element leaning against the
wall), there was no intermod, even though the temporary configuration was
on the end of the property nearest the cell tower. The man was intrigued.
He next pondered "I wonder if I can use this antenna permanently, so he
moved the antenna to the desired permanent location, but withheld the
final wiring until sure. Lo and behold, the antenna worked there as well,
though a little tweaking of the location was necessary to insure optimum
performance. The permanent location was to the rear of the house, and the
first trial location was to the front. The offending antennas would be on
an axis of about 45 degrees if the front of the house was assumed to be
zero degrees and the rear of the house 180 degrees.

Bottom line, when I got the radio, I had hoped to use my outdoor antenna,
but had the intermod problems as described above. I did not believe it
possible to use the outdoor antenna, since any form of indoor antenna
would produce grotesque intermod distortion from the cell tower and
television stations. ALL of my portables are plagued by intermod in this
location, though the Yaesu VR-500 and the Alinco DJ-X10T can handle the
intermod if their attenuation features are used. My Yaesu VX-5R is
hopelessly plagued by intermod in this location.

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 and
http://www.starkelectronic.com/scanner.htm and
http://www.valcoelectronics.com/scanner/scanner.htm though I purchased
it at Radio Shack several years ago.

I thought that if the telescoping antenna caused massive intermod that
there wasn't a chance to use an outdoor antenna, where I figured that the
signal levels would be higher. With the Uniden, the reception in the
upper 800 MHz range seemed dismal with this antenna, but I connected the
Alinco DJ-X10T and viola, great signal levels. Apparently my 785D isn't
very sensitive at those frequencies. Oh, and by the by, when I connected
the Alinco DJ-X10T to the antenna, I got surprisingly good shortwave
reception, at least on the several frequencies that I tried. The antenna
is fed by coax, so I don't believe the feedline helped with reception.
Any insights into why the scanner antenna would offer augmented reception
down to as low as the 7.415 that I tried it at? It is rated to 30MHz, I
guess that might be part of it.

Using the antenna, I am now able to use the Uniden Service searches, such
as aircraft, railroad, CB, FRS, and marine with a high degree of success.

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

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 in message
http://www.valcoelectronics.com/scanner/scanner.htm

This company doesn't respond to email inquiries sent to the email
address provided on their web site ). If
they don't reply to questions BEFORE I buy, I can only imagine how
responsive they will be once they get my money...

Soliloquy . ..

^ How about radiation patterns.

^ Is there an analogy to radiation patterns for reception (onmi-
^ directional antennas, still, as someone has suggested, perhaps
^ a degree of directionality) and could this be a factor?

I know that a Yagi is similarly directional for both transmit and receive so
it may be true for most antenna configurations.

You can make a very simple directional antenna by placing an element that is
slightly longer than your antenna's radiating element about one-eighth
wavelength from the antenna:

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The diagram isn't to scale -- the longer element should only be a few percent
longer. The shorter element is the antenna. In the diagram above, the main
lobe (the best reception) would be to the right. The longer element is a
reflector and tends to block signals from the left while enhancing signals
from the right.

If the additional element were a few percent shorter then it would enhance
the signal from the same side. This additional and shorter element is a
director.

If you place your antenna near other metal objects then a similar effect
might occur.

If you use both a reflector and a director

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each one-eighth wavelength from the antenna in the center, then you have a
Yagi. The antenna in the center should be a have-wave dipole. You could add
additional reflectors to the right, each a little shorter than the previous,
at one-fourth wavelength intervals:


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The spacing between elements is as stated but the antenna can be enhanced by
using precise measurements for the elements (reflector, antenna, and
directors) that consider the wavelength and the velocity factor of the
conducting material. But anything close will have some effect.

Frank

"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

Antennas act exactly the same on receive as transmitting as far as patterns
are concerned.

"Soliloquy" wrote in message
...
wrote in
om:

How about radiation patterns. I'm not familiar with how antenna design
affects reception, but I have seen the transmission patterns of various
antennas. Is there an analogy to radiation patterns for reception (onmi-
directional antennas, still, as someone has suggested, perhaps a degree
of directionality) and could this be a factor?

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

"Frank" wrote in
news:01c38a0d$08353ae0$0125250a@simkoxfheifcdjqq:


Thanks for the response, I have been away from this group for a couple of
days and didn't see your post. Apparently I am plagued by a pager/cell
tower less than 3/4 miles from my home, at 152.18 MHZ and the local PBS
broadcaster whose antenna/s are about 2 air miles away, simulcasting
their shows on TV channels 13 (at 213.00 MHz) and 16 (at 485.00 MHz).

Though the move to the outdoor antenna finally provided me with the
ability to monitor the local police using an antenna larger than a HT
stub antenna, and has provided additional "Service Search" capabilities
with the Uniden 785D that I was completely unable to do with the HT stub
(such as Aircraft and CB), the Marine and Railroad Frequencies are still
interfered with (to a degree) by the Pager (most of the interference
coincides with the presence of the 152.18 Pager signal), as well as a
permanent overload (even with the receiver attenuated) in the upper 400
MHz ranges (460 and up). It's quite horrible listening to the PBS
broadcast regardless of the frequency that the radio is tuned to.

I am considering the purchase of some PAR filters, the notch for the 152
MHz pager, and perhaps a more customized one for the 485 MHz PBS station.
Before I send the $160 for the filters, I wish I knew how to determine
the actual offenders, lest I end up with too many filters and too little
money.

I also looked at a Log Periodic scanner antenna,
http://users.cis.net/kingpop/Scan-log.htm hoping that most of the desired
signals are sufficiently forward of the antenna, wishing to orient the
antenna with the cell tower and PBS broadcasters to the rear. I don't
know if the Rear Rejection would be sufficient to accomplish the
interference relief that I need, nor am I entirely sure of the location
of the desired broadcast antennas (Police repeaters and such)

I'd like to have near 100% usage of my Uniden, but I seem to be in Radio
Alley. I have a Yaesu VX-5R Handheld Transceiver that I always carry. I
bought a Stub Antenna for it, hoping to reduce the intermod that plagues
it here. Funny thing is that it actually rejects the intermod better when
used with its original antenna. In either case, for the most part, while
driving throughout the city, the radio works fairly well, except of
course when I near my home, then the intermod starts. This is truly a
terrible area to live while attempting to pursue a scanner hobby.

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+
/|\
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My onmi-directional antenna looks like the above (assuming that the
pieces are re-assembled on your computer like I placed them on mine).
Part of the receiving element extends down the support bracket (but is
insulated from same), so that with the above image, at the center (+), it
would extend into the computer screen, the other part is connected to the
center radials. If I were to attempt to place a reflector on this
antenna, it would, I trust, be electrically connected to the center
radials but insulated from the brackets? How would I determine the size
and spacing?


Regards.





Soliloquy . ..

^ How about radiation patterns.

^ Is there an analogy to radiation patterns for reception (onmi-
^ directional antennas, still, as someone has suggested, perhaps
^ a degree of directionality) and could this be a factor?

I know that a Yagi is similarly directional for both transmit and
receive so it may be true for most antenna configurations.

You can make a very simple directional antenna by placing an element
that is slightly longer than your antenna's radiating element about
one-eighth wavelength from the antenna:

|
| |
|

The diagram isn't to scale -- the longer element should only be a few
percent longer. The shorter element is the antenna. In the diagram
above, the main lobe (the best reception) would be to the right. The
longer element is a reflector and tends to block signals from the left
while enhancing signals from the right.

If the additional element were a few percent shorter then it would
enhance the signal from the same side. This additional and shorter
element is a director.

If you place your antenna near other metal objects then a similar
effect might occur.

If you use both a reflector and a director

|
| |
| | |
| |
|

each one-eighth wavelength from the antenna in the center, then you
have a Yagi. The antenna in the center should be a have-wave dipole.
You could add additional reflectors to the right, each a little
shorter than the previous, at one-fourth wavelength intervals:


|
| |
| | | | | |
| |
|

The spacing between elements is as stated but the antenna can be
enhanced by using precise measurements for the elements (reflector,
antenna, and directors) that consider the wavelength and the velocity
factor of the conducting material. But anything close will have some
effect.

Frank