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?

Regards


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 . ..

^ 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

"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.

|
\|/
+
/|\
|

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

Soliloquy . ..

^ |
^ \|/
^ +
^ /|\
^ |
^
^ My onmi-directional antenna looks like the above ...

^ 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?

My comments were in response to your query on directionality and they are
probably not useful in your system. I am not an antenna expert, only an
amateur experimenter, and I have not seen any documentation on reflectors and
directors on anything except half-wave dipoles. Although, as I move around
town and examine TV antennas it appears to me that they are making some sort
of use of reflectors, directors, or both.

But the function of reflectors and directors, on the receive side, is to
increase the amount of the desired frequency reaching the antenna. They may
not be very useful if the intent is to scan a wide range of frequencies.

N2AH appears to have your answer.

Frank

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?