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radio and antenna questions for 108 to 136 MHZ



 
 
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  #1  
Old December 27th 04, 09:52 AM
Insert66
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Posts: n/a
Default radio and antenna questions for 108 to 136 MHZ


I would like to intercept the local airport frequencies
I tried a pro-82 with its default antenna and got little more than the
aircraft, .. no ATC . I returned the radio

I am now trying to decide if i should buy a better radio
and an add on / larger antenna

Or should i accept that local buildings are
going to make it impossible. I am 10 to 12 nm
from the airport. but have other buildings around me
Im on the 12th floor and have a balcony

would a discone do it?
is a base unit more sensitive than a hand held?

is the pro 82 considered sensitive?

what is the ideal antenna length for 108 - 136 MHZ ?

is it better to but aviation hand helds, as they may be
made for these frequencies?

would simply a taller antenna have made a difference with
the pro 82? It was simple enough to use




TIA for any replies
  #2  
Old December 27th 04, 02:19 PM
mikeFNB
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Posts: n/a
Default

most radios are the same ears wise
.. an external aerial will make all the diff.

mike



"Insert66" wrote in message
...

I would like to intercept the local airport frequencies
I tried a pro-82 with its default antenna and got little more than the
aircraft, .. no ATC . I returned the radio

I am now trying to decide if i should buy a better radio
and an add on / larger antenna

Or should i accept that local buildings are
going to make it impossible. I am 10 to 12 nm
from the airport. but have other buildings around me
Im on the 12th floor and have a balcony

would a discone do it?
is a base unit more sensitive than a hand held?

is the pro 82 considered sensitive?

what is the ideal antenna length for 108 - 136 MHZ ?

is it better to but aviation hand helds, as they may be
made for these frequencies?

would simply a taller antenna have made a difference with
the pro 82? It was simple enough to use




TIA for any replies



  #3  
Old December 27th 04, 05:08 PM
[email protected]
Guest
 
Posts: n/a
Default

It is more likely that you either did not have the correct ATC freqs,
or the Pro82
that you had was defective. Ilive about 15 miles from the Lexington
Bluegrass
airport and I have no trouble receiving ATC and even the low power
field utiltiy trucks/equipemnt/people on my ancinet Pro34. With the
stock antenna.
With a old RadioShack diskcone, the one without the center "stinger", I
can monitor airports in a 30 mile range.
At 12 floors up you ought to get a perfect signal. Even with other
buildijng between you and the airport, you shouldn't have any trouble.
An diskcone on you balcony ought to open up lots of reception
posibilties.
The only other thing I can think of might be a stong local signal that
is overlaoding yoru front end. I have a friend who lives 1/2 mile fro
the LFUC
police transmitter, 1/4 mile from 2 different paging towers. Even he
can
receive very well when all 3 are quite. And is pretty obvious whne his
sanner is being overloaded. Hard to sdicribe, but the garbled speach is
a givaway.
Terry

  #4  
Old December 27th 04, 05:34 PM
GLC1173
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Posts: n/a
Default

Insert66 wrote:
Or should i accept that local buildings are going to make it impossible. I

am 10 to 12 nm from the airport. but have other buildings around me
Im on the 12th floor and have a balcony


Hardly. Can't you hear the local police, fire, etc. base/repeater stations
from at least as far away? You just need a better antenna.

is a base unit more sensitive than a hand held?


Not necessarily. What a handheld usually lacks is a good antenna - as it
lacks a good ground and the antenna itself usually is a duck. Attaching an
external antenna to its jack will give you a big improvement.

would a discone do it?


I'd try any VHF mag mount first. Just stick it atop the refrigerator.
~~~~~~~~~~~~~~~~~~~~~
BDissident news - plus immigration, gun rights, weather, Internet Gun Show
IA HREF="http://www.alamanceind.com"ALAMANCE INDEPENDENT:
official newspaper of the Vast Right Wing Conspiracy/A/b/i

  #5  
Old December 27th 04, 06:51 PM
DougSlug
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Posts: n/a
Default

Sensitivity is probably not an issue, but the antenna type and location
definitely is.

A 1/4 wavelength for the range of interest is calculated with the equation:

length = 234 / frequency (in MHz)

and the resultant length is in units of feet. The frequency you choose
should be in the middle of the range of interest.

A very effective omnidirectional antenna design is the 1/4-wave ground
plane, which you can Google for simple plans. Another possibility is to
make a simple dipole antenna by taking a piece of the 300-ohm twin lead
antenna wire (cheap and readily available at Radio Shack or Home Depot),
then split the two wires with a utility knife for the distance of
1/4-wavelength you calculated and stretch the wires in opposite directions
(like a T). Leave some extra length to go to your receiver (no too much)
and connect the other end to a BNC connector, one wire to the center and one
wire to the collar. Hang this antenna on your balcony outside vertically
(away from metal objects) so that the center conductor wire is pointing up
to the sky and the ground end is hanging straight down. This makes an
effective omnidirectional antenna at and around the frequency you cut it
for, and it should work very well with your altitude.

"nsert66" wrote in message
...

I would like to intercept the local airport frequencies
I tried a pro-82 with its default antenna and got little more than the
aircraft, .. no ATC . I returned the radio

I am now trying to decide if i should buy a better radio
and an add on / larger antenna

Or should i accept that local buildings are
going to make it impossible. I am 10 to 12 nm
from the airport. but have other buildings around me
Im on the 12th floor and have a balcony

would a discone do it?
is a base unit more sensitive than a hand held?

is the pro 82 considered sensitive?

what is the ideal antenna length for 108 - 136 MHZ ?

is it better to but aviation hand helds, as they may be
made for these frequencies?

would simply a taller antenna have made a difference with
the pro 82? It was simple enough to use




TIA for any replies



  #6  
Old December 27th 04, 10:24 PM
GLC1173
Guest
 
Posts: n/a
Default

DougSlug wrote:
A very effective omnidirectional antenna design is the 1/4-wave ground
plane, which you can Google for simple plans.


He can check eBay for them already made in the $20-30 price range - and all
he will then have to do is stick it atop his refrigerator. By the time you
make one of these things - magnet, connectors, coax, rod - it will cost almost
as much.
~~~~~~~~~~~~~~~~~~~~~
BDissident news - plus immigration, gun rights, weather, Internet Gun Show
IA HREF="http://www.alamanceind.com"ALAMANCE INDEPENDENT:
official newspaper of the Vast Right Wing Conspiracy/A/b/i

  #7  
Old December 29th 04, 06:28 AM
Allan9
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Posts: n/a
Default

First off where are you?
Have you checked for the correct frequencies.
Transmissions are going to be line of sight. Chances are you are hearing
aircraft airborne rather than on the ground. I live appx 1 mile from our
airport and cannot hear the taxiing aircraft most of the time. I hear the
controllers most all the time.
You could also be hearing high altitude traffic talking to the enroute
facilities (Centers).

Al

"Insert66" wrote in message
...

I would like to intercept the local airport frequencies
I tried a pro-82 with its default antenna and got little more than the
aircraft, .. no ATC . I returned the radio

I am now trying to decide if i should buy a better radio
and an add on / larger antenna

Or should i accept that local buildings are
going to make it impossible. I am 10 to 12 nm
from the airport. but have other buildings around me
Im on the 12th floor and have a balcony

would a discone do it?
is a base unit more sensitive than a hand held?

is the pro 82 considered sensitive?

what is the ideal antenna length for 108 - 136 MHZ ?

is it better to but aviation hand helds, as they may be
made for these frequencies?

would simply a taller antenna have made a difference with
the pro 82? It was simple enough to use




TIA for any replies



  #8  
Old January 1st 05, 05:10 PM
Insert66
Guest
 
Posts: n/a
Default


thanks for the suggestions

I wound up with a BC92XLT
and made a dipole with major improvement


I have a few questions for the gurus

when we say 468 \ mhz = feet

and 234 / mhz = feet



dipole length is the total?

the element length is half that?



what equation would i use to find element length for a half wave?

what equation would i use for quarter wave?

would a full wave dipole be better?

what if the entire antenna was twice full wave long?
would i get better reception? or would i get interferance
as the 2 waves would interfere with one another?


i am only interested in receiving, and only intertested
at this point, in 108 to 136 mhz AM. I am surrounded by buildings
and a transmission towers of every type, in every direction


more questions .. heh

the question of matching the antenna for he frequency you are after

and the idea of matching the antenna ohms to the ohms of the wire
used to feed it

so for a dipole, with elements around 23 inches long
is it best to:

use the same material for the elemenst as the wire that
feeds the antenna?

use 75 oh lead to the antenna


use 50 ohm lead to the antenna

use 300 ohm 2 wire for just the antenna and 50ohms for the lead?

what if i make the dipole of 75 ohm lead and 75 ohm intact coaxial?






and finaly


after thinking about the idea that the lower ground
element and the top pointing driven element should be equal in
impedance, and that the lead should be the same, if one made a dipole
out of coaxial, and split the cable 23 inches from the end, and
dangled the mesh down and the center wirte up, the splitting
of the coaxial would change the ohms of the driven element and the
dangling mesh gound element

so

would you run 2 lines as the feed, one with the driven element as
the center wire and the other coax feed's center wire as the antenna's
ground pointing element? and then at the antenna merely point one
coax down and the other up? with all the bubber and sheilding intact?

I ask this because of this idea of having the same ohms for the
antenna and at the cable that feeds it


thought I'd ask







































TIA for any replies

  #9  
Old January 1st 05, 05:28 PM
Volker Tonn
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Posts: n/a
Default



Insert66 schrieb:


thanks for the suggestions

I wound up with a BC92XLT
and made a dipole with major improvement


I have a few questions for the gurus

when we say 468 \ mhz = feet

and 234 / mhz = feet


????


Do a search on "big wheel" antenna.
This is good for omidirectional reception with some gain over a dipole
and having a reletively wide bandwidth.

  #10  
Old January 1st 05, 08:44 PM
DougSlug
Guest
 
Posts: n/a
Default

"Insert66" wrote in message
...
I have a few questions for the gurus

when we say 468 \ mhz = feet
and 234 / mhz = feet


The 234 / frequency formula gives you the quarter-wave length. A dipole
consists of two quarter-wave length elements, one tied to the antenna input
and the other to the antenna ground. The total length, therefore, is twice
that number (also given by the 468 / frequency formula)

would a full wave dipole be better?


NO. The half-wave dipole is what should be used for best results. To get
better performance, beam antennas are made using a half-wave dipole along
with properly placed reflector and director elements giving the antenna
directionality and gain.

what if the entire antenna was twice full wave long?
would i get better reception?


Not if you're using a dipole configuration. There are other antenna types
based on full wave elements that would work better for certain
applications...that's a bit of overkill for your application.

i am only interested in receiving, and only intertested
at this point, in 108 to 136 mhz AM. I am surrounded by buildings
and a transmission towers of every type, in every direction


Then altitude is your friend. Another possibility is to use a beam antenna
(Yagi, for example) if you're monitoring a particular station for more gain
at the cost of directionality. Otherwise, an omnidirectional antenna with
no gain placed as high as possible should suffice. Since you're looking at
a fairly narrow range of frequencies, it will be easier to optimize (tune)
the antenna.

the question of matching the antenna for he frequency you are after
and the idea of matching the antenna ohms to the ohms of the wire
used to feed it


Ideally, your antenna should present a 50-ohm impedance at the receiver
antenna input. For receive only, matching is less critical. You can use a
75-ohm impedance about as successfully. If I recall correctly, a dipole has
an intrinsic impedance of around 72 ohms at resonance; that is, at the
frequency it is cut for. That should be fine for your purposes. If the run
to your antenna is long, you should use a quality 50-ohm coax between the
radio and the dipole. For best results, the dipole can be tuned using a
balun or a gamma match (among other things...you'll have to Google these
things since my theory is a bit weak in that area), but for your
application, I would say that matching is not critical as long as the
antenna is cut and built properly.

after thinking about the idea that the lower ground
element and the top pointing driven element should be equal in
impedance, and that the lead should be the same, if one made a dipole
out of coaxial, and split the cable 23 inches from the end, and
dangled the mesh down and the center wirte up, the splitting
of the coaxial would change the ohms of the driven element and the
dangling mesh gound element


Yes. That works, too, and is super easy to make. You can us a single piece
of coax, then extend the braid opposite the direction of the center
conductor to form a dipole. The impedance is determined by the combination
of the driven element and the ground; thinking of them as having separate
impedances is not really correct.

- Doug


 




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