"Michiel Kappeyne" wrote in
message ...
I want to build a Slim Jim antenna for receiving airband (118-136MHz)
out of 3/8" or 1/2" soft copper tubing. This antenna won't see any
ouside use--it will hang away from any metal against my window facing
the Hudson River towards Newark Airport, 15 miles away. Amzingly, my
little 8" helical rubber duck works as well as a ladder cable-based
125MHz Slim Jim I bought, so I want to step it up...!

There are many, many stories floating around the Internet about homebrew
Slim Jim projects, but what strikes me is that everybody seems to arrive
at different lengths for a 3/4 wave in the 144MHz band (where I find
most Slim Jims), even allowing for differing propagation speeds in the
various materials these builders use. What am I missing here? I thought
the calculation was as straightforward as the wave length in open air
adjusted by the velocity factor, about 0.96 for copper tubing. My
antenna should center around 125MHz, as the most interesting
frequencies, Tower, Ground, tend to group at the lower end of the
airband spectrum.

I understand that the bigger the diameter of the tubing is, the wider
the antenna's bandwidth. What would be a good trade-off diameter? Will
the resulting bandwidth be good enough?

I am leaning towards 3/8" as happy medium, unless one of you tells me
I'm crazy.

Anyway, I won't solder any elbows at the two ends; I will loop this
tubing using a bending spring. Should I compensate for these round ends
in my length calculations, or won't that matter? How critical is the
distance between the two parallel sides? I can't find any information on
that either. Can I leave the air gap bare, or should I treat the tube
endings to some kind of solid, perhaps adjustable, plug?

I'll devise a slide for the coax connection so I can adjust it for best
matching.

I am a complete novice when it comes to building antennas, and for now
can only contribute my thanks for your advice.

Cheers,
Michael

In simple terms the velocity factor of the antenna is determined by the
insulating material around the antenna.
There is another factor called the diameter to wavelength. That is how big
around the antenna is. That number usually runs from .94 to .98. The
larger the diameter the antenna is in relationship to the wavelength will
shorten the antenna also. When you go from a wire such as twin lead to
copper tubing for the slim jim there is a big ratio change. The graph here
may help you see that effect.
http://www.radio-electronics.com/inf...on-formula.php

So not only does the diameter effect the bandwidth, it also effects the
length.
For receiving I doubt that you will notice much differance in the bandwidth
as far as the signal strength going from a piece of number 14 wire to 3/8
inch tubing.

Public service radio - aircraft bands - are transmitted with massive amounts of power. Not to mention the fact that the airplanes are in the air when they are mainly transmitting. Not that there is much traffic to listen to anymore, with most things being computerized and digital.

There is no reason to use a 3/4 wave antenna, nor is there any benefit in using a antenna more then 5/8 wave long.

Might I suggest you get a copy of the ARRL antenna book, it will cover it better then I can.

The bottom line is - if all you want to listen to is local traffic, all you need is a 1/4 wave antenna - mounted outside, in as clear of a place as possible.
Some building materials - concrete reinforced steel - being one, is opaque to good radio reception.

All effective communications is line of sight.

The best scanner antenna for the money would be a Diamond Discone type antenna - which would cover anything from 10 meters to 999 MHz - all in one antenna.

Using a good high grade coax and connectors would be key to good reception.

As a example, Cleveland Approach is 120 miles from my location, with my Uniden 890XLT - I can hear Cleveland approach and the air traffic between Cleveland / Pittsburgh and Baltimore MD from my location in Central PA.

Not to mention the local air traffic.

Even a old VHF television antenna would get you some reception...

Copper J Poles are great for experimental purposes, but not real good when it comes to reliable reception - even though you are just going to use it inside of your house.
They do offer some front to back - directivity in one or more directions, but there is no reason to use a piece of copper pipe if all you want to do is receive.

You can acheive practically the same thing with a piece of 300 ohm twin lead - if assembled properly.

I have even seen antenna's that were made out of copper foil that worked for what you want to do.

Channel Jumper wrote:

Public service radio - aircraft bands - are transmitted with massive
amounts of power.

No they are not. Most aircraft radios are around 10 watts and ground
stations around 25 watts.

Not to mention the fact that the airplanes are in the
air when they are mainly transmitting.

Pretty much correct.

Not that there is much traffic
to listen to anymore, with most things being computerized and digital.

Nope, aircraft communications is still AM voice and there is lots of it.

There is no reason to use a 3/4 wave antenna, nor is there any benefit
in using a antenna more then 5/8 wave long.

Might I suggest you get a copy of the ARRL antenna book, it will cover
it better then I can.

The bottom line is - if all you want to listen to is local traffic, all
you need is a 1/4 wave antenna - mounted outside, in as clear of a place
as possible.
Some building materials - concrete reinforced steel - being one, is
opaque to good radio reception.

All effective communications is line of sight.

Well, you got something right anyway.

The best scanner antenna for the money would be a Diamond Discone type
antenna - which would cover anything from 10 meters to 999 MHz - all in
one antenna.

Giant overkill just to listen to airplanes.

Using a good high grade coax and connectors would be key to good
reception.

Actually, if the goal is to hear ground stations, the key is antenna
height.

As a example, Cleveland Approach is 120 miles from my location, with my
Uniden 890XLT - I can hear Cleveland approach and the air traffic
between Cleveland / Pittsburgh and Baltimore MD from my location in
Central PA.

You do know that things like Approach are not usually on the airport.

Not to mention the local air traffic.

You normally hear lots of air traffic but not the ground station.

Even a old VHF television antenna would get you some reception...

Copper J Poles are great for experimental purposes, but not real good
when it comes to reliable reception - even though you are just going to
use it inside of your house.

There is nothing about a J pole that makes in unrealiable for reception
though it is again overkill for the application.

They do offer some front to back - directivity in one or more
directions, but there is no reason to use a piece of copper pipe if all
you want to do is receive.

There is no reason to use anything other than a piece of coax with the
center conductor stripped out for about 20 inches for receive.

You can acheive practically the same thing with a piece of 300 ohm twin
lead - if assembled properly.

I have even seen antenna's that were made out of copper foil that worked
for what you want to do.

For listening at home with my aviation HT, I use a piece of RG-58 that
has the shield pulled back about 20 inches with about 20 inches of center
conductor exposed. The end of the center conductor is soldered into a small
loop so I can hang the thing from the ceiling with a piece of string.

In article ,
Channel Jumper wrote:

Public service radio - aircraft bands - are transmitted with massive
amounts of power.

You are an IDIOT, and your understanding of FAA Comms is ludicrous at
best. Most FAA Ground Transmitters are less than 100 Watts Output, and
Aircraft Radios are usually in the 25 to 50 watt Range, for Commercial
and 10 to 25 Watts for General Aviation Aircrat. All UniCom and MultiCom
Transmitters are 10 watts, unless an STA has been granted for some very
unusual reason. If you consider this "MASSIVE" you are a double Idiot....

Me One who actually KNOWS something about the subject.....