Building Slim Jim for Air Band

[Michiel Kappeyne]

Hi all,

New to this forum, I am so impressed with the level of expertise I find
here.

Thanks for welcoming me.

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




--
Michiel Kappeyne

[Ralph Mowery]

"Michiel Kappeyne" <Michiel.Kapp...@radiobanter.com> wrote in
message news:Michiel.Kapp...@radiobanter.com...

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/info/antennas/dipole/length-calculation-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.

[Channel Jumper]

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

[ji...@specsol.spam.sux.com]

Channel Jumper <Channel.Jum...@radiobanter.com> 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.

[Me]

In article <Channel.Jum...@radiobanter.com>,

Channel Jumper <Channel.Jum...@radiobanter.com> 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.....

[nm...@wt.net]

On Thursday, January 31, 2013 11:52:10 PM UTC-6, Michiel Kappeyne wrote:

> I want to build a Slim Jim antenna for receiving airband (118-136MHz)
>
> out of 3/8" or 1/2" soft copper tubing.

I'm not really a fan of those type of antennas. You would
likely be as well off to just make a simple 1/4 ground plane
with sloping radials. Easy match to coax, and super easy to
build. Can be as simple as a SO-239 connector with 5 stiff
wires attached.
All the bandwidth issues, etc are not worth worrying about
for receiving. You will likely never hear the difference
between a vertical length of wire, vs a fat copper tube
for receiving air band.
Another issue with most J-poles, and other antennas of it's
ilk are the total lack of decoupling from the feed line.
There is no point in worrying about extra gain or bandwidth,
when the design of the antenna does not take decoupling of
the feed line into consideration. The lack of decoupling will
skew the pattern up off the horizon, and make any extra gain
from a longer element length apply only to higher angles.
This could be good for airplanes in flight, but bad for
tower comms.
The decoupling of a 1/4 ground plane is not perfect, but it's
generally a good bit better than the usual J pole.
And it can be improved by adding more radials, or an extra
set of radials 1/4 wave below the feed.

Myself, I would just build a simple SO-239 ground plane,
"google for illustrations", hang up from the ceiling with
a short piece of fishing line and a tack, and be done with it.
It will likely function as well as most of the J pole designs,
with a lot less work involved. And it's a direct match to
coax, which means no matching losses. The sloping radial
version is a better match, than one with the radials at
90 degrees from the vertical element. And the gain of the
sloping radial ground plane will usually be within .3 db
of a 1/2 wave vertical. And this assumes the 1/2 wave is
properly decoupled. Most are not.. So it would not be totally
unusual to see the simple ground plane actually do better
in many cases.
If one is going to use a 1/2 wave vertical for VHF, one
needs to take decoupling of the feed line into consideration.
Most don't it seems.. :(

[Jeff Liebermann]

On Fri, 1 Feb 2013 05:52:10 +0000, Michiel Kappeyne
<Michiel.Kapp...@radiobanter.com> wrote:

>New to this forum, I am so impressed with the level of expertise I find
>here.

Oh-oh. We're in trouble now.

>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...!

Altitude will get you better results than gain in this case. The idea
is to get over the ground clutter so that you can hear aircraft and
service vehicles on the ground. 15 miles is a bit of a stretch but
possible. The trick is to create an antenna that will hear well
overhead but also has most of the gain in the direction of the
airport. Presumably, your station and the airport are not moving.

What I suggest is an aircraft band yagi antenna pointed at the
airport. That should take care of the ground stations. However, that
does nothing for hearing airplanes flying overhead. For that, I
suggest a simple coaxial antenna, as Jim Lux has suggested. A length
of coax with 1/4 wave of braid peeled back over the coax will sorta
work. If you want something better, a simple 1/4 wave ground plane
antenna or a horizontal 1/2 wave dipole. Don't bother with a J-Pole,
Slim Jim, or anything with gain. The problem is that such antennas
"borrow" gain from the upwards direction, and use it to improve gain
at the horizon. That's NOT what you want for the "overhead" antenna.
Obviously, it is necessary to combine the two antennas. A Wilkinson
combiner will have about 1dB loss on receive, which can be easily
tolerated.
<https://www.google.com/search?q=wilkinson+combiner&tbm=isch>
One potential problem is that if both antennas receive the same signal
180 degrees otto phase, there will be cancellation. To reduce (but
not eliminate) this problem, try to position the overhead antenna
where the path to the airport is blocked by some obstruction (i.e.
chimney).

>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?

The diameter of the elements has some effect on the cut length. So
does spacing between elements, coax impedance, where they're measuring
from, and how the end of the elements are cut. For example, a square
cut tubing will have a different effective length than one with the
end crimped and rounded, or with a hemispherical shaped end cap. The
idea behind the non-square tubing end is to increase the usable
bandwidth of the antenna.

>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.

Calculations are nice, but when it comes to actually sweeping an
antenna for where the minimum VSWR is located, I often get surprises.
If you had a sweep generator, direction coupler, detector, and scope,
you could see the VSWR sweep change as the antenna is waved around.
The calculations are generally under near ideal conditions. Real
installation tend to be not so ideal.

>My
>antenna should center around 125MHz, as the most interesting
>frequencies, Tower, Ground, tend to group at the lower end of the
>airband spectrum.

Yep. However, I think you'll find that with a receive only antenna,
you can have quite a bit of VSWR and it will still work just fine.
Therefore, the antenna cuts and trims are far less critical for a
receive antenna than one that's also used for transmit. You might be
trying to over-optimize the design without any obvious benefit.

>I understand that the bigger the diameter of the tubing is, the wider
>the antenna's bandwidth.

True.

>What would be a good trade-off diameter? Will
>the resulting bandwidth be good enough?

Well, the -3dB bandwidth of the aircraft band is about:
20/150 = 0.15 = 15%
The -3db bandwidth of my 2m J-Pole is about:
<http://802.11junk.com/jeffl/crud/144-Jpole-50-VSWR.jpg>
17/146 = 0.08 = 12%
I could recalculate the antenna for 125Mhz, but I'm lazy. The percent
bandwidth at 125 would be the same as at 146Mhz. The smaller
bandwidth for the J-Pole would seem to be a problem, but isn't because
receiver can tolerate a higher VSWR without (much) loss in gain.

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

You're crazy. How will you know that your antenna is actually
optimized? Without test equipment, you don't have much of a chance.
Better to build something really simple, where the tolerances are not
as important or critical.

>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?

The only way I know how to make sure your estimates are correct is to
either construct an NEC2 model, or to just build it and measure the
antenna gain and VSWR. Since a computer model tends to be less
expensive than copper, methinks that would be best.

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

Cut-n-Try? Well, good luck.

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

Well, you're off to a bad start. Got any RF test equipment?

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Ian Jackson]

In message <Michiel.Kapp...@radiobanter.com>, Michiel Kappeyne
<Michiel.Kapp...@radiobanter.com> writes

Some of the guys in this NG seem to be making things unnecessarily
complicated and confusing.

The Slim Jim / J-Pole is essentially just a halfwave, endfed via a
quarterwave stub (in HF terms, a "Zepp" antenna). It's a pretty good
antenna. For receiving, the dimensions shouldn't be that critical.
However, a quick Google brings up this calculator:
http://www.m0ukd.com/Calculators/Slim_Jim/index.php
--
Ian

[Jeff Liebermann]

On Sat, 2 Feb 2013 22:46:41 +0000, Ian Jackson
<ianREMOVET...@g3ohx.demon.co.uk> wrote:

>Some of the guys in this NG seem to be making things unnecessarily
>complicated and confusing.

That would be me. Thanks.

>The Slim Jim / J-Pole is essentially just a halfwave, endfed via a
>quarterwave stub (in HF terms, a "Zepp" antenna).

Sorta. The original Zepp antenna was intended for use on a Zeppelin
dirigible, because the entire antenna is at DC ground and the voltage
peak is as far away from the gas bags as possible. It was in the form
of a J-Pole, with the 5/8 wave section dangling downward. I just
Googled for pictures of Zeppelins but never found one showing a
trailing Zepp antenna.

I don't have a clue how the Slim Jim design arrived, but my guess(tm)
is that it coincided when people started making J-Poles out of
twinlead, and found that the extra wire would be tolerated. Using
Google (search by date range), it seems to have arrived in about 1990.

>It's a pretty good antenna.

I prefer a folded dipole and balun or a collinear dipole (as in
AMOS/Franklin) and balun antenna. I consider the complexity of
construction about the same, although I will concede that end fed
antennas are much easier to mount than center feed antennas.

>For receiving, the dimensions shouldn't be that critical.

Agreed. You could make the antenna out of barbed wire and it will
work. The problem is that you won't know how good or bad the antenna
operates without measurements, simulations, or comparisons. For
comparison, I carry a simple telescoping dipole with me. When someone
claims that they're "system" isn't working well, I compare it with the
dipole. I'm often impressed with the ability of some antennas (mostly
mobile antennas) to send most of the RF towards the sky, while leaving
little towards the horizon. However, that shouldn't matter here,
where a hemispherical pattern, that covers the entire sky, is the
goal. Maybe a discone?

>However, a quick Google brings up this calculator:
>http://www.m0ukd.com/Calculators/Slim_Jim/index.php

Ahem. Although the author mentions using twinlead and ladder line,
there's nothing in the calculator to compensate for the velocity
factor. The design shown is apparently for one built out of rod and
tubing.

Incidentally, our local radio club (K6BJ) had an antenna construction
session during the Oct 2011 meeting. The theme was how to build an
emergency antenna. The Slim Jim variety was the most common form.
There were numerous construction articles found online and in books
and magazines. Various pieces of test equipment were available. I
didn't participate because I wasn't sure I would arrive on time, but
eventually showed up near the end. I could almost cry. Most of the
antennas were hung from the suspended ceiling, which was metal. The
most sensitive point of the Slim Jim is near the top, causing
frequency sweeps to change drastically when moved. Several people
used the same design, the same roll of ladderline, but had various
interpretations of how to cut the antenna. That had some effect on
VHF, but was fatal at UHF. I gave a fast demonstration of how
proximity to metal, and construction variables can have a big effect.
There was plenty of head scratching and head shaking, but eventually
everyone threw together something that worked. I had an entertaining
diversion diagnosing a very nicely built Slim Jim, that had conductive
shrink tube slipped over the gap. The meeting was over before I had a
chance to connect these antennas to an HT and run a live test to a
distant repeater. My guess(tm) is that they wall would have worked,
but to varying degrees.

[nm...@wt.net]

On Saturday, February 2, 2013 6:10:44 PM UTC-6, Jeff Liebermann wrote:
I'm often impressed with the ability of some antennas (mostly
>
> mobile antennas) to send most of the RF towards the sky, while leaving
>
> little towards the horizon. However, that shouldn't matter here,
>
> where a hemispherical pattern, that covers the entire sky, is the
>
> goal. Maybe a discone?

Dunno.. I've done a lot of air band listening, and myself, I
think it's best to concentrate of the tower, and letting the
airborne chips fall as they may. Unless one is real close to
the airport, the tower, ATIS, and other ground signals will
usually be the toughest to hear.
The idea of using a *short* yagi actually works quite well in
most cases. You can hear the ground stations, and still most
of the others in the air also. Most of the airborne signals
will be fairly stout, and a rubber ducky would pick them up.
The more gain the yagi has, and the tighter the pattern,
the better the tower, but the worse the airborne.
This is why I would stick with short 2-3 el yagi's if going
that route. You don't want too much gain.

I've got a simple 3 el yagi cut for 2m, and I'd often use
it for air band. I could tweak the direction for best tower
signals, and with the antenna out of tune, the pattern is
not sharp, and would pick up most everything well enough.

Using that, I could receive ACARS from cruising jets up to
about 300 miles. From Houston, to about the OK border or so..
I could follow Southwest jets from KHOU to KDAL, and I could
hear them going into Dallas until they descended to about 9000
feet or so. And that's with the 2m yagi in no particular direction,
or aimed to favor Hobby.. With an out of tune yagi, sometimes
the max signal does not jive with the normal direction..

Anyway, I've found that it's best to favor the ground signals,
and not worry much about airborne. You will hear those with
little trouble. And this is the main reason I take the
decoupling into consideration if using a simple vertical.
You will want good decoupling to do well on the low angle
ground signals. A sky warmer will not be your friend in this
case.

[Michiel Kappeyne]

Thank you very much for all your responses! The spectrum of thought, and
wide array of opinions, are fascinating and educational.

There are clearly several roads that lead to Rome.

Distilling the narrative I extract the following:

- Antenna height dominates. It is hence a good thing I live in a
Manhattan apartment building at 250ft elevation, with a large window
facing West and an unobstructed view of New Jersey. (Of course I can
forget about listening to JFK or LGA.)

- With neither antenna-building expertise nor dedicated rf measuring
equipment, it is better to stick to a straightforward design such as a
dipole or Yagi with two or three elements, or even a piece of coax,
where dimensions either require less precision or can be realized more
easily. I'll definitely continue the experimentation. And oh, toss my
copper tubing lunacy...!

- Even so, I should not expect jaw-dropping improvement over my airband
rubber duck or even the $30 ladder line Slim Jim I bought. After moving
that around a bit it's working quite well now and pulls in airplanes
taxiing around Newark Airport, 11 miles out, depending on atmospheric
conditions. Newark Tower, which transmits with more power at higher
elevation, comes in intelligibly most of the time. Both the Slim Jim,
its horizontal gain notwithstanding, and the duck capture nearby
airplanes up to 8,000 ft or so. Most air-to-ground communications are at
lower altitudes anyway.

Many thanks again for all your comments! I'll post updates to this
thread in case anybody is interested in my novice attempts.

Cheers,
Michiel

P.S. On a different note, I'll be putting a perfectly restored Sony
Pro80 in as-new operational and cosmetic condition on eBay in the next
couple of weeks.

Michiel Kappeyne;801269 Wrote:
> Hi all,
>
> New to this forum, I am so impressed with the level of expertise I find
> here.
>
> Thanks for welcoming me.
>
> 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, 11 miles away. Amazingly, my

[Jeff Liebermann]

On Mon, 4 Feb 2013 02:25:15 +0000, Michiel Kappeyne
<Michiel.Kapp...@radiobanter.com> wrote:

>- Antenna height dominates. It is hence a good thing I live in a
>Manhattan apartment building at 250ft elevation, with a large window
>facing West and an unobstructed view of New Jersey. (Of course I can
>forget about listening to JFK or LGA.)

Ummm... you may have a different problem. Presumably, you're using an
indoor antenna as I doubt the windows open at the 250ft level. For
energy efficiency, such windows are coated with Titanium Dioxide and
other metallic coating designed to reflect infrared to improve the
energy efficiency of the HVAC system. Unfortunately, they also block
RF to varying degrees. If you're experiencing somewhat weak
reception, it might be the Low-E coating. Unless you can arrange for
antenna on the rooftop (unlikely), I can't offer a solution.

>P.S. On a different note, I'll be putting a perfectly restored Sony
>Pro80 in as-new operational and cosmetic condition on eBay in the next
>couple of weeks.

Good luck.