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Author Topic: Slim Jim (does it really have gain)  (Read 20292 times)
KB9WIS
Member

Posts: 180




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« on: December 27, 2004, 09:19:42 PM »

Ok,

The Slim jim, which is simply a 1/2 wave vertical end fed folded dipole design, is claiming 6db of gain on some websites.  Has anyone actually built one of these?  I wonder if the are better in low angle performance than a J-pole..  6db of gain seems like a lot (i believe its in reference to a 1/4 vertical with radials..)

If the vertical folded dipole (aka slim jim), really has 6db of gain, than perhaps I should make my 1/2 wave collinear elements out of this folded slim jim design..


Has anyone tried and/or compared them?
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AA4PB
Member

Posts: 12639




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« Reply #1 on: December 29, 2004, 10:57:06 AM »

6dB gain over what? I seriously doubt that it is 6dB gain over a 1/4 wave vertical on a good ground plane. Maybe they are quoting 6dB over isotropic - but even that's a stretch.

The only way that any omni directional antenna can get gain and remain omni directional is to concentrate the energy at a lower angle of radiation (you've got to take it from somewhere to put it where you want it - you can't create it). The low angle may or may not be a good thing, depending on the terrain and the relative height of the two stations.
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WA4MJF
Member

Posts: 1003




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« Reply #2 on: December 30, 2004, 09:14:55 AM »

Well, I know of two kinds of Slim Jims,
one is a meat stick and the other a
flat piece of metal with some cutouts
to open locked car doors.

The former would not work as an antenna
and the latter may well workas a
two meter antenna, but I doubt there
would be any gain.

Is there a third kinda Slim Jim?

73 de Ronnie
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KX8N
Member

Posts: 543




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« Reply #3 on: December 31, 2004, 08:21:06 PM »

Ronnie, here's a link about it:

http://www.hamuniverse.com/slimjim.html

Dave
KX8N
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KA0GKT
Member

Posts: 555




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« Reply #4 on: January 01, 2005, 02:33:14 PM »

One reason the ARRL doesn't allow antenna manufacturers to quote antenna gain in advertisments placed in QST is that it is easy to lie with deciBells.

Okay, lets look at the claim of 6dB over a 5/8-lambda antenna.

6dB equates a power gain of 4.  (9dB would be a power gain of 8, 10dB a power gain of 10)  A 5/8-Lambda antenna is the equivalent of half an Extended Double Zepp antenna fed over a ground plane.  This antenna, in a good installation exhibits about 1.6 dB over a halfwave antenna.  Lets give it the benefit of the doubt.  Lets say 2dB.  That means that the antenna, which is about a halfwave in height will have an omnidirectional gain of 8dB (6.31 power gain) over a halfwave dipole?  GET SERIOUS!

Now if you want REAL gain, I have a 12-inch rubber duck antenna which shows an omnidirectional gain of 12dB on 160 Meters*...I also have a really nice bridge for sale, it runs from Oakland into San Francisco.

*Then again, the gain is over a 2-inch piece of wet rotini.

73
de KA0GKT/7
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WB6BYU
Member

Posts: 12978




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« Reply #5 on: January 03, 2005, 10:13:17 AM »

A "Slim Jim" is simply a J-pole, and won't perform any
better than any other J-pole (that is properly built.)

If you build a J-pole out of 300 ohm twinlead, you have
a choice of how to connect the two wires in the radiator
section: connected at the bottom or not, connected at the
top or not, or remove the second wire altogether.  The
"Slim-JIM" design connects the two wires together at the
top and not at the bottom.  The truth is, it really doesn't
much matter how the second wire is connected to the first
one, since they are so closely coupled together that it
won't change the radiation.

Actually, one problem with some of the "Slim-JIM"
constructions is that they put an insulator across the
top of the matching section (and between the end of the
folded radiator and the top of the matching stub.)
This is a high impedance point, and insulators that are
acceptable at lower impedances will affect the tuning
and losses if used at this point.

But, in general, there are few times when this design
would have any advantages over a conventional J-pole.

Don't trust antenna gain claims that aren't backed up
by repeatable measurements, especially those based on
"S"-meter readings on VHF.  I had reason to measure the
S-meter on my mobile rig the other day, and the difference
between S5 and S9 on the meter is about 2.5dB.  To someone
measuring an antenna and using the "common wisdom" that
1 S-unit is 6dB, this would look like a 24dB change.
In fact, the whole meter has only 7 segments (marked as
2 S-units each on the display) and has a total range of
9dB from nothing to full scale.  And commonly someone
building a new antenna will compare it against an older
one, which may have poor joints, lossy coax, or be at
a lower height than the new one.  Although you can't make
an antenna work any better than theory, you certainly can
make it work much worse than that, and such an antenna
does not make a good reference.
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SMARTJIM
Member

Posts: 1




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« Reply #6 on: April 18, 2005, 03:19:38 AM »

YES !! it works great!
Speaking from experience.
Complete discription in a book written by F.C.Judd in 1980.(G2BCX)
He did labtests and also tests in the real world. Because of his positive testresult i build one for hf. Huge and big with wire hanging on a fishing "pole" (cant find the word, sorry) and tried also haning under a kite. Working in the best conditions near the sea with a batary. I was supriced of the capebillity of this antenna. I always compare with one other antenna to be sure what the new one does and this is the best ever. because of its low angle it is very useful for hf and f3 layer or air inversion skipping.
The labtest showed that the gain was comming from the low angle radiation compaired with a 5/8 wave antenna, whoos angle of radiation is pointed more up. 5/8 has about 3 db over a dipole and the slim jim has 3 db over the 5/8. question is how was the 5/8 measured,.. also at 0 degree angle?
Fact is that i experienced a verry nice time with this "simple to tune" antenna.
For VHF and UHF i would not use the insulator because is looses energy very easely.
Officially you should use a balun but if you make a role of the coax at the antenna and connect the outside to ground it should not matter really.
Lets cut the crap,.... just build one and try.

Best reguards, Bob.

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G4AON
Member

Posts: 511




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« Reply #7 on: May 01, 2005, 01:45:40 AM »

A slim jim can be made in just a few minutes, try one from 300 Ohm ribbon. Details at:
http://www.southgatearc.org/techtips/slimjim.htm

As for "6 dB gain", size matters and a slim jim is no bigger than a dipole...

To avoid feedline radiation/pickup, consider adding a few ferrite sleeves over the feedline to decouple it.

73 Dave, G4AON
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KK5VN
Member

Posts: 5




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« Reply #8 on: August 17, 2013, 02:10:12 PM »

Proof is in the pudding..I have built several out of 450 ladder line, and they are beasts.  Here is a good article by DU1ANV Joe explaining why the antenna works so well, why it is not a "j pole", and how the gain comes about. http://www.para.org.ph/membersarticles/DU1ANV/Slim%20Jim.htm
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K5LXP
Member

Posts: 4436


WWW

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« Reply #9 on: August 18, 2013, 01:06:51 PM »

Here is a good article by DU1ANV Joe explaining why the antenna works so well, why it is not a "j pole", and how the gain comes about.

What if the premise of his article is flawed?

"The resulting electromagnetic field of the fed signal is in-phase with the main radiator by virtue of induction where both elements are shock-excited by the feed stub."

"Shock excited"?  Two immediately adjacent wires in phase suddenly affect pattern?  Sounds like hooey to me.


Mark K5LXP
Albuquerque, NM
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WB6BYU
Member

Posts: 12978




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« Reply #10 on: August 18, 2013, 02:56:10 PM »

OK, continuing this thread after >8 years of dormancy...


I dug out one of the original G2BCX articles from Practical Wireless in 1978 that
explains the gain calculation:  he claims 6dB improvement over a 5/8 wave whip (possibly
with radials) because the latter has a higher angle of radiation above the horizon.  There
is no comparison to a standard J-pole, though he may be implying some because "everyone
knows" that the 5/8 wave whip has 3dB gain over a dipole.

What this displays is the poor understanding of the effect of common mode currents at
the time, along with common poor practices regarding 5/8 wave verticals.


Here's the deal:  a 5/8 wave whip, by itself, is simply a 1/2 wave radiator (just like
a dipole) with a little bit of extra conductor on one end where the current is OUT OF PHASE
with the rest of the antenna, causing a slight reduction in signal strength.  If you don't
believe me, look at the current distribution along the antenna and find the points of maximum
current:  there is only one, 1/4 wavelength down from the top.

Why does the 5/8 wave have a reputation as a gain antenna?  That's from studies of ground
mounted verticals
on MF and HF, where the Earth can be considered an infinite ground plane.
Over such a ground plane, 5/8 wave is the point where, as you increase the height of a radiator, the
gain peaks and starts dropping again.  This is due to the interaction between the radiating portion
(the upper 1/2 wavelength) and its ground reflection.  If you install the 5/8 wave antenna in any
configuration that doesn't include a near-infinite ground plane, it won't perform the same.
  This
includes at VHF, where even the top of a large van is nowhere near large enough to provide the
expected ground reflection.  (You need 20 to 50 wavelengths or so to get the expected gain at
low angles.)

At VHF the problem is compounded because a common way to mount a 5/8 wave whip for fixed
use was over a set of 1/4 wave sloping radials, which then are out of phase with the
main radiator and contribute to raising the angle of radiation.  (Volume 1 of the ARRL Antenna
Compendium
has a comprehensive article about the problems with 5/8 wave verticals at VHF.)


So the original article was comparing against an antenna design that is now known to be less
than optimal.  Looking at the plots in the original document, the only reason for the 6dB advantage
of the Slim-JIM was due to the tilt of the pattern of the 5/8 wave ground plane.  Which, because
it is not over an infinite ground plane, would be the same as a dipole or ordinary J-pole.

My opinion hasn't changed in 8 years - the Slim-JIM has no reason to work any better than a dipole,
standard J-pole, or a quarter wave ground plane with sloping radials.  Any variations measured among
them will more likely be to differences in common mode current, to which they are all susceptible.
Adding a second wire closely coupled to the first (as in a folded dipole, or the Slim-JIM) does not
provide any improvement in signal strength because the wires are coupled so closely that they act
as a single wire.

That's not to say that they can't work, or that they won't show an improvement over a poorly-built
antenna of some type, but there is nothing inherent in the design that will cause it to have any gain
over a dipole.
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N4JTE
Member

Posts: 1147




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« Reply #11 on: August 21, 2013, 03:05:07 PM »

Anyone who builds/sells antennas seem to have a problem with DB's, I pay no attention unless the "gain" is in DBD, gain over a dipole at the same height. There is no single vertical anywhere that has any gain over a dipole at the design frequency of both.
Bob
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KJ7WC
Member

Posts: 68




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« Reply #12 on: November 24, 2013, 01:40:08 AM »

Anyone who builds/sells antennas seem to have a problem with DB's, I pay no attention unless the "gain" is in DBD, gain over a dipole at the same height. There is no single vertical anywhere that has any gain over a dipole at the design frequency of both.
Bob

Really? So, if I were to make a 3.5m tall co-linear vertical on 2m (1/4w + 3x 1/2w), it wouldn't exhibit any appreciable gain over that of a 1/2w dipole on 2m? I suggest you familiarize yourself with the concept of capture.

On the topic of this thread, a Slim Jim *may* have 6dB of gain over a 1/4w vertical on a single vector. Bear in mind that a Slim Jim's radiation pattern is focused at about 10-degrees over ground, whereas a 1/4w ground-plane radiates at closer to 35-degrees elevation. Gain figures are always comparisons between two antennas on a given vector. When discussing directional antennas, that vector is assumed to be parallel to the boom, away from the reflector. It's not so easy with an omni.
« Last Edit: November 24, 2013, 01:48:04 AM by KJ7WC » Logged
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