Linear decoupling traps
 

After a bit of e-mail correspondence and the
exchange of a single, crucial, 1000-word, picture,
I think we've bottomed-out on this. I'm seeing two,
distinct, dipole-like responses, one at 3.6 MHz
and one at 10.2 MHz. The EZNEC outputs and
source file are at:

http://www.arizona-am.net/test/Antenna_Stubs_SWR.pdf

http://www.arizona-am.net/test/Anten...bs_picture.pdf

http://www.arizona-am.net/test/Anten..._10MHz_SWR.pdf

http://www.arizona-am.net/test/Anten...s_3MHz_SWR.pdf

http://www.arizona-am.net/test/Antenna_Stubs_wires.pdf

http://www.arizona-am.net/test/Antenna_Stubs_Patt.pdf

http://www.arizona-am.net/test/Ant_End_Stubs.EZ

Jim, K7JEB

Linear decoupling traps
 

Jim, K7JEB wrote:
After a bit of e-mail correspondence and the
exchange of a single, crucial, 1000-word, picture,
I think we've bottomed-out on this. I'm seeing two,
distinct, dipole-like responses, one at 3.6 MHz
and one at 10.2 MHz.

10.2/3.6=2.8 It appears that frequency ratios
between about 2.4 and 2.8 can be easily achieved.
That's 75m+30m, 40m+17m, 30m+12m.

Unfortunately, a 2:1 ratio seems difficult to
achieve.
--
73, Cecil http://www.w5dxp.com

Linear decoupling traps
 

Unfortunately, a 2:1 ratio seems difficult to
achieve.
--
73, Cecil http://www.w5dxp.com

In real life, at least in my case the stub will be made of twin-lead type
material which shortens it abt 30%.
If the shortening effect at lower frequency due to the inductive loading of
stub is less, then it would be possible.
Going one step further you could easily add another band to the design as
parallell dipole for 17m using unused parts of
douple-line. That should have litle interaction to the original double band
antenna.
http://farm4.static.flickr.com/3016/...fd3b1e6c_o.jpg


As Jim K7JEB verified, the total half length is not 20m but 15,5m due to the
loading effect of the stub.
The real stub length is 1/4 WL at higher frequency respecting the velocity
factor of material used.


73 Jouko OH5RM

Linear decoupling traps
 

On 16-Apr-2008, "Jim, K7JEB" wrote:

After a bit of e-mail correspondence and the
exchange of a single, crucial, 1000-word, picture,
I think we've bottomed-out on this. I'm seeing two,
distinct, dipole-like responses, one at 3.6 MHz
and one at 10.2 MHz.

I also played around with this yesterday on EZNEC. Started with a 75 M
center fed half wave at 30 feet high, added 20 M quarter wave stubs one
quarter wave (20 M) oout from the center. Got two low SWR points at ~3.5
MHz and ~14 MHz. By changing the distances to the stubs, the length of the
stubs, and the length beyond the stubs, I got the low SWR points to 3.9MHz
(2.8:1) and 14.2 MHz (1.2:1). Azimuth pattern on 14.2 MHz was sort of
omni-directional with major lobes at 45, 135, 225, and 315 degrees.
Pattern on 75 M was omnidirectional at high angles and max gain straight
up.

Conclusions: Might be useful as a way to add 20 Meters to a shortened 75 M
dipole. High SWR on 75 would still require a tuner. Losses were not
determined. More height might help.

Ken Fowler, KO6NO

Linear decoupling traps
 

Ken Fowler wrote:
I also played around with this yesterday on EZNEC. Started with a 75 M
center fed half wave at 30 feet high, added 20 M quarter wave stubs one
quarter wave (20 M) oout from the center.

That's a conventional trapped dipole with stub traps
instead of LC traps. Here's a graphic from my web page:

http://www.w5dxp.com/eznec.gif
--
73, Cecil http://www.w5dxp.com

Linear decoupling traps
 

On Wed, 16 Apr 2008 09:48:34 -0700, "Jim, K7JEB"
wrote:

Jouko, OH5RM wrote:

Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open end
is 7,25m from center and the shorted end pointing to the tip of antenna. One
side of stub is the 20m wire itself, like in the upper picture.

I did just that with EZNEC, but with a free-space dipole. Without the
stubs, the antenna resonated at 3.6 MHz. With the stubs, that dropped
to 2.75 MHz and additional low-impedance points were noted at 6.5 and
11 MHz. The patterns at 2.75 and 6.5 had the desired dipole shape,
but the 11 MHz pattern had multiple lobes.

Jim Bromley, K7JEB
Glendale, AZ, USA

I only have EZNEC 3 and the EZNEC 4 demo.

I don't remember the dimensions, but I remember that in the
instructions, EZNEC could not make accurate measurements with parallel
wires within a certain distance like 8-10 inches.

(someone correct me here.)

holding that assumption...

I have not modeled that antenna or made one, but I had an experience
with an 80 meter dipole that may relate. I cut a wire way too long
for 80 meters, I think it was something like 140 feet or so. I use
insulated stranded copper wire so I tried folding back the ends until
I shortened the antenna to 75 meters. What I discovered was, that the
change in frequency of the antenna did not match the reduced length of
the wire. After trying for quite some time, even after measuring the
antenna, I found it physically shorter than the calculated length, but
the center frequency, which changed a little, hadn't changed
significantly as expected. My conclusion was that I basically made a
linear-loaded dipole and the total electrical length of the antenna
was basically the wire length minus a small amount for interaction
between the folded back wire and the original leg. It wasn't until I
trimmed the wire itself that I raised the frequency of the antenna.

The wire I used had the ends loosely wrapped around the main wire of
the dipole, not tightly wrapped like the turns on a hangman's noose.
It was insulated THHN stranded copper.

Let's look at the 40/80 meter antenna cut for the CW portions (3.5 & 7
MHz). Assume, for the sake of argument that the 468/f = length in
feet accounts for the velocity factor and that it is the same for the
40 meter portion as the whole wire...

In theory, the Lattin antenna should be a total of 133.7 feet long,
with 66.85 (1/2) of the total length being the 40 meter wire and 1/2
on each end being the 40 meter decoupling trap. Overall there is a
total length of 200 feet of wire which theoretically would give you 40
and 80 meters. UNLESS the RF reacts according to my experience above
which would produce an antenna which would be tuned for 7.0 and 2.3
MHz (give or take reaction)

If this is true, and even with the reaction, the 40 meter dipole would
work, but you couldn't make the Lattin antenna work with the those two
bands. I might assume that 30 and 80 meters would work as the total
length of the 30 meter portion with trap would be less than 133 feet
and the 80 meter dipole would extend past the trap as an additional
extension of wire. Presumeably, that antenna would be resonant on 30
meters, 80 meters and somewhere else, maybe close to 6 or 7 mhz (Total
length of the wire from feed to end of trap.)

-------------------------------------------------------- O -------...
3.5 MHz | 10 Mhz
__________
trap plus ? Mhz


I don't know how the diagram shows, I hope you get an understanding of
what I am saying.

Comments?

=====================

PS, after writing all that, I re-read your post more carefully. I
think your EZNEC model reflected what i said all along. The
difference being that the continuation of the 80 meter dipole element
seems to be overlooked by EZNEC.

(ok, done this time :)






--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Linear decoupling traps, FINAL PLAN
 

"JN" wrote in message
...
I am planning a two band antenna for 30 and 80m using linear traps.
The idea is to build the antenna of zip-cord type line 140ohm vf 0.73.
Trap is made using 1/4 wave shorted stub, decoupling occurs at the open
end.

Thanks to everybody for valuable comments.
Special thanks to Jim K7JEB, who made the simulation and verified that the
antenna
works as expected.

After much confusion with the text graphics, I learned the lesson to never
use them again

So here is how the antenna works
http://farm4.static.flickr.com/3078/...7c03acc4_o.jpg

I am going to use this kind of material, which is very commonly used by
telephone companies here.
http://www.saunalahti.fi/hohtola/ham/killu/killu.html

And here is how it will be made:

http://farm3.static.flickr.com/2072/...5653e9b9_o.jpg

Why not normal traps? Easier to build
Why not parallell dipoles? Much shorter
You get three bands if needed very easily

73 Jouko OH5RM

Linear decoupling traps
 

Buck wrote:
. . .
I don't remember the dimensions, but I remember that in the
instructions, EZNEC could not make accurate measurements with parallel
wires within a certain distance like 8-10 inches.

(someone correct me here.)
. . .

That's not correct. However, it is essential to align the segment
junctions so they're directly across from each other when modeling
closely spaced parallel wires.

Roy Lewallen, W7EL

Linear decoupling traps
 

"Roy Lewallen" wrote in message
news:VfidnZbNEK-reZXVnZ2dnUVZ_tHinZ2d@easystreetonline...
Buck wrote:
. . .
I don't remember the dimensions, but I remember that in the
instructions, EZNEC could not make accurate measurements with parallel
wires within a certain distance like 8-10 inches.

(someone correct me here.)
. . .

That's not correct. However, it is essential to align the segment
junctions so they're directly across from each other when modeling closely
spaced parallel wires.

Roy Lewallen, W7EL

Roy,

One question:
Down the list is my posting FINAL PLAN.
Is it possible with EZNEC to simulate it?
It is made of transmission line (partly)

Yes or No is enough.
I have the DEMO version.

73 Jouko OH5RM

Linear decoupling traps
 

JN wrote:

Roy,

One question:
Down the list is my posting FINAL PLAN.
Is it possible with EZNEC to simulate it?
It is made of transmission line (partly)

Yes or No is enough.
I have the DEMO version.

73 Jouko OH5RM

Yes, a simulation should be reasonably accurate. The segmentation
limitation of the demo version will probably reduce the accuracy some,
but you'll be able to get a very good idea of how it will work.

Roy Lewallen, W7EL