Cobra multiband dipole (linear loaded) help pse
 

Hi
Can someone advise me of dimensions for a 40 meter version?

Here is original article:
http://www.hamuniverse.com/cobraantenna.html
Thanks
Jim K2TL

Cobra multiband dipole (linear loaded) help pse
 

wrote:
Can someone advise me of dimensions for a 40 meter version?

The 80m version is 1/2 the size of the 160m version.
The 40m version would be 1/2 the size of the 80m version.
--
73, Cecil, http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

Hi
Well, not exactly, but close. A half wave on 160 is 260 feet, half of
that is not 140 as in the Cobra and 80 meters half wave is about 134
feet, so half of that is not 70 feet, as in the Cobra antenna. But its
a good enough starting point to work with I suppose. Maybe I'll try 35
feet for 40 meters.
Thanks
Jim

Cobra multiband dipole (linear loaded) help pse
 

Oh wait, I see what you mean..you are correct Hi..So, 35 feet would be
correct anyway.

Thanks

Cobra multiband dipole (linear loaded) help pse
 

wrote:

Well, not exactly, but close. A half wave on 160 is 260 feet, half of
that is not 140 as in the Cobra and 80 meters half wave is about 134
feet, so half of that is not 70 feet, as in the Cobra antenna. But its
a good enough starting point to work with I suppose. Maybe I'll try 35
feet for 40 meters.

I didn't say anything about wavelength. The 140 ft. Cobra is alleged
to work on 160m. The 70 ft. Cobra is alleged to work on 80m. So a
35 ft. Cobra would be alleged to work on 40m.

Incidentally, I just modeled a 124 ft. Cobra version that has
performance on 160m that is superior to the 140 ft. version
because the SWR on the 450 ohm feedline drops from about 63:1
to about 47:1 according to EZNEC. Performance is comparable to
a 124 ft. dipole on all HF bands. This looks like a fairly
reasonable way to obtain 160m performance from a 124 ft. antenna.
--
73, Cecil, http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

wrote:
Oh wait, I see what you mean..you are correct Hi..So, 35 feet would be
correct anyway.

A word of caution about scaling of this antenna. Assuming that
the 70 ft. version does work on all HF bands, 80m scales to
40m, 40m scales to 20m, 30m scales to 15m, and 20m scales
to 10m. As you can see, scaling from an 80m version to a 40m
version doesn't necessarily cover the WARC bands.
--
73, Cecil, http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

As stated in the text, 73 feet.

wrote in message
oups.com...
Hi
Can someone advise me of dimensions for a 40 meter version?

Here is original article:
http://www.hamuniverse.com/cobraantenna.html
Thanks
Jim K2TL

Cobra multiband dipole (linear loaded) help pse
 

The website states 73 feet for the 40 meter version that can also be used on
80. One would then assume that a 20 meter version 36.5 feet would also work
on 40.

"Cecil Moore" wrote in message
. com...
wrote:
Can someone advise me of dimensions for a 40 meter version?

The 80m version is 1/2 the size of the 160m version.
The 40m version would be 1/2 the size of the 80m version.
--
73, Cecil, http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

Fred W4JLE wrote:
The website states 73 feet for the 40 meter version that can also be used on
80. One would then assume that a 20 meter version 36.5 feet would also work
on 40.

Yep, but not on 30m and 17m.
--
73, Cecil, http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

Thanks for the replies...I am trying to construct a multiband antenna
for a disabled friend who lives in a Townhouse. He has some room up in
the attic. I think I could squeez a 35 foot lnear loaded inverted vee
up. Its going to be fed with 300 ohm line. If it works, fine, if not,
then its butterfly collecting for him instead of Ham Radio. Hey, its
worth a try. Some of the reviews I have read are interesting. BTW, DX
Engineering sells 2 models of this antenna using 3 conductor power
cable and 300 ohm line. Check thier site. They even let you read the
manual in PDF.
Jim

Cobra multiband dipole (linear loaded) help pse
 

Don't know what a Cobra is.

But a 124-feet (38 metres) dipole, at a height of 40 feet (12 metres)
in conjunction with 60 feet (18 metres) of 450-ohm ladder line and an
L-match tuner has the following characteristics -

Freq MHZ Loss relative to ideal
1.9 8 dB
3.8 0.5 dB
7.1 0.9 dB
10.1 0.8 dB
14.1 0.6 dB
21.1 0.6 dB
29.0 0.6 dB

Loss includes, antenna, line, tuner and ground.

Data obtained using program DIPOLE3 from website below.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Cobra multiband dipole (linear loaded) help pse
 

Reg Edwards wrote:
Don't know what a Cobra is.

It's a linear loaded folding shaped like this for 1/2 of the antenna:

62 feet per side
---------------------------------------+
|
+--------------------------------------+
|
+----------------------------------------FP--- other side

Freq MHZ Loss relative to ideal
1.9 8 dB

The above antenna doesn't have nearly that amount of loss on 160m.
--
73, Cecil http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

"Cecil Moore" wrote in message
om...
Reg Edwards wrote:
Don't know what a Cobra is.

It's a linear loaded folding shaped like this for 1/2 of the
antenna:

62 feet per side
---------------------------------------+
|
+--------------------------------------+
|
+----------------------------------------FP--- other side

Freq MHZ Loss relative to ideal
1.9 8 dB

The above antenna doesn't have nearly that amount of loss on 160m.
======================================
Folding is quite beneficial then at 1.9 MHz. How did you find what
the loss actually is? Did you model it? How far apart are the 3
wires?
----
Reg.

Cobra multiband dipole (linear loaded) help pse
 

Reg Edwards wrote:
Folding is quite beneficial then at 1.9 MHz.

Not only beneficial for 160m but also acceptable for all of HF.
It's a win/not-lose situation.

How did you find what the loss actually is? Did you model it?

I modeled it using #14 copper wire and EZNEC and comparing the
maximum gain to the maximum gain of a dipole.

How far apart are the 3 wires?

EZNEC forced me to put them ~7 inches apart (minimum spacing on 160m).
--
73, Cecil http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

wrote:
You can clearly see
that they use a 3 conductor round cable for the antenna.

Seems to me a wider spacing would increase the efficiency.
--
73, Cecil http://www.qsl.net/w5dxp

Cobra multiband dipole (linear loaded) help pse
 

Cecil Moore wrote:
Fred W4JLE wrote:
The website states 73 feet for the 40 meter version that can also be used on
80. One would then assume that a 20 meter version 36.5 feet would also work
on 40.

Yep, but not on 30m and 17m.
--
73, Cecil, http://www.qsl.net/w5dxp

It's NICE to see OTHERS taking this Antenna IDEA......seriously !?

I asked about the design awhile ago. Good reading for me.

Paladin

Cobra multiband dipole (linear loaded) help pse
 

Well, its all built, awaiting installation in my friends attic maybe
this weekend. I built it out of 3 conductor round cable, number 14
stranded, 36.5 feet. That seems to be what DX Engineering is doing, so
its good enough for me. Its easy to build this way. Now lets see how
it plays.

Cobra multiband dipole (linear loaded) help pse
 

Paladin:
I have been using the Cobra ultra-lite antenna since
the fall season. I had to cut
the feedline to 80ft. of 450 ladderline,and then add a good 4/1 balun
at the window,and then
i had to add 50ft. of good grade coax to my T-network tuner.
I can tune all of 160 with no problems. I also can use
the amp, about 4-500watts
output WITH no sparks or rf on equiptment. I'm on the 2nd floor
here. There were some
"growing" pains at first,but no it's doing fine. It gets out rather
well,with or without the amp.
I'm still a bit confused though. I still seem to NEED that
feedline LENTH. TOTAL.....
80ft. of L.L. the balun,and then 50ft. of coax. That's 140ft.
total. Works great,though !

73's, Paladin

Cobra multiband dipole (linear loaded) help pse
 

Paladin wrote:
I'm still a bit confused though. I still seem to NEED that
feedline LENTH. TOTAL.....
80ft. of L.L. the balun,and then 50ft. of coax. That's 140ft.
total. Works great,though !

When the length of 50 ohm coax to the 50 ohm source
has a large effect, that is usually not good news.
We are usually aiming for a low SWR on that section
of feedline and if we are successful, length doesn't
much matter. Here's what can be happening:

The impedance looking into the ladder-line with an
SWR of 18:1 is 25 ohms because that point is a current
maximum point. The 4:1 balun takes that impedance
down to 6.25 ohms, a lossy condition for the tuner and
an impedance it may not be able to match. 0.15 WL of 50
ohm coax (SWR = 50/6.25 = 8:1) will transform that 6.25
ohms up to 17+j65 ohms, an impedance the tuner can handle.
Have you measured the SWR on the coax on the antenna
side of the tuner?

If one doesn't know what impedance is being seen by the
balun, a 4:1 balun may not be the best choice. It may
even be a bad choice.
--
73, Cecil http://www.qsl.net/w5dxp