Buyer Beware - Cobra Ultralight
 

Cecil Moore wrote:
Michael Coslo wrote:
Sounds like time for Cecil to join in. I know his system uses
switchable lengths of twinlead, but perhaps he might have something to
offer here in a one length feedline system.

Things are slow here at the GED office today so I modeled
the Cobra-Jr and Cobra-Sr. The Cobra-Jr suffers from
feedpoint impedance problems similar to a G5RV used on
75m. The feedpoint impedance on 3.7 MHz is about
16.5 + j459 ohms giving a 450 ohm SWR of 56:1.

For my variable feedline method to work with 450 ohm line,
the SWR must fall between 4.5:1 and 18:1. A dipole that is
1/2WL on the lowest frequency of operation usually meets
that requirement. The G5RV length doesn't and the Cobra-Jr
doesn't.

One thing interesting about the Cobra-Jr is that if it is
made 62 feet long instead of 73 feet long, it becomes
resonant around 3.85 MHz with a feedpoint impedance of
about 16 ohms. That's an SWR of only about 3:1 on coax.
or ~1.3:1 using a 1:4 balun. That's a half size antenna
for 75m that is only about 1.5 dB down from a full size
1/2WL dipole.
--
73, Cecil, http://www.qsl.net/w5dxp

Buyer Beware - Cobra Ultralight
 

On Fri, 09 Jun 2006 15:33:59 GMT, Cecil Moore
wrote:


One thing interesting about the Cobra-Jr is that if it is
made 62 feet long instead of 73 feet long, it becomes
resonant around 3.85 MHz with a feedpoint impedance of
about 16 ohms. That's an SWR of only about 3:1 on coax.
or ~1.3:1 using a 1:4 balun. That's a half size antenna
for 75m that is only about 1.5 dB down from a full size
1/2WL dipole.

Where are you observing resonance?

I created a model with ~73' length that does resonate (low Z) at the
dipole centre around 3.6MHz. Other factors affect resonance a little,
but you may want to check your model to see it is laid out properly
and connected up properly.

Owen
--

Buyer Beware - Cobra Ultralight
 

I have the Ultralite Senior and am very happy with it. I have it at 30
feet with the ends dropping down about 15 feet since my lot isn't big
enough. I bought it primarily as a local contact antenna since
everything else I have is a vertical of one type or another. It turns
out to work much better than I expected. I have worked the South
Pacific, Europe and Peter One on it with 100 watts

Yes, the Ultralite Senior is quite effective! Mine is at 60 feet at the
apex and the "Vee" droops down to about 20 or so. I get pretty nice signal
reports both from local and afar. You can't always win 'em all, but when an
antenna works *reasonably* well most anywhere you work, a fella can't ask
for much more! And I am perfectly happy with mine.

We had a second station in the 160 meter CQWW contest moving up and
down the band working people. That staion just used a 160 meter dipole
at 60 or 70 feet. I was amazed they could work anyone they called and
got through pileups very quickly, often on one call.

Of course when I compare that antenna to the main transmitting
antennas, it is typically 10-20 db weaker. Sometimes it is 40-50dB
weaker.

Of course I can work anything I can hear with my mobile using a 10 foot
tall antenna. I've worked Europe on SSB 160 meters and Australia
several times on CW, and I've worked all over the world on 80 and 75
meters. I have even worked Australia longpath on 40 meters in the late
aftenoon (VK6VZ) while sitting in rush hour traffic, and I've done that
more than once!

All of the above is absolutely true, and all of the above is totally
meaningless when it comes to quantifying how well an antenna actually
works. The only meaningful part is we are happy with the contacts, and
if I didn't have a better antenna I'd swear my 70 foot high 160 dipole
or my mobile antennas were the best thing on the planet.

73 Tom

Buyer Beware - Cobra Ultralight
 

On Fri, 09 Jun 2006 00:59:53 GMT, Owen Duffy wrote:

Mike, did you notice "Mystery's" comment re spending more money on a
"tuner extender": "Not the answer I wanted to hear". She/he may be
even less interested in the spend on the tuner you describe.

I agree with you that antenna looks worth a try / perseverance, but it
will probably need a wider range tuner than commonly fitted internal
to transceivers unless you experiment (what's that!) to find a
feedline length that the rig/runer accommodates on all bands, or
switch feedline lengths.

Owen


How is the "tuner extender" made? is it simple enough to duplicate
with a home-brew?


--
73 for now
Buck
N4PGW

Buyer Beware - Cobra Ultralight
 

On Fri, 09 Jun 2006 05:13:04 GMT, Owen Duffy wrote:


The Cobra Junior, a 72' long linear loaded dipole, looks interesting
in terms of a short antenna for low HF bands with reasonably low
losses. I have run some preliminary NEC models and analysed the feed
system loss with 25m of W551 ladder line, they look promising.

Well, the antenna system looks promising on some bands, but not
others. Draft article is at http://www.vk1od.net/cobra , constructive
comments appreciated.

The article includes a link to a graph of the modelled system losses
for the Cobra Senior, but it looks pretty uninteresting with ~9dB of
system loss at 160m... there is no shortage of poor performing
antennas on 160m.

Owen
--

Buyer Beware - Cobra Ultralight
 

Buck wrote:

How is the "tuner extender" made? is it simple enough to duplicate
with a home-brew?


A tuner extender is about worthless. It is an autotransformer. Nearly
every auto tuner has more impedance range than the ferrite core
autotransformer used to extend the tuner.

The problem really is the Cobra does NOT have resonances at the points
that are claimed, so impedance is off the scales on some Ham bands.

I measured a Cobra that was supposed to hit 80, and it missed by a few
MHz. A person would be much better off doing something else, although
I'm sure with a good tuner contacts could be made on any band.

After all.... I can work Europe on 160 SSB and VK's on CW with my
mobile antenna, and it is less than 1% efficient and that is on 160!!

73 Tom

Buyer Beware - Cobra Ultralight
 

Owen Duffy wrote:
On Fri, 09 Jun 2006 15:33:59 GMT, Cecil Moore
wrote:


One thing interesting about the Cobra-Jr is that if it is
made 62 feet long instead of 73 feet long, it becomes
resonant around 3.85 MHz with a feedpoint impedance of
about 16 ohms. That's an SWR of only about 3:1 on coax.
or ~1.3:1 using a 1:4 balun. That's a half size antenna
for 75m that is only about 1.5 dB down from a full size
1/2WL dipole.

Where are you observing resonance?

I created a model with ~73' length that does resonate (low Z) at the
dipole centre around 3.6MHz. Other factors affect resonance a little,
but you may want to check your model to see it is laid out properly
and connected up properly.


I'm modeling the thing in 4nec2 now. I get different results than both
of you, so I need to go back to the drawing board.....

I've always wondered though if some mfgr's fudge a little on their
lengths to cloud the issue a bit. But then I make my antennas, so I
never bought one to check.


- 73 de mike KB3EIA -

Buyer Beware - Cobra Ultralight
 

My trip down that road was called a "Gotham Vertical".
What it lacked in performance, it made up for in education!
Google it for an evening of fun reading...
de W8CCW John

On Fri, 09 Jun 2006 02:03:43 GMT, Cecil Moore
wrote:

wrote:
The problem is that antenna is advertised to work all bands.
Unfortunately it doesn't do very well on many bands.

Sounds like a G5RV.
John Ferrell W8CCW

Buyer Beware - Cobra Ultralight
 

On Mon, 12 Jun 2006 11:04:37 -0400, Michael Coslo
wrote:


I'm modeling the thing in 4nec2 now. I get different results than both
of you, so I need to go back to the drawing board.....

The NEC deck for my models is linked from the article. You will need
to replace the $freq symbolic variable though with a number.

Owen
--

Buyer Beware - Cobra Ultralight
 

Owen Duffy wrote:
On Mon, 12 Jun 2006 11:04:37 -0400, Michael Coslo
wrote:


I'm modeling the thing in 4nec2 now. I get different results than both
of you, so I need to go back to the drawing board.....

The NEC deck for my models is linked from the article. You will need
to replace the $freq symbolic variable though with a number.


Thanks Owen, I'll take a look after tonights club meeting.


- 73 de Mike KB3EIA -