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 -

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
--

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 -