On 4/6/2011 8:57 PM, tom wrote:
On 4/6/2011 8:16 PM, John - KD5YI wrote:
On 4/6/2011 2:51 PM, John - KD5YI wrote:

Hey Cecil -

I got:

Impedance = 85.26 - J 186.3 ohms
SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308

Let's see if we can determine why they don't match.

I adjusted the wire length for no reactance at 7.2 MHz.
I used 99 segments of #14 wire. I did not include insulation effects.

How about yours?

John

Are you using 9 segments?

John

I think may have missed something on how segmentation works. I thought
you needed an even number of segments to split a dipole evenly.

So while 99 is close to centered by my logic, if 9 was used it might
skew things a bit.

If I'm off base please tell me so I can adjust my model of how NEC works.

Thanks.

tom
K0TAR

Hi, Tom -

As far as I am aware, using an odd number of segments in a wire allows
the source to be placed in the center of a segment with equal numbers of
segments on each side.

Think of it this way: a source needs to be in the middle of a segment.
So, you need an even number of segments on each side to keep everything
balanced. If so, then the total number of segments needs to be odd.

If this does not make sense, please ask again and I'll try with more detail.

Cheers,
John

On Wed, 6 Apr 2011 03:56:21 -0700 (PDT), "
wrote:

Let's say I build a horizontal dipole, trimmed for minimum SWR @
7.200 mhz. I'll probably use 14-16 gauge insulated house wiring for
this antenna. It'd be a "nominal' quarter wavelength per leg.

This dipole will be fed with 50 ohm coax, and it will be erected
horizontally, 30 feet high over "average" ground.

What SWR reading could I expect to see from this antenna if I tried
to use it at 21.300 mhz........in the same environment as stated
above?

Thanks, folks......

FjLee KA0FPJ


Just put it up and see what happens.

Slap a tuner in the line.

Or check out a fan dipole.

Or buy a pair of 15 meter traps and use the design sheet that comes
with them to build a 40-15 trap dipole.
http://www.unadilla.com/?p=34

On Apr 6, 2:51*pm, John - KD5YI wrote:
How about yours?

I used an EZ file that I already had. It was 40 feet high and I
rounded off the feedpoint impedance. I thought that would be a close
enough approximation.
--
73, Cecil, w5dxp.com

On 4/6/2011 9:07 PM, John - KD5YI wrote:

Hi, Tom -

As far as I am aware, using an odd number of segments in a wire allows
the source to be placed in the center of a segment with equal numbers of
segments on each side.

Think of it this way: a source needs to be in the middle of a segment.
So, you need an even number of segments on each side to keep everything
balanced. If so, then the total number of segments needs to be odd.

If this does not make sense, please ask again and I'll try with more
detail.

Cheers,
John

Thanks! I guess I'll have to look into the docs a bit to educate myself.

tom
K0TAR

On 4/6/2011 11:19 PM, Cecil Moore wrote:
On Apr 6, 2:51 pm, John - wrote:
How about yours?

I used an EZ file that I already had. It was 40 feet high and I
rounded off the feedpoint impedance. I thought that would be a close
enough approximation.
--
73, Cecil, w5dxp.com

I understand. Of course, it's close enough. I was just wondering why my
answers did not agree with yours, but now I do.

Thanks,
John

tom wrote:
On 4/6/2011 8:16 PM, John - KD5YI wrote:
On 4/6/2011 2:51 PM, John - KD5YI wrote:

Hey Cecil -

I got:

Impedance = 85.26 - J 186.3 ohms
SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308

Let's see if we can determine why they don't match.

I adjusted the wire length for no reactance at 7.2 MHz.
I used 99 segments of #14 wire. I did not include insulation effects.

How about yours?

John

Are you using 9 segments?

John

I think may have missed something on how segmentation works. I thought
you needed an even number of segments to split a dipole evenly.

So while 99 is close to centered by my logic, if 9 was used it might
skew things a bit.

If I'm off base please tell me so I can adjust my model of how NEC works.



I always use an odd number of segments, and the middle segment is where
I feed it. The voltage source is essentially in series with the
segment, and if your segments are short enough, the current distribution
along the segment is effectively uniform, so it makes no difference.
OTOH, I'm running NEC4 with a lot of memory in double precision, so
using hundreds of segments isn't a big deal.