Rockinghorse Winner wrote:
I was checking the lengths for a 15 M dipole up 18 feet and the EZNEC
calculated length was about 10% longer than using 468/F. I was wondering if
the EZNEC calculation takes other factors into consideration and what these
may be? Thanks alot.




CUL8R



R*Horse





"Take a look behind you - upstream - now you begin to
recognize this country, don't you?"

"Yes, I do recognize it now. It is the most wonderful
thing I ever heard of; by a long shot the most
wonderful - and unexpected."

Mark Twain
Life on the Mississippi



http://rwinner.blogspot.com
Hi RH, The proximity to ground, the ground type, and the length will
be the factors that determine the source impedance of the dipole. I
just ran a dipole cut for 21.3 Mhz by the formula, 18' up over real
ground and found it to be off by 3%. Moved it up to 36' and it was off
by 1%. Your 10% sounds a bit high. The conductor size of a wire
dipole will have some effect at 15m, but I did not try anything
different than #12 wire.
Gary N4AST

wrote in message
ups.com...

Rockinghorse Winner wrote:
I was checking the lengths for a 15 M dipole up 18 feet and the EZNEC
calculated length was about 10% longer than using 468/F. I was wondering
if
the EZNEC calculation takes other factors into consideration and what
these
may be? Thanks alot.




CUL8R



R*Horse





"Take a look behind you - upstream - now you begin to
recognize this country, don't you?"

"Yes, I do recognize it now. It is the most wonderful
thing I ever heard of; by a long shot the most
wonderful - and unexpected."

Mark Twain
Life on the Mississippi



http://rwinner.blogspot.com
Hi RH, The proximity to ground, the ground type, and the length will
be the factors that determine the source impedance of the dipole. I
just ran a dipole cut for 21.3 Mhz by the formula, 18' up over real
ground and found it to be off by 3%. Moved it up to 36' and it was off
by 1%. Your 10% sounds a bit high. The conductor size of a wire
dipole will have some effect at 15m, but I did not try anything
different than #12 wire.
Gary N4AST

Just to be different, I tried #18 wire. Using 468/f gives a length of 21.9
feet at 21.3 MHz. According to EZneck, this is resonant at about 22 MHz. A
length of 22.4 feet gave resonance at just a shade above 21.3 MHz. Making
the wire 10% long put the resonance below 21 MHz. All at 18 feet.

I ran SWR from 21 to 22 MHz, with a step size of .05. Number of segments was
19.

Tam/WB2TT

"Rockinghorse Winner" wrote in message
...
On Sat, 22 Oct 2005 15:59:07 -0400, "Tam/WB2TT"
wrote:



wrote in message
roups.com...

Rockinghorse Winner wrote:
I was checking the lengths for a 15 M dipole up 18 feet and the EZNEC
calculated length was about 10% longer than using 468/F. I was
wondering
if
the EZNEC calculation takes other factors into consideration and what
these
may be? Thanks alot.




http://rwinner.blogspot.com
Hi RH, The proximity to ground, the ground type, and the length will
be the factors that determine the source impedance of the dipole. I
just ran a dipole cut for 21.3 Mhz by the formula, 18' up over real
ground and found it to be off by 3%. Moved it up to 36' and it was off
by 1%. Your 10% sounds a bit high. The conductor size of a wire
dipole will have some effect at 15m, but I did not try anything
different than #12 wire.
Gary N4AST

Just to be different, I tried #18 wire. Using 468/f gives a length of 21.9
feet at 21.3 MHz. According to EZneck, this is resonant at about 22 MHz. A
length of 22.4 feet gave resonance at just a shade above 21.3 MHz. Making
the wire 10% long put the resonance below 21 MHz. All at 18 feet.

I ran SWR from 21 to 22 MHz, with a step size of .05. Number of segments
was
19.

Tam/WB2TT



Yes, I was guessing when I said 10% The actual difference is closer to
2-3%
range. One glaring result of the EZNEC results, is SWR goes down from 1.8
to
next to nothing if I substitute a 75 ohm xmission line. I wonder if I
should
try some 75 ohm coax in place of the 50 ohm I've bought.

Lee K


In all likelihood you won't be able to tell the difference. A 1:1 SWR at 75
Ohms still leads to the radio thinking the SWR is 1.5,and the radio will be
seeing 75 Ohms at resonance. If you use 50 Ohm coax, the SWR will be
theoretically 1.5:1, but the actual impedance can be anywhere on the 1.5
circle of the Smith chart, depending on the length of the coax. Probably,
using 75 Ohm line would be more "proper". You probably know that you can
make the dipole impedance 50 Ohms by making the center higher, and drooping
down the ends. I just wouldn't let the ends go any lower than the 18 feet
that you have.

Tam




CUL8R



R*Horse


--




"Take a look behind you - upstream - now you begin to
recognize this country, don't you?"

"Yes, I do recognize it now. It is the most wonderful
thing I ever heard of; by a long shot the most
wonderful - and unexpected."

Mark Twain
Life on the Mississippi



http://rwinner.blogspot.com