Dave wrote:
better get out the snorkle... if you tie the braid and the center conductor
together you get a 25 ohm line, i use that all the time for my stack
matching boxes.

Nobody said anything about tying the braid and center conductor
together. If the side by side run of 50 ohm coax is used as a
parallel feedline, the braids are tied together and the Z0
is 100 ohms when the center conductors are connected across
the load.
--
73, Cecil http://www.qsl.net/w5dxp

"Sonny Hood" wrote in message
...
I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?

K4WYS

As I remember, common lamp "zip" cord (AWG 18 or 16) functions at about that
Z (I'll have to check)

gb

Cecil Moore wrote in news:x0N9g.86749$dW3.29542
@newssvr21.news.prodigy.com:

Dave wrote:
Sonny, the feedline cannot and does not have any influence on the
resonance of an antenna. Resonance is determined by the physical
dimensions of the antenna itself.

Feedline length has every influence on the resonance of
an antenna system if the SWR is not 1:1.

Why is a G5RV resonant on 3.7 MHz when a 102 foot dipole
is resonant on 4.6 MHz?

Truth is that the feedline can be used to resonate the
antenna system. I resonate my 130 ft. dipole on any HF
band by varying the length of the feedline. Check it out
at: http://www.qsl.net/w5dxp/notuner.htm

All true, but there isn't much difference between a G5RV and a 102 foot
dipole fed with 450 ohm line to a good balanced tuner.

Back years ago I had a very effective vee dipole antenna for 80m that was
composed of two identical wires from the top of my third story shack
window insulated and tied off to trees and shrubbery. I fed it with
home-made 600 ohm line, about six feet long and tapped it directly off
the tank coil of my amplifier. It was LOUD all over the Pacific, South
America, audible in Africa, but for some reason, nothing from Europe!

--
Dave Oldridge+
ICQ 1800667

On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote:

I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?
K4WYS


Is this a single band or multiband antenna? If I recall correctly,
you can use a different impedance feedline if it is a multiple of a
half wave. At the half wave points, the impedance is the same as the
beginning of the feed.

Cecil or others here will correct the length error if I am incorrect.
But I learned about a year ago that the impedance can be corrected by
the multi-fraction length feedline of a different impedance.

Maybe this is the principal of Cecil's feedline matching system.

73 for now


--
73 for now
Buck
N4PGW

"Sonny Hood" wrote in message
...
I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?
K4WYS

Does it have be twinlead? Certain LAN cable is made for 92 ohm Zo, which is
might close. Only downside I can imagine is the small size of the center
conductor: might limit XMIT power.

ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna
for 75 meters. The antenna wire is #12 multi-stranded tinned copper
with 600 volt TEW insulation. The antenna is fed within one foot of
the NW corner, via 25 ft. of 450 ohm twin lead to a 1:1 balun then via
air dielectric RG/8 (21.1 ft.) then to a AT-1500CV. The formula 1005/f
was used to figure the length and to cut the wire. However, due to
the low height about ground (24-26 ft.); the ground capacitance
changed the antenna resonance considerably. The target f (frequency)
was 3.9 MHz with 257.7 ft. of wire. After installation in a rectangle
configuration at 25+/- feet, the resonate frequency was 3.452 MHz.
With a tuner matching the antenna to the "rf" generator (transceiver),
the NVIS signal met the 100-200 mile range that was estimated.
The antenna was tuned - cut (shortened) to increase the
resonate frequency into the 75 meter phone band. After some
estimating of desired length at the low elevation and cutting and
pruning a resonate frequency of 3.790 MHz was obtained, that has a
VSWR of 1.6 to 1. The resonance was found with an MFJ-269 without the
tuner, 1.6:1 @ 3790 KHz. The tuner matches the load to 50 ohms for the
PW-1. The area of the loop decreased and the perimeter length is 229.5
linear feet. Again with the use of a tuner throughout the phone band
produced a NVIS signal range that was very good. K4WYS

On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?
K4WYS

Dave Oldridge wrote:
... there isn't much difference between a G5RV and a 102 foot
dipole fed with 450 ohm line to a good balanced tuner.

Not much difference in performance. Lots of difference in the
impedances seen by the tuner. For instance, built-in autotuners
will usually match a G5RV somewhere on 80m, 40m, & 20m. There
are usually certain troublesome lengths of 450 ohm line used with
a 102 foot dipole that will prohibit a match by built-in autotuners.
--
73, Cecil http://www.qsl.net/w5dxp

Buck wrote:
Is this a single band or multiband antenna? If I recall correctly,
you can use a different impedance feedline if it is a multiple of a
half wave. At the half wave points, the impedance is the same as the
beginning of the feed.

i.e. at the antenna. You're right, neglecting losses, the antenna
impedance is repeated every half wavelength. Neglecting losses,
that 100 ohm antenna feedpoint impedance will be repeated at half-
wavelength intervals for *any* feedline Z0, including 50 ohm coax.

Cecil or others here will correct the length error if I am incorrect.
But I learned about a year ago that the impedance can be corrected by
the multi-fraction length feedline of a different impedance.

Maybe this is the principal of Cecil's feedline matching system.

The principal of the feedline matching system that I use is that
every SWR circle crosses the purely low resistive point at a
current maximum point. For dipoles that are 1/2WL on the lowest
frequency of operation, the current maximum point is the ideal
point at which to feed the antenna system. Adjust the length of
the feedline until the current maximum point is at the transmitter
and you will usually not need a tuner at all.
--
73, Cecil http://www.qsl.net/w5dxp

Sonny Hood wrote:

I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?

Let's take a look at how the 450 ohm feedline "messed up the
match". The SWR on the 450 ohm feedline will be 450/100 = 4.5:1.
A feedline length equal to a multiple of 1/4WLs will transform
the 100 ohms to ~2000 ohms or a 50 ohm SWR of 40:1 which is
probably out of the matching range for built-in autotuners, for
instance. You can change the impedance seen at the tuner back
to 100 ohms by adding or subtracting 1/4WL of 450 ohm line which
will make it a multiple of 1/2WL.

As someone else said, one of the most common ways to match a
100 ohm antenna is to use 1/4WL of 75 ohm coax and then 50
ohm coax the rest of the way. Instead of "messing up the match",
it fixes up the match. The SWR on 75 ohm coax would be 100/75 =
1.33:1. The impedance at the end of the 1/4WL matching section
would be 75/1.33 = ~56 ohms, a good match to 50 ohm coax.

If you are wanting to use the antenna for multi-band operation,
change the length of the 450 ohm line until the match is not
"messed up".
--
73, Cecil http://www.qsl.net/w5dxp

you are great cecil, you cut out the original post to make it look like i
had nothing to do with the original statement... if you will remember he was
asking if he would get 25 ohms if he connected the braids together without
the center conductors... i simply say that if you connect both the braid and
center conductors you get a 25 ohm line... the implication being that if you
don't connect the center conductors you don't get a 25 ohm line.

thanks for keeping the fight thread going so well, now that you are riled up
i guess you can branch out into some other threads and bog them down for the
rest of the week.

"Cecil Moore" wrote in message
. com...
Dave wrote:
better get out the snorkle... if you tie the braid and the center
conductor together you get a 25 ohm line, i use that all the time for my
stack matching boxes.

Nobody said anything about tying the braid and center conductor
together. If the side by side run of 50 ohm coax is used as a
parallel feedline, the braids are tied together and the Z0
is 100 ohms when the center conductors are connected across
the load.
--
73, Cecil http://www.qsl.net/w5dxp