Buck wrote:

W5DXP wrote:
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.

So far, so good. If the antenna is fed with 50 ohm coax, there will
be great losses, ...

A 100 ohm antenna causes an SWR of 2:1 on 50 ohm coax. Lots
of hams don't even bother with a tuner when the SWR is 2:1. The
additional losses in 50 ohm coax due to an SWR of 2:1 are pretty
much negligible on 80m for reasonable lengths of feedline.

I have a yagi with about 25 ohms at the antenna. I connect 600 ohm
twin-lead and run it 1 1/2 wave lengths to my radio. This, I assume,
gives me the highest current at the end of the twin-lead. So I
connect it directly to the radio with a PL-259 and get an acceptable
load for the transmitter with low-loss in the feedline?

The SWR on the 600 ohm line will be 600/25 = 24:1

The SWR on 50 ohm coax will be 50/25 = 2:1

For a 25 ohm single band antenna, I would be inclined to go with
the coax.
--
73, Cecil http://www.qsl.net/w5dxp

The SWR on the 600 ohm line will be 600/25 = 24:1

The SWR on 50 ohm coax will be 50/25 = 2:1

For a 25 ohm single band antenna, I would be inclined to go with
the coax.

Ok, I learned something, but I would be inclined to go with the 1/2
wave of 75 ohm and then the 50 ohm


--
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

great! glad the good old measure and trim method worked adequately... now
if you are really smart you will stop reading more replies to this thread
and get on the air and make use of that nice new antenna!

"Sonny Hood" wrote in message
...
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

"Sonny Hood" wrote in message
...
ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna
for 75 meters. The antenna wire is #12 multi-stranded tinned copper
with ..............

Sort of like what I have. Full size 75m loop fed at one corner. I connected
a 1:1 current balun directly at the antenna , feeding a 1/4 wave section of
RG11 Foam coax. All 50 Ohms downstream from there. SWR at resonance is about
1.2:1. No tuners.

Tam/WB2TT

On Mon, 15 May 2006 08:26:39 -0400, Sonny Hood wrote:

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.

You would expect the feedpoint impedance of a full wave loop (cut
using your formula) to be around 90 ohms at resonance.

You have then connected 25' of 450 ohm line. Lets assume this is
Wireman 551 line, the input impedance to this line section at
(assuming 3.7MHz) should be around 141+j282 (and line loss would be
~0.1dB).

You then have an unspecified length of "air dielectric RG/8" (new to
me) which will transform the impedance further.

You seem to intend to then use an ATU to transform this impedance to a
satisfactory load for the transmitter.

Where did you make the measurements with the MFJ269? If it was not at
the antenna feedpoint, did you really expect resonance at the
measurement point to coincide with loop resonance, and is resonance at
either point important to efficient operation of the entire system?

Owen

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
--

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

Now, however did I know this thread would go on 'n on, 'n on 'n on......



Cecil Moore wrote:
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".