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

On Mon, 15 May 2006 14:26:11 GMT, Cecil Moore
wrote:

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.

So far, so good. If the antenna is fed with 50 ohm coax, there will
be great losses, however, if I am correct, there will be negligible
losses with 450, or even less with 600 ohm twin-lead. A 2:1 balun at
the end of this 600 ohm wire should net a low-loss, but good match for
his antenna?


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.

OK, I think that is a more technical explanations of what I said, so
we are in agreement. But for the sake of clarification, let me ask
this...

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?


--
73 for now
Buck
N4PGW

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

Buck wrote:

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

"1/2 wave of 75 ohm" would make matters worse for a 25 ohm
antenna since the SWR would be higher than for 50 ohms.
What you need is 1/4WL of 35 ohm coax like RG83.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 16 May 2006 13:24:37 GMT, Cecil Moore
wrote:

Buck wrote:

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

"1/2 wave of 75 ohm" would make matters worse for a 25 ohm
antenna since the SWR would be higher than for 50 ohms.
What you need is 1/4WL of 35 ohm coax like RG83.

OOPS, I went back to the original problem with 100 ohm



--
73 for now
Buck
N4PGW

Buck wrote:

Cecil Moore wrote:

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

"1/2 wave of 75 ohm" would make matters worse for a 25 ohm
antenna since the SWR would be higher than for 50 ohms.
What you need is 1/4WL of 35 ohm coax like RG83.

OOPS, I went back to the original problem with 100 ohm

Then you should use 1/4 wave of 75 ohm, not 1/2 wave.
--
73, Cecil http://www.qsl.net/w5dxp