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

On Tue, 16 May 2006 14:30:52 GMT, Cecil Moore
wrote:

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.
ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm
coax, it is 1.33:1 ??

If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax,
it will be a close match?

Why did the impedance change to 52 ohms?
--
73 for now
Buck
N4PGW

Buck wrote:

ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm
coax, it is 1.33:1 ??

If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax,
it will be a close match?

Why did the impedance change to 52 ohms?

Because of the energy bouncing back and forth between the 100 ohm load
(that mismatches the 75 ohm, so causes a reflection) and the 50 ohm
feed (that mismatches the 75 ohm section, so causes a reflection).

If the 1/4 wave section has an impedance that is the geometric mean of
the impedance on each end, it creates a perfect matching resonator.

So the perfect 1/4 wave matching section between 50 and 100 ohms would
have an impedance of the square root of (50 times 100) or 70.7 ohms.
75 is pretty close.

Buck wrote:
ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm
coax, it is 1.33:1 ??

Yes, and 75/1.33 = 56.25 ohms. For a perfect match to 50
ohms, one would need Z0 = SQRT(100*50) = 70.7 ohms.

If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax,
it will be a close match?

The SWR on the 50 ohm coax will be 56.25/50 = 1.125:1

Why did the impedance change to 52 ohms?

It didn't. It changed to 56.25 ohms. The load is 100 ohms
and the Z0 is 75 ohms so the SWR is 1.33:1 at the current
minimum/voltage maximum point. To find the impedance 1/4WL
away, divide Z0 by SWR, 75/1.33 = 56.25 ohms.
--
73, Cecil http://www.qsl.net/w5dxp