Lee wrote:
I`ll try making the feed RG58 with the choke balun and then feed that with
72ohm...

No need to do that. Just use 4 turns of the 72 ohm coax.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
om...
Lee wrote:
I`ll try making the feed RG58 with the choke balun and then feed that
with
72ohm...

No need to do that. Just use 4 turns of the 72 ohm coax.

Thanks Cecil, this is my second QFH !!!
the first worked a treat as designed with RG58, but 100feet is a bit
lossy.....
a preamp under pmr towers is asking a bit too much hence tv coax.....

Lee...de G6ZSG

--
73, Cecil http://www.qsl.net/w5dxp

On Mon, 14 Aug 2006 14:53:53 GMT, "Lee"
wrote:


There are alot of affordable amplifiers designed for TV that you could
use
at the base of your QFH. You might consider building your owm amplifier
to
fit in the base of the QFH.
I wouldnt recomend the use of a pre-amp at the antenna for NOAA satelite
station. They often cause more problems than they solve.
All Electronics has alot of ferrite tubes that can be used to fit over
the
coax so you wouldnt need the "4 turn choke".

`4 turn Choke Balun`.....typo....


Jerry

Thanks Jerry, i`ll give it some thought as i`m right under some pmr towers
which breaks through a little from 150megs pagers and a preamp may worsen
things...

Lee,

It is easy to build a preamp with high gain and low noise figure and
it will exhibit superb performance on a test bench in a shielded room
on a signal generator.

In a real world environment, you are unlikely to realise the full
sensitivity of the receiver due to:
- external noise; and
- intermodulation products generated within your receiver (preamp).

It is harder to build a preamp with low intermodulation distortion,
and one method of reducing the results of that intermodulation
distortion is front end filtering to reduce the level of undesired
signals reaching the non-linear devices.

Front end selectivity costs much more money than a low NF preamp
transistor or gasfet.

Whilst wideband preamps are available at low cost, it is quite likely
that they will actually degrade your receiver performance.

It may even be that adding an external filter will improve your S/N
ratio.

An interesting test to perform is to note the S/N ratio, add a small
attenuator to the receiver input, and again measure the S/N ratio. If
the S/N ratio improves, it is an indicator that you have significant
intermodulation distortion and front end filtering may improve the
sensitivity.

I listened last night and could hear NOAA 14 on a hand held scanner
(IC-R20) with a 130mm long rubber duckie off my 2m transceiver. It
wasn't good enough for pictures, but it could be heard... so it
shouldn't take a lot of receiver sensititivity to decode it well.
(BTW, I could not hear the bird using a 200mm whip on the scanner...
to much noise from intermod products).

I know you asked about coax and you are seeking a low loss coax
situation, coax loss might be less important that adequate receiver
front end filtering so that you can realise most of its potential in
the presence of other strong signals. In the absence of that, coax
loss might actually improve S/N!

Owen

PS: I recently performed some tests on the new Icom IC-7000 on 144MHz
to determine the usable sensitivity on a wideband antenna, and
although the specified sensitivity is -126dBm, the sensitivity when
connected to a Diamond D-130 at this location was -96dBm, that is 30dB
poorer than spec, and the main contibution was IMD within the IC-7000.
Putting a 10dB attenuator inline improved the sensitivity by 14dB!
--

On Mon, 14 Aug 2006 14:58:32 GMT, Cecil Moore
wrote:

Reg Edwards wrote:
I don't know, never did know, how to use an old fashioned, mid-20th
century Smith Chart.

Reg, I'm curious how you would solve this stub problem
without a Smith Chart.

| 45 deg | 45 deg |
Source====Z01=========Z02====open

Stub sections are lossless. Z01 = 600 ohms and is 45
degrees long. Z02 = 50 ohms and is 45 degrees long.
What is the impedance looking into the stub from the
source?

I missed the significance of this problem Cecil.

Is it principally a theoretical (being lossless) problem that a Smith
chart can solve, or does it have some other significance?

Whilst a Smith chart is great for visualising transmission line
problems, a great way for visually mapping impedance over a range of
frequencies, it isn't the most practical way to solve practical
problems when we have access to the computing power commonly available
to designers today.

Owen

PS: I think the problem you have given can be solved with simple trig:
find the reactance of the Z02 section using one trig term, find the
length of Z01 that would deliver that reactance using one trig term,
add that length and the actual length of Z01 section, find the
reactance of the Z01 section using one trig term. I could do that in a
flash with a scientific hand calculator while you were sharpening your
pencil.

It is a trivial problem either way, and can only ever be an
approximation of a practical problem.
--

"Owen Duffy" wrote in message
...
On Mon, 14 Aug 2006 14:53:53 GMT, "Lee"
wrote:


There are alot of affordable amplifiers designed for TV that you could
use
at the base of your QFH. You might consider building your owm
amplifier
to
fit in the base of the QFH.
I wouldnt recomend the use of a pre-amp at the antenna for NOAA
satelite
station. They often cause more problems than they solve.
All Electronics has alot of ferrite tubes that can be used to fit over
the
coax so you wouldnt need the "4 turn choke".

`4 turn Choke Balun`.....typo....


Jerry

Thanks Jerry, i`ll give it some thought as i`m right under some pmr towers
which breaks through a little from 150megs pagers and a preamp may worsen
things...

Lee,

It is easy to build a preamp with high gain and low noise figure and
it will exhibit superb performance on a test bench in a shielded room
on a signal generator.

In a real world environment, you are unlikely to realise the full
sensitivity of the receiver due to:
- external noise; and
- intermodulation products generated within your receiver (preamp).

It is harder to build a preamp with low intermodulation distortion,
and one method of reducing the results of that intermodulation
distortion is front end filtering to reduce the level of undesired
signals reaching the non-linear devices.

Front end selectivity costs much more money than a low NF preamp
transistor or gasfet.

Whilst wideband preamps are available at low cost, it is quite likely
that they will actually degrade your receiver performance.

It may even be that adding an external filter will improve your S/N
ratio.

An interesting test to perform is to note the S/N ratio, add a small
attenuator to the receiver input, and again measure the S/N ratio. If
the S/N ratio improves, it is an indicator that you have significant
intermodulation distortion and front end filtering may improve the
sensitivity.

I listened last night and could hear NOAA 14 on a hand held scanner
(IC-R20) with a 130mm long rubber duckie off my 2m transceiver. It
wasn't good enough for pictures, but it could be heard... so it
shouldn't take a lot of receiver sensititivity to decode it well.
(BTW, I could not hear the bird using a 200mm whip on the scanner...
to much noise from intermod products).

I know you asked about coax and you are seeking a low loss coax
situation, coax loss might be less important that adequate receiver
front end filtering so that you can realise most of its potential in
the presence of other strong signals. In the absence of that, coax
loss might actually improve S/N!

Owen

PS: I recently performed some tests on the new Icom IC-7000 on 144MHz
to determine the usable sensitivity on a wideband antenna, and
although the specified sensitivity is -126dBm, the sensitivity when
connected to a Diamond D-130 at this location was -96dBm, that is 30dB
poorer than spec, and the main contibution was IMD within the IC-7000.
Putting a 10dB attenuator inline improved the sensitivity by 14dB!
--

Hi Owen

Not that it makes alot of difference, but, you could have been hearing
NOAA 17 on 137.62. Its coordinates may have been different from where you
were anticipating while orienting your 200mm scanner whip.
And, as you probably know you'll need about 30 KHz minimum if you want to
produce images from the NOAA satellites. I think the IC R20 selectivity
is either too narrow or too wide for producing APT images even when the
signal strength is adequate. But, you probably knew that.

Jerry

On Tue, 15 Aug 2006 00:17:40 GMT, "Jerry Martes"
Not that it makes alot of difference, but, you could have been hearing
NOAA 17 on 137.62. Its coordinates may have been different from where you
were anticipating while orienting your 200mm scanner whip.
And, as you probably know you'll need about 30 KHz minimum if you want to
produce images from the NOAA satellites. I think the IC R20 selectivity
is either too narrow or too wide for producing APT images even when the
signal strength is adequate. But, you probably knew that.

It was actually NOAA 15 that I heard (sorry for the typo) and I was
listening to the APT signal on 137.5MHz in USB mode. I am pretty
confident it was the bird, the doppler shift changed direction at the
right moment.

I was more interested in the strength, I couldn't find a ready source
of link budget calcs on the 'net.

Owen
--

Owen Duffy wrote:
Cecil Moore wrote:
Reg, I'm curious how you would solve this stub problem
without a Smith Chart.

| 45 deg | 45 deg |
Source====Z01=========Z02====open

Stub sections are lossless. Z01 = 600 ohms and is 45
degrees long. Z02 = 50 ohms and is 45 degrees long.
What is the impedance looking into the stub from the
source?

I missed the significance of this problem Cecil.

Is it principally a theoretical (being lossless) problem that a Smith
chart can solve, or does it have some other significance?

It's just a mental exercise with a hidden significance. This
is the type of problem that I would solve with a Smith Chart.

How about a solution? What impedance does the source see? The
physical length of the stub is 90 degrees. What is the electrical
length of the stub in degrees?
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:



It's just a mental exercise with a hidden significance. This
is the type of problem that I would solve with a Smith Chart.

How about a solution? What impedance does the source see? The
physical length of the stub is 90 degrees. What is the electrical
length of the stub in degrees?

KISS

But if you did that you couldn't get this thread to last forever.

tom
K0TAR

"Cecil Moore" wrote in message
. ..
Owen Duffy wrote:
Cecil Moore wrote:
Reg, I'm curious how you would solve this stub problem
without a Smith Chart.

| 45 deg | 45 deg |
Source====Z01=========Z02====open

Stub sections are lossless. Z01 = 600 ohms and is 45
degrees long. Z02 = 50 ohms and is 45 degrees long.
What is the impedance looking into the stub from the
source?

I missed the significance of this problem Cecil.

Is it principally a theoretical (being lossless) problem that a Smith
chart can solve, or does it have some other significance?

It's just a mental exercise with a hidden significance. This
is the type of problem that I would solve with a Smith Chart.

How about a solution? What impedance does the source see? The
physical length of the stub is 90 degrees. What is the electrical
length of the stub in degrees?
--
73, Cecil http://www.qsl.net/w5dxp

Hi Cecil

Is it posible that the length of the "stubs" change? I'd have thought the
length of the stub is always the same. 45 degrees should always be 45
degrees, shouldnt it?? An open circuit, 45 degrees back along a 50 ohm line
looks like 50 ohms capacitive. That 50 ohms looks like something like 500
ohms inductive as viewed 45 degrees back along a 600 ohm line.
I'd guess your point is that 500 ohms of pure inductive reactance is never
seen 90 degrees back from an open, no matter what the Zo of the line

Jerry

Tom Ring wrote:
KISS

Keep It Simple Stupid?

But if you did that you couldn't get this thread to last forever.

I have kept it as simple as possible. Wonder why nobody
has ventured an answer?
--
73, Cecil http://www.qsl.net/w5dxp