"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