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Old August 14th 06, 03:53 PM posted to rec.radio.amateur.antenna
Lee Lee is offline
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Posts: 36
Default QFH Antenna and 72ohm TV Coax


"Jerry Martes" wrote in message
news:hU%Dg.12256$hH1.9718@trnddc08...

"Lee" wrote in message
k...
I`m building a QFH NOAA weathersat antenna and wish to use low loss t/v
coax
which is
72ohm; but is more manageable and less lossy at VHF than the 50ohm RG58
specified
over a 100ft run!.... ( RF pre-amps aint cheap! ).......

What are the pros and cons... can i use CT100 72/75ohm..... i believe i
can
but, will the impedence affect
the 4 turn choke ????...

Thanks.

Lee......de G6ZSG.....


Hi Lee

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....de G6ZSG....





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Old August 14th 06, 10:46 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jul 2006
Posts: 168
Default QFH Antenna and 72ohm TV Coax

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!
--
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Old August 15th 06, 01:17 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jul 2006
Posts: 173
Default QFH Antenna and 72ohm TV Coax


"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


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Old August 15th 06, 01:54 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jul 2006
Posts: 168
Default QFH Antenna and 72ohm TV Coax

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
--
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Old August 15th 06, 08:00 AM posted to rec.radio.amateur.antenna
Lee Lee is offline
external usenet poster
 
First recorded activity by RadioBanter: Aug 2006
Posts: 36
Default QFH Antenna and 72ohm TV Coax


"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`ll consider your advice very carefully Owen, Thanks....


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).


Yes, i`ve done that also....and the handy was quite strong too but the
scanner front end was awful ..... AOR2002..... Ugh!!!..


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!


Surprisingly, i get quite a good picture using my Yaesu 857D also!!! ( with
attenuator in, of course)....


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!
--


I have the Icom IC-E90 which has good audio on the NOAA`s too ... also on
the ducky...

Lee.....G6ZSG....




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