"Ralph Mowery" wrote in
:
"Owen Duffy" wrote in message
...
Dave Oldridge wrote in
:
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I used to have an FT-221 tricked out with a hot front end. Solar
noise would run the S meter up to well over the S9 mark and you
could even see the galactic plane passing through the antenna
pattern. Needless to say, it heard well on terrestrial 2m SSB.
Firstly, this was actually Howard's statement.
That is no mean feat!
And my comment.
I think ambient noise temperature at 144MHz for an antenna pointed at
cold sky is somewhere around 200K to 250K, when you add a pretty good
receiver at say 30K, you are talking 230K to 280K total system noise,
and the sun is probably around 800K with a low end 4 bay EME antenna
setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or
16dB.
A single yagi of gain around 15dBi is much poorer, not only is the
sun noise reduced proportionately to the gain reduction, but the
ambient noise increases with higher gain in the side and back area of
the antenna, but it still should be possible to reliably 'see' the
sun with a very good receiver.
Ambient noise temperature for a beam at zero elevation here in
suburbia varies from 1000K to 6000K depending on the day and time...
so a very low temperature receiver is wasted for terrestrial
contacts.
Owen
Owen
I would like to see what mods are made to the 221 to do that and also
what kind of antenna system. I have a 221 I am using with a gasfet
preamp in the shack that should be less than 1 db of noise fugure and
about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913
type of coax. I can just see some sun noise with the antenna aimed at
the sun. It sure does not deflect the smeter several sunits. The
antenna is on an azel mount. I am sure the system is working as I
compaired it to an Icom 706 and another antenna that is mounted on a
tower and I am getting about the differance in signal levels I would
expect at the horizon.
So, lets take some guesses about things here. FT221 native NF ~8dB, line
loss 0.2dB, preamp 1dB NF, 20dB gain, 9913 loss 1.2dB (75', load end VSWR
1.5). On my reckoning, Teq at antenna connector is 200K. The antenna is I
understand a 22 element crossed Yagi, let's assign it 15dBi for the
purposes of discussion.
Looking back at the earlier scenario, ambient (cold sky and earth) of
225K (ignoring the prospect of worse spillover with the smaller antenna)
and sun noise of 160K, sun noise rise would be (225+200+160)/(225+200) or
1.4dB. That is not going to be very noticeable on an S meter.
O&OE!!!
At higher points in the solar cycle or if the sun is disturbed, the rise
will be greater, but genuine quiet sun measurements should not capture
disturbed sun, should they? Antennas more sensitive off the back / sides
will be worse.
In that scenario, moving the preamp to the antenna would improve G/T from
-11.3dB to -10.1, yielding a 1.2dB improvement in S/N ratio. At zero
elevation, the ambient noise is typically much higher and the improvement
would be much less.
Ambient noise is pretty easy to measure on 144MHz in a simple station. If
you know the noise figure of your receiver and loss to the antenna, and
providing ambient noise is not sufficient to cause AGC, note the audio
power output change between antenna and a dummy load, you can calculate
the ambient noise, see
http://www.vk1od.net/sc/anc.htm for more
information. In fact, with this tool, I can measure the change in audio
output on my TS2000 by switching the internal 12.1dB attenuator in, and
calculate Ta (try Example 3).
I hear sun noise rise bragged about, but the bragger often cannot quote
the solar flux prevailing at the time, in which case it is rather
meaningless, especially when their method is to capture the largest rise
rather than largest minimum rise whithout artificial compression.
The reason I wrote the online calculator is that a number of people I
have discussed sun noise rise with were using solar flux figures that
were old and / or the wrong frequency.
Owen