Thanks, Jim. That cleared that up nicely? How are you at explaining
the details of quantum mechanics, to be precise, entanglement and dark
energy? Certain facets elude me...
Patrick NJ5G
On 2/19/2015 8:39 AM, Jim Lux wrote:
On 2/19/15 5:15 AM, Patrick Greenlee wrote:
Will someone please explain why what I'm about to write is not true (if
that is the case.) I'm trying to follow the "debate" regarding coax
losses and their effects on transmit and receive.
The receiver has an inherent internal noise floor which can be
qualitatively observed by shorting the input coax fitting and cranking
the RF and AF gain up. If the signal portion within the "signal +
noise" coming through the coax to the rcvr goes sufficiently low it will
be irretrievably lost in the receiver's internal noise irrespective of
other considerations. Coax losses on receive could in some weak signal
situations attenuate the signal sufficiently to reduce it below the rcvr
internal noise and make it unrecoverable.
Often true on VHF and higher..
However, on HF and lower, the atmospheric and environmental noise
sources are often 10-20 dB higher than the receiver noise. For HF
receivers, a 10 dB input Noise Figure is quite good, most are in the
15-25 dB range.
Looking at the noise powers:
The kTB noise (what you get from a good load at 290K) is -174 dBm/Hz.
If the Rx noise figure is 10dB, then the noise power at the receiver
input is 10dB more: -164 dBm/Hz.
Let's ignore atmospheric noise for a minute..
What this means is that if you have a signal at -160 dBm, it will be 4
dB above the noise floor(in a 1 Hz bandwidth). If you add in 5 dB of
coax loss, now your -160dBm signal is -165 dBm, and it's -1dB SNR.
(note that you can consider the coax as part of the receiver, and move
the measurement plane to "the antenna", so now the noise figure is 15
dB, instead of 10dB. The SNR works out the same.. Signal at -160dBm,
Noise Floor of receiver+coax is -159 dBm/Hz.. -1 dB SNR in 1 Hz BW)
So coax loss, where the noise floor of the receiver is low makes a
difference... But with atmospheric noise that is 20-30 dB above the
receiver noise floor:
-164dBm/Hz -> receiver noise
-144 dBm/Hz -> atmospheric noise
If your signal is at -160dBm, no change in the coax loss will help..
you're under the -144, which is essentially *at* the antenna. Better
coax increases the level of your desired signal and also increases the
level of the noise. It's like using a low noise preamp.
The only way to solve the problem is to "get a better antenna".. most
atmospheric noise is directional to a certain degree, and if you're
lucky, the noise is coming from a different direction than your
desired signal. OTOH, if you're on the west coast trying to work a
station in Missouri in the middle of the afternoon in summertime.. the
station in MO is in the middle of the noise generator<grin>, and no
clever antenna scheme will work
Clearly, I'm no expert, just trying to understand the topic under
discussion. Critiquing the above could help me understand the matter
better.
Patrick NJ5G
On 2/19/2015 6:52 AM, Jim Thomson wrote:
Date: Wed, 18 Feb 2015 21:22:02 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Coax cable economics. (was Mosley
Antenna Question)
On Wed,2/18/2015 8:51 PM, Larry Loen wrote:
The point of low loss coax is to hear them.
Not really -- that's only true with very small signal work in very
quiet
locations. For the vast majority of us, and for nearly all real world
conditions, what we can hear is limited by atmospheric and man-made
noise. It's easy to figure this out -- if you start with no antenna
connected to your RX, then plug one in, and the noise increases by 10
dB or more, you are limited by the noise coming in on the antenna, and
lower loss coax (or even a more sensitive RX) won't help. What CAN
help,
sometimes a lot, is an antenna that rejects some of that noise while
not
rejecting (or rejecting less of) the weak signal you want to copy.
What low loss coax DOES do is make us louder in the other guy's radio.
73, Jim K9YC
## Excellent point. So if one has say an S-5 noise level on 20m,
and even
lossier coax is then used, and noise level drops to S-4, the SN ratio
on RX
has not changed at all ? You could simulate the same thing by adding
some attenuation in on RX.
## You are correct though, 3db loss in the coax means we just lost
3db on
TX..and RX. The 3db loss on RX wont be noticed, with any amount of
noise on
RX present. The 3db loss on TX is a huge amount. All these losss
add up
though...and fast.
## reduce coax loss by 1-2 db. Make the boom longer, add more eles.
perhaps
another 1-3 db. Install yagi higher, perhaps another 2-6 db.
Increase power on TX
by 1-3db. All of a sudden, we are now 5-14 db louder on TX..and
that’s nothing to
sneeze at.
## On CW, with narrow filters being used, the RX noise level will
drop by a huge amount,
vs SSB filter widths. On 40-10m, that’s when u can get the RX
noise level way down to the
noise floor..at least on cw mode.
## Drives me nuts seeing power being burned up in the coax on TX. Use
what ever method you want
to measure coax loss, but the method of wattmeters on both ends of
the coax is a real wake up call.
Jim VE7RF
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