[TowerTalk] Coax cable economics. (was Mosley

[Jim Lux]

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 at audiosystemsgroup.com>
>> To: towertalk at 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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