[600MRG] Rx noise 630m

Mon Mar 20 10:20:07 EDT 2023

Hi Dave,

I am pretty sure think I have read know the antenna you have. I cannot 
find the article now. Maybe you can link it for me?

I generally agree with most of the construction I recall and had a 
favorable view of that antenna but I probably would not use the balanced 
line. The balanced line link doesn't offer anything over just using 
isolated coax and might in many cases be worse. Remember a balanced line 
no matter how well balanced has the electric and magnetic fields outside 
in space around the pair. There really isn't anything to be gained by a 
balanced line, and there is a whole lot that might be lost.

A shield several skin depths thick has no troublesome ingress except at 
a shield break. If the shield and coaxial connections are good all the 
way to the receiver, the only point of ingress for common mode will be 
at the antenna.

One of the most difficult tests in the electronics world is for common 
mode pickup in a receiving system. The antenna itself is generally 
involved in common mode,  the feed line connection at the antenna is 
very often the injection point. Thus when we perturb that connection by 
shorting it, we can actually block or stop the common mode.

A Wellbrook loop I tested (as part of another company's research) a 
dozen or more years ago had significant common mode response all through 
HF. I know this because the antenna pattern is asymmetrical. The nulls 
are not placed 180 degrees apart and are not symmetrical in null depth. 
If I got inside the loop and shorted the antenna terminals that common 
mode response would disappear, because the loop terminals were the point 
of common mode ingress.

If I pulled my hair out while trying to think of a good test for common 
mode, I would have lost all my hair 40 or 50 years ago. Common mode is a 
tough thing to measure because the point of ingress to the receiver 
should be at the antenna, and normally is there at the antenna 
terminals. It is not at the receiver, unless the feed line or feed line 
connections are "poor".

73 Tom

On 3/19/2023 2:17 PM, Dave Riley via 600MRG wrote:
> TNX again, Tom,
>
> The tuned 5 foot, 7 turn loop feeds a toroid transformer, then 50 feet 
> of 72 ohm twin lead to the receiver.. No amplifier..
>
> The RX end of the twin lead sees an isolation toroid and the twin lead 
> itself lays flat upon the ground..
>
> The secondary of the termination toroid enters directly in to the R75 
> receiver with preamp set on #2.. No S meter reading yet..
>
> I use no AGC and monitor the audio line out and set volume control to 
> -10 db on the volt meter with no antenna selected.. Normally I see +13 
> with antenna on and no signals..
>
> So I tried the Cushman which shows -112 db with a short or dummy load 
> at the input..
>
> Then I apply the loop ( no pre-amp ) and the Cushman says -105db, or 7 
> db of antenna noise.. ( 1 S unit roughly )
> If I tune to 460 or 490 I only get -111, still 1 db above Cushman 
> receiver noise..
>
> This may be the end of the road for local noise study and that I must 
> be happy with whatever the number is..
>
> Still thinking, 73s fer sure.. DaveR @ W1FRV
>
>
>
>
>
> on 3/19/2023 1:30 PM, Tom W8JI wrote:
>> Hi Dave,
>>
>> I see what you are doing now. I thought you were looking at site or 
>> band noise changes.
>>
>> You are trying to determine if your loop antenna is setting the noise 
>> floor of your system or if the amplifier and receiver system are 
>> limiting the noise floor.
>>
>> The ideal test of course would not be to dead short the loop 
>> amplifier input, but to substitute a dummy load with an equivalent 
>> impedance to the loop. If the noise drops several dB at the quietest 
>> time of day in the most quiet season at the narrowest receiver 
>> bandwidth you know the loop external signal pickup (absent any 
>> common  mode) is setting the noise floor.
>>
>> The problem with a short is mostly in the amplifier behavior. We 
>> don't know how the amplifier and any matching system reacts reacts to 
>> a dead short, so that may or may not skew results enough to mislead 
>> us. Removing the antenna and substituting a more similar inductive 
>> load might be better.
>>
>> I was going to put a loop or a voltage probe out back in the woods. I 
>> mostly receive on my Beverages, and I don't use the receiver filter 
>> bandwidth WSJT is based on, so the already nearly useless SN reports 
>> in WSJT are skewed a lot more than they could be.
>>
>> My inverted L antenna noise level measurements like any antenna 
>> system noise level measurements are influenced by the inverted L 
>> efficiency and pattern. Since this is a full size antenna the only 
>> real losses are ground related losses. Matching losses are zero.
>>
>> I know I'm close to limiting by ionosphere propagated distant noise 
>> at night on the L because because on the quietest winter nights by 
>> noise floor increased about 3dB rather consistently. I'd like to see 
>> that be 10dB or more.
>>
>> If I consistently saw no change day or night then I would know local 
>> noise is dominating my system.
>>
>> At 0600Z last night my L antenna's 3.1 kHz bandwidth noise level was 
>> -68dBm average with peaks over -20 dBm from distant lighting storms. 
>> The 530 kHz AM BC channel was -23.5 dBm average.
>>
>> My daytime local solar noon noise floor was  -72dBm and the 530 kHz 
>> channel (all sky wave propagated) signal level is -62 dBm.
>>
>> I have to understand my ambient and propagated noise so I know how 
>> much negative antenna gain I can tolerate when building a directional 
>> receive antenna. I am limited to 1000 feet or so in every direction 
>> except NE/SW.  I have to use a "compact" array since I have less than 
>> 1/2 wave of space going NW and SE, and that means significant 
>> negative antenna element gain. This is why I am trying to understand 
>> the band noise floor here.
>>
>> 73 Tom
>>
>>
>>
>>
>>
>>
>>
>> On 3/18/2023 5:08 PM, Dave Riley via 600MRG wrote:
>>> TNX, Tom for another look through your prism..
>>>
>>> I just set the Cushman 24 up on 475kc.  With a shorted input it 
>>> reads -111 db ( not sure of cal )
>>>
>>> Then I plug in the loop and it goes up to -5 on the Cushman or +6 db 
>>> over -111 noise.
>>>
>>> I'd guess that the -111 is the noise floor on that meter.  It was on 
>>> 3.1 kc
>>>
>>> I tuned off to 460 and then 490 and the noise indicated -110 db on 
>>> the edges.
>>>
>>> Tuned to 476 kc. and it was -5  ' fairly good loop Q '
>>>
>>> Best I can do is 'relative'...  Think I should be using a 50 ohm 
>>> load instead of a short..
>>>
>>> TNX summore de DaveR W1FRV
>>>
>>>
>>>
>>>
>>> On 3/18/2023 4:09 PM, Tom W8JI wrote:
>>>> Hi Dave,
>>>>
>>>> Relative performance or noise performance is very difficult to 
>>>> measure in a meaningful way.
>>>>
>>>> 8dB is meaningless. 8 dB is just a ratio, what is the 8dB ratio 
>>>> against and under what condition?
>>>>
>>>> The 13dB difference between a short and an antenna doesn't mean 
>>>> very much of anything either. It only means the antenna is picking 
>>>> up 13dB more noise than a short. That doesn't mean anything about 
>>>> ambient noise or antenna receiving performance. It only means is 
>>>> the antenna system has 13dB more background noise than a short in 
>>>> some test. It doesn't tell us anything about how well something is 
>>>> working, how poorly it is working, or what the ambient noise level 
>>>> is unless there is some good reference.
>>>>
>>>> Now if we have some reference signal it might mean something. My 
>>>> full size inverted L antenna in January was about -73 dBm in 3.1kHz 
>>>> bandwidth daytime at this time of day, and -70 dBm average on a 
>>>> quiet night just before sunrise.
>>>>
>>>> I just measured it now and it is -72 dBm in the same 3.1 kHz 
>>>> bandwidth. Some digital signal on right now is -86 dBm, 14 dB below 
>>>> the 3.1kHz noise floor.
>>>>
>>>> My guess is that is N4WLO, my level meter can't decode digital 
>>>> signals.
>>>>
>>>> Even this does not mean much.
>>>>
>>>> 73 Tom
>>>>
>>>>
>>>>
>>>>
>>>> On 3/18/2023 3:11 PM, Dave Riley via 600MRG wrote:
>>>>
>>>>> Saying good bye to seasonal DX..
>>>>>
>>>>> Think the bulk of QRN has been nulled out here but still, the 
>>>>> difference between antenna and a short is about 13db
>>>>>
>>>>> Not much difference in noise level with loop spun around either..
>>>>>
>>>>> Maybe there is 8 db ambient noise average, some say that is low..
>>>>>
>>>>> What you say??
>>>>>
>>>>> TNX Dave @ W1FRV
>>>>>
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