[600MRG] Rx noise 630m

Ed Cole kl7uw at acsalaska.net
Mon Mar 20 12:30:27 EDT 2023 

No comment on the loop antenna but a common (popular) method of blocking 
common-mode is by use of ferrite beads.  At 630m not sure what mix would 
be best too use.  My 6m LFA yagi balun uses a "whole bunch" of them.

I agree Tom regarding using coax over balanced line.

Perhaps one could connect a 50-ohm load at antenna end of the coax to 
determine common-mode rejection.  Good shielding should not be subject 
to any variation of noise by movement or bringing something near the 
coax with such termination.

You could also use a weak signal source to determine isolation with load 
vs being connected to loop.

I did this with my 2m eme preamps to determine receiver sensitivity 
(-155 dBm).  If shielding is not good then leakage will reduce how weak 
a signal can be measured.  I had to place my signal source inside a 
metal enclosure with internal battery to reduce leakage enough to reach 
-155 dBm.  Double-shielded coax was used with 50-dB coaxial attenuation 
with my -107 dBm signal source.

73, Ed - KL7UW

On 3/20/2023 6:20 AM, Tom W8JI wrote:
> 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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