Historically in the early days of coax, 50 Ohms was preferred for better power handling and 75 Ohms for lower loss.

Over time 50 Ohms became the de-facto standard for transmitters and 75 Ohms for domestic receivers and baseband video distribution.

The technologies were often developed and maintained by independent sets of engineers, possessing contrasting skills and training, which partially explains why this difference has perpetuated to this day.

Regards,

Martin

Hello Simon,

Saturday, January 1, 2022

Who is Mary and will your wife mind, that is what I want to know? ;) Happy New Year. The divorce courts are backlogged so you will have time to hide some of the expensive stuff..... ;)


Best regards,
Chris mailto:chris@...


S> Quote

S> “Whilst I realize there's no such thing as perfection in an imperfect world,”

S> I beg to different..my Wife is allowing me to put up a top band
S> 20m vertical with all its associated stuff, plus all the rx
S> antennas etc at our new location in Cornwall when we move there this year..
S> This makes her perfect..( plus Mary will be helping, though she does not know this yet.)

S> Simon

S>

Quote

“Whilst I realize there's no such thing as perfection in an imperfect world,”

I beg to different..my Wife is allowing me to put up a top band 20m vertical with all its associated stuff, plus all the rx antennas etc at our new location in Cornwall when we move there this year..
This makes her perfect..( plus Mary will be helping, though she does not know this yet.)

Simon

Hello Alan and Martin,

Saturday, January 1, 2022


I should be able to give this a go tomorrow and will report back. Thanks. One downside of the loop's relocation and my making it a smaller circumference (vertical elements of the rectangle remain the same, horizontal ones were shortened by about half) is LF performance, which was very good, has now taken an apparent dive...

I guess you win some and you lose some?

I have borrowed a 12V accumulator from a race car and have it on trickle charge tonight, so I can avoid the wrath of her indoors with a full size, wet cell car battery in the house. Race car batteries go in carrier bags, and what she doesn't know can't trigger her ;) I should be able to run the loop injector from this tomorrow, too.

Whilst I realize there's no such thing as perfection in an imperfect world, I am up for a bit more fiddling to see if things can be further improved :) To think I went about four or more years with this loop only really any use at all on LF.... Ignorance can be bliss though.



Best regards,
Chris mailto:chris@...


Mvgi> On Sat, Jan 1, 2022 at 02:39 PM, Alan wrote:
Mvgi> As Andrew said shorting the input is not a good idea and will
Mvgi> not do what you think it will. Leaving the terminals open will
Mvgi> but you need to disconnect both terminals.
Mvgi> Hi Alan,

Mvgi> OK, if this is problematic, then I'm not sure how the test I
Mvgi> was thinking of can be achieved.

Mvgi> My intention was to leave the loop in place with the ends
Mvgi> shorted, so that the differential mode is suppressed, leaving
Mvgi> only the common mode contribution from the loop and coax cable.

Mvgi> This method seemed to work OK for me when testing with a
Mvgi> Wellbrook type design, but I can certainly see the flaws
Mvgi> associated with doing it this way.

Mvgi> Regards,

Mvgi> Martin

On Sat, Jan 1, 2022 at 02:39 PM, Alan wrote:

As Andrew said shorting the input is not a good idea and will not do what you think it will. Leaving the terminals open will but you need to disconnect both terminals.?

Hi Alan,

OK, if this is problematic, then I'm not sure how the test I was thinking of can be achieved.

My intention was to leave the loop in place with the ends shorted, so that the differential mode is suppressed, leaving only the common mode contribution from the loop and coax cable.

This method seemed to work OK for me when testing with a Wellbrook type design, but I can certainly see the flaws associated with doing it this way.

Regards,

Martin

Martin

As Andrew said shorting the input is not a good idea and will not do what you think it will. Leaving the terminals open will but you need to disconnect both terminals.? The amplifier is a Zwischenbasis type, as you have plotted in the past the input reacts to the signal source impedance. This form of noiseless feedback is the complement to the Norton Amplifier where the output impedance is a transformed version of the input impedance, here the output impedance hardly matters but you get to? optimise the matching to the source over a much wider range than a conventional? "dumb" input , it is NOT a virtual earth amplifier in the normal sense ( which also goes nuts with a shorted input !) but a much cleverer configuration due to Cantz from 1953.

However before disconnecting the loop? I would remove the power to the injector then disconnect the coax to the injector. That would show what is leaking into to the coax and then what remains after the coax is removed. It seems futile to disconnect the loop when you can just power it down.

I used a naked ALA100LN until recently, it works very well from LF to HF and beyond. The IMD performance is outstanding six miles from Brookman's Park. I now use a new arrangement? that drops BP signals an additional 30dB. I spot to PSK reporter from a tiny urban back garden, Don't do too bad for an antenna 3m high surrounded by switchers although BT has blocked 30m very effectively.

73, Alan G8LCO.

On Fri, Dec 31, 2021 at 03:35 PM, Chris Wilson wrote:

NEVER has this Wellbrook loop performed so well,

Hi Chris,

Happy New Year.

I managed to take a listen today, and it's pretty good, and certainly a lot more sensitive than it originally was.

The signal levels seem correct and the noise floor is now much more in line with what I'd expect in an semi urban environment.

Two bands of noise remain at a fairly low level.

2-4MHz has some noise with a 100Hz buzz.

17-25MHz has some white noise and mush with a bit of 100Hz buzz between 22-24MHz.

If the noise remains at a similar level when the DC to the bias tee is removed, then it's possible it's still getting in via the coax cable run, and moving the cable may help diagnose where this is occurring.

If you wish to experiment even further, I'd try shorting the input of the loop amplifier with the loop still connected. This will effectively eliminate the wanted differential mode signals that are produced by the loop, and any remaining signals are likely to be as a result of unwanted common mode signals, being carried by current on the outside of the coax screen, and these could probably be further reduced by adding suitable ferrite cored chokes to the cable.

Happy 2022,

Martin

On Fri, Dec 31, 2021 at 09:26 PM, Larry Davis wrote:

I highly recommend this double-shielded RG-58 substitute,

Hi Larry,

Yes that's a good cable.

My main reason for suggesting 75 Ohm sat TV cable is that it's about the cheapest double screened coax that is readily available.

But if you are really keen to use 50 Ohm stuff for whatever reason, the type you suggest is a very good choice.

Regards,

Martin

On Fri, Dec 31, 2021 at 06:59 PM, Tomasz Lemiech wrote:

What about connectors for these cables?

Hi Tomasz,

I have standardised on using 'F' connectors on all cables that carry both DC and RF, as this helps prevent accidentally connecting cables with DC onto a receiver input.

Sometimes I use adaptors from F to BNC (or other types) and vice versa, but generally when not carrying DC, I use standard BNC connectors that are available for 75 Ohm CCTV cables such as RG6.

All modern 50 & 75 Ohm BNC connectors are interchangeable, as the pin size has been standardised, and the impedance of the connector (which in this application is not important) is controlled by the amount of dielectric material surrounding the conductors. You may occasionally find old style BNC connectors at surplus and junk sales with a different taper on the pin, and narrower contact fingers on the socket, but these are now becoming increasingly rare and are best avoided.

A few hints if you are using F connectors.

When you make you make up the plug, try to ensure that the dielectric insulation protrudes a short distance past the hole it goes through in the bottom of the connector shell. This helps prevent electrolytic corrosion tracking across the gap and also forms a seal against the edge of the socket receptacle where the spring clips retain the coax inner conductor.

Do not let the inner conductor of the coax protrude too far beyond the open end of the connector shell. If it is excessively long, it can touch the rear of the socket housing and short out the connector, and any DC that may be present.

Two turns of PTFE plumbers tape wound around the screw thread of the F socket, before the plus is screwed onto it will help moisture finding it's way into the connector via the screw thread. It will not affect electrical continuity, as the ridges of the screw thread will cut through the thin PTFE material.

If you join cables outdoors with an in-line back to back socket (bullet), waterproof it with self amalgamating tape and another layer of PVC tape over the top.

Regards,

Martin

Hello all,
???? Just want to jump in regarding RG-58, etc...
I highly recommend this double-shielded RG-58 substitute,



???? Very good quality coax, and and for me a significant improvement in noise floor over regular RG-58.???
Be sure to order their connectors also, standard RG-58 connectors likely won't fit.
Can be ordered direct from the manufacturer, in Italy, and I believe Bonito also sells it.

??? Best New Year's wishes....

Larry Davis
Geneva,IL
USA

Good question

But why not just use a 50r plug? Unless missing something here..obviously does not matter about the slight swr difference, just as long as 50r plug fits 75r cable.

Simon

On Fri, Dec 31, 2021 at 03:35 PM, Chris Wilson wrote:

NEVER has this Wellbrook loop performed so well, NEVER has it pulled in anything much on 20 meters, and as for the lower bands it is darn near as good on 40 meters as my high, full size dipole.

Hi Chris,

That's excellent news, your perseverance has paid off, so well done for sticking with it.

The downside is that your KiWi has now become more popular, so it's difficult to obtain an available receiver slot.

The other interesting thing is that as you have discovered, you don't always need the loop to be particularly large, as lots of other factors come into play, and size isn't always important :-)

The local noise floor, loop location and orientation can often be more significant factors than the absolute level of received signals.

Lets hope your postings will be encouraging to others, and perhaps inspire them to experiment and improve their antenna systems too.

Best wishes to all for the quickly approaching New Year of 2022.

Martin

On Fri, Dec 31, 2021 at 03:58 PM, Simon wrote:

Opps how did i get that wrong!!!?? I was even looking at the filters at the time..

Hi Simon,

Try G3TXQ

Don't worry, it happens to the best of us.

Rather too often in my case........

Regards,

Martin

Hi Martin

Opps how did i get that wrong!!!?? I was even looking at the filters at the time..


Apologies to g3tso, espically if still with us!!

G3TSQ NOT G3TSO..

Sorry peoples ( getting old i guess)

Hello Martin, Alan, Andrew Ikin and all that contributed to this lengthy thread.

Friday, December 31, 2021

Sincerest thanks to all, I moved the loop into the woods yesterday and today found 135 feet of RG-58 on a drum in the attic, so joined it with good branded (Telegartner) connectors to the existing coax. Loop is half the circumference of the original, same original injector box, same linear supply, same Kiwi. Unbelievable difference, the satellite stuff is switched on save for the SMPS for the PA to which I am adding ferrites. NEVER has this Wellbrook loop performed so well, NEVER has it pulled in anything much on 20 meters, and as for the lower bands it is darn near as good on 40 meters as my high, full size dipole.

See for yourself:

Remarkable and thanks for all the support, suggestions and patience. All the best, let's hope for a less restrictive and a healthy New Year, all the very best to all.


Best regards,
Chris mailto:chris@...

For receive purposes there is really very little difference between the two.

In practice neither the signal source (generator) or the receiver (load) are likely to be an exact 50 Ohm match, and in real life here will be some slight amplitude variations as a result of changes in mismatch loss across the required frequency range.

You can simulate the use of different coax types using tools like SimSmith.





If we model a 30m Length of RG58 to feed a 50 Ohm load from a 50 Ohm source we get this result.




The purple trace shows the line loss in dB relative to the 1w source on the left hand vertical axis, and the blue trace shows the SWR on the right hand vertical axis.

The loss is about 2.5dB at 30MHz and the worst case SWR is about 1.1:1

If we now swap the coax for 75 Ohm RG59 we see these changes occur.






The average loss over the 1-30MHz frequency range has increased slightly, but the loss at 30MHz has reduced by about 0.5dB.

The SWR is also noticeably worse, but doesn't exceed 2.2:1, which is still well within acceptable bounds for such excursions. It's really only when you get ratios above say 6:1, that the mismatch losses start to have a noticeable effect, with >3dB additional variation in signal levels.

If you can't see the graphs embedded in this post, I've added them to the "Antenna Noise reduction" album in the photos section.

Regards,

Martin

开云体育

Martin,?

I was thinking of asking about using 75ohm sat cable
when I saw your preference for this.

I have read elsewhere that such coax can be used
in lieu of standard 50hm.

How would you use it for rx and tx?

"Better" NewYear to all!

Michael
2E0IHW