On Sun, 22 Dec 2024 12:57:54 -0500, ehsjr <ehsjr@verizon.net> wrote:

>
>Maybe you should first build a dim bulb tester. Among other
>things, it would enable you to re-form the power supply caps
>it old gear and save an explosion or three. :-)
>
>Many hits on a Google search, here's the first one:
>https://antiqueradio.org/dimbulb.htm
>
>Ed

I did actually build one a few years ago with swappable bulbs for
different loads. Two problems, though:

1) I'd have to remember to use it
2) I'd have to somehow find it.

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
> Cursitor Doom <cd@notformail.com> wrote:
>
>> On 21 Dec 2024 22:30:23 GMT, Allodoxaphobia <trepidation@example.net>
>> wrote:
>>
>>> On Sat, 21 Dec 2024 21:37:59 +0000, Cursitor Doom wrote:
>>>>> in series with your 50 Ohm source.
>>>>
>>>> Yes, but you know what a *bitch* it is to find a purely resistive 10
>>>> ohms....
>>>
>>> Maybe just a length of nichrome wire?
>>
>> Still got parasitics. Avoiding them is a craft in itself.
>
> I wouldn't worry about the inductance of an ordianry 1/4 -Watt resistor
> at the frequencies involved. The only point where it might make any
> difference is the VHF input and oscillator tuning - and I can't think
> you need a great level of accuracy to align a domestic portable radio.
>
> It is even possible that they specified a 60-ohm sig gen because they
> knew that most of the available ones would be near enough at either 50
> or 75 0hms. Another possibility would be to try to make you think you
> had to return the radio to their specialised service department for
> repair. The avarage radio shop (if there are any left) simply wouldn't
> care and would use whatever they had on the shelf above the workbench.
>
>

60 ohms was a popular impedance for RF test gear in Europe in the 1950s and
60s

--
piglet

On 22/12/2024 22:50, Cursitor Doom wrote:
> On Sun, 22 Dec 2024 12:57:54 -0500, ehsjr <ehsjr@verizon.net> wrote:
>
>>
>> Maybe you should first build a dim bulb tester. Among other
>> things, it would enable you to re-form the power supply caps
>> it old gear and save an explosion or three. :-)
>>
>> Many hits on a Google search, here's the first one:
>> https://antiqueradio.org/dimbulb.htm
>>
>> Ed
>
> I did actually build one a few years ago with swappable bulbs for
> different loads. Two problems, though:
>
> 1) I'd have to remember to use it
> 2) I'd have to somehow find it.

I think that you might have to add a third one in future - finding an
incandescent bulb to use!

--
Jeff

On Mon, 23 Dec 2024 08:16:15 +0000, Jeff Layman <Jeff@invalid.invalid>
wrote:

>On 22/12/2024 22:50, Cursitor Doom wrote:
>> On Sun, 22 Dec 2024 12:57:54 -0500, ehsjr <ehsjr@verizon.net> wrote:
>>
>>>
>>> Maybe you should first build a dim bulb tester. Among other
>>> things, it would enable you to re-form the power supply caps
>>> it old gear and save an explosion or three. :-)
>>>
>>> Many hits on a Google search, here's the first one:
>>> https://antiqueradio.org/dimbulb.htm
>>>
>>> Ed
>>
>> I did actually build one a few years ago with swappable bulbs for
>> different loads. Two problems, though:
>>
>> 1) I'd have to remember to use it
>> 2) I'd have to somehow find it.
>
>I think that you might have to add a third one in future - finding an
>incandescent bulb to use!

Yes, getting harder as time goes by, so I stocked-up in advance and
put a couple of dozen aside while they were still widely available. I
would imagine the oven types will still be around for a few more
years, anyway - and they tend to be more robust.

In message <vka5ni$r5r3$1@dont-email.me>, piglet
<erichpwagner@hotmail.com> writes
>Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
>> Cursitor Doom <cd@notformail.com> wrote:
>>
>>> On 21 Dec 2024 22:30:23 GMT, Allodoxaphobia <trepidation@example.net>
>>> wrote:
>>>
>>>> On Sat, 21 Dec 2024 21:37:59 +0000, Cursitor Doom wrote:
>>>>>> in series with your 50 Ohm source.
>>>>>
>>>>> Yes, but you know what a *bitch* it is to find a purely resistive 10
>>>>> ohms....
>>>>
>>>> Maybe just a length of nichrome wire?
>>>
>>> Still got parasitics. Avoiding them is a craft in itself.
>>
>> I wouldn't worry about the inductance of an ordianry 1/4 -Watt resistor
>> at the frequencies involved. The only point where it might make any
>> difference is the VHF input and oscillator tuning - and I can't think
>> you need a great level of accuracy to align a domestic portable radio.
>>
>> It is even possible that they specified a 60-ohm sig gen because they
>> knew that most of the available ones would be near enough at either 50
>> or 75 0hms. Another possibility would be to try to make you think you
>> had to return the radio to their specialised service department for
>> repair. The avarage radio shop (if there are any left) simply wouldn't
>> care and would use whatever they had on the shelf above the workbench.
>>
>60 ohms was a popular impedance for RF test gear in Europe in the 1950s and
>60s
>
For many purposes you can ignore the difference between the 50 and 75
ohm impedances (and 60 if you ever come across any). However, be very
aware only BNC 50 and 75 ohm connectors are mutually mechanically
mateable (yes - they really are!). I don't know if any others that are.
--
Ian
Aims and ambitions are neither attainments nor achievements

On 2024-12-23 16:22, Ian Jackson wrote:
> For many purposes you can ignore the difference between the 50 and 75 ohm impedances (and 60 if you ever come across any).
> However, be very aware only BNC 50 and 75 ohm connectors are mutually mechanically mateable (yes - they really are!). I don't know if any others that are.

Not recommended.
The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75 ohm BNC.
I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV company), the rest is 50.

Arie

In article <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, noreply@ademu.nl
says...
>
> On 2024-12-23 16:22, Ian Jackson wrote:
> > For many purposes you can ignore the difference between the 50 and 75 ohm impedances (and 60 if you ever come across any).
> > However, be very aware only BNC 50 and 75 ohm connectors are mutually mechanically mateable (yes - they really are!). I don't know if any others that are.
>
> Not recommended.
> The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75 ohm BNC.
> I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV company), the rest is 50.
>
>
>

It is the N connector the center pin is different. The BNC pin is the
same. It is the ammount of dialectric (insulation) in the BNC that
makes it a 75 or 50 ohm connector.

On Mon, 23 Dec 2024 10:31:04 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>On Mon, 23 Dec 2024 08:16:15 +0000, Jeff Layman <Jeff@invalid.invalid>
>wrote:
>
>>On 22/12/2024 22:50, Cursitor Doom wrote:
>>> On Sun, 22 Dec 2024 12:57:54 -0500, ehsjr <ehsjr@verizon.net> wrote:
>>>
>>>>
>>>> Maybe you should first build a dim bulb tester. Among other
>>>> things, it would enable you to re-form the power supply caps
>>>> it old gear and save an explosion or three. :-)
>>>>
>>>> Many hits on a Google search, here's the first one:
>>>> https://antiqueradio.org/dimbulb.htm
>>>>
>>>> Ed
>>>
>>> I did actually build one a few years ago with swappable bulbs for
>>> different loads. Two problems, though:
>>>
>>> 1) I'd have to remember to use it
>>> 2) I'd have to somehow find it.
>>
>>I think that you might have to add a third one in future - finding an
>>incandescent bulb to use!
>
>Yes, getting harder as time goes by, so I stocked-up in advance and
>put a couple of dozen aside while they were still widely available. I
>would imagine the oven types will still be around for a few more
>years, anyway - and they tend to be more robust.

In the US anyway, there is an alternative, to use only bulbs intended
for industrial uses, meaning other than 120 Vac. Like 130 Vac.

Joe Gwinn

On 12/23/24 18:58, Ralph Mowery wrote:
> In article <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, noreply@ademu.nl
> says...
>>
>> On 2024-12-23 16:22, Ian Jackson wrote:
>>> For many purposes you can ignore the difference between the 50 and 75 ohm impedances (and 60 if you ever come across any).
>>> However, be very aware only BNC 50 and 75 ohm connectors are mutually mechanically mateable (yes - they really are!). I don't know if any others that are.
>>
>> Not recommended.
>> The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75 ohm BNC.
>> I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV company), the rest is 50.
>>
>>
>>
>
> It is the N connector the center pin is different. The BNC pin is the
> same. It is the ammount of dialectric (insulation) in the BNC that
> makes it a 75 or 50 ohm connector.
>
>

N connectors are good, but the fact that 75 and 50 Ohm connectors
are similar enough to mate, but not nearly enough to do so without
damage or malfunction is perverse. When I took charge of the
electronics lab, it took a while to sort them out and to eliminate
the victims of mismatched matings. (I was always amazed at the
sheer quantity of connectors in a busy lab.)

For BNCs, there *is* a difference, but it doesn't usually matter.
I kept 50 and 75 Ohms separate, but when put to the test, many
75 Ohm connectors turned out to really be 50 Ohm ones in disguise.

In a pinch, BNC and N can even mate between them. I'm not sure if
that's deliberate or accidental.

Jeroen Belleman

In message <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, Arie de Muijnck
<noreply@ademu.nl> writes
>On 2024-12-23 16:22, Ian Jackson wrote:
>> For many purposes you can ignore the difference between the 50 and 75
>>ohm impedances (and 60 if you ever come across any). However, be very
>>aware only BNC 50 and 75 ohm connectors are mutually mechanically
>>mateable (yes - they really are!). I don't know if any others that are.
>
>Not recommended.
>The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75
>ohm BNC.
>I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>company), the rest is 50.
>
For most of its length, the male BNC 50 ohm pin is actually the same
diameter as the 75. However, the tip of the 50 pin is fairly 'blunt',
while the 75 is more pointed.

I guess that if you are particularly clumsy while you are inserting a 50
male into a female 75, it might just be possible to have the blunter
male pin a bit off-centre, and catch the side of the female receptacle,
and splay it. However, despite 43 years working in the CATV industry, I
failed to achieve this!

The real difference between the two impedances is that the amount of
PTFE dielectric in the 75 has been minimised in order to increase (with
some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
more-uniform structural RLR, so it the better connector at the higher
frequencies.
--
Ian
Aims and ambitions are neither attainments nor achievements

In message <MPG.41d36ed7f88483b798a01a@news.eternal-september.org>,
Ralph Mowery <rmowery42@charter.net> writes
>In article <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, noreply@ademu.nl
>says...
>>
>> On 2024-12-23 16:22, Ian Jackson wrote:
>> > For many purposes you can ignore the difference between the 50 and
>> >75 ohm impedances (and 60 if you ever come across any).
>> > However, be very aware only BNC 50 and 75 ohm connectors are
>> >mutually mechanically mateable (yes - they really are!). I don't
>> >know if any others that are.
>>
>> Not recommended.
>> The center pin differs, a 50 Ohm BNC pin is thicker and may damage a
>>75 ohm BNC.
>> I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>company), the rest is 50.
>>
>>
>>
>
>It is the N connector the center pin is different.

Indeed. A 75 ohm N male (thin pin) inserted into a 50 ohm female
(fatter receptacle) results in a non-connection. A 50 ohm N male (fat
pin) inserted into a 75 ohm female (thin receptacle) results in a
splayed (and usually wrecked for future use) receptacle.

> The BNC pin is the
>same. It is the ammount of dialectric (insulation) in the BNC that
>makes it a 75 or 50 ohm connector.
--
Ian
Aims and ambitions are neither attainments nor achievements

In message <vkckph$1cvdj$1@dont-email.me>, Jeroen Belleman
<jeroen@nospam.please> writes
>On 12/23/24 18:58, Ralph Mowery wrote:
>> In article <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, noreply@ademu.nl
>> says...
>>>
>>> On 2024-12-23 16:22, Ian Jackson wrote:
>>>> For many purposes you can ignore the difference between the 50 and
>>>>75 ohm impedances (and 60 if you ever come across any).
>>>> However, be very aware only BNC 50 and 75 ohm connectors are
>>>>mutually mechanically mateable (yes - they really are!). I don't
>>>>know if any others that are.
>>>
>>> Not recommended.
>>> The center pin differs, a 50 Ohm BNC pin is thicker and may damage a
>>>75 ohm BNC.
>>> I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>>company), the rest is 50.
>>>
>>>
>>>
>> It is the N connector the center pin is different. The BNC pin is
>>the
>> same. It is the ammount of dialectric (insulation) in the BNC that
>> makes it a 75 or 50 ohm connector.
>>
>
>N connectors are good, but the fact that 75 and 50 Ohm connectors
>are similar enough to mate, but not nearly enough to do so without
>damage or malfunction is perverse. When I took charge of the
>electronics lab, it took a while to sort them out and to eliminate
>the victims of mismatched matings. (I was always amazed at the
>sheer quantity of connectors in a busy lab.)
>
>For BNCs, there *is* a difference, but it doesn't usually matter.
>I kept 50 and 75 Ohms separate, but when put to the test, many
>75 Ohm connectors turned out to really be 50 Ohm ones in disguise.
>
>In a pinch, BNC and N can even mate between them. I'm not sure if
>that's deliberate or accidental.
>
Yes - the 'innards' are essentially the same. IIRC, the BNC innards also
mate with C-connectors. I think it's deliberate. After all, its the
innards that carry the RF.
--
Ian
Aims and ambitions are neither attainments nor achievements

On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>The real difference between the two impedances is that the amount of
>PTFE dielectric in the 75 has been minimised in order to increase (with
>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>more-uniform structural RLR, so it the better connector at the higher
>frequencies.

I don't think that's quite right. The diameter of the inner and outer
conductors has more influence on Zo than the dilectric thickness.

Ian Jackson <ianREMOVETHISjackson@g3ohx.co.uk> wrote:
> In message <vkckph$1cvdj$1@dont-email.me>, Jeroen Belleman
> <jeroen@nospam.please> writes
>> On 12/23/24 18:58, Ralph Mowery wrote:
>>> In article <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, noreply@ademu.nl
>>> says...
>>>>
>>>> On 2024-12-23 16:22, Ian Jackson wrote:
>>>>> For many purposes you can ignore the difference between the 50 and
>>>>> 75 ohm impedances (and 60 if you ever come across any).
>>>>> However, be very aware only BNC 50 and 75 ohm connectors are
>>>>> mutually mechanically mateable (yes - they really are!). I don't
>>>>> know if any others that are.
>>>>
>>>> Not recommended.
>>>> The center pin differs, a 50 Ohm BNC pin is thicker and may damage a
>>>> 75 ohm BNC.
>>>> I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>>> company), the rest is 50.
>>>>
>>>>
>>>>
>>> It is the N connector the center pin is different. The BNC pin is
>>> the
>>> same. It is the ammount of dialectric (insulation) in the BNC that
>>> makes it a 75 or 50 ohm connector.
>>>
>>
>> N connectors are good, but the fact that 75 and 50 Ohm connectors
>> are similar enough to mate, but not nearly enough to do so without
>> damage or malfunction is perverse. When I took charge of the
>> electronics lab, it took a while to sort them out and to eliminate
>> the victims of mismatched matings. (I was always amazed at the
>> sheer quantity of connectors in a busy lab.)
>>
>> For BNCs, there *is* a difference, but it doesn't usually matter.
>> I kept 50 and 75 Ohms separate, but when put to the test, many
>> 75 Ohm connectors turned out to really be 50 Ohm ones in disguise.
>>
>> In a pinch, BNC and N can even mate between them. I'm not sure if
>> that's deliberate or accidental.
>>
> Yes - the 'innards' are essentially the same. IIRC, the BNC innards also
> mate with C-connectors. I think it's deliberate. After all, its the
> innards that carry the RF.

A type N male will also mate with a BNC female.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

On 23/12/2024 10:31, Cursitor Doom wrote:
> On Mon, 23 Dec 2024 08:16:15 +0000, Jeff Layman <Jeff@invalid.invalid>

>>> I did actually build one a few years ago with swappable bulbs for
>>> different loads. Two problems, though:
>>>
>>> 1) I'd have to remember to use it
>>> 2) I'd have to somehow find it.
>>
>> I think that you might have to add a third one in future - finding an
>> incandescent bulb to use!
>
> Yes, getting harder as time goes by, so I stocked-up in advance and
> put a couple of dozen aside while they were still widely available. I
> would imagine the oven types will still be around for a few more
> years, anyway - and they tend to be more robust.

I doubt they'll be able to replace the oven illumination easily (unless
they mount the bulb on the outside and use fibreoptics to bring light to
the inside!). I was surprised a few months ago when I bought a new
microwave oven to find it had an LED lamp which illuminated the inside.
I would have thought that with around a kW of microwave energy being
generated that would have been a pretty hostile environment for an LED
lamp. Obviously, though, they had the screening well sorted out. I
suppose that I shouldn't have been surprised as the timing and display
circuitry have been around for years and hasn't been affected by the RF
energy or switching spikes.

--
Jeff

In message <6ftjmjppf4421dl2ec0ek4mvfht74lmnu2@4ax.com>, Cursitor Doom
<cd@notformail.com> writes
>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>
>>The real difference between the two impedances is that the amount of
>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>more-uniform structural RLR, so it the better connector at the higher
>>frequencies.
>
>I don't think that's quite right. The diameter of the inner and outer
>conductors has more influence on Zo than the dilectric thickness.

It's right all right.

The outer diameter is the same for the 50 and 75 ohms. For the 75, I
presume it's not practicable to make the pin diameter smaller and retain
its robustness, so the only way to increase the Zo is to remove as much
of the dielectric as possible. If you compare the 50 and the 75, you
will see what I mean.
--
Ian
Aims and ambitions are neither attainments nor achievements

On Tue, 24 Dec 2024 22:42:17 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>In message <6ftjmjppf4421dl2ec0ek4mvfht74lmnu2@4ax.com>, Cursitor Doom
><cd@notformail.com> writes
>>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
>><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>>
>>>The real difference between the two impedances is that the amount of
>>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>>more-uniform structural RLR, so it the better connector at the higher
>>>frequencies.
>>
>>I don't think that's quite right. The diameter of the inner and outer
>>conductors has more influence on Zo than the dilectric thickness.
>
>It's right all right.
>
>The outer diameter is the same for the 50 and 75 ohms. For the 75, I
>presume it's not practicable to make the pin diameter smaller and retain
>its robustness, so the only way to increase the Zo is to remove as much
>of the dielectric as possible. If you compare the 50 and the 75, you
>will see what I mean.

I still maintain the principal determinants of the impedance are as I
stated previously. The formulas for line impedance are shown on this
page and the aforementioned determinants are key.

https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator

On Tue, 24 Dec 2024 10:13:29 +0000, Jeff Layman <Jeff@invalid.invalid>
wrote:

>On 23/12/2024 10:31, Cursitor Doom wrote:
>> On Mon, 23 Dec 2024 08:16:15 +0000, Jeff Layman <Jeff@invalid.invalid>
>
>>>> I did actually build one a few years ago with swappable bulbs for
>>>> different loads. Two problems, though:
>>>>
>>>> 1) I'd have to remember to use it
>>>> 2) I'd have to somehow find it.
>>>
>>> I think that you might have to add a third one in future - finding an
>>> incandescent bulb to use!
>>
>> Yes, getting harder as time goes by, so I stocked-up in advance and
>> put a couple of dozen aside while they were still widely available. I
>> would imagine the oven types will still be around for a few more
>> years, anyway - and they tend to be more robust.
>
>I doubt they'll be able to replace the oven illumination easily (unless
>they mount the bulb on the outside and use fibreoptics to bring light to
>the inside!). I was surprised a few months ago when I bought a new
>microwave oven to find it had an LED lamp which illuminated the inside.
>I would have thought that with around a kW of microwave energy being
>generated that would have been a pretty hostile environment for an LED
>lamp. Obviously, though, they had the screening well sorted out. I
>suppose that I shouldn't have been surprised as the timing and display
>circuitry have been around for years and hasn't been affected by the RF
>energy or switching spikes.

If they placed the LEDs in the corners of the inner cabinet then they
would be quite safe. All the RF energy is concentrated towards the
center with very little indeed misdirected.

On 12/25/24 15:48, Cursitor Doom wrote:
> On Tue, 24 Dec 2024 22:42:17 +0000, Ian Jackson
> <ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>
>> In message <6ftjmjppf4421dl2ec0ek4mvfht74lmnu2@4ax.com>, Cursitor Doom
>> <cd@notformail.com> writes
>>> On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
>>> <ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>>>
>>>> The real difference between the two impedances is that the amount of
>>>> PTFE dielectric in the 75 has been minimised in order to increase (with
>>>> some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>>> more-uniform structural RLR, so it the better connector at the higher
>>>> frequencies.
>>>
>>> I don't think that's quite right. The diameter of the inner and outer
>>> conductors has more influence on Zo than the dilectric thickness.
>>
>> It's right all right.
>>
>> The outer diameter is the same for the 50 and 75 ohms. For the 75, I
>> presume it's not practicable to make the pin diameter smaller and retain
>> its robustness, so the only way to increase the Zo is to remove as much
>> of the dielectric as possible. If you compare the 50 and the 75, you
>> will see what I mean.
>
> I still maintain the principal determinants of the impedance are as I
> stated previously. The formulas for line impedance are shown on this
> page and the aforementioned determinants are key.
>
> https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator
>

Very poor web page indeed. No educational value whatever.

They lump together the values of the dieelectric constant
and the permeability of free space, the geometry of the
configuration and the conversion from natural to base-ten
logarithms all together into one magic factor, without any
hint of where it all came from.

Shame! That's not 'everythingRF': It's almost nothing!
Oh, and there is no such thing as "impedance per unit length".

Jeroen Belleman

In article <8i6omj1vidqoch9421207205iajrb0h5ue@4ax.com>,
cd@notformail.com says...
> will see what I mean.
>
> I still maintain the principal determinants of the impedance are as I
> stated previously. The formulas for line impedance are shown on this
> page and the aforementioned determinants are key.
>
> https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator
>
>
>

I could not get the calculator to come out for me. Getting negative
numbers and about 10 times what they should be.

However if you change the Relative Permittivity you will see how much it
will effect the impedance of the coax. Go from 1 as air and then 2.1 to
2.6 for teflon and other common insulator/dialectric material and see
how much the impedance changes.

I doubt that you have ever actually ran the numbers or you would see the
impedance change as the material is changed.

The main thing is the ratio of the diameters of the cables, but you
still have to account for the material between them just as you will for
a capacitor..

You can often find the Relative Permittivity tables where capacitors
are.

On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>In message <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, Arie de Muijnck
><noreply@ademu.nl> writes
>>On 2024-12-23 16:22, Ian Jackson wrote:
>>> For many purposes you can ignore the difference between the 50 and 75
>>>ohm impedances (and 60 if you ever come across any). However, be very
>>>aware only BNC 50 and 75 ohm connectors are mutually mechanically
>>>mateable (yes - they really are!). I don't know if any others that are.
>>
>>Not recommended.
>>The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75
>>ohm BNC.
>>I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>company), the rest is 50.
>>
>For most of its length, the male BNC 50 ohm pin is actually the same
>diameter as the 75. However, the tip of the 50 pin is fairly 'blunt',
>while the 75 is more pointed.
>
>I guess that if you are particularly clumsy while you are inserting a 50
>male into a female 75, it might just be possible to have the blunter
>male pin a bit off-centre, and catch the side of the female receptacle,
>and splay it. However, despite 43 years working in the CATV industry, I
>failed to achieve this!
>
>The real difference between the two impedances is that the amount of
>PTFE dielectric in the 75 has been minimised in order to increase (with
>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>more-uniform structural RLR, so it the better connector at the higher
>frequencies.

The critical part of the connector here is a fraction of an inch long,
so none of this stuff matters below a few GHz.

On Wed, 25 Dec 2024 13:40:19 -0500, Ralph Mowery
<rmowery42@charter.net> wrote:

>In article <8i6omj1vidqoch9421207205iajrb0h5ue@4ax.com>,
>cd@notformail.com says...
>> will see what I mean.
>>
>> I still maintain the principal determinants of the impedance are as I
>> stated previously. The formulas for line impedance are shown on this
>> page and the aforementioned determinants are key.
>>
>> https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator
>>
>>
>>
>
>I could not get the calculator to come out for me. Getting negative
>numbers and about 10 times what they should be.

There are a lot of silly equations around, that people plug into apps.

Try a wide microstrip in some online calculators. Many use the
equation that's in the old Motorola ECL book, and a wide trace reports
a negative Z.

In message <l7vomj55u59o3cu753v7de77t0j4n20jtp@4ax.com>, john larkin
<JL@gct.com> writes
>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>
>>In message <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, Arie de Muijnck
>><noreply@ademu.nl> writes
>>>On 2024-12-23 16:22, Ian Jackson wrote:
>>>> For many purposes you can ignore the difference between the 50 and 75
>>>>ohm impedances (and 60 if you ever come across any). However, be very
>>>>aware only BNC 50 and 75 ohm connectors are mutually mechanically
>>>>mateable (yes - they really are!). I don't know if any others that are.
>>>
>>>Not recommended.
>>>The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75
>>>ohm BNC.
>>>I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>>company), the rest is 50.
>>>
>>For most of its length, the male BNC 50 ohm pin is actually the same
>>diameter as the 75. However, the tip of the 50 pin is fairly 'blunt',
>>while the 75 is more pointed.
>>
>>I guess that if you are particularly clumsy while you are inserting a 50
>>male into a female 75, it might just be possible to have the blunter
>>male pin a bit off-centre, and catch the side of the female receptacle,
>>and splay it. However, despite 43 years working in the CATV industry, I
>>failed to achieve this!
>>
>>The real difference between the two impedances is that the amount of
>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>more-uniform structural RLR, so it the better connector at the higher
>>frequencies.
>
>The critical part of the connector here is a fraction of an inch long,
>so none of this stuff matters below a few GHz.

Yes. Obviously.
I've been retired now for many a year but, IIRC, the 50 ohm was
considered 'good' to around 1000 MHz, and the 75 to around 500MHz. [A
Google on the individual manufacturers' specs is recommended.]
Regardless, both are often used to higher frequencies.
>

--
Ian
Aims and ambitions are neither attainments nor achievements

On Wed, 25 Dec 2024 22:18:21 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>In message <l7vomj55u59o3cu753v7de77t0j4n20jtp@4ax.com>, john larkin
><JL@gct.com> writes
>>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
>><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>>
>>>In message <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, Arie de Muijnck
>>><noreply@ademu.nl> writes
>>>>On 2024-12-23 16:22, Ian Jackson wrote:
>>>>> For many purposes you can ignore the difference between the 50 and 75
>>>>>ohm impedances (and 60 if you ever come across any). However, be very
>>>>>aware only BNC 50 and 75 ohm connectors are mutually mechanically
>>>>>mateable (yes - they really are!). I don't know if any others that are.
>>>>
>>>>Not recommended.
>>>>The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75
>>>>ohm BNC.
>>>>I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV
>>>>company), the rest is 50.
>>>>
>>>For most of its length, the male BNC 50 ohm pin is actually the same
>>>diameter as the 75. However, the tip of the 50 pin is fairly 'blunt',
>>>while the 75 is more pointed.
>>>
>>>I guess that if you are particularly clumsy while you are inserting a 50
>>>male into a female 75, it might just be possible to have the blunter
>>>male pin a bit off-centre, and catch the side of the female receptacle,
>>>and splay it. However, despite 43 years working in the CATV industry, I
>>>failed to achieve this!
>>>
>>>The real difference between the two impedances is that the amount of
>>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>>more-uniform structural RLR, so it the better connector at the higher
>>>frequencies.
>>
>>The critical part of the connector here is a fraction of an inch long,
>>so none of this stuff matters below a few GHz.
>
>Yes. Obviously.
>I've been retired now for many a year but, IIRC, the 50 ohm was
>considered 'good' to around 1000 MHz, and the 75 to around 500MHz. [A
>Google on the individual manufacturers' specs is recommended.]
>Regardless, both are often used to higher frequencies.
>>

Here's a BNC tdr/tdt. It's really not so bad.

https://www.dropbox.com/scl/fi/1yklmtb4gxanwn7c5ldlx/BNC_TDR_TDT.JPG?rlkey=f8ro6nzy7n2nvdfjqfu1b36rf&raw=1

https://www.dropbox.com/scl/fi/oacgzpebcjbhxg396hytq/BNC_TDR_ZOOM.JPG?rlkey=8j0ke6s8rw424t6q0ik6ohzp0&raw=1

https://www.dropbox.com/scl/fi/gldt1avq6edxqzvgnitvx/BNC_TDT_RISE.JPG?rlkey=lc3w3zb8naegvf1x5pmq8r11g&raw=1

In message <8i6omj1vidqoch9421207205iajrb0h5ue@4ax.com>, Cursitor Doom
<cd@notformail.com> writes
>On Tue, 24 Dec 2024 22:42:17 +0000, Ian Jackson
><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>
>>In message <6ftjmjppf4421dl2ec0ek4mvfht74lmnu2@4ax.com>, Cursitor Doom
>><cd@notformail.com> writes
>>>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
>>><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>>>
>>>>The real difference between the two impedances is that the amount of
>>>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>>>more-uniform structural RLR, so it the better connector at the higher
>>>>frequencies.
>>>
>>>I don't think that's quite right. The diameter of the inner and outer
>>>conductors has more influence on Zo than the dilectric thickness.
>>
>>It's right all right.
>>
>>The outer diameter is the same for the 50 and 75 ohms. For the 75, I
>>presume it's not practicable to make the pin diameter smaller and retain
>>its robustness, so the only way to increase the Zo is to remove as much
>>of the dielectric as possible. If you compare the 50 and the 75, you
>>will see what I mean.
>
>I still maintain the principal determinants of the impedance are as I
>stated previously. The formulas for line impedance are shown on this
>page and the aforementioned determinants are key.
>
>https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator
>
I'm sure that the principal determinants of the impedance are absolutely
correct. As Scotty said, "Ye cannae change the laws of physics".
However, we're talking about the practical problems and constraints in
the construction of a connector so that it is mechanically viable, and
at the same time attempting to maintain the most constant Zo throughout
the complete male-female junction.
--
Ian
Aims and ambitions are neither attainments nor achievements