dbm to volts (humax F2foxT)

Mark Carver wrote:

I wonder what makes them so bad ?

They often appear to be designed as mechanical interfaces only, with
little or no regard to maintaining anything like constant 75 ohm
impedance. The diameter of the centre line may not be reduced where it
passes through the supporting dielectric material, and the latter is
usually a big lump of nylon - a polar dielectric with a relatively high
dielectric constant. PTFE would be too expensive for these things,
which leave China priced at a few cents. There's also something of a
conflict between mechanical robustness and RF return loss specs.

Invariably the internal construction gives an internal Zo of well under
75 ohms, so they look like a shunt capacitor across the transmission line.

I need an X-Ray machine !

No, just a vector network analyser and a hacksaw :~)

--
Andy

The message
from Andy Wade contains these words:

Johnny B Good wrote:

That 3 digit value of 273mV is 'correct' only in the Microsoft sense

In what context do Micro$oft truncate numbers rather than round them?

In the context of disk drive capacities (3 significant digits). Eg.
7.999GB drive reported as 7.99GB or 291.956GB reported as 291GB. Worse
still is taskmanager's 2 digit accuracy of CPU% usage truncating 0.999%
as zero%!

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

In message , Andy Wade
writes
Mark Carver wrote:

I wonder what makes them so bad ?

They often appear to be designed as mechanical interfaces only, with
little or no regard to maintaining anything like constant 75 ohm
impedance. The diameter of the centre line may not be reduced where it
passes through the supporting dielectric material, and the latter is
usually a big lump of nylon - a polar dielectric with a relatively high
dielectric constant. PTFE would be too expensive for these things,
which leave China priced at a few cents. There's also something of a
conflict between mechanical robustness and RF return loss specs.

Invariably the internal construction gives an internal Zo of well under
75 ohms, so they look like a shunt capacitor across the transmission
line.

I need an X-Ray machine !

No, just a vector network analyser and a hacksaw :~)


It's true that many '75 ohm' connectors are too capacitive, and
therefore the RLR goes to pot at high frequencies. A TDR shows this even
more convincingly than a vector network analyser. A constant 50 ohms is
easier to maintain through the connector. However, for domestic TV
applications, I doubt if the so-called 75 ohm connectors cause many
actual problems below (say) at least 1GHz.
--
Ian

On 06/08/2007 23:53, Ian Jackson wrote:

The ones I have are all one piece (at least, I'm pretty sure they are -
there's no need for them to be two pieces.)

If they are one piece, there's no "slip" in them and when you tighten
them they'll put twist into your cable, ok if the other end is belling
lee, but what if you have F at both ends?

In message , Andy Burns
writes
On 06/08/2007 23:53, Ian Jackson wrote:

The ones I have are all one piece (at least, I'm pretty sure they are
- there's no need for them to be two pieces.)

If they are one piece, there's no "slip" in them and when you tighten
them they'll put twist into your cable, ok if the other end is belling
lee, but what if you have F at both ends?

Are we talking at cross-purposes? I'm referring to simple twist-on
F-connectors, which are usually a single metal tube (about 3/4" long),
threaded internally at one end, and knurled externally at the other. As
available in Homebase, B&Q etc.
--
Ian.

"Ian Jackson" wrote in message
...
Are we talking at cross-purposes? I'm referring to simple twist-on
F-connectors, which are usually a single metal tube (about 3/4" long),
threaded internally at one end, and knurled externally at the other. As
available in Homebase, B&Q etc.

How would you screw it onto the socket? You'd need to rotate the cable and
dish about twenty times! That would be a real nuisance.

Bill

In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
Are we talking at cross-purposes? I'm referring to simple twist-on
F-connectors, which are usually a single metal tube (about 3/4" long),
threaded internally at one end, and knurled externally at the other. As
available in Homebase, B&Q etc.

How would you screw it onto the socket? You'd need to rotate the cable and
dish about twenty times! That would be a real nuisance.

Bill


You're right (of course)! Well, I did say that I didn't use them very
often. It's been a while......
--
Ian.

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
Are we talking at cross-purposes? I'm referring to simple twist-on
F-connectors, which are usually a single metal tube (about 3/4" long),
threaded internally at one end, and knurled externally at the other. As
available in Homebase, B&Q etc.

How would you screw it onto the socket? You'd need to rotate the cable and
dish about twenty times! That would be a real nuisance.

Bill


You're right (of course)! Well, I did say that I didn't use them very
often. It's been a while......

You've set me off! Just for a laugh I've superglued a few and left them on
the bench where father assembles the head-ends. Watch this space.

Bill

In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
Are we talking at cross-purposes? I'm referring to simple twist-on
F-connectors, which are usually a single metal tube (about 3/4" long),
threaded internally at one end, and knurled externally at the other. As
available in Homebase, B&Q etc.

How would you screw it onto the socket? You'd need to rotate the cable and
dish about twenty times! That would be a real nuisance.

Bill


You're right (of course)! Well, I did say that I didn't use them very
often. It's been a while......

You've set me off! Just for a laugh I've superglued a few and left them on
the bench where father assembles the head-ends. Watch this space.

Bill


Some time ago, my favourite RS 4" adjustable spanner (ideal for
F-connectors) got superglue spilled on the adjusting screw. The jaws
just too narrow to be of any use for Fs. It needs a blow-torch (or
something hot) to free it. If Father comes up with an alternative method
of freeing the connectors, please let me know.
--
Ian.

Johnny B Good wrote:

In the context of disk drive capacities (3 significant digits). Eg.
7.999GB drive reported as 7.99GB or 291.956GB reported as 291GB. Worse
still is taskmanager's 2 digit accuracy of CPU% usage truncating 0.999%
as zero%!

Bugger all that, I still use 22/7 for Pi.


--
Mark
Please replace invalid and invalid with gmx and net to reply.