Edwin Spector wrote:


The Natural Philosopher wrote:

Edwin Spector wrote:


I have just done some measurements, and it hardly makes a gnat's cock of
difference!

...snippetty...
What was teh VSWR?


Return loss of better than 20dB over 650 to 750 MHz, which is equivalent to a
VSWR of 1.22:1. Theoretical loss due to this mismatch is 0.043dB (I looked it
up).



In which case. your, my and others conclusion that 'you won't notice any
difference' is justified. Way under the sort of passband ripple you
would get from the antenna anyway.


Edwin.

Edwin Spector wrote:


Edwin Spector wrote:

I have just done some measurements,...snippetty...
Results
Original return loss: 27dB (Very good)
Original insertion loss 0dB (relative, not absolute)
Crude join: IL = 0.3dB at 650MHz, 0.31dB at 750MHz.
Better join: IL = 0.1dB over 650 to 750MHz.


Footnote:
It's probably worth spending a couple of minutes making the joint as 'nice' as
possible. A discontinuity in a TV feed could produce a reflection which could
give a ghost on the telly image.



Unlikly given

(a the small refelction that has been demonstrated = 20dB
(b teh fact that teh distances involved are small. you need a fair few
meters to give a discernile ghost.

a line is 60Us long, roughly, so for 600 dots per line rough image
quality, 100ns or less is indiscernible.

That equates to 3meters of cable. Quite a bit


Edwin.

In message , Edwin Spector
writes


Edwin Spector wrote:

I have just done some measurements,...snippetty...
Results
Original return loss: 27dB (Very good)
Original insertion loss 0dB (relative, not absolute)
Crude join: IL = 0.3dB at 650MHz, 0.31dB at 750MHz.
Better join: IL = 0.1dB over 650 to 750MHz.

Footnote:
It's probably worth spending a couple of minutes making the joint as 'nice' as
possible. A discontinuity in a TV feed could produce a reflection which could
give a ghost on the telly image.


Except that he's mentioned several times that it's not for TV
--
geoff

The Natural Philosopher wrote:

Edwin Spector wrote:
...
It's probably worth spending a couple of minutes making the joint as 'nice' as
possible. A discontinuity in a TV feed could produce a reflection which could
give a ghost on the telly image.


Unlikly given

(a the small refelction that has been demonstrated = 20dB
(b teh fact that teh distances involved are small. you need a fair few
meters to give a discernile ghost.

a line is 60Us long, roughly, so for 600 dots per line rough image
quality, 100ns or less is indiscernible.

That equates to 3meters of cable. Quite a bit

Good points. However, just to clarify, the 20dB return loss was for the
reasonably good joint. The bad joint had a return loss of about 11.6 dB. That's
about 7% of the power being reflected. (20dB reflection corresponds to 1%). I'm
not sure how noticable that ghost would be.

Also, don't forget that signals travel more slowly down cable than in air, by a
factor of (say) 0.5 to 0.7 (.) So, it would only take 2m of cable to give a
discernable ghost (by your test).

I added the note about reflections after a colleague's account of a bad ghost
which was traced to a cable joint made using a 30A mains junction box. Who ya'
gonna call?

Edwin.

In article , Edwin Spector
writes


The Natural Philosopher wrote:

Edwin Spector wrote:
...
It's probably worth spending a couple of minutes making the joint as 'nice'
as
possible. A discontinuity in a TV feed could produce a reflection which
could
give a ghost on the telly image.


Unlikly given

(a the small refelction that has been demonstrated = 20dB
(b teh fact that teh distances involved are small. you need a fair few
meters to give a discernile ghost.

a line is 60Us long, roughly, so for 600 dots per line rough image
quality, 100ns or less is indiscernible.

That equates to 3meters of cable. Quite a bit

Good points. However, just to clarify, the 20dB return loss was for the
reasonably good joint. The bad joint had a return loss of about 11.6 dB. That's
about 7% of the power being reflected. (20dB reflection corresponds to 1%). I'm
not sure how noticable that ghost would be.

Also, don't forget that signals travel more slowly down cable than in air, by a
factor of (say) 0.5 to 0.7 (.) So, it would only take 2m of cable to give a
discernable ghost (by your test).

I added the note about reflections after a colleague's account of a bad ghost
which was traced to a cable joint made using a 30A mains junction box. Who ya'
gonna call?

Edwin.

In fact there a very well written article on a subject called pre echo
where there is a possibility of direct pick up off poor cables directly
from the transmitter and the delayed signal via a distribution system.
Very good site this is if you're into TV etc...

http://www.wrightsaerials.tv/Resources/Pre-Echo.pdf
--
Tony Sayer

Edwin Spector wrote:


The Natural Philosopher wrote:

Edwin Spector wrote:
...

It's probably worth spending a couple of minutes making the joint as 'nice' as
possible. A discontinuity in a TV feed could produce a reflection which could
give a ghost on the telly image.


Unlikly given

(a the small refelction that has been demonstrated = 20dB
(b teh fact that teh distances involved are small. you need a fair few
meters to give a discernile ghost.

a line is 60Us long, roughly, so for 600 dots per line rough image
quality, 100ns or less is indiscernible.

That equates to 3meters of cable. Quite a bit


Good points. However, just to clarify, the 20dB return loss was for the
reasonably good joint. The bad joint had a return loss of about 11.6 dB. That's
about 7% of the power being reflected. (20dB reflection corresponds to 1%). I'm
not sure how noticable that ghost would be.



3db is about 50% reflection, so probably less than 1db


Also, don't forget that signals travel more slowly down cable than in air, by a
factor of (say) 0.5 to 0.7 (.) So, it would only take 2m of cable to give a
discernable ghost (by your test).


I don't think its that much reduction.



I added the note about reflections after a colleague's account of a bad ghost
which was traced to a cable joint made using a 30A mains junction box. Who ya'
gonna call?



I never advocated a bad joint in a 30A mains junction box :-)

Its been a long time, but certainly one TV installation I saw, had lost
the entire connection to the aerial earth lead...it worked, but certain
stations were vastly better than others - big drop - 10 meters plus.

No ghosts tho - or very few. Possibly because the discontinutity was at
the antenna.



Edwin.