Ga Uli,
Good to hear from you. If I am able will scan a page from one of my telephone
books about twisted pair, I'll send it. It may prove to be helpful. It's the
shunt capacitance between the two conductors that causes the attenuation. You
have the same problem any time one exceeds the bandwidth of the cable design
specifications. Telephone transmission requires 3 Kc bandwidth, anything passed
that limit of 3 Kcs requires conditioning, which means the cable must be
equalized on the high frequency end limit. Bare twisted pair , no conditioning
will have a bandwidth of 19-20 kcs with loss. When we equalize we knock the low
frequencies down to the same level as the high frequency energy amplitude. This
maybe done in a number of ways. Resistance, capacitance, etc...or loading coils
in series with the twisted pairs. ...Why do we want a equalized transmission
line??? To transmit voice or radio programs from the studio to the transmitter
location we want the line to have the same characteristics as the voice or
music being transmitted as their were in the studio over the transmission
facilities.... Therefore all the frequencies from the very low ones 100 cps as
an example to 15 kcs, we want all of them to have the same amplitude level, we
want the response to be flat. Radio loops as the BTC (BELL Telephone Co.) calls
them all must be flat. Especially, if a Stereo Radio Loop, so we equalize all
the frequencies......Now many times transmission of data stream are transmitted
over twisted pair, we have the same problems as in audio transmission. However,
we must deal with other serious impairments during transmission, jitter, phase
distorting, amplitude distortion, etc.....data transmission is much more
demanding. Conditioning on data lines are much more
expensive....too....Television is even worst....BTC used WE754 balance 124 ohm
cable.....and lead cable having the same characteristic impedance....video
conditioning is from at least 500 kcs to 6 Mcs. It has to be withing +/-0.1DB
over the entire bandwidth. Intially, WE (Western Electric) started with A2A
tube amplifiers which had large equalizers and then GE came out with solid
state video conditioners.....All this was done employing analog
techniques...which nowadays are being supplanted by ditigal and lightguide,
equalization is no longer a problem. Light guide has the largest bandwidth and
now they use multi-mode light transmission, similar to mode theory as used in
microwave wave guides......Digital technques allow better bandwidth usage
depending on the speed of transmission.....It was also very interesting upon
using microwave to transmit video and audio together and meeting the same
specifications for equalization on land line, here it also depended on the
tuning of the Klystron.....later replaced with solid state oscillators.
Well, enough of the short history lesson on transmission....
I will catch up with you later....and I am probably going to send this out to
the reflector, since everybody thinks the telephone company is just
telephones.....
Best Regards,
Walter
----- Original Message -----
From: Ulrich Weiss
To: Walter Schulz
Sent: Friday, July 30, 2004 3:53 AM
Subject: Re: twisted pair
good morning Walter,
I'm quite happy about the discussion, as I had often thought of trying that
easy method, but in the hurry before the contest weekends I never found the
time to make the necessary "boxes"... so I confined myself to what I had from
the year before...
I have always suspected that attenuation was the critical issue here, but I
had never expected that bad figures for this (mechanically) excellent twisted
pair...
Jacques' figures were a little better (but ot significantly) than mine...
due to the line losses of my 67 m sample the half-wave 1:1 "transformation"
turned out to be a 1:1.5 causing an SWR of 1.5... using a pair of matching
transformers is surely the more scientific approach...
when you suggested the two twisted pairs I remembered that I didn't send all
my answers to Rys via the reflector... you'll be probably amused about the my
(private) mail to Rys that I will forward to you in a minute...
I hope all is ok on your side, just as we have the good luck to report
here... from Aug 10 to Aug 30 Irmgard and myself will go on a tour of Namibia
together with DJ6SI and two more families... just sight-seeing - no radio...
will tell you about it after returning...
my best wishes to the States (looking hopefully for a new president???) from
a sunny (at last!) Germany...
regards
Uli, DJ2YA
----- Original Message -----
From: Walter Schulz
To: dj2ya@t-online.de
Sent: Thursday, July 29, 2004 11:22 PM
Subject: twisted pair
Hi Uli,
I was following the twisted pair story here on the reflector. The thought
occurred to me rather than un-twist the pair, why not just use two twisted
pairs as two single wires. Just as one does using one twisted pair as a single
wire, why not do the same for the second wire in a two wire Beverage antenna.
Usually, telephone wire that is used for outdoors is composed of steel
conductors with copper plating. The insulating covering is neoprene. The stuff
is so strong you can tie the wire to a car's bumper and then tie it to a second
car and pull it with the wire.
The reason for the twist is to place the conductors 180 degrees out phase
so there is no cross talk --- its a method of cheap shielding.
73
Walter
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|