Topband: Re: twisted pair

[Walter Schulz]

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 at 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