SWR causing line radiation?

Discussion in 'Feedback and Suggestions' started by stancomm, Jun 21, 2010.

  1. stancomm

    Thread Starter New Member

    Jun 21, 2010
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    1
    To whom;

    So far, this book is a winner and I've already referenced a link to the Volume II, "3 phase vs, single phase" on another forum.
    However I see something, that as an amateur radio operator, kind'a stuck in my throat.

    From; vol_2/chpt_14/6.html

    "Also, a transmission line with a high SWR tends to act as an antenna, radiating electromagnetic energy away from the line, rather than channeling all of it to the load. This is usually undesirable, as the radiated energy may “couple” with nearby conductors, producing signal interference. An interesting footnote to this point is that antenna structures -- which typically resemble open- or short-circuited transmission lines -- are often designed to operate at high standing wave ratios, for the very reason of maximizing signal radiation and reception."


    From all that I've read, high swr in itself is not the cause of line radiation, but may be a clue as to what is wrong with a particular installation.

    All references I've come across state that it is rf energy present on the outside of the co-ax cable's shield which is responsible for said radiation.

    As a thought experiment, take a piece of your RG-58 and completely short the load end center conductor to the shield so that the center conductor cannot be seen from any angle, completely encapsulated. This will yield maximum swr, but will not radiate (unless the source end is improperly shielded). Skin effect will (if the shield is well enough constructed) keep everything inside the line.

    When feeding a dipole antenna with co-ax cable, there can be an unequal division of currents between that which is flowing over the inside of the shield and the antenna element, the remaining energy flowing down over the shield's outside surface. Typically a device known as a "balun" or line choke is introduced to isolate the cable's shield from this phenomenon.

    Also, if the down-lead is placed in such a way that some of the radiated energy from the antenna can induce a parasitic voltage on the shield you will see such misbehavior.

    This line radiation condition can be observed to cause a change in the apparent swr as the operators hand moves about the measuring instrument, adding a position varying amount of body capacitance, effectively parasitically tuning the antenna system.

    As regards parallel conductor transmission line; is it not line current imbalance which is the culprit? Caused by the load (assuming a that it is an antenna) which is not symmetrically positioned with regard to its environment.

    I believe both Lewallen and Cebic have commented on this issue.

    Unfortunately if my assertion is correct, it does mess up the flow of the segue from undesirable swr to desirable swr. I leave that up to your capable hands to remedy.

    I have been involved in our radio club's training of new hams and was planning to mention this work to them as a place to go to learn more about the basis of this hobby. I hate to have to make recommendations with caveats.

    Stan, K2STN
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    One of the nice things about open source is it is open. If you wish to suggest corrections (as in do a rewrite) the option is available. I shot our resident HAM ebook writer an email, then read his post where he going to be gone for a while (writing a book full time).

    It will be addressed eventually, it may take a while though.
     
  3. stancomm

    Thread Starter New Member

    Jun 21, 2010
    5
    1
    Bill, thanks for the reply.

    I did not consider myself as being up to the task, taking note of the obvious greater learning exhibited here than what I posses.

    I was more hoping to prompt the original author to re-investigate his understanding in the light of new data, seeing that he would have to reorganize the segue regarding desirable and undesirable swr.

    Stan
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You may have noticed the book isn't complete. Several members such as myself have stepped forward to try to finish long term. The original author finished a huge chunk, most of it in fact, before moving on. I'm currently swamped, but over time this project will get done.

    Part of the reason Tony started this book is errors in existing text books never get fixed. Open source (which isn't quite what this is, but close) is one way to address that.
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
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    Greetings, all:

    The standing wave ratio on a transmission line has nothing to do with whether the line radiates or not. Line radiation is caused by UNBALANCED currents in the transmission line. In certain installations, a high SWR can be INDICATIVE of line inbalance, but is not the cause of line radiation itself.

    Antennas with open wire feeders can have very high SWR and yet have no line radiation if the currents are balanced.

    In the case of a coaxial transmission line, if the current on the inner conductor is the same as the current on the inside of the outer conductor there will be no radiation. If there is current flowing on the OUTSIDE of the outer conductor, there will be radiation. A balun is a device that prevents this undesirable current from flowing, either by brute force choking, or by more elegant balance to unbalanced transformation.

    By the way, the definitive source for transmission line theory is Reflections by Walt Maxwell, available here:
    http://www.cq-amateur-radio.com/

    This should be required reading by every R.F technician and engineer.

    Eric
     
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