rf everywhere

Discussion in 'Misc Electronics' started by RichD, Mar 6, 2013.

  1. RichD

    Guest

    On Fri, 15 Mar 2013 05:50:02 -0700, dave <>
    wrote:

    >On 03/14/2013 11:10 PM, wrote:
    >> On Thu, 14 Mar 2013 08:16:45 -0700, dave <>
    >> wrote:
    >>
    >>> On 03/12/2013 05:50 PM, Michael A. Terrell wrote:
    >>>>
    >>>> dave wrote:
    >>>>>
    >>>>> On 03/12/2013 09:09 AM, Michael A. Terrell wrote:
    >>>>>>
    >>>>>
    >>>>>> IR losses turn current into heat, the frequency doesn't matter. RF
    >>>>>> power costs a lot more than power from the utility company, and using a
    >>>>>> bigger transmitter to turn that RF into heat costs more for the hardware
    >>>>>> & air conditioning.
    >>>>>>
    >>>>>
    >>>>> The reason we use 6" line for 20 KW at 500 MHz is the skin effect, which
    >>>>> is quite frequency "dependent".
    >>>>
    >>>>
    >>>> No, it's because of losses caused by the corrugated conductors. The
    >>>> conductors aren't that thick to start with that the depth of the skin
    >>>> effect cause all the attenuation. I've had to scrap thousands of
    >>>> dollars worth of internally oxidized Heliax.
    >>>>
    >>>> Dielectric losses go up with frequency, as well. Compare the loss
    >>>> similarly constructed teflon & foam coax at 1.2 GHz. The silver plating
    >>>> helps reduce the resistance of the braid, but the main purpose is to
    >>>> prevent the copper from oxidizing due to trapped air and ingression of
    >>>> oxygen through the plastic jacket.
    >>>>
    >>>> That's why the rigid 6.125" O.D. coax isn't corrugated. Neither can
    >>>> withstand a hard vacuum, so they need to be properly pressurized with
    >>>> dry gas and Nitrogen gas is not only cheap, but it won't oxidize the
    >>>> conductor surfaces inside the coax.
    >>>>
    >>>> Look at the losses in braided VS foil 75 Ohm coax as frequency
    >>>> increases. The rough surface of the braid has higher losses because the
    >>>> path is longer and the resistance is slightly higher.
    >>>>
    >>> Why is there less loss with bigger air dielectric line at UHF?

    >>
    >> In a coaxial cable the EM field propagates in the dielectric, not
    >> in/on the inner and/or outer conductor. The outer surface of the inner
    >> conductor and the inner surface of the outer conductor will just
    >> confine the EM field into the dielectric as does the inner surface of
    >> a waveguide.
    >>
    >> For this reason, the quality of the dielectric is critical, dry air is
    >> nearly as good as vacuum at lower frequencies (below a few hundred
    >> GHz).
    >>
    >> In addition, the metallic surfaces should have infinite conductivity,
    >> but of course, skin effect and oxidation will degrade the performance.
    >>
    >> The 15 cm coax is not very usable at frequencies well above 1 GHz,
    >> since the EM field starts to propagate in some strange waveguide mode
    >> above these frequencies.
    >>
    >> For lower frequencies, coaxal cables work OK even with much larger
    >> dimensions, such as 50 cm coaxial "cables" at a 500 kW short wave
    >> station.
    >>

    >What is the advantage (if any) to using co-ax v. open-wire feed for
    >above HFBC example?


    When you have several (sub)megawatt HF transmitters and a few
    directional antennas aimed on different continents, you really need a
    big switchyard. with coaxial relays. I do not understand, how to do
    this with open-wire systems.
     
    , Mar 15, 2013
    #21
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  2. RichD

    Tim Williams Guest

    <> wrote in message
    news:...
    >>What is the advantage (if any) to using co-ax v. open-wire feed for
    >>above HFBC example?

    >
    > Coax has the fields contained within the cable, open-wire does not.
    > Structures, people, and critters in proximity to coax don't matter.
    > It's a different matter with open-wire. Open wire usually requires a
    > BalUn, as well.


    Why not?
    http://en.wikipedia.org/wiki/File:Solec_Kujawski_longwave_antenna_feeder.jpg\

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://seventransistorlabs.com
     
    Tim Williams, Mar 15, 2013
    #22
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  3. RichD

    Guest

    On Fri, 15 Mar 2013 18:30:19 -0500, "Tim Williams"
    <> wrote:

    ><> wrote in message
    >news:...
    >>>What is the advantage (if any) to using co-ax v. open-wire feed for
    >>>above HFBC example?

    >>
    >> Coax has the fields contained within the cable, open-wire does not.
    >> Structures, people, and critters in proximity to coax don't matter.
    >> It's a different matter with open-wire. Open wire usually requires a
    >> BalUn, as well.

    >
    >Why not?
    >http://en.wikipedia.org/wiki/File:Solec_Kujawski_longwave_antenna_feeder.jpg\


    No one home.
     
    , Mar 16, 2013
    #23
  4. RichD

    Tim Williams Guest

    Oops, nix the trailing backslash.

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://seventransistorlabs.com

    <> wrote in message
    news:...
    > On Fri, 15 Mar 2013 18:30:19 -0500, "Tim Williams"
    > <> wrote:
    >
    >><> wrote in message
    >>news:...
    >>>>What is the advantage (if any) to using co-ax v. open-wire feed for
    >>>>above HFBC example?
    >>>
    >>> Coax has the fields contained within the cable, open-wire does not.
    >>> Structures, people, and critters in proximity to coax don't matter.
    >>> It's a different matter with open-wire. Open wire usually requires a
    >>> BalUn, as well.

    >>
    >>Why not?
    >>http://en.wikipedia.org/wiki/File:Solec_Kujawski_longwave_antenna_feeder.jpg\

    >
    > No one home.
    >
     
    Tim Williams, Mar 16, 2013
    #24
  5. RichD

    Fred Abse Guest

    On Fri, 15 Mar 2013 12:44:18 -0400, krw wrote:

    > Coax has the fields contained within the cable, open-wire does not.
    > Structures, people, and critters in proximity to coax don't matter. It's a
    > different matter with open-wire. Open wire usually requires a BalUn, as
    > well.


    The WWII, British, CH radar (circa 50MHz), used open wire feeders to the
    transmitting antennas. The transmitters were a Marconi-EMI television
    design, of a few hundred kW, modified for pulse modulation, one of the
    reasons for killing the early British TV service.

    The guy I knew who'd worked on it described how the routing at one site,
    around the edge of the field, with several corners, to conceal the feeders
    from the air, couldn't be made to work, and they had to go direct, making
    a beautiful sighting line to the camouflaged transmitter.

    --
    "For a successful technology, reality must take precedence
    over public relations, for nature cannot be fooled."
    (Richard Feynman)
     
    Fred Abse, Mar 16, 2013
    #25
  6. RichD

    Guest

    On Sat, 16 Mar 2013 10:34:59 -0700, Fred Abse
    <> wrote:

    >On Fri, 15 Mar 2013 12:44:18 -0400, krw wrote:
    >
    >> Coax has the fields contained within the cable, open-wire does not.
    >> Structures, people, and critters in proximity to coax don't matter. It's a
    >> different matter with open-wire. Open wire usually requires a BalUn, as
    >> well.

    >
    >The WWII, British, CH radar (circa 50MHz), used open wire feeders to the
    >transmitting antennas. The transmitters were a Marconi-EMI television
    >design, of a few hundred kW, modified for pulse modulation, one of the
    >reasons for killing the early British TV service.


    Good grief, I didn't say open-wire was never used, just that it is
    more difficult.

    >The guy I knew who'd worked on it described how the routing at one site,
    >around the edge of the field, with several corners, to conceal the feeders
    >from the air, couldn't be made to work, and they had to go direct, making
    >a beautiful sighting line to the camouflaged transmitter.


    Perhaps Coax wasn't available? Seems like a perfect opportunity for a
    ruse. ;-)
     
    , Mar 17, 2013
    #26
  7. RichD

    Fred Abse Guest

    On Sun, 17 Mar 2013 09:52:27 -0400, krw wrote:

    > Perhaps Coax wasn't available?


    This was 1942 Britain.

    --
    "For a successful technology, reality must take precedence
    over public relations, for nature cannot be fooled."
    (Richard Feynman)
     
    Fred Abse, Mar 17, 2013
    #27
  8. RichD

    Guest

    On Sun, 17 Mar 2013 12:07:38 -0700, Fred Abse
    <> wrote:

    >On Sun, 17 Mar 2013 09:52:27 -0400, krw wrote:
    >
    >> Perhaps Coax wasn't available?

    >
    >This was 1942 Britain.


    Duh, really? <boggle>
     
    , Mar 17, 2013
    #28
  9. On Sun, 24 Mar 2013 01:34:41 -0400, "Michael A. Terrell"
    <> wrote:

    >
    >dave wrote:
    >>
    >> On 03/15/2013 02:07 PM, wrote:
    >> > On Fri, 15 Mar 2013 05:50:02 -0700, dave <>
    >> > wrote:
    >> >
    >> >> On 03/14/2013 11:10 PM, wrote:
    >> >>> On Thu, 14 Mar 2013 08:16:45 -0700, dave <>
    >> >>> wrote:

    >>
    >> >> What is the advantage (if any) to using co-ax v. open-wire feed for
    >> >> above HFBC example?
    >> >
    >> > When you have several (sub)megawatt HF transmitters and a few
    >> > directional antennas aimed on different continents, you really need a
    >> > big switchyard. with coaxial relays. I do not understand, how to do
    >> > this with open-wire systems.
    >> >

    >>
    >> Apparently...
    >>
    >> http://hawkins.pair.com/voaohio/voab5.jpg

    >
    >
    > That station is long gone,


    Well, a lot of it is. THAT antenna array Is still there, however.

    > and the site it sat on is now a golf
    >course.


    No, it isn't. it isn't even a Frisbee golf course.

    That particular array is one of the only remaining historical elements
    of the site. It is now a learning center for radio broadcasting.

    > I got the nickel tour of that station in '70.


    And your alzheimer's set in in the mid '90s.


    > SOme of the
    >switching was done in underground vaults at that time.


    And some was done by hand on that array in the picture.

    > They were in the
    >process of a total upgrade of that plant at the time. 10 new 50 KW
    >custom National .5 to 30 MHz transmitters.


    3 transmitters were replaced.

    > A complete new control room,


    Original footprint.

    >and they were replacing the wood poles with small towers.


    Never happened.

    > The only
    >thing that wasn't replaced was the Curtain antenna that was aimed at
    >Europe & Russia.



    VOA BBC still exists.
     
    DecadentLinuxUserNumeroUno, Mar 24, 2013
    #29
  10. In sci.electronics.misc dave <> wrote:
    > On 03/23/2013 10:34 PM, Michael A. Terrell wrote:
    >>
    >> dave wrote:
    >>>
    >>> On 03/15/2013 02:07 PM, wrote:
    >>>> On Fri, 15 Mar 2013 05:50:02 -0700, dave <>
    >>>> wrote:
    >>>>
    >>>>> On 03/14/2013 11:10 PM, wrote:
    >>>>>> On Thu, 14 Mar 2013 08:16:45 -0700, dave <>
    >>>>>> wrote:
    >>>
    >>>>> What is the advantage (if any) to using co-ax v. open-wire feed for
    >>>>> above HFBC example?
    >>>>
    >>>> When you have several (sub)megawatt HF transmitters and a few
    >>>> directional antennas aimed on different continents, you really need a
    >>>> big switchyard. with coaxial relays. I do not understand, how to do
    >>>> this with open-wire systems.
    >>>>
    >>>
    >>> Apparently...
    >>>
    >>> http://hawkins.pair.com/voaohio/voab5.jpg

    >>
    >>
    >> That station is long gone, and the site it sat on is now a golf
    >> course. I got the nickel tour of that station in '70. SOme of the
    >> switching was done in underground vaults at that time. They were in the
    >> process of a total upgrade of that plant at the time. 10 new 50 KW
    >> custom National .5 to 30 MHz transmitters. A complete new control room,
    >> and they were replacing the wood poles with small towers. The only
    >> thing that wasn't replaced was the Curtain antenna that was aimed at
    >> Europe & Russia.
    >>
    >>

    > The BBC used motorized balanced feeder switches that were about two
    > stories tall. They actually convey a lost sense of whimsy that once made
    > pre-CAD technology interesting.
     
    Cydrome Leader, Mar 25, 2013
    #30
  11. RichD

    josephkk Guest

    On Tue, 26 Mar 2013 13:18:42 -0400, "Michael A. Terrell"
    <> wrote:

    >
    > The original transmitters at that site were made by Crosley. Green
    >metalflake paint and big glass doors in front of the finals. Beautiful
    >examples of '30s industrial design, but built before Television. They
    >tried numerous methods to eliminate the TVI, and were replacing them
    >with custom designed National radio transmitters. When you changed
    >frequency, the transmitter reduced power and self tuned every stage.
    >All 10, or any combination could be fed from one MFO for power levels
    >from 50 to 500 KW. The self tuning was done with detectors to measure
    >the signal and large servos to drive the variable capacitors & roller
    >inductors.


    Once upon a time i had a USC-30, 2 to 30 MHz 1kW transceiver, in my care.
    It auto-tuned in the same way. I got to watch it once (with the covers
    off) and it was a thing of beauty to behold. It had its own low power (50
    W) dummy load and tuned up into that. Then it commanded the auto-tuner
    of the selected antenna to tune up. Just beautiful, but 30 seconds for
    any frequency change from 100 Hz to 28 Mhz. You used a different receiver
    and antenna to find an empty spot in an authorized band to transmit.

    ?-)
     
    josephkk, Mar 28, 2013
    #31
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