[RFI] An RFI solution report

i4jmy@migate.ampr.org i4jmy@migate.ampr.org
Sat, 10 Jan 98 19:33:37 +0000

From: i4jmy@n8it.#nwmi.mi.usa.noam 
To: rfi@contesting.com

  Hi Guys,
    here some of my practical solutions to a number of interferences I
  had to front with. Probably not everitying I've done it's exactly
  against RFI in the strict sense of the word since a part my work was
  devoted to avoid HF RFI to PC and other devices, while usually RFI is 
  intended as the interferences to HF from other devices, but I believe
  finally the two things are overlapping.
    As a preface, my shack is at 5th floor of a concrete building just
  12 meters below the KT34 and less than 2 meters away from the
  supporting tower used also as a shunt feed vertical, for 40 80 and 
  160m. The VHF UHF antennas, used for packet, are equally close to the 
  shack, having the UHF 20el on a 1.5 meter pole placed some couple
  of meters from the main tower and a dualbander vertical in a
  very similar situation.

    This description has been made to explain that the distance to
  ground is more than 20 meters and consequently a connection to the
  ground, if good for safety reasons is definitely bad to keep "cold" 
  my shack by RF.
  The symple connection of devices to the ground heavy wire doesn't 
  help leaving unsolved 95% of the interferences.

    My goal was to have VHF/UHF TNCs and computer working in the same
  while without damages first (...had to repair a TNC2 blown because of
  RF), and free by any mutual interference, even if using the home made
  Amp with the 3CX1200.
    What I got, after a number of tries, is a situation where this is 
  nearly 100% obtained unless few restricted HF spots (a total amount
  of 15-20 KHz in the whole HF spectrum) where some birdies and noise, 
  due to computer, monitor and one of the TNC, is present. 
  The level of this interferences is not very big since none of them 
  is strong enough to move the s-meter needle of my rig, when most of 
  the times QRM does it.

    The first "good idea" was to discard a TNC in a plastic box, the
  second to place a ground bus between all the metal cabinets of all
  the devices including Computer, PS and rotators control boxes. 
  Most of these devices has no ground screw and GND passes only through
  power cables. Altough this is good for safety reasons, often this 
  cables are too long, in term of fractions of wavelengts so resulting 
  inefficent for effective RF screening purpose.

    While the above operation took away most of RFI from TNCs toward
  HF and some of the PC "noises", it was definitely insufficient to 
  prevent RFI from HF (40 80 & 160) transmitting to VHF/UHF rigs and 
  to TNCs.

    Since my tower (including rotator and KT34) work also as a shunt fed 
  vertical, the only effective way I found to solve this trouble was 
  to create an artificial earth on its base.
  Due to the omega match I used, SWR was not a problem even when I
  used, as counterpoise, only a group of 16 radials each of them only 
  8 meters long.
  Altough maximizing the counterpoise current didn't give appreciable
  benefits in the performances of the antenna itself, the upgrade of
  implementing 3 separate ground radial systems, each one raised in
  current flow (I used an RF ammeter) on its band with variable
  discrete components, gave a sparkling improvement in RFI creating an 
  RF nearly-no-voltage point at the antenna (tower) base.
  This action reduced the rf flows, trough antenna cables, to the shack,
  and was the only effective one I could find.
  To be precise I have to tell that coaxial cable shield of KT34 and
  all the rotator cables (by use of 10,000 pF cap.) are connected to
  the base of the tower, likewise the gnd of the 3 omega matches.
    Concerning RFI from PC to HF the great improvement was obtained
  with the use of RF chokes along the cables to TNCs, monitor and
  Having the HF antennas at a nearly zero distance from the shack,
  any radiation by any of the cable between PC, monitor, TNC and
  PS is big enough to be received. 
  Here it is the need of stopping the rf flow and cosequent radiation,
  along the interconnecting cables. The only possible (practical) way
  to do it is to keep 'em short, and placing chokes as close as it is 
  possible to their ends.
    To realize effective chokes in few turns it is a need to use
  materials whose permeability is very high, 2000 or more, I used
  5000 toroids where I could.
  Some of the cables, i.e. that one of the monitor, are pretty big
  or some other cable had sealed connectors beeing unpractical to 
  wound, also for the biggest ferrite toroids I could find. 
  Trying to solve this nasty problem, it occurred to me I saw ferrite
  elevation transformers (TV ones) realized by cores in two pieces
  in a form of C then kept toghether, to ensure the concatenated flux,
  by long screws and bolts.
  They are pretty easy to find out, at least here. Shops or TV repaiar
  are very happy to sell the old models at symbolic prices.
    I had two doubts, the first about the losses of a ferrite material, 
  good at 19KHz, when used at 30 MHz, the second about the detriment in
  the magnetic flux because of placing the two halves faces one against
  the other. Infact, if the two couples of faces are not perfectly 
  matching one against the other, there is a space of air in which the
  concatenated flux drops dramaticly.
  I wasn't also very enthusiast to use the original metal fixture to 
  combine the two halves the meanwile keeping the windings. 
  Thinking about an alternate metod, I saw a little plastic bottle 
  containing Loctite (TM) liquid.
  Since the cyanoacrylate bonds effectively only if the surfaces are 
  perfectly matching, if the two halves of the ferrite core would have
  solid joined by Loctite (TM), then the magnetic flux won't have been
  too much reduced and the problem of fixture inherently solved.
  I wounded the monitor cable on the two halves of the ferrite core, and
  after 60 seconds from bonding I tried, with all my forces, to separate  
  the halves but it was not possible any more by symply use of hands.
  By tests, this rudimental but easy to manage cores, are enough to
  effectively stop RF in the HF spectrum in big cables.

    I hope this report can help someone in solving problems similar
  to mines.

  Mauri I4JMY

  E-mail i4jmy@migate.n8it.ampr.org

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