Static bleed off for dipole / doublet

Discussion in 'Tips and Tricks' started by K7JOE, Jun 7, 2021.

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  1. AG5CK

    AG5CK Premium Subscriber QRZ Page

    Looking at the typical T match schematic it seems there is no dc continuity between the center conductor of the input and output.

    Some manufacturers may have a static bleeder of some sort in their tuners. I would check rather than assume.

    This is another advantage of the double balanced L with a doublet. A resistor or choke can be placed across the (50 ohm) input of the tuner.

    Even if the expensive moden rig has an internal bleeder would still feel better about having something redundant, or at the very least shorting the coax before connecting it.

    When using my link coupled tuner the coil can't be grounded with a center tap because the coil has been split for series configuration. In that situation I have chokes on each leg of the feedline. You can roll your own or use plate chokes.

    For link coupled tuners in parallel configuration a husky ground strap can be used to DC ground the entire antenna.
     
    K7JOE likes this.
  2. K7JOE

    K7JOE Subscriber QRZ Page

    Yes, that config works too. I just put them at the feedpoint since it's easy to place "set and forget"...but you get the idea.

    Ebay.. 10 pieces for $5.90 and free shipping
    https://www.ebay.com/itm/274256728767

    just search for 3.3M ohm, 2 or 3 watt ...
     
    N1VAU likes this.
  3. N1VAU

    N1VAU XML Subscriber QRZ Page

    Thanks!

    I always forget about eBay for electronic components, DOH! :confused:
     
  4. KU3X

    KU3X Ham Member QRZ Page

    First put a UHF T connector inline with your coax. That will give you an external connection to your coax. Next take an FT 240-43 ferrite core and wrap 32 turns of either #16 or 14 enamel wire around the core. Now shunt that across the T. Connector. Make sure the shield of coax is connected to ground.
    That choke is transparent to the HF bands. It would take a direct hit from lightning to destroy it.
    It does not take too much of a static discharge to destroy a resistor.

    Barry, KU3X
     
    N5AF and AG5CK like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    I have a 40ft irrigation 2" alum irrigation pipe standing vertically on my metal building, which puts the tip at ~63ft agl. To equalize the static-induced voltage potential between the pipe and the building (in the event that the coax is disconnected or open), I placed a static-bleed network consisting of a 1mH Pi-wound RF choke in series with a 10K 1W carbon resistor between the monopole and the building (effectively across the antenna feedpoint). I had the choke and resistor in my junk-box; I would have had to mail-order a 3.3Meg 3W resistor...

    I can run 1kW and because the antenna is non-resonant on some bands, the RF voltage across that choke/resistor combination can reach >1500V. Since the RF impedance of the RF choke is so high, the RF current while transmitting is essentially zero, but at DC, the 10K resistance prevents any voltage difference between the vertical pipe and the building (effectively between the coax center conductor and shield at the other end of the coax), and the 10K limits the peak discharge current.

    As Joe correctly pointed out, if you ever connect an expensive antenna analyzer (like my RE AA-600) to that coax run, you want to make sure that any static-induced voltage difference between has been neutralized before connecting the coax to the instrument. If you forget to do it manually (with a clip lead or even your finger), and there is no static bleed network, it can get very expensive...
     
  6. KN4HGI

    KN4HGI XML Subscriber QRZ Page

    Joe, as the ZS6BKW or its bent end brother the G0GSF uses the twin-lead as part of the “system,” is it possible to place the resistor or choke at the bottom of the T feed line…where the antenna is actually “fed?” Thanks for the video and making me think about improving my favorite antennas! KN4HGI Alan
     
  7. WZ7U

    WZ7U Ham Member QRZ Page

    I'm going to put a stick it note near my vna to remind me to do this every time. Thanks for the reminder!

    Will that mix play well below 20 meters? I only ask because of what I saw here https://www.amidoncorp.com/ft-240-43/
    Thanks!
     
  8. KU3X

    KU3X Ham Member QRZ Page

    "Will it play well below 20 meters?" YES......as I said before, it is transparent on ALL of the HF ham bands. That means above and below 20 meters.
    Some time ago Amidon required a $50 minimum order. Mouser does not.

    https://www.mouser.com/ProductDetai...ha2pyFaduhTDKNerYvfZzlAD%2BWSg7drO4v8J4lYOV8=

    Don't be misled by the picture on mouser's web page, it is in fact an FT 240-43 core. Download the data sheet from the web page and it shows the correct picture. On the mouser web page the price for this core was around $5, the last I checked. I see the price went up ! Outch !
    Barry, KU3X
     
    WZ7U likes this.

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