[TowerTalk] more on 43ft matching nets

[Jim Lux]

Ran some test numbers to sort of frame the discussion.. After all, we're not really talking about a perfectly generalized matching situation, but with a 43 foot radiator, which is "special" (i.e. it's not a half wavelength, so the feedpoint Z isn't way high).. Two different models for a 43 ft vertical give feedpoint Z at 3.75 Mhz of 12.5-185j (perfect ground) or 50+156j (real ground)..  (didn't model radials or anything.. just a stick in the ground)

Trying several types of matching network, and assumed 1500W through the network

For the first case, a low pass L of 1470pF and 8.77uH does it
9.2A, 375V on C, 10.28A, 3kV on the L

For the second case, a low pass L of 6.6uH and 493pF, or a low pass pi of 260 pF, 6.9uH, 500 pF does it.
for the L, 5.4A, 1200V on the L, 10.3A, 1250V on the C
for the pi, 1.7A, 390V on C, 5.6A, 1300V on L, 10.5A, 1300V on C

Those are all RMS, so peak voltages are 1.4 times higher (and good design practice would be to use 2.5 or 3 times RMS voltage)
And, obviously, a real antenna will be different, with different values, but I think it gives an idea of the voltage and current ratings required for the components.


For comparison, a typical RF silver dipped mica (e.g. CDE CDV16 type at 500pF, 1000V) is rated at about 2.2A max current. Lest one think you could parallel a pair of 250pF caps, the rating on the 250pF units is half: 1.2A (the cap is physically smaller, and it's basically a thermal dissipation thing)

So, you'd have to series parallel a whole raft of these babies to get the kind of ratings you need.  They're probably a $0.50 to $1 each, but you'd need a fair number.

A more conventional ceramic doorknob type (e.g. a CDE 291/292 series) would be 500pF @ 6kV and rated at 6.8A at 3MHz.  But those cost about $30 each.

Building an L to take 5-10A and stand off a couple kV from end to end is trivial. A half dozen or so turns of AWG12 enameled wire will do nicely. (and if you look inside those autotuners, that's what they do)

    -----Original Message-----
    From: Dennis OConnor
    Sent: Nov 24, 2008 9:30 AM
    To: jimlux
    Cc: towertalk at contesting.com
    Subject: Re: [TowerTalk] Was "43ft Vertical Feeding

    HI Jim, thanks for the comments...
    Yes, the relays will handle the voltage and current at 1.5kw... The ones I use are ~ $2.50... I buy typically 2 dozen at a time for the price break..




    You have to remember that the voltages are not that high at the base of a 43 foot vertical - and they are low on the 50 ohm coax side and not much more at the antenna feed point, 1200-1500 volts maybe at worst case...

    >>> A bit more voltage, I think.. a couple kV perhaps, depending on the band.  If you want to run 160, then a lot more voltage.

     
    I build tuners, for converting 50 ohm coax to 600 ohm open wire line and the voltages there are considerably more, and the relays work..  My tuners handle 1500 watts without blinking... I have 7 out in the field (woods actually) at the moment and #8 is on the bench...
     
    We make our own condensors (motorized in my case) from aluminium sheets; and 0.200" spacing handles 1500 watts in the tank circuit feeding 600 ohm line with unknown (but high) impedences and reactances..  A conservative estimate is that it will handle 5KW minimum, though I have no way to test that... The dielectric breakdown for air is 3000v per millimeter... In a 1500 watt antenna system you are not going to see those kind of voltages... A Tesla coil is a another story..

    >>>> Tesla coils see around 10kV or so in the primaries, which is where you're building parts. Dielectric breakdown for air, in a uniform field is 3kV/mm, but you're hardly a uniform field. The issues would be breakdown along the insulator path, and corona off the edges of the plates. If you're using 0.060 thick aluminum, and you nicely round the edges, the radius of curvature is 0.03 inches. At 71kV/mil for air, when the voltage gets to around 2kV, it's going to start to breakdown. In reality, around 1kV, you're probably getting significant breakdown (OTOH, if the duty cycle is low, the ozone concentrations might not get high, so you'll get some loss, but at least all the rubber parts won't fall apart in a day).  the 0.2" spacing works out to a breakdown of 14kV in a uniform field, but along a creepage path (e.g. if you have a spacer between the plates), you would probably get breakdowns at around 4-6kV.  If the spacer has a stack of different materials, it gets trickier.

     
    I have one I just whipped up to convert an 80 meter base fed vertical into a halfwave base fed vertical for 40 for CQWW - so I needed a parallel tuned tank circuit with a tap for the coax feed... ten feet of copper tubing and 2 pieces of aluminium from the scrap box to make a 71 pf condenser and I was in business... Took all of an hour, with lots of coffee time and lolly gagging around... I gave this one 0.300" spacing because the voltages across the parallel circuit will be high at 1500 watts... No signs of arcing so far and this is in a plastic box laying on the ground - and has a half inch of snow on it as I type this......
     
     
    Be aware that building a single L-match for one band, as I described, is a trivial matter... And taken one band at a time, and later integrating it into an enclosure with relays, etc., is not daunting when taken one step at a time - over time...
     
    Basically, your analysis says you don't want to build... That is fine - your choice...

    >>> well, the original poster said "if one's available I'd buy it".. so we digressed into an exercise of what such a product would cost (and actually do cost, since there are such things being made).  Clearly, one can trade time for money, as you say.  Especially if you have a well stocked junk box and enjoy fabrication and packaging, then it's a good way to go.

    >>Jim, W6RMK

    (excuse the weird formatting.. the webmail client is funky)