[600MRG] Vacuum variable capacitor, low-pass/band-pass filters

[John Langridge]

Hi Frank,

RF Part's stocks the values they need but will be pricey:
https://www.rfparts.com/capacitors/capacitors-vacuumvari.html

Some guys find them on ebay with good success.  Im  not a fan,
however.  Sometimes the envelopes can leak even if the glass is
intact.  You can usually tell by looking at the copper.. if its
shiney, you are probably ok.  If tarnished, move on... Oxygen free
copper is typical to avoid outgassing which can significantly lower
the voltage handling capabilities (and create a cold cathode X-ray
tube at appropriate voltages and vacuum pressures)...

Yeah, silver mica TX caps.. You really want the big boxy brown
ones...RF parts probably has a few.. I usually find them at ham
fests.. if there is a specific value you need, ask around, as one of
us might have one...



check out the chebyshev filter value's from WA3ETD that are posted on
my site:  http://njdtechnologies.net/630-meter-low-pass-filter-values-from-wa3etd/

failing that, Jay, W1VD,  has a great design that I use at my station
that was built by WW2R before he moved back the England... Like
everything Jay designs, its built like a tank...You can see the
details and schematic link here:  http://w1vd.com/LPF.html


Hopefully that will get you pointed in the right direction or at least
stimulate some discussion that will...

73!

John..


On 6/21/19, Frank O'Donnell <600 at inkbox.net> wrote:
> Hi all,
>
> I'm a fairly new ham and very new to 630m, currently in the process of
> getting a station together.
>
> I have a new Elecraft K3S (major birthday this month), and recently
> bought a 100w 630m class D amp from Ken Roberson K5DNL. Due to the great
> number of trees and lack of clear space for radials on our city lot, I'm
> following an excellent suggestion to look into putting up a vertical
> loop antenna rather than the inverted-L that I first had in mind.
>
> At this point, I'm working on the following questions -- any advice or
> suggestions would be greatly appreciated:
>
> 1) Depending on how it's hung, the final circumference of the loop could
> vary between about 100 and 130 feet. Using AA5TB's loop antenna
> spreadsheet, it appears that I will need a range of about 1548 pF to
> 2136 pF capacitance to tune the loop across the entire 630m band. I have
> zero experience so far with vacuum variable capacitors, and although I
> see them listed for sale on eBay and elsewhere, I'm reluctant just to
> pick one and hope it's suitable. Does anyone have suggestions for a
> source, possibly even for a specific cap?
>
> 2) In case I need to supplement the above capacitances for any reason,
> is there a type of fixed cap that works well for this? For HF work I've
> been using silver micas in transmitter antenna tuning circuits.
>
> 3) Although I have a fairly good external low-pass filter for receiving,
> I often run several SDRs with different antennas at a time, and would
> like to get some additional low-pass filters and experiment with
> bandpass filters. I saw a favorable reference to filters by Clifton
> Labs, which apparently was acquired by DX Engineering, but I'm not clear
> on which of their current offerings would be best for 630m and below.
> Any suggestions on filters, either prebuilt, kit or homebrew plans?
>
> It would also be interesting to hear from any K3/K3S owners -- as a
> newbie on that front I have a few things I'm wading through as well.
>
> Thanks for any guidance,
>
> Frank K6FOD
>
>
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