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

[Frank O'Donnell]

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