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

[Dave Riley]

Hi Frank,

Those HV silver micas may be rated high enough to parallel your vacuum 
variable with and transmit with..
In that case you can get by with a lower value of vacuum cap., say 500 pf..

If not then you can add some low loss inductance in series with it until 
you come in to the new loop resonance 'window'..
Or, make the loop longer, even twice around...

In AM broadcast arrays, placing an inductor in series with a cap. makes 
the cap look like a larger value...
It sounds odd but is a good $$ saver..

I add a decade capacitor box across this loop to get down to resonance 
at any lower frequency..
Even 17.2 khz to hear SAQ in Sweden this month..

Good luck with your new phase of real hands on ham radio..

73s from DaveR aa1a


On 6/21/2019 2:52 PM, Frank O'Donnell 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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