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

[Gedas]

Hi Frank. I see you have received a good number of excellent tips and 
leads. I just wanted to chime in and tell you that when I started 
listening to 630m I was using my stock Flex 5000A. It was absolutely 
horrible and all but useless due to all the overwhelming spurious & 
strong intermod signals I was detecting down there. Then there were the 
AM stations mixing in that made matters even worse. I ended up whipping 
together a very fast 7 pole LPF to use in the RCV loop of my receiver 
and the difference was like night and day. Completely knocked out all 
the crud !

You already have several leads for schematics for filters, I just 
quickly designed my own using free filter design software off the net 
(ELSIE) and within an hour had built up a prototype filter.

It was, bar none, the ugliest, most pitiful looking perf board circuit 
you could ever imagine but down at these extremely low frequencies lead 
lengths and components are extremely forgiving. The filter provided, if 
memory serves my right, around 40-50 dB of stop band attenuation and 
made SWL'ing a dream. My first night alone I had decodes from KH6 and VK 
land and over a thousand stateside as well !!!!

So don't be afraid to build something up. No case required and looks 
don't matter. Of course if I were to do another I would not be in such a 
rush and would make something I could show someone but a year ago I was 
an eager-beaver and just had to get on the band !

The 2nd point was about your required capacitors. I have used high 
voltage coax as a capacitor in a pinch with great success. For example 
you can find used 7/8" heliax coax which averages around 23 pF/ft and 
has a breakdown voltage over several thousand volts. I have also used 
smaller Teflon based coax for the same applications. Even 450-ohm ladder 
line can be used, just make sure nothing can come in contact with it esp 
people etc.

So if you have some laying around and don't have any kV rated silver 
mica's etc keep coax in mind.

Gedas, W8BYA

Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

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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