CQ250 bandpass filter

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M0YDH
Posts: 154
Joined: Sat Dec 31, 2016 5:59 pm

CQ250 bandpass filter

Post by M0YDH » Sat Sep 02, 2017 12:07 pm

Hi
I'm making the 146.5MHz bandpass filter in CQ250 described by Shaun. Can anyone confirm the coil diameter? The type doesn't get any clearer when enlarged. I've guessed it's 3/8" diameter and wound 4 off 6.5 turns on a 9.5mm drill

The rush is on to get filters ready for CAT17 workshop day.

73
David M0YDH

M0SKM
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Joined: Sun Jul 13, 2014 3:15 pm

Re: CQ250 bandpass filter

Post by M0SKM » Sat Sep 02, 2017 12:45 pm

From the original article 9.5mm internal or 3/8".

I built one in a diecast box and they work extremely well. A friend found the copper clad board design unstable due to flexing.

http://www.m0skm.com/index.php/2m-band- ... er-another

73
Steve

g8vpg
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Re: CQ250 bandpass filter

Post by g8vpg » Sat Sep 02, 2017 2:36 pm

David,
I've just measured my prototype and it is 10mm inside diameter, so 9.5mm or 3/8inch is just right. We've built several down here in Bristol and they do work well.
73 Shaun G8VPG.

M0YDH
Posts: 154
Joined: Sat Dec 31, 2016 5:59 pm

Re: CQ250 bandpass filter

Post by M0YDH » Sun Sep 03, 2017 7:24 pm

Thanks Steve and Shaun.
Pictured is progress to date. I'm stuck for what to do with the 0.5pF "capacitor" wire. It's not connected yet. Hopefully at Saturday's workshop session at CAT17 all will be put right.

73
David M0YDH
Attachments
20170903_CQ250_bandpass_filter_part_built.jpg
146.5MHz bandpass filter under construction
20170903_CQ250_bandpass_filter_part_built.jpg (179.52 KiB) Viewed 2175 times

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M5TXJ
Posts: 181
Joined: Wed Jul 05, 2017 8:34 pm

Re: CQ250 bandpass filter

Post by M5TXJ » Sun Sep 03, 2017 8:29 pm

Would a 0.5pF capacitor made from FR4 (approx 2.6pF/sq cm) be too lossy?

73 Dave.

M0SKM
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Joined: Sun Jul 13, 2014 3:15 pm

Re: CQ250 bandpass filter

Post by M0SKM » Mon Sep 04, 2017 7:22 am

My capacitor is a length of RG405 semi rigid.
At one end the inner only connected to the cap/coil junction; the other has the outer connected to cap/coil.

Steve

Edited as first post too early in the morning Oops.
Last edited by M0SKM on Mon Sep 04, 2017 9:47 am, edited 1 time in total.

g8vpg
Posts: 307
Joined: Fri Oct 11, 2013 9:35 am

Re: CQ250 bandpass filter

Post by g8vpg » Mon Sep 04, 2017 8:21 am

I didn't use a capacitor. I drilled a 3mm hole in the partition and just put a length of wire through, protruding about 6mm into the second cavity. You can adjust the coupling by the length and position of this wire. You really need a spectrum analyser with tracking generator to set it up properly.
73 Shaun.

G8GKQ
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Re: CQ250 bandpass filter

Post by G8GKQ » Mon Sep 04, 2017 8:34 am

Hi David

If you were to bring the filter to Finningley, we'll have a spectrum analyser and sweep generator suitable for setting it up there. Please bring the lids so that we can close it up when we get it right!

Dave, G8GKQ

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M5TXJ
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Re: CQ250 bandpass filter

Post by M5TXJ » Mon Sep 04, 2017 8:35 am

Steve M0SKM,

If your coax is shorted inner to outer, you haven't a capacitor, you've a direct connection.

73 Dave.

M0YDH
Posts: 154
Joined: Sat Dec 31, 2016 5:59 pm

Re: CQ250 bandpass filter

Post by M0YDH » Mon Sep 04, 2017 9:58 am

Thanks folks. I'll make some coax capacitor samples and see how they measure on the LCR meter.
CAT17 workshop day is very important to me as no working spectrum analyser is to hand. I'll have plenty for Dave to examine! E.g. the QRP-LABS low pass filters for 4m and 2m as well as the new M0HMO power meter that I built at GQRP club Buildathon on Saturday.

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