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Reflector :: Helical Filter for 2m

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Post by G3CWI on 23rd October 2011 at 08:17
A while ago I wrote about a helical filter that I had made to reject pager interference on 2m. I was curious to see how stable that was over time. To test it, it has been carted about all over the place, dropped, knocked and otherwise abused. Yesterday at the G-QRP Convention I finally got round to retesting it. To my great surprise, it has not drifted at all and the peak is still at 145.5MHz.

73

Richard
G3CWI
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Post by G8JSM on 25th October 2011 at 16:23
In reply to G3CWI:
I'm surprised that more people haven't built these filters, I've just put together a four section helical filter using pcb to make the cavities, most of the rest of the bits came out of my junk box. Provided you can cut the pcb reasonably accurately and can solder, the hardest bit is getting it on frequency, I used an antenna analyser.The filter measures 150mm * 51mm * 38mm excluding sockets and adjusting screws.Big enough to be easy to work on and light enough to carry. Cost, less than 5.00 Weight 175 grammes, bandwidth for less than 1.5: 1 vswr about 1MHz. Build time about 4 hours not including design time (using Helical software) and rooting for bits. I can now use my Wouxun handheld on Billinge hill without any pager problems. I do not have lots of expensive equipment to test this filter, but it works!!! and was reasonably simple to put together.

Regards,
Colin
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Post by M0JLA on 25th October 2011 at 17:56
In reply to G8JSM:

I am interested in this, having had problems on several summits with masts. The only details I can find at present are at:-

http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/144MHz_High-Q_High-Power_BPF.gif

Which looks easy enough to make but is perhaps more complicated than what Colin suggests. No dimensions given for the actual box, so I'm not sure whether I will get it to work.

or at:-

http://www.dj9kw.de/dj9kw/projekte/afu/2m_lowpass/lowpass.htm

which is presumably more like it. Would this need a metal box or a non-conductive one?

Grateful for any suggestions.

73,
Rod
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Post by MM0FMF on 25th October 2011 at 18:56
In reply to M0JLA:

>The only details I can find at present are at:-

Or search the SOTA reflector with the search facility....

http://www.sotawatch.org/reflector.php?topic=6041

The VA3IUL link is taken from the RSGB VHF/UHF handbook (3rd or 4th edition).

Andy
MM0FMF
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Post by G8JSM on 25th October 2011 at 19:15
In reply to M0JLA:
Go to www.tonnesoftware.com and download Helical. This will allow you to design the resonators, I would suggest that you put a small bolt down through the top of the resonator to enable you to fine tune the resonators, I used M3 nuts and bolts. The size of the box and the coils are calculated, make sure that the box fits together well and only try to tune the resonator with everything bolted down.

I don't think that the lowpass filter would give you enough reduction in the 153 MHz signals.

Make sure that you use high Q capacitors if you try the bandpass filter

Regards,
Colin
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Post by M0JLA on 25th October 2011 at 20:26
Many thanks to Andy and Colin for the suggestions; I will follow these up and see how I get on.

73,
Rod
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Post by G8JSM on 25th October 2011 at 22:11
In reply to M0JLA:
I've put some pictures in http://www.flickr.com/photos/g8jsm to give you some idea of how I made the resonators ,for some reason its not letting me organise the photos, so you will have to find the 4pole helical resonators. You might decide to only use two or three poles to suit your needs.

Hope this helps,
Colin
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Post by M0JLA on 27th October 2011 at 08:13
In reply to G8JSM:

Colin,
Thanks very much for these; most useful and in conjunction with Helical (and the other references pointed to by Andy) give me a pretty good idea what to do. Done lots of interesting graphs and think a 4 pole looks just the job but might start with a 2 pole to get my eye in and also to leave room in the bandwidth for 144.300 ssb. Off to Maplin later today.
I will report back later.

73,
Rod
This thread is closed
Post by G8JSM on 27th October 2011 at 16:55
In reply to M0JLA:
I would suggest making the bandwidth greater, maybe 3MHz and looking at the VSWR plot, but staying with 4 pole, this should give at least 40 dB at 153 MHz assuming that there are imperfections in the resonators (tin/lead solder joints, no silver plating etc)

I used a piece of 1/2" square aluminium bar clamped to the longer side of the box to get the cross members in position and perpendicular, using double sided pcb for the crossmembers allows you to tack solder one side, move the aluminium bar and solder the other side, then sort out the tacks. Check that any connectors will clear the coil, it may also be easier to fit the connectors before soldering the box together, I fitted them after I made the box, very fiddly. Bear in mind that the coil goes in the centre of the cavities.

Have fun,
Colin
This thread is closed
Post by M0JLA on 29th October 2011 at 08:09
In reply to G8JSM:
Colin,
Many thanks for these further suggestions. Progress continues; I have now spent a fair bit of time with Helical and notice that the dimensions may remain constant and the bandwidth is set by tapping point and coupling so there is scope for some experimenting here. We shall see what it does when the Mk 1 version is complete. I don't have a sweep generator but do have a VHF SWR meter so should be able to set the range to something suitable, allowing for construction defects etc.

73,
Rod
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