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
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
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:-
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
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
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
Many thanks to Andy and Colin for the suggestions; I will follow these up and see how I get on.
73,
Rod
In reply to M0JLA:
I’ve put some pictures in Colin | Flickr 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
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
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
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
As the subject of interference from adjacent masts has popped up again I thought that reviving this topic might be helpful. The filter I made allows a VX-7 with MFD to operate on Cyrn-y-brain; without it the receiver is completely swamped. I assume M6BWA used it when we did CyB; activation point was the fence corner above the carpark where the ground drops away very steeply.
The filter is not hard to make although I remade it twice, each time with heavier gauge wire as it did not remain stable as the coils distorted during rough handling.
Definitely worth having if you must use a 2m HH on RF saturated summits.
73,
Rod
Thanks Rod for reviving this thread - All very useful information.
73
Nick
That’s interesting, because I’ve never had a second’s trouble on Cyrn-y-Brain with any configuration of 2m FM operating - including the VX7 handy (with wideband rx) and RD. Great Orme and Billinge Hill are notorious for problems, and I’ve had them there, but never on NW-043.
Cyrn-y-Brain used to be clean, I contested from there on 4/2/70 many times. We did get breakthrough on a harmonic of the Band II FM on the 70cms preamp but a coax stub was enough to kill the in-band harmonic.
The last time I was there I noticed things were worse that before. My VX-170 is fairly bullet-proof but I was being de-sensed regularly such that noisy S1-S2 signals would register on the S meter but not open the squelch. I can only assume it’s got worse. This was operating from the shelter nearest the 1st TX site. As BT have removed most of the dishes from the far tower, it may be worth the 10mins walk to operate from there or the trig.
It appears to be 5 years since I was last there. Can whoever stole all those years from me please return them!
A good reason for doing unique summits - you never get that “how long ago was it since I was here” feeling. 
Our filters are finally here after a two week delay thanks to ParcelForce. Billinge Hill tomorrow perhaps.
I spent a good few hours up on Billinge Hill today filming for a video on combatting 2m blocking. No shortage of it up there. That video will go live after Christmas.
That will make interesting viewing. Especially if you have included sections showing that without the filter a radio can be blocked totally so it doesn’t hear responss, and the user is unaware of them.
73 Andrew VK1DA/VK2UH
I have published a video today on this topic on my “Radio Adventures” YouTube channel.
Forgot the link:
Ideas or questions welcome!
