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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 20th Jan 2019, 3:17 pm   #1
Martin G7MRV
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Default Helical Filters - fooling them to a lower frequency?

I have a small problem with a vintage Pye PF8 im converting. Although I can change the various capacitors to move it from U0 to T1 band, to cover 70cm, the receive front end uses a double and a triple helical filter.

There is no chance at all of finding T1 band filters, so im stuck with the U0 band ones fitted. These are pushed right down to their limit! Theres also no way, with the tools I have, of being able to remove them from the radio to modify them.

Does anyone know of any way that helical filters can be 'fooled', without direct physical modification, to work at a little bit lower frequency than they are designed for?


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Old 20th Jan 2019, 4:41 pm   #2
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Default Re: Helical Filters - fooling them to a lower frequency?

You won't be able to do anything with the coupling loops, input and output. If your tuning screws are fully in, then that's that for easy tricks.

THe next step would be to fit new helices with more turns and finer pitch to keep the overall lengths. Makes the slow-wave structure slower.

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Old 20th Jan 2019, 8:20 pm   #3
Martin G7MRV
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Default Re: Helical Filters - fooling them to a lower frequency?

Yes, thats what I was afraid of!

Unfortunately, as ive found trying it on my one and only 'donor' PF8, its impossible to remove these filters without destroying them. And having broken open the one I removed (the 3-pole unit from the receive side) even if it was possible to remove them without damage it looks near impossible to modify.

Years ago 2 and 3 pole filters like this could be had for 2m and 70cm from Toko, but ive no idea if they are still available, and if they are they probably wont fit!

Probably my only option really will be to remove them both and replace them with something homemade. Since the radio is a single UHF channel, I wont need much tuning range! The big worry though is that there is also a 2-pole helical filter in the Tx side.

Cheers
M
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Old 20th Jan 2019, 9:47 pm   #4
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Default Re: Helical Filters - fooling them to a lower frequency?

Is there any way you can add some capacitance to the resonator?
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Old 21st Jan 2019, 9:51 am   #5
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Default Re: Helical Filters - fooling them to a lower frequency?

Out of the box thought, fill them with a dielectric fluid/solid paraffin, epoxy, wax
 
Old 22nd Jan 2019, 4:39 pm   #6
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Default Re: Helical Filters - fooling them to a lower frequency?

Don't give up - there may be a way!

I remember some US mobile-radios [Bendix-King?] that had helical filters - these radios were on the 'federal' band up around 470MHz and would not tune-down to 430MHz.

The helicals had the lower couple of turns (including the input/output-tap where necessary) wound on a spiral-grooved translucent plastic former [PTFE? Polythene?] but the upper couple of turns were in free-space - presumably because that was the highest RF voltage bit and you didn't want anything with dielectric losses reducing the Q.

They were tuned by a cylindrical brass cap that screwed down over the top, about the same as the Pye type.

Someone worked out that if you drilled a small hole through the end of the brass cap and silver-soldered into it a short length of brass rod this would then poke down into the open top of the helical resonator coil and add sufficient extra capacitance so you could tune it to 430.

Could be worth a try.
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Old 22nd Jan 2019, 5:08 pm   #7
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Default Re: Helical Filters - fooling them to a lower frequency?

BONEX were the people for helicals (and other stuff) back in the Days of Yore, but since they modernised and became B.E.D they seem so have moved to ceramic UHF resonators for mobile phone applications etc. I was lucky in getting their last few helicals for 23cm (1300 MHz) when I was homebrewing a 23cm transverter. They were very helpful.
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Old 22nd Jan 2019, 9:28 pm   #8
Martin G7MRV
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Default Re: Helical Filters - fooling them to a lower frequency?

Unfortunately these are plastic adjuster caps of some sort, and all contained within the can, they dont screw out all the way. To attempt any sort of modification, i'd need to remove the filters from the board, which looks to be impossible without damage. I'll try with my donor set, although ive already destroyed the 3-pole unit trying to remove it! Maybe the 2-pole units will be a bit easier to remove. I might have to try hot air, but its so compact that it'll take some careful placement of kaptan tape to keep the rest of the parts from coming off!

I used to use Bonex as a teenager for RF parts. Ive taken a look at BECs online shop but they no longer list prices, which suggests they will be extortionate! Back when Bonex did a catalogue it was easy to find what you wanted. Although, at the time I didnt understand 'ex-VAT' pricing - I probably still owe them the VAT for some parts!

Ive got printouts of the datasheets from BEC for all the 2- and 3-pole helicals in the ranges suitable and electrical replacements, so its a case of seeing which will physically fit - and whether I can afford them!
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Old 23rd Jan 2019, 10:01 pm   #9
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Default Re: Helical Filters - fooling them to a lower frequency?

Ah yes ... right pain these are.
Also found in M and MX296s I reckon.

I have rewound them in those and in PF8s - fun it isn't.
Have to be very careful...

The bits were made by Neosid I think - i doubt they have any now.
(PF8 ferrites same source by the way)

Once upon a time I had some of the parts, formers, adjusters and cans.
The cans had some custom tooling which once soldered together as pairs or triples would nip out the coupling slots.

Polystyrene former material it could well be - the melting point is very low.

They were re-workable, but only just, with 60/40 solder. No way with unleaded!!

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Old 24th Jan 2019, 9:06 pm   #10
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Default Re: Helical Filters - fooling them to a lower frequency?

Just remembered another dodge on these if a rewind was really out of the question.

On some helicals it was possible to ding the can with a centre punch so that the capacitance to the top of the helix would increase and so pull them a bit lower in frequency.

Generally on Pye PF8 and M or MX296 ones that would not work because inside the can was a threaded polystyrene (or what?) former, in which the tuning slug ran. Dinging the can could never have much effect as the former would not compress.

The tuning slugs could not be removed until the resonator was dismantled - they would not pass through the holes in the can.

ISTR the slugs could be screwed all the way to the bottom and some fine threaded metal object could be screwed in behind them. If that was grounded to the can then it would increase the top capacitance and lower the frequency somewhat - it was a pretty hit or miss technique.
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Old 25th Jan 2019, 7:56 am   #11
Martin G7MRV
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Default Re: Helical Filters - fooling them to a lower frequency?

Hit and miss might well do in this case! I'll have a rummage see what I have about, and play with inserting things into the tops of the filters!
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Old 23rd Feb 2019, 5:27 pm   #12
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Default Re: Helical Filters - fooling them to a lower frequency?

I have recently come across this with an MX296, I have another that will cover 70, but as it has a 40 channel head the EPROM would need reprogramming and at present I cannot do that. Based on past experience, the prospect of swooping the resonators over set my impending doom alarm off so an alternative was sought. In the great spirit of bodgery I discovered that placing a small bolt into the top improved things so after a spot of trial and error and the 2950 test set I found the best combination and held them in with some Blutack.

Scientific or workmanlike it is not, but it is a noticeable improvement in the short term. Which as a railwayman means it will still be like that in a decade.....

73 Ian
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