VHF filter, Aerial Facilities Ltd

[sidspop]

I have a filter made by the above company.
H.O Part number FLO122MN.
It is 17" x 6.5" x 4.25", made from aluminium sheet, and reasonably heavy. Internally four sections, with large fixed coils to ground, and the other side has 7 adjustment screws for tuning, 4 of which go down centres of coils, 3 adjust gap to separation plates.
By use of a wide band noisy diode generator and spectrum analyser, it appears to be peaked at 156 Mhz. I want it tuned for 144 to 146, to try to limit out of band noise I think is upsetting my radio.
There are 4 plastic standoffs with holes in, which must have done something. The manufacturers would not have fitted them for no reason. I suspect some kind of inter stage coupling loop, but whether it is one length, or to separate pieces I do not know.
Does anybody have any information, or pictures of the internals of this device?

[Radio Wrangler]

It is a "Helical filter"

The coils are not connected at the top end and are self resonant within their cavity. The cavity with helix has a lower resonance frequency than the cavity alone would give. Tuning screws add an adjustable C at the hot end for fine trimming. The PTFE bits can be used to trim coupling through the apertures in a fairly fine way.

The input and output are to taps low down on the end helices.

David

[dsergeant]

Something like this (reviewed in December RadCom)?
https://www.sotabeams.co.uk/bandpass-filter-for-2m/
Can't help you with your filter though.

73 Dave G3YMC

[Radio Wrangler]

I don't know. Sotabeams don't show the innards of their box. A helical filter would be a good choice of the way to realise a filter with that spec. The 5W rating would fit with the size. There are other methods. It's still a bit small, judging for the relative size from the SMA connectors for interdigitated filter on even a high Er PCB. Ceramics are another possibility. The cost also needs to be low. It seems a bit too good for plain old LC style.

The use of slow wave structures means that cavity sizes can be rather small for the frequency involved.

Toko made some PCB mount helical filters for 2m and for 70cm as well as for a few other services.

Anyway, sidspop's filter is definitely a helical, we do get to see the innards.

David

[G6Tanuki]

They were typically used at shared antenna mast sites to protect receivers against overload from the transmitters on the same mast. Yes it's a helical filter, just what you need if you have a Kilowatt of pager transmitters blasting away a couple of Megahertz from your receive frequency.

Aligning them can be tricky, everything seems to affect everything else!!

[Jon_G4MDC]

It's been got at.

Originally the PTFE standoffs supported the ends of the coils. Somebody has tried to lower frequency by compressing the turns.

What it really needs is more turns on coils of the original length.

[Radio Wrangler]

Originally Posted by G6Tanuki

Aligning them can be tricky, everything seems to affect everything else!!

Exactly right. Everything does indeed affect everything else. Tricky as well, so to align them, you need to know the trick. Short all the resonators except one. Use very light coupling. Adjust to the geometric centre frequency. Do all the resonators. If you don't get the designed shape, then the coupling factors are off.

David

[dodgy-dxer]

I had an idea that you might know!



Fred

Originally Posted by Jon_G4MDC

It's been got at.

Originally the PTFE standoffs supported the ends of the coils. Somebody has tried to lower frequency by compressing the turns.

What it really needs is more turns on coils of the original length.

[Jon_G4MDC]

I might know a little bit ;-)

Here is a spec sheet for 165MHz versions and a picture of the HP8505 rig which was used to tune them up.

[Cruisin Marine]

Because the coils have been compressed the aperture for coupling has also been reduced, the only way to get it working properly on 2 metres is to wind new coils with more turns on.

[Jon_G4MDC]

I think there might be another method but it does need the coils to be returned to the original shape more or less.

As helical resonators they must have been almost exactly on the resonant frequency, as they were wound. Just a little high. The effect of an M6 tuning screw entering down the middle of a 30mm diameter coil would be very small.

If the diameter of the tuning screw could be beefed up by a disk or sleeve or some such (inside the box) 146 MHz might be in reach. Tuning stability will suffer however. And I'm not saying about what the effects on the couplings would be.

Somewhere I have a 50 MHz one (originally the lowest design frequency) which I made to tune to 29MHz by adding ceramic trimmers (they were precision beehives - expensive!).

[Cruisin Marine]

Padding the coils with much more C will work, but at the great expense of increased bandwidth, which defeats the point of the whole thing really.
Winding the inductors with more turns is the way to go really.
There should be enough physical space to do that.
Remember, those screws if placed inside the inductor will make the inductance lower and therefore INCREASE the resonant frequency, if done right it can give swing either way HF or LF.

[AdrianH]

A long time ago I used to work for Aerial facilities, since then they have had a few other names as they were taken over and taken over again.

Yes those coils have been compressed and the ends would have been going through the PTFE holes.

It was that long ago when I used to tune up systems I was using a Polyskop II.

I had thought the tuning screws down the centre added capacitance to the top of the coil and would lower the frequency. The coupling between sections was controlled by the height of the aluminium plates between the coils. The tuning screws on these would give limited change in the coupling. We used to have stacks of aluminium plates of various heights that we would try to get the required overall bandwidth required for the order.

The wire should be silver plated, although for a few percent in loss of Q it may not matter if copper is used if you can get the correct dia wire?

We never had to do much design work as that had been done by some graduates at the the time at Bradford Uni I believe.

Think my memory is correct but I am thinking back some 40 plus years.

Adrian

[Radio Wrangler]

Design formulae and some nomographs for helical filters are in Zverev.

There are procedures to go from the normalised tables of "3dB down k and q' to patterns of coupling factors and end-stage Qs. I found the coupling iris size to be more empirical than mathematical, though.

David

[Cruisin Marine]

Here is some great info. on Gerald David and Aerial Facilities and how the company was built around Worthington Beer Barrel cavities https://www.ingenia.org.uk/Ingenia/I...vity-resonator

[g4wim_tim]

In a previous job circa 1981 we had a bunch of those beer barrell cavities to combine a bunch of tx's into a single antenna operating circa 400MHz. As I recall each one had about 50kHz bandwidth and were surprisingly stable.

[Wendymott]

Hi Peeps. Reading the above I thought I may add my "two pennorth" . I acquired an Alinco DR150 a few years back, whos front end resembled a very wide barn door. Living in Huddersfield there is a local FM transmitter that cares not for Alinco DR150's. I was at the Rally in Blackpool that year and spied a trader with a very large 4 coiled 2 metre bandpass filter, who ... very kindly had a perspex front fitted. I studied and looked and studied some more, until I had the dimensions fixed in my brain. I acquired some 7mm copper tube and the results you can see in the photo. Not the nicest build, but "by eck" it did the job. To tune it up, I made a 2 metre sweep oscillator , and the rest, and the FM station were history. The tuning caps were 2 pence pieces. It never realised its power potential, local ragchews dont need 50 W