Cavity resonator - construction?

[FuZZ1L0G1C]

I found this hand-drawn sketch from several decades ago, which was given to me by a TV repair technician.

The resonator was intended as a "notch" or "narrow bandpass" filter to receive only one terrestial TV station (in those days, I wanted to try pick up a weak TV broadcast, BOP TV).

As it turned out, I never got around to building the resonator, as BOP ceased transmission.

However, the idea now is to use such a device for receiving (DX'ing) VHF AM transmissions (Air bands being the goal), so around 118-130 MHz or so.

What is not clear in his sketch is how the input and output are coupled to the cavity.

Would this require coax, with the center insulation being the "feed-through" spacer, or just a small hole, signal conductor centralized through the hole?
In the cavity itself, do the 'antennas' have to loop, as seen in sketches of cyclotron 'pickup' probes, or are they just straight wires?

How would I calculate dimensions for say, 132 MHz?

I remember he said the 'coin' depth was to fine-tune the resonance.

Thank you. C.

 

[davenn]

However, the idea now is to use such a device for receiving (DX'ing) VHF AM transmissions (Air bands being the goal), so around 118-130 MHz or so.

What is not clear in his sketch is how the input and output are coupled to the cavity.

Would this require coax, with the center insulation being the "feed-through" spacer, or just a small hole, signal conductor centralized through the hole?
In the cavity itself, do the 'antennas' have to loop, as seen in sketches of cyclotron 'pickup' probes, or are they just straight wires?

How would I calculate dimensions for say, 132 MHz?

for 120 - 130MHz odd the cavity is going to be BIG. around 0.5 metre long and around 100mm diameter eg one for 144MHz

those dimensions seem smaller than I remember seeing at repeater sites, but it will give you something to go by

 

[FuZZ1L0G1C]

Thanks for the prompt reply, Dave.

Now...
In your supplied PA2MRX schematic, the signal conductors from (BNC?) connectors are drawn as if both shorted to ground, on left.
Is this correct?
If so, then is BP resonance controlled by adjusting the air gap between signal and ground (if 6mm top shaft-bush is conductive)?
Looking at how big the CuZn (?Copper-Zinc?) cavity is, I'll first verify that material is obtainable.
Even if I don't build the VHF resonator, I'll Google further on cavity resonators, for interest..
1 mm "dik" CuZn - 1 mm thick?

 

[duke37]

Cu/Zn is brass.
There is coupling between the input and output wires via the resonating central rod which is tuned by the variable capacitor at the end.

The size can be reduced with some loss of Q by changing the central straight rod to a helical coil.
Constuction from copper would be a bit better than brass but silver plating would be best.

Copper sheet is available, I would use copper from a leaking hot water cylinder. Bend the sheet round a wooden dowell and solder the edges to make a tube. The current runs up and down the tube so the solder connection will not reduce the Q significantly. The bottom and top connections will be more important and little solder should be visible.

 

[davenn]

In your supplied PA2MRX schematic, the signal conductors from (BNC?) connectors

BNC could be used. those used by him would more likely be N type

drawn as if both shorted to ground, on left.
Is this correct?

Yes, DC short, but isn't a short at RF frequencies ... the 2 L shaped lengths will be an electrical 1/4 wave from memory

If so, then is BP resonance controlled by adjusting the air gap between signal and ground (if 6mm top shaft-bush is conductive)?

Yes

Looking at how big the CuZn (?Copper-Zinc?) cavity is, I'll first verify that material is obtainable.

Cu/Zn is brass.

Yes

 

[hevans1944]

MMMM! Resonate cavities at frequencies reachable by ordinary amateurs not having microwave-deep pockets. What's not to like, except maybe the unfamiliar theory-of-operation, where circuits appear to be dead shorts or grounded but really are not. I recall seeing the Dayton Amateur Radio Association (DARA) repeater traps installed on a local FM radio station tower that the tower owners kindly shared with the club. However... large signs shouted: Hands off! Do not touch the tuning adjustments! They must have been a real bear to get adjusted properly, but once set they seemed to stay that way for many years.

I never got involved with that, but from what I have read, silver plating greatly improves the Q of this type of filter. Brush-electrode silver-plating kits have been available for many years that work just fine for this purpose, as well as obvious jewelry applications. Silver plating appears to work quite well at lower frequencies, too, minimizing parasitic I²R power losses in loading coils and other resonate tank circuits. You might want to invest in a silver plating brush kit if your notch filter doesn't have quite enough Q for whatever it is you are doing.

Dimensions are critical for this kind of work. Make sure nothing wobbles when you are finished assembling.

 

[FuZZ1L0G1C]

Thanks for info.
Plating is no problem, as there are numerous businesses around here that offer this service.
I'll bribe our local plumber/electrician to sell me an old, discarded geyser with copper vessel.
Or sheeting may be available at a metallurgist.
There may even be ham / waveguide dealers locally - will research suppliers via the web.

 

[hevans1944]

It is my understanding that commercial silver plating is a dip-process involving cyanide-based chemistry, heated tanks, and "trade secret" recipes that is not at all suitable to the thin layer of "spot" silver plating required for radio frequency conduction enhancement, which only requires a "skin depth" layer of plating. Most commercial shops here in the USA won't even talk to you because it isn't worth their time. The actual skin depth for RF is a function of radio frequency, decreasing with increasing frequency. Also, skin depth is not a "brick wall" boundary but instead is a continuously varying resistivity that is a non-linear function of depth. Very little is gained by plating excessively thick layers of silver. Hence my recommendation that you try using inexpensive brush plating systems first.

Soldering up, and then plating thin copper sheeting with silver, is probably the least expensive way to learn about tuned RF cavities. Start small and work your way down in frequency to larger resonant cavities. I would avoid trying to excite cavities with magnetrons salvaged from microwave ovens until you have more hands-on experience at lower power levels and lower frequencies. As always, avoid exposing your body to intense near-field radiation that heats (and destroys) tissue without your realizing the damage that is occurring. Have fun with this!