EMRFD Message Archive 6381

Message Date From Subject
6381 2011-06-15 09:40:35 wb8yyy_curt PA Circuit Enclosed in Altoids Tin ?
My question is about air circulation for locating a 2N3866 PA circuit inside an altoids tin. To hopefully cure the VFO isolation problem on my ugly weekender transmitter, my thought is to move the PA/driver/bias switch circuitry all inside a tin, and then place this tin inside the existing enclosure. Knowing the PA is only cooled by convection from its attached free-standing heatsink, should I aim air circulation holes and will these circumvent the isolation I am trying to achieve?

I did try a couple vertical shields. The first one was a wall separating the PA and its matching network. This slightly increased the 'error' in the VFO spot. Then remembering that ferrite coils can be especially susceptible I added a second wall between the driver circuit and the VFO. This merely further increased the VFO spot error! Perhaps if nothing else this tends to verify the leakage is electromagnetically coupled and not conducted in wires via bias and controls.

Another tidbit on this PA circuit - I discovered it was later applied to a NJQRP kit called the Fireball 40. I did look curiously at the tightly packed 'ultimate transceiver' wondering how its VFO is shielded. In my case the VFO is a lot of stuff as I used a pair of metal variable capacitors. Starting from scratch one might make a much smaller VFO using varactors and ease the implementation of shielding. But my project has a 'historical preservation' aspect to it. The VFO now with NPO's is reasonable stable so I sense it is worth the effort to use it.

73 Curt
6382 2011-06-15 11:40:58 Wes Re: PA Circuit Enclosed in Altoids Tin ?
Hi Curt, and gang,

Thanks for the update on your experiments.

The Altoids tin would probably work, although only if done with care. I would recommend that you put the 2N3866 inside the can, along with the related output network and even driver, and then put a coax output connector that goes through the wall of the tin AND through the wall of the rig to the outside world. Heat sink the 3866 to the wall of the Altoids tin that then connects to the rest of the metal. This will require some sort of electrical insulation.

It's interesting to look at this very old Ugly design and compare it with the things that folks are doing today with SMT. RF power amplifiers are certainly built with SMT components. The key to handling the heat is to attach the device collectors (or drains) to moderately large surface board areas. I've built some transmitters that are very much like the original weekender that use parallel SOT-89 SMT parts. A couple of square inches of circuit board area is enough of a heat sink to keep the transistors cool, or at least cool enought that they can be touched. The capacitance is not an issue, for it is absorbed within the output network.

The vertical shields that we used in our vacuum tube rigs in earlier times rarely do much good in the low impedance world of solid state electronics. The vertical shields worked because the coupling mechanism from one stage to the next was simple capacitance. If you put a grounded shield between two "hot" points, the electric field lines ended up running between the hot points and ground. The hot-to-hot path is destroyed, or greatly attenuated. The coupling mechanism is different with typical solid state electronics. Rather than being capacitive, it is usually by way of shared current. In the case of the Ugly Weekender, the PA (Power Amp) causes significant current to flow in the ground foil around the PA transistor. The output network contributes to this current. If some of that current also flows around the VFO, there may be problems. Putting the VFO in a box separated from the PA forces the current from the PA to flow on the outside of the VFO enclosure, which is usually well isolated from the inside that contains the VFO tank.

It's amazing how many holes you can drill in an enclosure without causing a shielding problem. The holes must be small with regard to a wavelength, but hey, this is a 40 meter rig so it's really easy to satisfy that criterion. A small hole could be a problem if there was a wire or cable going through the hole that shorted "ground" on the outside to "ground" on the inside. But the holes themselves are not usually a problem. I often reuse aluminum boxes, often many times.

This is an interesting problem that really emphasizes some fundamental concepts. Careful measurements and observations will be the key.

It is not important that the two boxes worked for us when the UW was originally built. What is important are the reasons why they worked. Avoid lore at all cost, or if you must resort to the applicati
6383 2011-06-15 17:13:44 wb8yyy_curt Re: PA Circuit Enclosed in Altoids Tin ?

Thanks for the detailed explanation on the EM problem that lurks amongst the circuit problem - and it explains the mystery I sensed when RF showed no sign of traveling 'in the air.' Also the thermal matter deserves attention as I sensed. So I will work the design concept before I do any soldering, which includes refining the enclosure concept and also considering other PA options - for example a pair of BS170 MOSFETs can be attached to a large enough heat sink without electrical insulation. Plus I need to test this Hybrid Cascode amplifier I ugly constructed (very carefully that is). I was going to put it into the receiver, but now I am sensing I really should test it individually - which I can do with a xtal oscillator and the ancient scope here - yes you would recognize the 543A.

73 Curt