Grounded grid VHF front-end

That is very true. I have a 60 year old tube receiver and up to 10 MHz I can hear any signal on it as I can on my year old Icom 7610. However the modern receiver does much better selecting a signal when there are many signals on the band. My 50 year old Heathkit tube and transistor receivers work just as well up to 30 MHz as the Icom as far as hearing a weak signal if there are no strong stations near by.

It does not do much good to have a receiver with a sensitivity of .1 uV when there is a noise of 1 uV all aound you if not more.

With all the man made noise in many locations 50 MHz is getting to be very noisey and 150 MHz is starting to get that way. At one time I had a modem that put out a signal that was strong enough to lock up my scanning receiver on one of the 2 meter repeaters I used. I was out in the country and not much around to give problesm but my own house.

Remarkably commutating switch mixer like diode rings etc can be made almost lossless if the source and load impedances are manipulated right. British ham Peter Martinez the inventor of varicode psk31 has shown that if the mixer RF port is fed from a parallel tuned circuit and IF port loaded with a series tuned circuit in the right ratio then the 6dB loss always assumed to be unavoidable goes.

piglet

Or a parallel tank with taps. The Q killers are the radiation resistance of the antenna and, a little bit, the ohmic component of the grid impedance from electrons being ornery.

A cheap MMIC from MiniCircuits will get down to 1 dB, and it's 4 pins, in out 2 grounds, fabulously simple.

I use their MMICS in time domain, as pulse amplifiers. Most - not all

- work fine down to DC. As is some Federal law with RF parts, the DC specs are absurd so you have to characterize them yourself.

If a mixer is just made of switches (diodes, phemts, relays) there is no loss mechanism.

You do get sum and difference outputs, but any single mixer does that.

The point I was making about grounded-grid operation is that the input impedance of the valve is very nearly the characteristic impedance of the co-ax (voltage ratio 3:2 for a triode-strapped EF91 drawing 6mA from a 200V HT line). A Pi network or a 3:2 winding on a ferrite core could be used to match them

With some VHF triodes, the gm is around 13 mA/V so the ratio is 1:1 without any additional matching componments and you can join the cathode through an isolation capacitor directly to the aerial socket, if you don't mind the lack of selectivity..

If you want selectivity, insert a series-tuned circuit which will be high impedance to unwanted frequencies and low impedance to the wanted ones. The input impedance of the valve and the characteristic impedance of the feeder and dipole appear in series as the resistive component of the series tuned circuit, so the lower they are, the higher will be the 'Q'.

Another possibility is to use two EF91s in parallel to bring the impedance down to 80 ohms. The phasing errors between different valves is unlikely to be significant at 144 Mc/s.

Why?

Sounds like a band pass filter you're describing here, mon cher piglet.

Arduinos are very cheap and don't have all the overhead the Pi has in terms of video capabilities. Plus they're piss-easy to program. A Pi 5 plus accessories for 120 bux doesn't sound bad at all, though. Or if you're really that much of a skinflint, you could consider PIC programming. :-)

A tuned circuit into the grid has voltage gain, but the grounded-grid with ohmic matched impedance throws away at least half the available signal voltage. Impedance matching isn't good when it throws away signal.

It's the voltage difference between the grid and cathode that gets amplified against the tube's inherent noise.

Of course you can never get a better s/n than what the antenna provides, and that will be pretty bad, so working hard to get a very low noise fig in a HF receiver is entertaining but not terribly useful.

At some wavelengths, in the microwave, looking at things way overhead with a very directional antenna, low noise figures are worth the hassle. The effective temperature of the universe is low.

A very directional antenna is a big win on s/n. It ignores a lot of junk. I don't think it improves the inherent thermal background if it's receiving terrestrial transmitters. That would violate COE.

I wonder if one can tell the difference in thermal noise by aiming an antenna north or south from the USA or Europe. It's certainly less aiming up.

I guess a good antenna feeding a matched resistive load will heat up the load; steal power from the BBC. Or aim up and cool it.

Under 5Mhz is where it gets particularly bad. 80m is often unusable.

Again, a very directional antenna will help a lot. I guess that's tricky at 80m, but some sort of phased array with signal processing would be interesting.

Hasn't cell phones and the internet made ham radio and shortwave listening mostly obsolete? Are kids becoming hams?

It's about power noise and power gain, not voltage noise and voltage gain., a tuned circuit doesn't have power gain. At lower impedances the noise voltage and the signal voltage are reduced equally so the S/N power ratio isn't altered. Matching the feeder and aerial impedance to the circuit impedance is what matters, that wsy you get maximum signal power transfer.

Grounded grid circuits were common above 150 Mc/s and gave good S/N ratios, the reason they weren't much used below that frequency was because specialised valves in conventional circuits could do the job adequately. I am trying to push a general purpose valve to do the job of a specialised one. (Like a BC109 being used as a VHF oscillator - it can work if you get it right!)

Ham radio seems to be alive and well AFAIK, despite very few kids getting into it currently. Curiously, Morse code is still extremely popular and has very many adherents, some of whom spend a small fortune on fancy keys. Speeds have gone up massively over the last half century and I struggle to copy much of what's being sent nowadays it's become so fast.

In message <1r2rj8l.msi28f14weovyN% snipped-for-privacy@poppyrecords.invalid.invalid>, Liz Tuddenham snipped-for-privacy@poppyrecords.invalid.invalid> writes

I built a 2metre down converter in 1971. It used an E88CC ( gold pins!) cascode grounded grid front end followed by an ECC81 mixer. Another ECC81 was used as xtal Local Oscillator/multiplier.

I still have it.

The design was in UK publication Practical Wireless. I had quick look on World Radio History to see if I could find it , but no joy. Similar designs might be available.

Brian

I've found one similar to your description in the VHF/UHF Manual by Jessop, published by the RSGB. Knowing Practical Wireless, the version you saw was probably copied from there and published under the name of a notorious electronics 'guru' with a few component changes (not usually for the better).

No idea what you mean, Jan. I'm 100% against censorship!

I bought Raspberry Pi 1B when they appeared, but after that used Chinese alternatives. Orange Pi used to be cheap, most is more expensive now. But Orange Pi Zero 3 is reasonably priced and powerful enough for my purpose. You apparently want PC class machine, for this I want real PC. For light use mini-PCs may be enough and are quite cheap. I got one for equvalent of $70, 6GB RAM, dual core Celeron N3350, 64 GB solid state disc, 2 USB 3.0 slots (+ 2 USB 2.0), LAN, Wifi, of course in case and with included power supply. For me important advantage is that there is no fan (passive cooling only). Less powerful used mini-PCs can be as cheap as equivalent of $5.

Supposedly some "TV boxes" are cheap, resonably powerful and can be programmed with Linux. But I did not try one.

Pi-s are better for electronics/automation thanks to available interfaces, but that needs much less compute power (camera is the only high bandwidth interface that I use). When you are satified with lower compute power there are some cheap ones. I am trying now Milkv Duo. Radxa ROCK also seem to be reasonably priced. But once you want faster CPU, more RAM, EMMC, etc they are getting more expensive. I am not sure why, memory modules for PC seem to be cheaper than price of adding memory to SBC-s (possibly this is just pure marketing).

I mostly depend on storing data on multiple HDD-s (my PC have mirrored pair of discs and I have extra discs for backup). In last several years I did not burn any DVD-s, but maybe I will do some with importand data for extra safety (DVD are too small for bulk data).

I depend on data stored on HDD, most is fetched from Internert but things vanish randomly from the net and I have my own indices of interesting data, so I normally use local copy from my disk. Also, have some compute intensive stuff.

USB can do milliseconds, ethernet hundreds of microseconds, small micros can do much better. Theoretically with a micro connected via USB one can synchronize clocks of the micro and PC with microsecond accuracy, I plan to try this but do not know how this will work.

Nice. I have avoided PICs, using now mostly STM32 and coding in C. One can create quite small and efficient programs in C. I use assembler when I feel it is better but currently that is mainly for delay loop. Doing all in efficient assembler would be large effort for moderate gain (maybe 20% efficiency/size improvement), and IME "easy assembler" tend to be less efficient than C.

I dislike bloat but OTOH thanks to bloat powerful PC-s are available at affordable price. Otherwise they would be an expensive industry/ corporate items.