Untuned antenna preamplifier with EF80 pentodes? - Page 1
Forum section: electronics / rf-microwave

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 24, 2020, 05:16:16 pm
content: I have a few parts that I have accumulated over a couple years that I simply cant find much use for.The EF80 is basically a UHF pentode and I dont have any equipment that could benefit these tubes.Being a UHF pentode I guess they might have quite a lot of high frequency gain and by high frequency I mean broadcast AM radio from 2000m up to 16m band.This sounds all simple but lack of knowledge on my side about RF stuff let alone tube RF stuff is the big limitation.If anyone can help me out here I will be gratefull. Also I expect that I will have to wind my own coils so I am prepared for that very relaxing wire winding session.Technical parameters available :voltage B+ 290VDCfrequency range from 2000m to 16mGain: As much as I can have without distortion..preferably variable gainThe purpose of this antenna amplifier would be to sit inside one of my old radio recievers and really boost its performance quite a bit so I can just sit down when I have time and coast trough the bands maybe recieve something from quite the distance and so on..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2020, 07:20:57 pm
content: Pretty straightforward if you have a well defined system impedance. Transform 50 ohms up to whatever the grid needs (it'll be a few kohms), bias it at average conditions (say 10-15mA; 290V is kind of a lot but you could drop it down with a zener, or resistor if you don't need variable bias for AGC), and transform the plate back down, with reflected load impedance suited for either bandwidth, power or gain.Short form data says 7.5pF input, 3.3pF output, and 10-15kohm input resistance at 50MHz. So, that's cold capacitance, but it's higher than that due to space charge. This is not well documented for some reason, but it makes sense, the space charge has mass and that mass interacts with the grid voltage as an equivalent capacitance. The resistance is due to the space charge of the beam current, effectively the work done in controlling the flow.I would estimate 13pF input equivalent under bias. (Which has a reactance of 244 ohms at 50MHz, meaning a Q of ~50, not bad at all. You could run this at scary high gain -- stability limits notwithstanding -- but the bandwidth will be quite small.)You have a range of choices. Full bandwidth requires Zg1 = Xg1 at cutoff, i.e., for 20MHz BW you need R = 1 / (2*pi*(20MHz)*(13pF)) = 612 ohms. From a 50 ohm system, that needs a transformer with 1:3.5 turns ratio, give or take. Which is pretty reasonable, actually (super high impedances aren't realizable, but ~2x impedance-of-free-space isn't crazy).Likewise for the plate, 3.3pF plus strays, call it 5pF, the bandwidth limits you to a maximum load of 1.6kohms (or a 5.65:1 turns ratio).At 7mS gain, you'll get 11.2 voltage gain (grid to plate), or overall about 7x overall  gain (50 to 50 ohms). And, let's see, 7.07 would be about 10/sqrt(2), so that's 37dB overall gain. Not bad.You can get more gain with narrower bandwidth, but then you will need switchable or pluggable coils to cover the whole band. The maximum gain condition is matched input resistance (grid resistance at 20MHz and below will be quite high indeed), and output resistance matched to plate resistance (100s kohms). At some point, feedback capacitance takes over and you get a tuned-plate, tuned-grid oscillator; neutralization can extend maximum stable gain a bit further, and also some other things I don't remember about maximum stable gain, the definition of unconditional stability and so on. In any case, because of limitations of physical coils and stability, you probably won't get anywhere near this gain, but it would also be pretty ridiculous gain (like 80dB in a single stage..?) so who cares.Also, you can push the band edges with peaking networks (the narrow band version being double tuned resonators, or even higher order networks), which improves bandwidth marginally (about doubling it, if you don't mind the flatness or group delay too much, which, radio work usually isn't picky about), though the extra phase shift also reduces stability, so maximum GBW doesn't go up by quite as much.Probably, super high gain wouldn't be very useful to you, so a tee coil each for the grid and plate circuits to push the GBW slightly higher at say 800-1000 ohms grid circuit and 2-2.5k plate would do to guild the lilly. Or you might not even bother with that, and use somewhat lower impedances just to get the bandwidth easy with less compromise on dynamic range (due to the high gain). At some point, pentode noise should be considered as well (however much that is).The real question is: are all your antennas 50 ohms matched or whatever, or can they be? Or are they, just, whatever the hell they are -- and that's going to be your real challenge? Electrically small antennas have impedances very different from Zo and I'm just going to guess you aren't the proud owner of a 500m tower.(Not that I would be shocked if that guess is wrong, there are some crazy hams out there!)The preamp probably doesn't matter all that much anyway..? Atmospheric noise is pretty strong below 20MHz. I guess, depending on time of day and year, and varying with frequency, a noise factor of 20dB or more isn't even a big deal? If your radios happen to lack overall gain, it would help out though, yeah.Tim

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 24, 2020, 08:49:37 pm
content: Well Tim, you never dissapoint.I have two EF80s (and a couple more EF800s but id like to keepthem for my old VTVM). A two stage amplifier would be fun too. I am really looking for frequencies up to 20MHz not really more cause I dont have a CB or a reciever of any kind that could do above that (yet).37dB does sound a lot damn. You assumed a single stage amplifier but yeah I have two of those tubes. Antenna to tube into another tube and then the reciever itself. I guess I could go with a tuned preselector at that point too but I would like to avoid those big variable capacitors. I do have an old spare AM radio that is basically dead to me since there is no documentation on the internet (and really all components turned into fossils inside) so I could sacrifice that.As for the antenna...weelll.. You werent all that far off with that anythings an antenna logic. I do plan on building 50 and 75 ohm antennas in the future. (Matching antennas with toroidal transformers and so on and actually build a 40 meter long antenna for proper recieve.....).I do have a question if I could use ferrite cores from old computer power supplies and wind my own stuff on those (I have a lot of enameled wire...A LOT).Maybe I could do with multiple band selection. Build speciffic coils for speciffic requency or just do a multitap and switch those.All I have is an LCR meter, lots of ferrite cores, lots of enameled wire, two EF80s and a load of time to wind those things...The radio I own wasnt really built for extreme distance recieving since it was built in the soviet era.. The soviets didnt want you to listen to open broadcasts of somwhere elsewhere out of the union..the biggest crime back in the day was owning a CW transmitter. It has the potential it just needs an insane antenna.So if you could perhaps help out give some inductance values for the coils I would be gratefull.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 24, 2020, 11:49:43 pm
content: Here's a quick schematic how I am envisioning this and also so I am not making stuff up so we can point to exact things on a schematic.I thought about using the second pentode as a cathode follower for low impedance output with a regular oh I don't know 100uH coil in the cathode and that would then be coupled with a capacitor to the antenna input of the reciever.. However I find this pointless cause I can just wind the transformer and it would give me the same impedance anyways maybe even more optimal and have shitloads of gain with that second stage too.My only concern is winding those transformers. If I could find those on toroidal ferrite coils from old PC power supplies (which I have a lot of) then I need an inductance value for at least one winding and then I could figure out the amounth of turns on the other side by simply calculating it based on what ratio I need. It would be much neater on those toroids.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2020, 01:27:32 am
content: Do you know that you would need 74dB of gain?That will definitely overload on a lot of signals, say if you have a nearby AM transmitter and any kind of bandwidth in your antenna.It's probably a good idea to have matching suited to each antenna, and treat the antenna-network system as its own filter. It in turn presents some impedance to your amplifier, being near 50 ohms in the passband but who knows what elsewhere.You may consider bandpassing them to ensure no accidental passbands outside of what's intended. Which, because filters also depend on terminating impedances, should have damping, or use a diplexer design, or something like that, to ensure behavior. This should all be confirmed with a network analyzer of course (which, at these frequencies, is easily enough done by hand with a sweep generator and scope).Accidental passbands are a funny thing, for example this network I built on a 20m subminiature set (in terms of the tubes used, not its overall size as you can see!),is a tapped coil (autoformer) into the 5702 grid. A somewhat lengthy link to a tuning capacitor sets the center frequency; this link would be series resonant near the tube's cutoff frequency so a ferrite bead puts an end to that (without, it oscillated ca. 400MHz). The grid is grounded through 10k, the dominant load impedance at this frequency (the internal grid resistance is probably 100k).Anyway, the interesting thing about this is it's unexpectedly wideband. This must be due to leakage inductance serving as an accidental matching or peaking (though it doesn't look peaked; it's quite broad and soft, implying low Q). It's not due to self-resonant modes of the coil; it's too small for that, those are 100MHz+.Probably with some tweaking, I'd be able to determine what's responsible for that passband, and, well in this case tweak them out since the extra bandwidth allows images in this receiver, but also optimize them for a flat passband when that is desirable.So that's interesting, and potentially applicable here; though not really very useful I'm afraid, as the starting point is literally a tapped coil in a photo...Also somewhat relevant is this,This is an FM radio front end, 88-108MHz with the coils shown, and a 33MHz IF output. 6688/E180F preamp, 6J6 mixer, 6C4 osc. The input is a double tuned coil, the input being series tuned to match the low (50 ohm) impedance, the second parallel tuned to match the grid (which is about 500-1000 ohms resistance at this frequency). Plate coil is the larger red hoop in the middle, over the mixer's grid coil (which is balanced).This is a pretty canonical design, being made of coupled resonators and known impedances; the difficulty I had with it is, the preamp only seems to be doing 7 or 10dB gain, hardly even worth it. The grid capacitance was markedly higher than the datasheet says (including bias condition; yes, one of the few tubes to fully specify this!), and I guess it's like transconductance is just lower than expected. That or the plate load impedance is too low (I don't remember offhand how much capacitance -- and thus GBW -- there was to spare there). Could also be parasitics like stray inductance (this is in a regular vertical socket?), or the cathode degeneration (though it's the recommended 10 ohms -- this affects grid impedance, by the way; something about its flatness with respect to bias or frequency).Also, the coils are pluggable; I currently have a set for, hmm, think I was still working on the 120MHz set (aviation comm) and didn't start the 144MHz (2m) set? -- anyway, that was a motivation in this design. Critically, I also made a set of 'null' coils: wideband (or wide-ish, anyway), controlled impedance (50/100 ohms), and also normal/common/differential mode for the mixer (so I can check LO feedthru and diff performance separately -- incidentally, the LO is delivering almost 0.25W here, it's quite overkill). This way I can directly measure the insertion gain and filtering of each coil.Anyway, you'd want neither of these, exactly; the first example is closer, fitted with a ferrite core to vastly increase inductance, extending the LF cutoff.SMPS cores are okay, but mind which kind you have. There are three or four types:- Line filter, some ferrite beads: high mu, low cutoff frequency. Good for pulse transformers. Kinda mediocre here, but would suffice.- Power transformers: medium mu (~2000?), medium-low cutoff frequency. Good for power transformers, obviously; not as good for pulse transformers because the LF cutoff won't be as great. Mediocre here; the large size somewhat makes up for the low inductance, but then again, the large size keeps inductivity (uH/t^2) kind of low. Gapped cores are out (flyback transformers, switching chokes).- Choke cores: low mu (~60), low to medium cutoff frequency. Good for energy storage, shite for signal transformers. Many types: gapped ferrite shapes, powdered iron toroids, powder alloy toroids, sometimes powder shapes also. Composite molded cores too, these days (most common in PoL converters).- Other ferrite beads: medium mu, medium to low cutoff frequency. Can be good for RF and pulse transformers, when the cross section is large. Thick cable beads for example. Nice is they can be small, which keeps wire length short, even if the TL impedance isn't especially great.Best are pot cores, particularly in wideband materials. The thick cross section ensures high impedance, requiring a minimum number of turns. Here's a 4W linear using a few:Circuit is set up as a pair of small preamps (cascode + follower), with the spooled transmission line and the (equal length) line-on-bead delivering complementary inputs. (The beads happen to be hi-mu material, so have excellent impedance, even if it's dropping off a bit beyond 20MHz.) Followers drive IRF510s (curled around the bottom edge). The drains are tied with two transformers: one for enforcing balance (a TLT wired as a CT choke, also feeding supply power), one for output (a step-up balun, matching the, I think 12.5 ohm? drain-drain load to the 50 ohm output). Think the transformers are like 4 turns, but they're good for 10 or 20V at, hmm, I forget what the LF cutoff on this was, 100kHz? Also used pot cores on my distributed amplifier, which I don't have a closeup of, but you can see the same 22mm size pot core at the input (right board, top-left corner), and a bigger one (bottom right) for output, delivering over 20W at 100kHz-27MHz:https://www.seventransistorlabs.com/Images/DistAmp2.jpgRegarding your schematic: the transformers can be autoformers; well, the interstage can be isolating, but it can also be wired in parafeed (use a generously sized choke to supply plate current, then a coupling cap to the transformer) so that a more optimal TLT can be used. (Not that optimal TLTs will be a huge priority at these frequencies.)On that note, the extra space on a ferrite power transformer may prove helpful. You can space the windings apart (from adjacent turns and from the neighboring winding), keeping the transmission line impedance high (imagine you were winding 600 ohm ladder line, aye?). The large physical size sucks because you simply need more wire length (= transmission line length), and that limits performance. It can well be better to use a smaller core, and more turns, but still end up using many times less wire and therefore get comparable or better performance.The interstage needs a load resistor, otherwise plate resistance and grid resistance are the only termination and gain will be poorly defined (meaning, not very flat, and perhaps prone to oscillation). Probably 1-10k would be effective. Can also be rolled into the bias network (e.g., 1.6k plate supply resistor instead of a parafeed choke; coupling cap to interstage).Also, if grid and plate circuit impedances are close enough anyway, there's not really much point in matching at all, and you might simply use a peaking network (if anything at all) between them. What's wrong with an audio amp style circuit if your audio accidentally happens to be 20MHz, and the amp works fine up there?Also, note these pentodes are quite happy to function at 400MHz or thereabouts, from whatever accidental parasitic path you've put in. And these are most often series-resonant tanks from stray wiring stubs, by themselves or against bulk capacitors (like the wire link and tuning cap in my first example). Up at those frequencies, socket and pin lengths matter, as does the electrode structure of the tube itself (one of the reasons VHF and UHF transmitter pentodes provide oodles of cathode/suppressor/shield pins, or the 5702 for example has all its leads in a row with the plate and grid the first and last respectively). Equivalent circuits start to get quite complicated and hard to anticipate up there, and damping them with resistance (stopper resistors, L||R snubbers, ferrite beads) can be a good idea.Which is really just to say, FB on one or both grids might be handy.Also mentioned that cathode degeneration resistance can be helpful, but it's with respect to grid impedance, and that's really only important at very high frequencies -- like I said, my receiver with the 6688 has a grid resistance of some 500ish ohms up there; yours will be 10s kohms at least, so it probably won't be noticeable. You can employ this to help reduce noise* and gain, I suppose. YMMV. In any case, that's just putting a resistor in series with the cathode bypass cap (a R || (R+C) bias network), or the cathode itself (a R + (R || C) network -- pretty much equivalent), nothing hard about that.*Although, if pentode noise is mainly due to partition noise, that won't be carried by the cathode, so degeneration won't do squat. Which is a good reason why, if you're going to use a pentode, you probably should be needing the maximum gain you can get from it. If you don't need so much gain, consider a triode. I suppose it could be triode-strapped, if you're heart-set on using the EF80s forsomething...Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: March 25, 2020, 02:23:47 am
content: 74dB of gain is a huge amount!The problem with wideband gain is thateverythingis amplified, so that you are hitting your poor, embattled mixer stage with the nearby 50Kw MF broadcaster, the grot from all your (& other people's) SMPS, & the hapless, fully legal Ham down the road.(who, will inevitably get blamed when things go "pear shaped!")You are talking about using it with "old receivers".Preumably these are domestic type multiband receivers, which may, or may not, have a tuned RF stage.If the former, you have a degree of protection, if the latter, the input tuned circuit selectivity will definitely be overwhelmed .Untuned RF preamps are used, but normally only on what are very inefficient antennas like "car radio" typetelescopiic whips,& even ten, are mainly impedance matching devices, with nowhere near the gain you are looking at.If you use that amplifier with a large antenna, I guarantee you will have problems.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 25, 2020, 08:23:02 pm
content: Well I dug trough my box of history and found a dual ganged tuning capWhat a miracle. Its really a 12 som odd puff up to 420 puff potato slicer. I wonder if this could be to any help.Most of the power supply ferrite toroids I found turn out to be 60-70-80 or thereabout.At this point I would really like to build this thing even if just for experimenting but yeah I would need some turns ratio or inductances and stuff. I really dont have the knowledge to do this all myself I would need at least a couple EXACT given values. Also if the schematic could be improved you can modify it to your will.

author: m3vuv
author_rank: Super Contributor
timestamp: March 26, 2020, 11:03:08 am
content: isnt it funny how hams get the blame for everything,the restaurant owner next door stopped me the other day and said my antenna was affecting his point of sale machines,its funny as i go on air most mornings before 8am for a net yet the resto doesent open untill 6pm,its a strange thing is rf!! m3vuv.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 26, 2020, 11:19:20 am
content: Quote from: m3vuv on March 26, 2020, 11:03:08 amI dont know how this is relevant, one solution is to have such a big transmitter that the person who is complaining will recieve a lightning from it.There arent any HAM people arround my area possibly not my country anymore. Its a dead thing these days at my location. Only remnance is the stuff that the 80 years old from my country posted on the internet and I dont even know if they are still alive.After all my country isnt really ham friendly because in the past soviet union the biggest crime you could ever do was to own a transmitter of any kind and so this hobby didnt really catch on

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 26, 2020, 11:36:50 am
content: Normally it's not even necessary to amplify signals up to about 20Mhz if your receiver is not deaf by design. If you can hear a static noise increase when you connect the antenna you don't need any amplification because the S/N ratio will not improve, you can only make things worse by adding the noise from the amplifier and overloading your receiver.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 26, 2020, 11:45:51 pm
content: Quote from: PA0PBZ on March 26, 2020, 11:36:50 amI dont understand why is it so difficult to build that amp.. Everyone telling me why it wont work only one knows how to actually do it but is not responding anymore well this place aint such a helping place.. More like a "dont bother why would you do it anyways" place.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 26, 2020, 11:46:40 pm
content: I wonder how many forums will I have to go trough to actually find someone who would help me do it instead of telling me why it wont work..

author: vk6zgo
author_rank: Super Contributor
timestamp: March 27, 2020, 03:52:21 am
content: Quote from: SK_Caterpilar_SK on March 26, 2020, 11:46:40 pmWhy on earth would we help you do something that is going to be ineffective, & in fact, harmful to your radio reception?You will not find any forums which will do that!There is an idea out there that "you learn by making mistakes".I've never believed in this approach---------if other people have the knowledge to save you the trouble of "making your own mistakes", listen to them, & learn.Finally, we are not "employed" by the forum to do your bidding--------we do this because we like to try to help, & will do so, as much as we can, but you cannot expect to conscript us into doing your bidding.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2020, 07:07:46 am
content: Not much of a mistake, trying something that merely doesn't work? Nothing's going to explode, or even cause milk to spill. The parts can be repurposed for the next project and so on, nothing lost really, just time.Not everyone has the inferential power to learn a skill entirely by book. What's left must be covered by experiment, in person....Also a correction, 7.07 is 17dB, not 37.So your two stage cascade would be 34dB, still quite a lot, but not... absurd.So, you might consider a bypass switch for the 2nd stage, to pull extra gain as needed. You might also now find it useful to have switchable or tunable networks, since for example a 2.8MHz bandwidth (1/7th of the original figure) could give about +17dB gain, equivalent to that added stage alone.If you expect to use gang tuning with a multisection cap, mind that you'll have to align that so they all track reasonably well. May be a PITA. Start small, make switchable/pluggable bands instead.Quote from: SK_Caterpilar_SK on March 25, 2020, 08:23:02 pmFor what, antennas? The amp seems perfectly feasible, either whole band, or in several bands. Sounds like plenty for matching the lower frequencies.Quote60---what?QuoteNeed much more info. What band plan do you want? What gain do you want?Exactly what parts do you have available? Don't know what they are? Measure them.Need to know how to measure them? What equipment do you have that could help? There are many circuits/jigs, setups and articles on how to do this, or it's worthy of its own thread (I think there are several already on exactly such a subject?).Dig down to the first and smallest step, figure it out, then recurse up the stack until you've solved the whole thing top to bottom, and all that...Reality of the forum being what it is -- I'm not particularly inclined to do, like, a full design for free -- not that that's what you're asking for, anyway -- but even if I did it would simply be using off-the-shelf components wherever possible. Which, I mean, if you don't mind buying, say, a few #43 toroids or binoc cores, that'll be a great help for anything else like this you're planning on doing, no need to qualify/quantify scrapped parts or work around their limitations.Tim

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 28, 2020, 12:18:58 am
content: Thank you Tim for your reply despite my frustrated last reply..Most of the ferrite cores have a "Relative magnetic permeability" ranging from 60µ up to 80µ. I definitely woul not mind buying some off the shelf parts but I would rather avoid if possible. I would like to use the loads of scrap I have but if it makes life easy then sure I will take the buy method. Just a small note - my source of components is TME and there are ferrite cores and bobbins and other stuff there so I could look for things over there.(all tho no ferrite toroids)I do have proper equipment to measure almost every electrical parameter possible.The bands I would need to cover would be :Band 1: 150kHz-395kHzBand 2: 520kHz-1650kHzBand 3: 1.95MHz-5.9MHzBand 4: 5.7MHz-18.45MHzIf I would have to switch multiple band circuits that would be fine aswell.. As for the gain I'd say 25-30dB range.For the parts I have the tubes some unidentified toroidal ferrite cores and all the rest (resistor and caps...).I understand that none of the forum member are paid to make a schematic for me or spend extra time above work which is unpaid. I cannot expect anyone to spend time on my silly ideas. I just a kind soul would be nice enough to help with it. If you anyone does not want to help me then fine..thats alright too I cant force anyone to do this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2020, 01:45:08 am
content: Oh, well those are no good then. Sure you don't have any meaty ferrite beads with higher mu?Not that they're expensive or anything, a few bucks will buy what you need. Just the bother of shipping, and if they do minimum orders you'll probably wait until you have other things to buy with it, so it may be a while before you finally get down to building something I guess?...Turns are driven by LF cutoff. This is where XL = RL, so L >= RL / (2 pi F), and you may want an excess to account for the roll-off of multiple stages. For example, 1.6kohm at 150kHz needs at least 1.7mH.On a typical power ferrite shape of say 1uH/t^2, you'd need 40 turns. And with a 5.65:1 ratio, 7 turns secondary (actually a 5.714:1 ratio, close enough eh?). Which is a fair amount of wire, enough that you'd probably not be able to meet the high band at the same time, i.e. available GBW will be reduced due to winding length.Maybe noteworthy that, lower mu cores (including power cores, gapped cores and powder) handle more magnetization (DC bias). Saturation can be calculated, given core parameters of course. You may yet find those low-mu cores useful, if they have reasonable impedance at high frequencies and can take advantage of the high saturation current. Although, 40 turns at 10mA is only 0.4 At, not much magnetization; most power cores need 5-10At to saturate, an order of magnitude in favor of supplying DC through the transformer without using a parafeed connection.And just to be perfectly clear, if it's not already -- what I mean by GBW is, the maximum available product of bandwidth and gain is approximately constant, as you vary the input and output impedances. Need more gain? Use less bandwidth. Need more bandwidth? Use less gain. Most importantly, the relationship isproportional.It's an incredibly useful approximation, easily understood from for example a simple Norton equivalent (consider the plate/collector/drain as an ideal current source, in parallel with the internal resistance and the electrode capacitance). You might wonder if the capacitance can be "resonated out", but in fact the HF cutoff will rise at exactly the same rate as the LF cutoff, so the maximum GBW remains constant, even for a tuned amplifier. The proof follows easily from this setup (granted, easy ascircuit analysisgoes).And it's a quite reasonably accurate model for tubes and BJTs.Notably, it doesn't always apply to MOSFETs. RF MOSFETs are quite nice, but general-purpose and power MOSFETs often have a diffusion characteristic, which means they don't follow a constant-GBW rule -- a 2N7002 might have 200MHz GBW when measured at 20MHz, but 400MHz when measured at 80MHz. That is to say, a gain of 20dB at 20MHz, and 12dB at 80MHz. Put another way, it drops off, but not as aggressively as a single-pole rolloff does.Anyway:Your bands 1-3 take up a tiny fraction of the bandwidth of band 4, so it's more likely you'd shoot for similar gain across bands, and use the same coils for bands 1-3, but divide band 4 into two or three segments. Or, I mean, you can do whatever you want -- equal bandwidth assumes equal gain, but if you need more gain in the low bands, that'd be a good reason to restrict them to narrower bandwidths. Or make band 4 with tracking tuning, and the others can be fixed/broadband because who cares.So, this should be enough info for you to set the band LF and HF corners, gain and bandwidth, and, at least the inductance and number of turns; then we can discuss what parts to use for them, whether you still need more gain (e.g., cascading two 15dB stages at nearly full bandwidth will get you there with no band selection at all, assuming transformers good enough; or single stage 30dB with the above band plan, give or take), and how to wind the transformers and such.Tim

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2020, 11:56:04 pm
content: Quote from: SK_Caterpilar_SK on March 26, 2020, 11:19:20 amAny of hundreds HAMs in your country would tell you: ".. a good antenna is the best amplifier .."PS: the placing a preamplifier in front of a shortwave receiver is generally a not good idea, as you will make the life of the first mixer much harder (overloading) - an overloaded mixer is something nobody wants to have in his/her receiver..Normally, on shortwaves the preamplifier is sometimes required (switched "on") above ~20MHz - ie. sometimes on 18MHz, 21MHz, 24MHz, and more frequently on 28MHz band (and almost always at 6m).For bands below ~15MHz you most probably would rather use an attenuator instead (especially in the urban areas)..

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 07, 2020, 11:24:28 pm
content: There are antennas with high impedance, as I recall there was one made with twin lead TV line which is 300+ ohms?? ( I really do not remember,maybe someone else does).Perhaps it would be easier to have the amplifier have a high impedance output and use one of these high impedance antennas?Just a thought.Wally KC9INK

author: iMo
author_rank: Super Contributor
timestamp: April 08, 2020, 12:25:44 am
content: A "random wire" antenna for the bands up to 18MHz may have 5ohm-10kohm impedance (it is a complex Z).You have to tune and match such an antenna to the input of the receiver (typically 50ohm) to get max out of it.Also the efficiency of shorter random length wire at lower bands is questionable.We do not know how Z is the input of OP's vintage radios (unless it is a vintage communication receiver the input impedance is rather random).Instead of the pre-amplifier he/she may build a tuned antenna (ie. an Loop) - it is directional and tuned and matched, thus much better choice, imho.https://en.wikipedia.org/wiki/Loop_antenna

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 10, 2020, 09:14:09 pm
content: I built a Magnetic loop such as imo suggested. It works well in its freq range. It is a real challenge to have one that works for more than a few ham bands. Mine is a Tx/Rx loop and works from maybe 4 MHx to 44 MHz. The receive loops that are available commercially have a preamp built in.Maybe that is a way to go, possibly look into how the commercial receive loops are built and modify their design for his pentodes.In my application, without preamp and for use as RX and TX:You have to tune the loop and also tune the matching circuit. Typically the match is set but some folks do have a tune able match. The 50 ohm set match will change even between ham bands. The loop tuning is quite sharp, Impressive even. It is a LC circuit.If you get lower in frequency you need to get much bigger, It becomes a real challenge for frequency less than 80 meters. I think he wanted something for AM Band.Ancient loops were multiple turn antennas on wooden frames and did work well for receive. As I understand these loops, they were essentially part of the tuning apparatus of the radio, part of the tune-able LC circuit. But I am not sure how much bandwidth you can get and be efficient.However looking at the input for the preamp as a simple LC circuit and making it as physically big as possible would be interestingMaybe a big tapped coil and an old variable cap, Sounds like fun. Would look like schematic for an old radio.Maybe a little math to figure out the resonant frequency range.