Active antenna for a shortwave radio (schematic with N-FET) 

You don't need that resistor but but you can wire it in. In theory it gives signal loss. As far as I know the Gate needs no negative bias. But your point is very important. On a long wire antenna hanging outside due to static voltage buildup the FET easily gets damaged. For that reason I have in my shortwave radio a relay contact between Gate and Ground that opens (relay activated) when I switch the radio on. When you don't do that the FET gets damaged every 3 months or so (my experience). With an indoor antenna there is no problem. I have pinned your comment. 5 August 2020.

On the lowest frequencies (10 KC-100 KC) the antenna's differ substantially (construction) compared to the range of 100 KC - 600 KC and 600 KC-1.8 MC. They often react on the magnetic component of the EM field (ferroceptor or air coil). The next band is 2 MC-30 MC. The circuit in this video is useful as a whip antenna with a length of (say) 1 meter. Best idea is too hook it up to a BF 199 (HF amplifier with a series capacitor to the base) and take the amplified signal from the BF 199 out via a coupling capacitor at the collector (grounded emitter circuit, both in the 100 pF range). In that case it is usable for frequencies between 1 KC and 30 MC. There is a series of video's about VLF reception on my channel. Search via the "looking glass" on my channel trailer with keywords like "VLF". Via that looking glass you can find more info about antenna - and radio circuits with keywords like "radio", "shortwave" etc.

@@radiofun232 I really appreciate your time to clarify the subject with more details. I'll check out your videos right away.

When you connect the floating Gate to an outdoor antenna, the FET can be easily damaged by static charges. So, in that case, when the active antenna is not in use the Gate must be shortcut to ground. When that Gate is connected to an indoor antenna this is not a problem. Biasing is not necessary, but you can solder a resistor in the order of 1 M - 2 M from the Gate to the minus/ground. Of course it will "damp" the sensitive input on the Gate of the FET somewhat. Sometimes you see two antiparallel Silicium diodes going from the Gate to the ground/minus. In that case the voltage to the Gate cannot go higher than 0,7 Volt, that is the standard barrier voltage of a Silicium diode. Thanks. 23 feb 2023.

I don't know because I never worked on such high frequencies. Perhaps, with a special HF FET.

The tank circuit at the input is a piece of wire, maximum length 1 meter. You can also take a longer antenna but that antenna must be connected to the gate of the FET via a tiny (2 pF or so) capacitor. When you want to use it as a free HF pre amp, the "out" must be connected to the antenna socket of the radio. The coupling capacitor must be found experimental in that case, my first idea would be 100 pF in such a case. ("C" in the drawing).

By the way: this is the "better" circuitru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-i8uAbKUV6Tc.html

It depends on what you want and need. The schematic showed is a so called "aperiodic circuit". That means it will/can receive all kinds of radio stations on Shortwave. Say between 2 MC and 12 or even 18 MC (I tested it for that purpose, never got higher than 18 MC). But: such a 30 cm to 1 meter "spriet" (Dutch word= rod) antenna is of course not ideal, due to its length. It is directly connected to the gate of the FET. In fact it is far to short to receive radio stations in an ideal way (say tuned to their resonance) between (say) 2 MC and (say) 30 MC. Though it works OK! Due to the amplification of the FET and its endless input impedance on its gate. When you do radio calculations, this is (say) the main thing: antenna length = frequency in Megahertz / (divided) by wavelength in meters. Long wave (150 KC-345 KC) means the ideal antenna (full wave) = 2000 meter to 857 meters long. Far too long to hang out of your room or in your garden! But you can also use a half (1/2) wave or a 1/4 wave lenghth antenna. SW = 187 meter to 10 meter, divided by 4 gives usable lengths of wires to hang outside in your garden or whatever. Or in your room: also a very good option. Connect such a 4 meter long antenna inside your room with a cap. of 14 pF to the radio receiver antenna input. Works nice, that setup (my experience). Anyway: my answer to your question is yes: by making a few turns around a ferrtite antenna the circuit gets non-periodic, it means that it gets tuned to a certain frequency band received on the ferrite antenna. That could be a surprise, where it works at its best. Test, try, evaluate. Thanks for your reaction, I can eleborate when you can describe the exact radio situation. 23 jan 2023.

Thanks, Carl. I used the second circuit with a wire antenna from 10 meters.

It depends where the antenna (arrow head) is connected to the coil. That is why I drawed it that way. The radio theory says: there is more selectivity when you connect the antenna low on the coil and less selectivity when it is connected to the top. But experiments in a real situation show what is best, because it also depends on the lenghth from te antenna. A very long antenna can give messy reception results because it takes the Q from the antenna coil down. That is why an antenna capacitor (15 pf or so) is used.

Thanks so much for the explanation!

You can use a 1 uF electrolytic capacitor there. Acts a a kind of smoothing capacitor. When you give that capacitor a too high value (say 100 uF or so) that capacitor keeps on holding so much charge that the regulation with the volume/level control (node of the 2K2 or 1 K potmeter) does not work properly any longer. With "not properly" I mean: the FET works far too long, even when you bring the amplification level down it keeps on working for a long (... is set by the R-C ) time. So when that capacitor has a too high value the FET keeps working on and on, with no effect (or a very delayed effect) of the control potentiometer. 11 may 2021. That is why I used the dashed line in the schematic. 1 uF is a good value to be used here. The FET and amplification regulation works properly with that capacitor value.

@@radiofun232 Ok, thank you very much for the clarification.

Ja die heb ik wel. Maar wat wil je? Wil je AM detecteren? Ik wacht nog even met het geven van meer informatie en links, maar op mijn YT kanaal staan veel geaarde emittorschakelingen die goed voor een vervolg HF of AF schakeling kunnen voldoen.

say: 10 windings loosely around the 60 KHz coil, one to ground and the other one of these 10 windings coupled via a 100 p capacitor (the C in the circuit) would be my first idea. Loosely coupling is necessary to take the frequency of 60 KHz not down.

Yes, in general, when you want to go to the same (one) frequency. But the bandwidth from the coil with more windings and the Q (quality factor) will differ. It is not so simple: say divide 500 pf by 2 does not mean that the coil automatically needs 2 times more windings. Much more info & calculations in these cases (parallel resonance L-C) are on the www. Keyword: "parallel resonance". 19 March 2023.

28 vc por aqui abraços...

+André Quintal kkkk pois eh. O ex-futuro tecnico em eletrônica aqui fica acompanhando os canais. O Mr. Tillmann é o cara q monta com a técnica de pregos q te indiquei. Lembra? Sempre com videos legais também. Abraços

Eu montava também é mais prático e funciona.... eu lembro dele sim e já comprei os livros deles...

Abraços

+André Quintal bacana! Comprou em algum site do br?

Yes.

Hoi Rob, bedankt voor de links. Heel informatief. Ik heb wel een paar HF Fets met 2 ingangen liggen, op de beurs gekocht. Voor het geval dat.... Links to schematics from other active antenna's in the comment from R.Rob.