Hello all. I'm looking how to make narrow band active antenna for 7 or 10.1mzh. My idea: I will use magnetic antenna with one loop. A one-turn loop of
3.14m cupper pipe with diameter 15mm has 2.687uH. With Varicaps of
192pf it will have resonance frequency of 7MHz. I will load this LC tank directly to Gate one of dual gate MOSFET.
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Should it work? I'll never seen such schematics. Usually people use transformers. But I will place this transistor directly inside gap in the loop of cupper pipe. The Gate of MOSFET will add only additional
2pf to Varicaps and it will be very easy to compensate.
You should probably download Reg Edwards' RJELoop3 and/or RJELoop1 programs, which will tell you that the conductor loss resistance of your loop will be about ten times the radiation resistance, so not even counting the relatively low Q of varactor diodes, you'll have quite a bit of loss. On the other hand, even at 7MHz the atmospheric noise is so high that the loss won't be a significant problem so long as your amplifier is reasonably low noise. I'd recommend you use a C0G ceramic or possibly silvered-mica capacitor for most of the tuning capacitance, to keep the Q as high as possible (the losses as low as possible). Reg suggests the Q for use at 7MHz will be around 2000, so the bandwidth is quite narrow. If the Q really is that high (polish the copper and coat it with a protective varnish or paint...), the parallel resonant impedance will be up around 200k ohms, so it should be a decent match to your FET amplifier. Make sure any loading at the gap is well above 200k ohms resistive, to keep from introducing significant additional loss. If you want the antenna to also do a good job rejecting locally generated E field noise, you need to keep things well balanced. But at 7MHz, this is probably of marginal utility since any noise generators whose noise would be rejected would have to be very close to the antenna--within a few tens of meters.
-- I've done similar amplifiers for multi-turn loops for LF, down around 150kHz, using a balanced JFET design directly across the loop, with good success.
You can find the suggested programs at
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And you likely would get some additional replies if you cross-post to rec.radio.amateur.antenna.
----------------------------------------------- Hello all. I'm looking how to make narrow band active antenna for 7 or 10.1mzh. My idea: I will use magnetic antenna with one loop. A one-turn loop of
3.14m cupper pipe with diameter 15mm has 2.687uH. With Varicaps of
192pf it will have resonance frequency of 7MHz. I will load this LC tank directly to Gate one of dual gate MOSFET.
formatting link
formatting link
Should it work? I'll never seen such schematics. Usually people use transformers. But I will place this transistor directly inside gap in the loop of cupper pipe. The Gate of MOSFET will add only additional
2pf to Varicaps and it will be very easy to compensate.
oops.htm for info on calculating the radiation resistance. I believe that will be helpful. I assume Reg's RJELoop1.exe uses essentially the formula you'll find there. For your loop, that program says inductance is
2.69uH, conductor loss resistance is 46.2 milliohms, and radiation resistance is 5.7 milliohms. Although the conductor loss resistance (essentialy the same as you calculated above is "not bad," you need to consider it with relation to the very low radiation resistance. A current in the loop will dissipate far more power in the resistive loss than in the radiation resistance. You ideally will keep the loss resistance small compared with the radiation resistance, though for receiving (because of the very high level of atmospheric noise on HF and lower frequencies) it matters not nearly so much as for transmitting. When transmitting, you want your power to go into radio waves, not heat. When receiving you only need signal greater than noise, and it is relatively easy to make an amplifier with low enough noise figure that even an inefficient antenna will result in an amplifier output whose noise is dominated by the atmospheric noise received by the antenna.
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So 0.02 ohms sounds like a small amount, but it's almost half as much as the resistance of the copper loop. This may not be a bad thing, because the Q is so high that the bandwidth will only be about 3.5kHz assuming a lossless capacitor, and with the added loss the Q will be lowered to perhaps 1400, allowing a slightly wider bandwidth. With so narrow a bandwidth you need to be concerned about the stability of the varicap diodes' capacitance. Still, I would think a very high Q fixed capacitor supplying most of the total capacitance would be a good thing. Use only enough varicap to cover the tuning range you want. So for example, with 2.69uH inductance, if you want to cover 7.00MHz to 7.30MHz, you need 192.2pF at the low end and 176.7pF at the high end, a range of a little less than 16pF. You should be able to do that easily with two of your suggested varicap diodes, perhaps a couple of fixed 82pF high Q caps, and a high Q trimmer such as a piston trimmer to trim the center of the range.
It has more to do with the symmetry of the way the antenna is mounted. You want to make sure that the capacitance to ground from each side is as nearly the same as possible. You need to put the gap in the loop (the feedpoint) either at the top or at the bottom of the antenna, and for mounting it's often easier to put it at the top. That way you can clamp onto the middle of the bottom of the loop to mount to a pole... But if you have a balanced amplifier at the bottom and bridge the gap symmetrically across the box that amplifier is mounted in, it should also work well. I recommend to you the discussion about small loop antennas in King, Mimno and Wing's "Transmission Lines, Antennas and Waveguides."
At this frequency skin effect 0.028mm. It's uneasy to explain for me because my English is not enough. But It's more short way by plane territory that through mountains. For RF if very simply: mountains must be no more than 0.028mm at 7MHz. IMHO.
On Mar 13, 1:18 pm, Artem wrote: ... (snipping out a lot to keep this a reasonably length...)
I understand, but my point is that an antenna is reciprocal: there is the same percentage loss in the copper resistance whether receiving or transmitting.
...
I would think almost any modern RF mosfet would be fine. You do not need very good noise figure for HF reception, even with an antenna with modest efficiency. Just make sure the amplifier input has high effective shunt resistance at the operating frequency-- greater than
200kohms--to preserve the high Q. ...
It should not be that bad. C0G dielectric (also called NPO) capacitors have a _maximum_ temperature coefficient of 30ppm/C. The diameter of the loop itself, and therefore its inductance, will change with temperature in the same range, I believe. C0G capacitors, especially surface mount type, also have extremely low effective series resistance. I've found some C0G SMT caps that seem to have very close to zero temperature coefficient--it varies from lot to lot, apparently depending on the exact mix of the dielectric. What do you suppose the temperature coefficient of the capacitance of varactor diodes is? Note: 100ppm change in capacitance causes 50ppm change in resonant frequency. That's 350Hz at 7MHz. You probably wouldn't even notice that. It's only about 10 percent of the 3dB bandwidth of the antenna.
Citation would be an article written by an engineer at Andrew and published 10-15 years ago that I unfortunately can't find. "Polishing" (implying a mirror-like finish) certainly isn't necessary, but I do think it's worthwhile getting it reasonably clean and then protecting it so corrosion doesn't set in. In the case of the antenna presented here, the copper resistance is about ten times the radiation resistance, so there is some incentive to keeping it as low as practical. In the case of a typical resonant dipole, the copper resistance is such a small percentage of the radiation resistance that it's much less concern.
Yes, that should be fine. I suppose a transformer will be a convenient way to better match the FET amplifier output to a transmission line. The transformer can be physically quite small. MiniCircuits and some others (CoilCraft; M/A-Com; ...) sell appropriate transformers, and of course they are easy to make if you have an appropriate core. Also you can feed power to the amplifier through the transmission line. The transformer secondary can return to a bypass capacitor instead of directly to ground, and your DC feed appears across that capacitor. The circuit I used for 150kHz loops used a shunt voltage regulator in the amplifier, and by feeding the other end with a controlled variable current, I could avoid problems with uncertain voltage drop in the line plus connectors, and also use the current through the regulator to control the tuning voltage on the varicap diodes. So everything was done through the transmission line.
I mean that I will use you suggestion about balanced amplifier. I'll use two FETs. I'll place one more tube from middle of the loop to the gap. And I will use this point as ground point for balanced amplifier. In this case I will have differential signal at FET Drains and I will need transformer for put this signal to coaxial cable.
The transformer can be physically quite small.
My cable only 10m long. So I this it would be unnecessary. And I will operate with three signals:
Yes...very good. That should help keep the loop nicely balanced, especially if you build it all very symmetrical. That one more tube can also then help support the loop mechanically. Good luck with your project!
In the HF MHz range (3 to 30 MHz) Litz wire is a better improvement to RF signal conduction than pipe. Pipe does not become all that useful until VHF and over 100 MHz, and becomes very much less helpful by 500 MHz.
Yes...very good. That should help keep the loop nicely balanced,
Hi. I have some problem. I can't receive nothing except noise And self oscillation frequency. I have some photos. Could any give me some suggestions? Antenna:
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