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My PA0FRI active loop receiving antenna
Tom - VE3PSZ:The first time I saw the '2222 being used in the differential amp, I had strong reservations.? That device, while it has bound a multitude of "homes" in a multitude of equipment, it is not known for low noise figure or high dynamic range (intermod).? In that respect, I MUCH prefer the old tried-and true J-310 family of JFETs which I've been using since Siliconix introduced the U-310 long....long ago in a metal package.? While I also live in a relatively RFI quiet location with only a few AM BCB "blast furnaces", I do not have a problem with IMD from those broadcasters as the closest is roughly 20-miles as the crow flies distant.?? The problem with the PA?RFI amp for me is the HPF filter placed right at the differential input.? Maybe others require good coverage at MF for shortwave, but I do not.? I could build the thing, but would eliminate the HPF and any other LF limiting components.? If anything, I'd incorporate a LPF with the corner set at 500 kHz, but those are my requirements.? Further, I would NOT use the 2N2222 or variants.? I've had far too many good experiences with the U-310 family in home brew projects to try a "garden variety" BJT.? Unless the specific BJT is designed and manufactured for reasonable noise performance and expecially, dynamic range, just say no.??? Dave - W?LEV ?? On Wed, Jan 29, 2025 at 6:49?PM Tom - VE3PSZ via <thomas.b.seeger=[email protected]> wrote:
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Dave - W?LEV |
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If I must use a BJT, my "go-to" has always been the 2N5109.? But they have been discontinued for unknown reasons - likely low demand.? My back-up is the 2N3553, but its noise performance is not outstanding.? However, as a UHF power device, its dynamic range is quite acceptable for at least having a good shot at a low IMD design.? Most BJTs fall down badly when high dynamic range is required in a design.? In my searching, I have not found equivalents other than those with high ft like 6 or 8 GHz.? In ELF, VLF, MF, and some VHF designs, taming them is a real challenge.? Usually a loss element is required in the collector which, by the time the monster is tamed, the gain is significantly reduced. ? In addition, the average home brewer does not have the RF/uW design experience and skills (or test equipment) to tame these devices. Dave - W?LEV On Wed, Jan 29, 2025 at 7:12?PM W0LEV via <davearea51a=[email protected]> wrote:
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Dave - W?LEV |
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The PA0FRI version that I built did not include the HPF. Maybe it should have and then the IMD would have been much better. But since I am interested in everything from 20 kHz to 30 MHz any HPF is a non starter for me. The LZ1AQ circuit variant I use is from Everett Sharp N4CY, which uses much faster transistors than the 2N2222. Even though it uses UHF transistors, it is stable, well behaved, and works well even down to 20 kHz. Highly recommended.
A link to the late PA0FRI's work is here:
https://www.pa0fri.com/Ant/Actieve%20ontvangst%20antenne/Actieve%20loop%20ontvanger%20antenne.htm |
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Hi Folks,
Thank you for the lively discussion and the warm welcome! I just started working on a LZ1AQ Active Loop Antenna Amplifier based design. It will be tested and open-sourced as well in due course. I have some questions: (which uses 2N2222 with the 'original' design) says: --- According to the simulation, 3dB bandwidth is 200 kHz to 10 MHz. Yes, this is a bit disappointing at the high end, but still usable up to around 20 MHz. The 6dB bandwidth is 180 kHz to 18 MHz. Above that, gain drops off fast. But the good news is that you won’t need to worry about bleed through from strong local FM stations. And besides, it is going to be a few years before there is much to hear at upper HF. --- Coming to the year 2025 (now), 10m is super active and we need the Active Antenna to work well there. Can we use PZT2222A in the modified LZ1AQ + N4CY design (posted in this group earlier) to overcome this gain-dropping problem? I don't trust my hobbyist level RF skills to be able to use the BFU590 device successfully (just yet), hence the desire to use the PZT2222A transistor which was tested to perform well by Steve (AA7U). Also, can someone kindly share their LTspice simulation files for the common active amplifier projects? Thanks for the help! Cheers, Dhiru |
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The 2N2222 always had an ft around 200 to 300 MHz.? There is no reason that specific BJT should limit HF response.? It's fine for the differential circuits I've seen online so long as you don't live near an AM broadcast transmitter.? As I stated in a previous email, BJTs are prone to IMD (intermodulation distortion). However, for better response in the higher HF frequencies, I'd have a serious look at the ferrite material and windings used in the originals.? In several of the online pictures, I see a white ferrite for the output transformer (bifilar wound).? That is a mix 7 which is a powdered iron composition and is optimized for 3 to 35 MHz.? HOWEVER, these cores - powdered iron - are specifically designed at supporting Hi-Q inductors over their frequency range.? Hi-Q is NOT what is required for broadband performance!? They are not the best choice for broadband transformers.? Again, I'd blame the lack of good response at the higher HF frequencies on the material choice(s) of the toroidal transformer(s) material.?? Broadband transformers are better wound using materials 31 (the low end), 43 (a good mid-range choice), and 61 (for the higher HF and low VHF frequencies).? For the "really" low frequencies at and below the submarine comms. including WWVB at 60 kHz 73 or 75 material would be a good choice. ? Dave - W?LEV ?? On Wed, Jan 29, 2025 at 9:38?PM Dhiru Kholia via <dhiru.kholia=[email protected]> wrote: Hi Folks, --
Dave - W?LEV |
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Here are the links to a schematic by Everett Sharp, and sweeps by Steve Ratzlaff. I may be wrong but I believe Steve's build did not include the input LPF. The LPF actually boosts the input impedance and sensitivity of the amp above 25 MHz.? Above 30-40 MHz there is a rapid decrease.
/g/loopantennas/photo/0/18910.13034.1?p=Created%2C%2C%2C20%2C2%2C0%2C0
/g/loopantennas/photo/296351/3804913?p=updated%2C%2Clz1aq%2C20%2C2%2C0%2C0 |
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Hi Dhiru,
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The main problem in this circuit is the two output transistors don't have a high enough gain-bandwidth product if using the PN2222 or PZT2222--gain droops badly at 30 MHz. This afternoon I quickly ugly-built by hand the modified LZ1AQ circuit, for the standard 10 volts supply, first using PZT2222's for all four transistors, measuring the gain. Everett and myself found that using a 1 uH choke in the base of the two output transistors helps the top-end? gain a little. Note I do not install the two 220 ohms across the transformer primary, for added gain. I then added the 1 uH chokes (in series with the existing 12 ohms) and swept again--the top-end gain helped a little but as with the first sweep, the gain at 30 MHz falls off a lot due to the PZT2222s in the output. I then changed the two output transistors to BFU590QX (keeping the 12 ohms and 1 uH to the bases). This time there was no fall off in 30 MHz gain, in fact there was some peaking of gain at 30 MHz.? Plus there was no VHF/UHF oscillation (we found use of the 1 uH seems to stop any oscillation). I've made a folder in Photos and uploaded the sweeps plus a picture of my ugly-build. /g/loopantennas/album?id=300337????? The album is called "Modified LZ1AQ using PZT2222" and there are 4 files. Note that I'm 78 and only have one functioning? eye, and I can still homebrew using SMD devices such as the small BFU590QX (though it does take using magnification, and working? very carefully). So I suggest at least putting the BFU590QX's in the two output transistor locations--you can use the PZT2222 for? the first two transistors as only the outputs need a transistor with higher gain-bandwidth product. 73, Steve AA7U On 1/29/2025 12:55 PM, Dhiru Kholia via groups.io wrote:
Hi Folks, |
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On Wed, Jan 29, 2025 at 05:11 AM, Martin - Southwest UK wrote:
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The LZ1AQ also has high Z at 30 MHz due to the input LPF and yet its CMRR is ~25 dB below PA0FRI.
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Yes, but not the original form.
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On IMD, I am totally with you because the LZ1AQ is the hands down winner in this dept. See OIP3 column in the attached table: /g/loopantennas/photo/296366/3880256
However, I will allow that there are possibly locations which don't need good IMD. Hence, the zealots at the FB MLA-30 group despite its abysmal IP3 (~32 dB below LZ1AQ).?
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Digression: LZ1AQ achieved high IP3 by running the transistors really hot and I am concerned about long term reliability (in the tropics). A big mail-order retailer has TO-19 clip-on heatsinks, but at price gouging level. Is there a more affordable solution?
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:-) Of course, you are a LZ1AQ evangelist!
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Hi Fred,
Errr...wrong to direct this question at a newbie who has just finished assembling his preamps but hasn't even one working loop aerial. Nevertheless, I will chance a guess: CMRR is like IMD, in the sense that different locations need different numbers? Really, my knowledge about aerials is nearly zilch.?
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As for the choice of amp topology, I am just regurgitating Horowitz's AoE (pg. 98) - Need good CMRR? Choose differential amp.
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Hi Mike,
1. LZ1AQ Zin isn't low at all because of the input LPF. ?/g/loopantennas/photo/296366/3805464
2. Perhaps, your observed problem has nothing to do with CMRR, but PA0FRI's & M0AYF's significantly poorer IMD (~15 dB) than LZ1AQ? The two parameters are independent.
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Hi Pedro, the link leads to PA0FRI's evaluation of the PA3GZK single-ended INA-02186 preamp (though, the HP part may make W0LEV happy :-) ). It is totally different from the PA0FRI balanced preamp that we are currently discussing.
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On Wed, Jan 29, 2025 at 01:38 PM, Dhiru Kholia wrote:
According to the simulation, 3dB bandwidth is 200 kHz to 10 MHz. Yes, ?
Namaste, Dhiru,
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The LZ1AQ's gain will peak at ~28 MHz when used with the obligatory 1m dia. loop. See fig. 6 in "" in your Github's Study folder.
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AFAIK, PZT2222A and 2N2222A are similar die in different packages.?
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See Fig. 4 of the same document for the Spice circuit.
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73, Leong, 9M2LCL, ex-9W2LC
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On Thu, Jan 30, 2025 at 02:42 AM, <biastee@...> wrote:
:-) Of course, you are a LZ1AQ evangelist! ... by training.
The experienced experimenters here have built, measured and evaluated lots of different loop amplifier concepts. To mention the highly praised variants of the ALA1530 with its high sensitivity due to the noiseless feedback technique, MMIC or Norton-Amp based mid-Z Amps, with and without balancing transformer at the input, and also the PA0FRI differential amp.? I myself have built various modern RF-OpAmp based balanced loop-ampfifiers, that do a real great job with much less power consumption compared to the LZ1AQ. But the reason why i meanwhile also tend to become a fanboy of the LZ1AQ concept (in the improved version with better transistors), is its unbeatable price/effort/receive-performance relation. ?
You will find out soon :-)
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regards
Fred |
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Hi Leong,
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What is the test configuration you are using to measure the CMRR ?
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Regards,
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Martin
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On Thu, Jan 30, 2025 at 02:42 AM, <biastee@...> wrote:
The LZ1AQ also has high Z at 30 MHz due to the input LPF and yet its CMRR is ~25 dB below PA0FRI. |
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Same here.
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I like the LZ1AQ because it has proven to be a repeatable design, that is easy to build, and provides excellent results at moderate cost.
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It is also tolerant of using different parts, whilst still maintaining good performance, even with less than ideal construction.
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That's why I recommend it to folks who are new to broadband loop construction. It's a good reference point to start with, and difficult to get wrong.
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Regards,
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Martin
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On Thu, Jan 30, 2025 at 06:57 AM, Fred M wrote:
But the reason why i meanwhile also tend to become a fanboy of the LZ1AQ concept (in the improved version with better transistors), is its unbeatable price/effort/receive-performance relation. |
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Hi Fred,
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I think I'd agree with your comments.
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I'm not sure if the usual common mode test, with the test source connected to both input terminals strapped together, and referenced to ground, is a true representation of an actual loop / amplifier combination.
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The other question, of "how much common mode rejection do you need ?", is a good one.
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I suspect that due to factors such as re-radiation from nearby objects, and using a typical maximum value of null depth as a guide, 20 to 30dB may be about as much as can actually be achieved in practice.
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Regards,
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Martin
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On Wed, Jan 29, 2025 at 03:19 PM, Fred M wrote:
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I'd argue as a generalisation, that common mode rejection is generally more of an issue on the low frequency bands <10MHz, as localised electrical noise and interference seems to be more prevalent on those frequencies.
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There are instances when this is not the case, but noise sources such as Switched Mode Power Supplies, which tend to be the main culprits, usually produce much higher levels of conducted noise at frequencies below 1MHz, which is when good common mode rejection is most useful. It is much less useful when the noise is radiated, which is often the main problem on the higher frequencies.
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Regards,
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Martin
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On Thu, Jan 30, 2025 at 02:42 AM, <biastee@...> wrote:
The LZ1AQ also has high Z at 30 MHz due to the input LPF and yet its CMRR is ~25 dB below PA0FRI. |
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Hi Dave,
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Not all white coloured cores are the same.
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A lot of coated ferrites, of typically European manufacture, tend to use a white, or pastel coloured pink or blue "plastic" looking coating, irrespective of the actual material type.
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A few examples.
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Regards,
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Martin
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On Wed, Jan 29, 2025 at 10:18 PM, W0LEV wrote:
I see a white ferrite for the output transformer (bifilar wound).? That is a mix 7 which is a powdered iron composition and is optimized for 3 to 35 MHz |
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On Thu, Jan 30, 2025 at 01:32 AM, Martin - Southwest UK wrote:
Exactly, as you described - " test source connected to both input terminals strapped together, and referenced to ground"
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There is an official config in IEC315-1 (1988), fig. 7, pg. 64. Initially, I thought of using it, but gave up when I couldn't figure the resistor values. The resistors are in a T-network & followed by a divider network and so, will introduce some loss. I figured that, due to the loss, the CMRR measured with the official config will appear better than the hacky usual config. Then I won't be able to compare my results with others. |
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I can't find a free version of that standard on-line.
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Have you got a weblink, or can you upload a scan of the picture ?
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Thanks,
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Martin
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On Thu, Jan 30, 2025 at 01:05 PM, <biastee@...> wrote:
There is an official config in IEC315-1 (1988), fig. 7, pg. 64. |
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My thought is that it would be more appropriate to connect two 5pF caps in series across the loop's input. Then connect a signal source between the junction of those two caps and ground. Some loop amplifiers like the Wellbrook don't like a zero impedance input because it interferes with its negative feedback.
Tom |
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