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My PA0FRI active loop receiving antenna
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Hi Folks,
I recently built the PA0FRI active loop receiving antenna for HF (shortwave) use. The reception results have been highly impressive, especially considering the extremely constrained space where the antenna is installed. I have made the entire project open at the following URL: Total cost: Roughly 10 dollars (including professional PCB, BOM, IP65 casing, aluminum loop and all). I hope this work is useful for you. Feedback is welcome! Cheers, Dhiru (VU3CER + WQ6W) |
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Please.? Could you post to something more "friendly" than github.? Just a bunch of numbers and tables to me. Dave - W?LEV On Tue, Jan 28, 2025 at 4:03?AM Dhiru Kholia via <dhiru.kholia=[email protected]> wrote: Hi Folks, --
Dave - W?LEV |
开云体育Boy I am with you !? To me github is unbelievably user hostile.Gedas, W8BYA EN70JT Light travels faster than sound..... This is why some people appear bright until you hear them speak. On 1/28/2025 2:07 PM, W0LEV via
groups.io wrote:
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Seems OK to me. The files are Gerber's and other schematic info, so that you can get your own boards made and build them as required.
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However, as I've said before, there isn't much additional effort required to build an LZ1AQ type circuit, but the improvement in performance is substantial.?
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Apart from the novelty of building something, the PA0FRI, and the G8CQX it is based upon isn't that good, and personally I wouldn't bother.
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Regards,
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Martin |
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Gerbers are fine if you have boards built and them populate them yourself.? I do only dead-bug style, so the Gerbers are only confusing to me.? Give me a CLEAR schematic.? I can take it from there! Dave - W?LEV On Tue, Jan 28, 2025 at 8:33?PM Martin - Southwest UK via <martin_ehrenfried=[email protected]> wrote:
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Dave - W?LEV |
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So, where is the schematic in the github mess?? I can not find anything labelled "schematic".? That's all I need. Dave - W?LEV On Tue, Jan 28, 2025 at 8:36?PM W0LEV via <davearea51a=[email protected]> wrote:
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Dave - W?LEV |
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On Tue, Jan 28, 2025 at 12:33 PM, Martin - Southwest UK wrote:
I can think of 3 situations where the PA0FRI may be preferable to the LZ1AQ. ?
1. Long (read: lossy) coax between active loop & receiver, e.g. 1st floor of a 30-storey flat.? PA0FRI has significantly higher gain over 6-18 MHz, which can overcome the coax loss.?
/g/loopantennas/photo/297909/3880257 The OP Dhiru's version which uses a 4:1 output transformer has even higher gain that the above graph which is based on the older 1:1 version.
2. Where better CMRR is required, the PA0FRI's differential amp has an edge over LZ1AQ's common base. ?
3. Eliminating the plastic enclosure. PA0FRI's smaller PCB can be shoved inside the PVC pipe.
73, Leong, 9M2LCL, ex-9W2LC. |
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On Wed, Jan 29, 2025 at 06:12 AM, <biastee@...> wrote:
2. Where better CMRR is required, the PA0FRI's differential amp has an edge over LZ1AQ's common baseI am not sure wether this proves true in general terms. The CMRR of the LZ1AQ type amplifier depends on the symmetry of the pcb-layout, the gain balance, the balance of the Push-Pull transformer and the performance of the common-mode choke (Balun) at the coaxial output. ?
regards
Fred? |
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Hi Dhiru Great things on your github! Very impressive and informative. Keep up the good work! And thanks for sharing! 73 Andrea IN3IWZ Il mar 28 gen 2025, 05:03 Dhiru Kholia via <dhiru.kholia=[email protected]> ha scritto: Hi Folks, |
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On Tue, Jan 28, 2025 at 11:42 PM, Fred M wrote:
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Hi Fred, Good point. I agree with you. However, the same person (me) wound the output transformers and laid out the PCBs (as a smorgasbord on one panel).? All things being equal, the differential amp has an edge over the common base where CMRR is concerned.?
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Hi Leong,
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I'm not sure if I agree with your comments.
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The PA0FRI may have significantly higher gain over 6-18 MHz, but this just demonstrates the poor frequency response of the circuit, when used with a typical loop. Do you actually need this additional gain to overcome coax loss ? I don't think so. A 30-story apartment block is likely to be in the region of 30 x 3m tall, approximately 90m or 300ft. This shouldn't be a problem. If you were to use RG-58, the loss at 30MHz for a 90m length is around 6dB. The LZ1AQ or Wellbrook, will have more than enough gain in hand to overcome such losses.
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The PA0FRI, and the G8CQX, have a moderately high value of input impedance, so the overall common mode impedance when used with a loop is likely to be higher than that achieved with an amplifier with a much lower input impedance. In practice, I don't think the PA0FRI / G8CQX, is particularly good in terms of CMRR.
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The LZ1AQ can be made quite small if SMD parts are used.
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However, the main issue with the PA0FRI / G8CQX design, apart from the very poor frequency response, and lack of loop impedance tracking, is that the third order Intermodulation performance is quite poor. Even the modified versions with additional output buffer stages are not much better.
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Without saying it out loud, I think you can tell what my opinion of the design is :-)
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Regards,
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Martin
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On Wed, Jan 29, 2025 at 06:12 AM, <biastee@...> wrote:
I can think of 3 situations where the PA0FRI may be preferable to the LZ1AQ. |
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On Wed, Jan 29, 2025 at 01:05 PM, <biastee@...> wrote:
All things being equal, the differential amp has an edge over the common base where CMRR is concerned.? Hi Leong,
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OK, due to your research, the differential amp seems to achieve a by far better CMRR.
The CMRR of a loop amplifier is responsible for the null depth, a distortion of the radiation pattern, and for the "antenna effect", the sensitivity to common mode voltages caused by electric fields. That leads to the question, what is, from the practical point of view,? the minimum CMRR tolerable at a certain frequency or what is a sufficient or recommended CMRR? ?
regards, Fred |
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Hi all,
some thougts with regards to the CMRR of a low-Z loop amplifier for an aperiodic broadband loop:
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- the desired differential reception voltage of a loop is the magnetically induced voltage across the loop clamps.? This voltage drives a current into low Z transimpedance amplifier (aperiodic broadband loop) or feeds a resonance circuit formed of the loop inductivity with an external capacitor (Hi-Z selective tuned loop)
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- the common mode voltage ist a voltage induced mainly by the electric field. The voltage occures equally at both clamps with reference to earth.
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- the Z of the source for the common mode voltage consists - equivalent to a short monopole - of a verly low resistance in series to a (frequency dependant) high capacitive reactance. (some milliohms in series to a few picofarads)
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- because of the latter, the CMRR measured with a source impedance of 50 Ohm must not be comparable to the CMRR voltage of a loop acting as a short monopole, as a reactive voltage source connected to a low -Z amplifier input impedance. A short monopole dummy antenna should be used for the measurement.
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What is your opinion? regards, Fred |
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A real life data point here. I live in a low density urban area, fairly close to a bunch of high power AM transmitters. Low input impedance of the loop preamp is absolutely critical here, because it effectively shorts the common mode source. The loop is a relatively high impedance whip for the common mode. I tried M0AYF (exact same topology as PA0FRI, don't see how it can be different from the point of view of CMRR), and it was unusable with a 1m diameter loop. AM intermod was all over the low HF band. The null was also not as good as with the LZ1AQ clone.
LZ1AQ works ok if the current through transistors is sufficient.
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73, Mike AF7KR |
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Dhiru, thank you for sharing. I can see that you went to quite a lot of work bench testing and it is well constructed. Nice work.
But I built one of those a year or two ago. I used it for a couple of days. I have a several strong local BCB stations. The intermod distortion was horrific. It might be a good amplifier in a rural area but not here. It was also quite deaf on LW and NDB bands presumably because of its higher input impedance. My Wellgood and LZ1AQ clones are infinitely better in terms of IMD, and sensitivity. The PA0FRI might have produced more S-units at some frequencies, but SNR was the same. I for one can't stand listening to a rare or un-identified station for many minutes, just to find out I'm listening to a harmonic or spur. This one is now in my junk box. Sorry.
But please keep building, testing and posting. Tom |
Larry Naumann
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