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LZ1AQ: iMDs from local AM stations in Dipole mode
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Hello,
I recently finished my cross parallel loop (commercial LZ1AQ amp). It works well in A+B mode. But looks like local AM broadcast stations cause an overload in Dipole mode. Antenna is mounted on my townhouse balcony as you can see on a picture. It's a semi-urban quite RF noisy environment (SF Bay Area in California). Using it with SDRPlay RSPDx. From what I see all the intermodes are from the loops high Z amplifier, not from my SDR. I see none of that in Loop mode. I continue seeing that on 40m band. Now 20m and 10m are clean from IMDs but sensitivity seems to be much lower compared to loop mode low-Z amp. Which is probably a result of de-sensing caused by amp being oversaturated from MW. And I had hope for Dipole mode on higher bands but looks like it's quite useless in my conditions. Now, since RSPDx is missing a dedicated 160m bandpass filter it can easily be saturated with any antenna that I have on 160m. I tried multiple loops, my 10-80 EFHW. A dedicated preselector like MFJ 1046 placed between antenna and SDR takes care of that. Same story with this new cross looped LZ1AQ: no issues on 160m in any Loop mode if I use a preselector after it. Which tells me that local AM MW stations don't saturate loop mode amp, only saturate a preamp in SDR. And of course preselector doesn't help in Dipole mode at all. So, is that kind of behaviour to be expected in relatively noisy environment from LZ1AQ Dipole amp? Or am I missing something in my setup? Thank you, Simon P.S. Links below will show a picture of my antenna and Dipole mode reception on 80m: https://photos.app.goo.gl/CC993JVz7Q2gTQcQ8 |
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wn4isx
IMD can be caused by sources external to your antenna. A neighbor installed a cheap chain link fence, sections rusted within a year. The two strongest local stations mixed and produced spurs up to at least 10 times their fundamental. The fence looked so ugly after a winter they tried painting it and it looked even uglier, lipstick on a pig ugly. They took it down and installed a fancy wood privacy fence and my low level, wide band grunge, decreased by about 10dB. ? Owen Duffy has an article on intermod and colorbond. Colorbond is an Australian product, paint baked on sheet metal. ? https://owenduffy.net/blog/?p=12086 ? He also has an article on "how much power does it take to produce a S9 signal 1km away." https://www.owenduffy.net/bpl/S9QQ.htm 3 microwatts EIRP. ? Very weak RF mixing can produce some interesting signals. ? When tracking down external intermod, one must use at least two receivers with different mixer/IF scheme to insure any signal isn't generated in the receiver. |
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On Mon, Jun 24, 2024 at 05:18 PM, rfsam wrote:
local AM broadcast stations cause an overload in Dipole mode.From your notes, I think you say that the pre-selector, when used after the loop and dipole, doesn't fix the problem, which suggests it is occurring in the dipole amplifier. Plus, the problem is less significant when used in loop mode, so it's not likely to be externally generated. If you put a 10dB attenuator between the loop output and your receiver, do the unwanted signals reduce by 10dB, or by a larger amount, say 20 to 30dB ? It is possible that the amplifier is being overloaded in dipole mode, but the signal would have to be very strong. Chavdar has these notes on the website. Regards, Martin |
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Hi Martin, Thanks for your reply! My explanation was probably a bit confusing. There are two non related issues: 1. I need to use pre-selector between ANY antenna (including passive long wire above the roof) and my SDRPlay but only on 160meter band as internally RSPDX doesn't have a dedicated bandpass filter for 160m. It only has a low pass filter that allows all the MW in unless MW notch is selected in SDR uno software. But if notch is selected then it also rejects 160m as it's a part of a MW band. So an additional preselector in front of RSPDX is the only way to use it here on 160m. Without preselector I see an "overload" indication with ANY antenna, including a long wire passive inverted L. Of course I can refuse RF gain but then SNR goes to drain. New LZ1AQ in A+B or single loop mode works with preselector great on 160m and same way I saw my older Wellgood loop working, my MLA30+, my passive wire antenna. No issues here. Once I enable a to preselector that's placed at the SDR input all the overloading notifications go away and I can see 160m band. So there are no issues with any active loop amplifiers that I used. They are not getting saturated by local MW AM. It's the SDR that was doing it without an additional preselector. And it's only happening on 160m due to the RSPDX design. I tried using same adjustable multi and preselector on 80m/40m/20m/10m enabled/disabled and there was zero difference. MFJ 1046 has a bypass button, so it's very convenient to do an A/B comparisons. 2. Now, Dipole mode is very different compared to what I described in #1 above. When I enable it on 80m and 40m bands I immediately see an overloading picture on the screen similar to what I saw on 160m ONLY before in non-dipole mode. And what very important I also see NO Overloading notifications from my SDR! RF Gain doesn't do a thing at all. That brought me to a conclusion that all the overloading and massive IMDs I see are all coming from Dipole HighZ amp, not from RSP! Additional pre-selector between Antenna and RSP doesn't help at all. 160m band is a complete mess with or without a preselector (which was not the case in Loop mode or any other antenna). 20m and 10m look clean on Spectrum/Waterfall but SNR is very bad, sensitivity is low. Antenna seems to be just dead. Again that only in Dipole mode. In Loop mode everything looks great on all higher bands. On Tue, Jun 25, 2024, 7:34?AM Martin - Southwest UK via <martin_ehrenfried=[email protected]> wrote: Plus, the problem is less significant when used in loop mode, so it's not likely to be externally generated. It's not less significant, it doesn't exist at all as I indicated above. It may look similar only on 160m but thats different and has a different root cause (no bandpass filtering o Inside RSPDX on 160m). Nothing is effecting 80m and 40m in Loop mode WITHOUT preselector at all and I can see all the band clearly with great SNR especially in A+B mode as loop amp (LNA) does not get into saturation at all.? So everything I described above leads me to believe that HighZ amp just gets overloaded much easier in my conditions compared to LowZ loop amp inside LZ1AQ box. Maybe (and it's probably it) signal amplitude at the HighZ amp input is so much higher even with short Dipole? Thank you, Simon km6mul
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On Tue, Jun 25, 2024, 8:52?AM rfsam via <1rfsam=[email protected]> wrote
I meant to write "deaf", not "dead", sorry. I still see some strong signals which go away completely when I shut down the power supply, so it works in Dipole mode on 20m and 10m, just not as sensitive as in Loop mode and any other antenna I have. Which can be explained if HighZ amp is IN or close to be in saturation. And this is an opposite I hear from other people who use this amp on higher bands. If I'm not mistaken some people were having a better sensitivity and SNR in Dipole mode on Higher bands compared to the Loop mode. I was actually hopping for that. Regards, Simon
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I have a suggestion.
One 'trick' you could try, is to connect a resistor (initially a variable one, perhaps 50K ohm) between the two loops, across the dipole input. This will shunt the dipole input impedance and reduce the LF gain. The exact amount will depend on the resistor value. However, it will affect the higher frequency bands much less than the LF bands, and it will not affect the performance when used in loop mode. Once you have found a suitable value, you can replace it with a fixed resistor wired inside the amplifier case. I'd anticipate that a value of somewhere in the region of 1k ohm, is likely to be in the ballpark. Regards, Martin |
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Hi Simon,
Did you insert jumpers in order to use the? dipole amp. input balun? What are you using for dipole antenna?? What is your e.m. environment? If you suspect saturation use oscilloscope at the amp. output and measure the signal level.? Below 4 v pp? the amp is not saturated. The wideband output level can be measured also with milliwat meter (W7ZOI type)? +10 dBm is a safe upper limit. ? Read ?Read also? . Recently I have published several new articles in The sensitivity of dipole mode is proportional to the lengths of dipole arms so if you are using long arms the total wideband signal might becomes quite strong.?? On LF the antenna factor in dipole mode is almost 10 dB better that that of loop mode when using 2x1 m dipole and 1 m diam single loop ( see the specifictions in Fig. 1.4)? . Also if there is? insufficient feeder CM choking,? the feeder acts as an additional antenna and this effect might be quite strong.? Usually I use 2 x 2 m arms? or 2 m vertical GP and feeder CM chokes with grounding as described in and the results are quite good .? The Hi-Z amp with very short arms is very susceptible to CM asymmetry. The CM asymmetry and parasitic feeder antenna is very closely related.? Using "fat dipole" or GP? (with input balun inserted) is a better solution.? Increasing the capacitance of the antenna? reduces CM asymmetry so? dipole or short GP with small capacitive hats is also a useful solution - ? . There will be always a difference in reception between loop and dipole modes but the antenna gain of the amp. in both modes is chosen to be similar when 2 cross 1m diam loops (in one plane ) and the same loops as dipole? arms? are used. Regards, Chavdar LZ1AQ |
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Hello Martin, Thank you for the tip. Might be a good experiment to try! Thanks again! Regards, Simon On Tue, Jun 25, 2024, 11:51?AM Martin - Southwest UK via <martin_ehrenfried=[email protected]> wrote: I have a suggestion. |
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Hello Chavdar, Thank you for you reply! I need to double check the jumpers positions. I'm not sure I changed them from the default position (the way you shipped an amp). Need to verify. I remember changing a position only on the jumper that allows amp to have power. I have a simple vertical dual loop configuration (A and B loop). Each loop is about 1m diameter and 16mm thick aluminum pipe. So, it's vertical 2x1. You can see the antenna installed on my balcony (Google drive links in my first post), but I will try to attach a picture to this post. I did use one mix31 clip on ferrite on SFTP cat8 cable right next to the amp. I'm definitely in the noisy environment (Silicon Valley here in California), so lots of houses next to each other with many of them having solar systems. And that was one of the reasons I chose active loop antenna :) Having a Dipole mode in dual loop antenna is always a plus! Thank you for those links, I will look through them. I was assuming that there is a higher antenna factor in Dipole mode. That can lead to saturation but I just started to validate this new antenna and will try to do more checks per your suggestions. I assume that cable going down vertically cause some asymmetry for the Dipole as it's crossing lower 1m loop. Here is a Google link showing my antenna installation: Best Regards, Simon On Tue, Jun 25, 2024, 1:18?PM §é§Ñ§Ó§Õ§Ñ§â §Ý§Ö§Ó§Ü§à§Ó via <lz1aq=[email protected]> wrote: Hi Simon, |
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Hi Simon,
Ok now things are more clear.? The balun that you are using has insufficient impedance - just several uH. You need a balun with hundreds of uH? which can be achieved with toroidal core and more windings. Try to use #31 core T240 and wind the FTP cable with as much windings as possible. The minimal diameter of FTP bending is somewhere? at 5 cm but I? manage to wind it very tightly - just be careful not to disrupt the aluminum folio. In any case for balun to be effective? in dipole mode you need a good grounding point. Look at fig. 7 in? for proper cabling.? This balun will work quite well for loop mode even without grounding point.? If you still have IMd problems? check for nearby FM or AM stations.? The most useful test is to go somewhere in the nature? and test the loop and dipole mode with battery powered equipment thus avoiding any impacts from surroundings. PLace the antenna 1 m above the ground and use several meters of feeder laying on the ground. There you will not need any baluns. If you have more questions you can mail me to lz1aq at abv.bg . Regards , Chavdar lz1aq |
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I can agree with Chavdar..
I use directional array using 3 LZ1AQ preamps ( home brew using 5551¡¯s) Chavdar design phasing array. YOU MUST use very good cm choke..I use design on LZ1aq website..or noise from mains power etc, upsets everything It worked in LONDON, noise etc virtually removed. Now in Devon its a very good antenna array.. I used crossed parallel loops, now here in Devon will be investing in 3 new Chavdar commercial preamps to be able to use verticals ( noise floor here on 40m full size vertical 1/4w is s0, where as in London s7-9.) once Wife releases cash. Lol. Now I can here in Rural Devon utilise a vertical rx array, no point in urban land.. Maybe try loop?? ( high noise floor, vertical not so good.) Search LZ1aq website for crossed parallel loops. With a loop you can null out offending interference maybe?? Impossible with a vertical. Just saying, have a rethink? Simon g0zen |
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Hi Simon, As I mentioned few times I do use the loops with good success in my noise environment. Regards, Simon On Wed, Jun 26, 2024, 12:58?PM Simon via <ohhellnotagain=[email protected]> wrote: Maybe try loop?? ( high noise floor, vertical not so good.) Search LZ1aq website for crossed parallel loops. |
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Hello Chavdar,
On Wed, Jun 26, 2024 at 12:28?PM §é§Ñ§Ó§Õ§Ñ§â §Ý§Ö§Ó§Ü§à§Ó via groups.io <lz1aq@...> wrote: Ok now things are more clear. The balun that you are using has insufficient impedance - just several uH. You need a balun with hundreds of uH which can be achieved with toroidal core and more windings.Chavdar, are you referring to the balun at the Dipol mode amplifier's input (mentioned here: ) or to the CM choke placed on SFTPs CAT cable near an amplifier's output? Or both? I haven't played with additional baluns at the Amp HiZ input yet but I can try FT240-31 core at CAT cable (at amp's output end) with several windings. I also have FT240-43 but -31 maybe better for MW/LF rejections. I have just a clip on (bat pretty massive) ferrite bead there right now. My cable is pretty thick (~8-10mm) but flexible, so doing few windings should be no problem. Try to use #31 core T240 and wind the FTP cable with as much windings as possible. The minimal diameter of FTP bending is somewhere at 5 cm but I manage to wind it very tightly - just be careful not to disrupt >the aluminum folio. In any case for balun to be effective in dipole mode you need a good grounding point. Look at fig. 7 in for proper cabling. This balun >will work quite well for loop mode even without grounding point.Thank you! May try something like that if just using FT240-31 won't help. Although heaving a good GND point maybe a bit of a challenge in my case. Best Regards, Simon |
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Looks like massive amount of FT240-31 cores with as many turns as possible do really solve this problem. I used a stack of 2xFT240-31 on CAT cable with about 8 turns right next to antenna and 3xFT20-31 stack next to control board. Even have one single core in the middle of my CAT cable (although I'm not sure if that's FT240-31 or FT240-43). Also some on 50 Ohm coax that goes to my receiver (next to control board) and a core on control cable (next to control board) that goes to mode switches/power. I have control board in a separate box sitting outside of my "shack", so I only have 50Ohm and "control lines" coming into the shack.
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So, my initial thoughts about Hi-Z amp being power overloaded by local AM stations were wrong. And thinking more about that I see where I was wrong initially: if Hi-Z amp was overloaded (in saturation) it should produce quite a high level at the output that should overload my SDRPlay but that wasn't happening!
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I must say that almost anywhere Loop low-Z amp still outperforms "Dipole" Hi-Z amp. I haven't tried 10m band as I see almost zero activity there but 80m - 20m Loop mode works just better (better SNR) in most cases. But then again I live in Silicone Valley noisy environment.
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Regards,
Simon
KM6MUL |
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