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Author Topic: This might be the start of something big  (Read 7163 times)

Offline jFarley

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Re: This might be the start of something big
« Reply #15 on: August 14, 2017, 1406 UTC »
If you could actually measure the Q from a screenshot, that would be cool and helpful.

Measuring the Q of a resonant loop which is operational and under power can be problematic.  I use this technique which seems to jive with both theory and alternative measurements with a scope and signal genny.

I tune the loop to a vacant frequency F, and observe the resonant peak in the SDR's FFT window, scaling both the amplitude and frequency display to get the best rendering of the peak.  Generally, you are seeing the noise peak up to expose the resonance curve.  I like to have this peak at 20 dBm higher than the noise outside of the resonance range.  This may not be possible for lower Q loops.  If necessary, I couple in a wideband noise generator to give me the desired peak height. 

I measure the bandwidths of the response at the following power points around the frequency F:

BW3 at -3 dBm if possible - this is usually tricky and error prone for higher Q loops
BW7 at -7 dBm
BW12 at -12 dBm
BW18 at -18 dBm.

Given these, apparent Q can be computed from each measurement:

Q= F / BW3
Q = 2F / BW7
Q = 4F / BW12
Q = 8F / BW18

I generally take the loop Q to be an average of 3 or 4 measurements.  This method has worked for me well while evaluating loop designs.
Joe Farley, Near Chicago
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Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #16 on: August 14, 2017, 2231 UTC »
Earlier, I took the following screenshots, fortunately vacant and tuned (315 kHz), although unfortunately perhaps not with optimal resolution on the frequency and signal scale.

If I eyeball the 12 dB width it looks like about +/-2.5 or a total of 5 kHz. So if I am following your formula correctly, the Q would be 4 * 315 / 5 = 252.  That seems too high?
 
I do notice that the resonant peak is not symmetrical, it is wider on the high frequency side.



« Last Edit: August 14, 2017, 2234 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
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netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline jFarley

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Re: This might be the start of something big
« Reply #17 on: August 15, 2017, 1206 UTC »
The response curves, especially the bottom one, seem to indicate that your loop is operating at a significantly high Q.  I speculated, based on some previous attempts here, that the loaded Q down at LF might be low.  I must have been using a bum amp or screwed up something else, because the Q suggested by the curves is definitely not low.  It is significantly higher, and I came up with values similar to what you calculated, which blows me away...

The shape of the response is very similar to what I have routinely seen with the ferrite loop I use, which routinely measures out at 200-300 Q midband LF.  If true, this suggests a very high effective height for your loop.

I also generally note the sharper response on the low side of the peak.

Joe Farley, Near Chicago
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Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #18 on: August 15, 2017, 1430 UTC »
OK, thanks for confirming the Q. I was sure I made an error in my calculations.  This is, as Adrian Monk might say, both a blessing and a curse for my DGPS DXing.  On the plus side, the high Q should result in good sensitivity. On the negative side, I can only see a small portion of the DGPS band at a time. My experience is that the DX openings are often very brief, sometimes lasting just a minute or two. So I potentially miss most of them if I can only look at a few channels at a time. OTOH if I have excellent sensitivity during that time, I might pick up stations that I would otherwise miss on another antenna. 

I'm going to try and run DGPS recordings each night both with the AFE822 and 500 foot beverage (which isn't really a beverage on the DGPS band), as well as the SDR-14 and resonant PVC loop, and compare the results. I believe I can modify the control box for the resonant loop so the tuning voltage slowly ramp over time, that is, superimpose a sawtooth on top of the DC tuning voltage. I can then sweep it through the DGPS band at a fixed rate. Not sure what that rate should be. It should be fairly slow, due to the message rate. Perhaps something in the range of 1 kHz per minute. O maybe sweep the entire 40 kHz band every half or or hour?

I will orient  the loop in a fixed direction for each night's DXing session. I think there are two positions I want, which happen to be roughly perpendicular to each other, one towards Europe, the other towards the Pacific Northwest.  While the nulls are fairly sharp, the peaks seem to be broad? I could try nulling out one local or semi local pest with each orientation.

One potential issue is the Annapolis station on 301 kHz. It is extremely strong. When the loop is resonated on or near it, something saturates. The clipping LED on the SDR does not turn on, so I don't think it is that. However, enabling the 10 dB attenuator setting in the SDR seems to prevent it. So I am not sure what is going on. The orientation favoring Europe should put Annapolis in a null, so that might not be an issue for those nights. But Pacific NW nights will result in maximum signal for Annapolis. I could run with the 10 dB attenuator on those nights.

That's the plan, anyway.  ;D
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Pigmeat

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Re: This might be the start of something big
« Reply #19 on: August 15, 2017, 1616 UTC »
A chain-link backstop, T-ball League size should work nicely as a directional Faraday Cage and reflector. Many have a hitch and wheels so you can pivot them. You'll be the envy of the neighborhood with a 15 ft. chain-link backstop in your yard. Plus, it will mask that PVC contraption so your neighbors won't have anything to complain about.

There's some truth to the backstop as a Faraday Cage, but it involves some yahoo's I know who used park on what was left of an old baseball field behind a shopping center. The only thing left of the field was the area around home plate, an alley that allowed access and a 20 plus ft. backstop that loomed overhead. They'd park there to drink beer and smoke weed. I lived nearby and would walk my dog through the fields out there. I'd stop by to talk to them when I would see them. They constantly complained about not being able to hear the local FM rock station, whose stick was dead across the valley about a mile and a quarter away. I'd say, "It's damned odd, isn't it?" and get back "It sure as Hell is, you get back out on the main road and there it is!" I'd say "It must be this low spot coming off that little bank, the cold air collects down here, probably causes an inversion layer that messes with the reception from over that way." and get back "Yeah, that's it!" or something like that. Like any of those health class hero's knew what an inversion layer was!

I miss the 70's. Today's nimrods can look up "inversion layer" on their phones.

Offline Josh

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Re: This might be the start of something big
« Reply #20 on: August 15, 2017, 1635 UTC »
Quit farting around, you know you want to roll a wullenweber.
http://www.premium-rx.org/ref/wullenweber.pdf
Conveniently located near Vincennes Indiana.

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #21 on: August 15, 2017, 1832 UTC »
I posted the results from last night's spur of the moment recording of the loop on the DGPS band, comparing them to the AFE822 and beverage. Very interesting results: https://www.hfunderground.com/board/index.php/topic,36708.msg137349.html#msg137349

Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #22 on: August 19, 2017, 2136 UTC »
I've reconfigured the PVC loop from resonant mode, it is now connected to the LZ1AQ amplifier.

I ran a quick test of it vs the beverage, recording at the same time, here are the results. The antenna was aimed roughly at a 50 degree angle, to New England and Europe.

Slightly better on stations in the beam. It did not see Kensington or Detroit at all, but they are further out of the beam, closer to the null. It's daytime, so it is not really the best time to test out reception on DGPS. I am going to record overnight, and see what I get.
 
Beverage:
Count    ID   ref1 ref2 kHz   Baud City                           Country              Lat      Lon      km     Deg
66       771  196  197  294.0 100  New Bern, NC                   United States        35.181   -77.059  503    181
95       844  094  095  324.0 200  Hudson Falls, NY               United States        43.272   -73.542  489    35 
131      772  198  199  306.0 200  Acushnet, MA                   United States        41.749   -70.889  561    64 
202      806  012  013  289.0 100  Driver, VA                     United States        36.963   -76.562  307    173
252      803  006  007  293.0 100  Moriches, NY                   United States        40.794   -72.756  378    70 
455      847  058  059  301.0 200  Annapolis, MD                  United States        39.018   -76.61   82     158
457      804  008  009  286.0 200  Sandy Hook, NJ                 United States        40.475   -74.02   266    70 


PVC Loop:
Count    ID   ref1 ref2 kHz   Baud City                           Country              Lat      Lon      km     Deg
24       778  192  193  292.0 100  Kensington, SC                 United States        33.491   -79.349  723    198
55       771  196  197  294.0 100  New Bern, NC                   United States        35.181   -77.059  503    181
87       844  094  095  324.0 200  Hudson Falls, NY               United States        43.272   -73.542  489    35 
94       838  116  117  319.0 200  Detroit, MI                    United States        42.306   -83.103  590    301
114      772  198  199  306.0 200  Acushnet, MA                   United States        41.749   -70.889  561    64 
161      806  012  013  289.0 100  Driver, VA                     United States        36.963   -76.562  307    173
241      803  006  007  293.0 100  Moriches, NY                   United States        40.794   -72.756  378    70 
404      804  008  009  286.0 200  Sandy Hook, NJ                 United States        40.475   -74.02   266    70 
443      847  058  059  301.0 200  Annapolis, MD                  United States        39.018   -76.61   82     158
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #23 on: August 20, 2017, 1547 UTC »
I switched the PVC loop to the LZ1AQ amplifier yesterday, and ran it overnight. Originally oriented to New England and Europe, but a storm early in the evening appears to have rotated it to NNW. Not really an ideal orientation. Plus lots of static last night, so not really ideal conditions either.

I posted some DGPS logs here, along with a comparison against the beverage: https://www.hfunderground.com/board/index.php/topic,36779.0.html

More and more I think I need to run the PVC loop in non resonant mode. I miss using the LZ1AQ amp with my hula hoop antenna for SW though, for the NRD-545.
« Last Edit: August 20, 2017, 1549 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #24 on: August 20, 2017, 2221 UTC »
I spent some time rotating the loop, while monitoring the waterfall on SdrDx (I have an iPad app I wrote that displays the waterfall by connecting to SdrDx to get the data, very handy for working on antennas outside, without having to run back inside to the shack to see what happened when I changed something). I can get some pretty good nulls with it. This is in non-resonant mode, using the LZ1AQ amp. Hopefully tonight I can get it to stay in one orientation and not rotate part way through the night. Now to decide if I want to aim it towards New England / Maritimes / Europe or to the Pacific NW.

What I need to be able to do is rotate the antenna. I am considering using the rotor for the 250 MHz satcom helical antenna I built a number of years ago (the antenna was destroyed in a snowstorm), you can see it near the bottom of this page: http://www.radiohobbyist.org/blog/?p=933

Most of the weight of the loop antenna is supported via a rope that goes up and over a tree branch. So the rotor won't need to support the antenna, just rotate it. I'll need to mount the motor at an angle so the shaft is vertical, but I think that can be done. 
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #25 on: August 22, 2017, 1523 UTC »
I posted last nights DGPS logs here: https://www.hfunderground.com/board/index.php/topic,36830.0.html including a comparison of the loop (non resonant, with the LZ1AQ amp) vs the beverage. The loop is certainly directional, which is what I wanted. Lots of storms last night again. Things should calm down later this week, and perhaps the comparisons will be more interesting. At least the loop stayed put last night and did not rotate on its own  ;D  Another project, perhaps on the weekend, will be to get the rotor attached to it.

On another front, I am closer to having a version of Amalgamated DGPS that can do live decoding, directly talking to the SDR. This will let me see the effects of rotating the loop in real time. This will only work with the RF Space and AFEDRI SDRs.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

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Re: This might be the start of something big
« Reply #26 on: August 26, 2017, 2303 UTC »
A short update. I have a rotor attached to the PVC loop. That itself was a saga, which I will eventually write up separately...

Being able to adjust the loop inside while monitoring the signals is very educational, and it turns out necessary. The nulls are very sharp, just a degree or two wide it seems. And fairly sharp, I can put a -35 dB dent into the signal from local DGPS station Annapolis MD on 301 kHz. Various other DGPS stations can be completely nulled out, as can those very annoying NDBs that pollute the DGPS band  ;D

The loop is not nearly as directional on the MW band, and the pattern seems very funky when you get to the X band, presumably because it is way too long. The LZ1AQ amp I use has two inputs, right now the loop has a pair of 9 turn windings, from when I first ran it in resonant mode. I am now sold on using the LZ1AQ amp instead, so I can reduce the number of turns on one of the windings, and maybe get something that works well on MW.  Another question is whether I want more turns on the LW winding?

Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 120 ft T2FD/ 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ultravista

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Re: This might be the start of something big
« Reply #27 on: September 07, 2017, 1343 UTC »
Perhaps too big.

Try one of these - the bigger ones have two heavy springs @ a 8-9 foot collapsible center post. The legs are ~ 4 feet long, can fold up, or down as needed. The wind load is pretty good.

I have a few that I use two hold 20 feet of galvanized fence post + a 20 foot fiberglass pole.



http://www.roadtrafficsigns.com/windmaster-dual-spring-roll-up-and-metal-sign-stand/sku-wm-0006

I would like to build something similar for HF/MW/LF work - what do you suggest?
« Last Edit: September 07, 2017, 1349 UTC by ultravista »

Offline Pigmeat

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Re: This might be the start of something big
« Reply #28 on: September 07, 2017, 1932 UTC »
I've seen road crews in adjoining states using those things with a MW tx in a metal box near the base as shortened antennas to advise drivers of changing conditions for years. The combo seems to have a roughly two mile radius, so my guess is they're allowed to exceed to 100 mw maximum of most consumer grade MW tx imposed by the FCC?

Offline redhat

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Re: This might be the start of something big
« Reply #29 on: September 07, 2017, 2322 UTC »
That falls under TIS rules which I think allow 10-20 watts depending on the license.

*edit*
TIS falls under CFR 47 part 90.242 which says 10 watts with a maximum antenna of ~49 feet.  A basic rundown of this and a search tool from the commission...
https://www.fcc.gov/media/radio/travelers-information-stations-search
The actual rules pertaining to this service...
https://www.law.cornell.edu/cfr/text/47/90.242

+-RH
« Last Edit: September 08, 2017, 0531 UTC by redhat »
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