HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on August 11, 2017, 2304 UTC
-
Perhaps too big.
Test fit of the PVC pipe for the 10 ft by 10 ft loop antenna. That little patio umbrella stand, as heavy as it is, is no match for the loop frame. But that is just temporary, the loop is actually going to hang from a tree limb, at least for initial prototyping and testing.
The weight of the pipe itself is about 14 lbs, plus throw in the fittings, my guess is 16 or 17 lbs total.
(http://i.imgur.com/ZptXgqn.jpg)
-
Be sure to paint it red & white and put a light on top.
-
Perhaps this will be incentive to get my MW TX running?
+-RH
-
Nice!
When it's up in the tree, are you going to rotate the entire tree? If so, Fanco makes a nice ultra heavy duty rotator which will do the job. I assume you already have the required 480V 3 phase service to your shack?
-
These things are a sure fire wife pleaser!
-
Be sure to paint it red & white and put a light on top.
But then I gotta worry about getting one of the peeling paint NOVs.
Nice!
When it's up in the tree, are you going to rotate the entire tree? If so, Fanco makes a nice ultra heavy duty rotator which will do the job. I assume you already have the required 480V 3 phase service to your shack?
I got a surplus nuclear reactor from a Russian sub, so I should be all set.
As I mentioned earlier, this loop frame is a test bed vs a known path. I'll try both a resonant loop with a jFarley amplifier, as well as perhaps with a tuning cap and pickup loop. And maybe with the LZ1AQ amplifier in non-resonant mode. I am somewhat torn as to use of resonant mode. While it has huge advantages, my DGPS data shows that the spectacular openings are inherently brief. Meaning you want to be looking at the entire band at once, not just a few channels. I guess I could run with a very low Q, so the entire DGPS band is passed, but I am not sure if that will be useful. Again why this is more of a journey than a destination.
Rather oppressive out today, but the dew points are supposed to be lower tomorrow, so maybe I can get back to it then. I'm going to set up a little table right next to the loop (more like under it I guess) with the AFE822 and laptop, so I can evaluate it in real time as I rotate it and fiddle with the settings and loop itself.
-
Perhaps too big.
Test fit of the PVC pipe for the 10 ft by 10 ft loop antenna. That little patio umbrella stand, as heavy as it is, is no match for the loop frame. But that is just temporary, the loop is actually going to hang from a tree limb, at least for initial prototyping and testing.
The weight of the pipe itself is about 14 lbs, plus throw in the fittings, my guess is 16 or 17 lbs total.
(http://i.imgur.com/ZptXgqn.jpg)
What size PVC pipe are you using there?
-
What size PVC pipe are you using there?
1 - 1/2" Schedule 40. Some (hopefully all) of the sagging is because I did not completely insert the pipe into the fittings yet. The wire may help to tighten things up as well. No idea how many turns or what gauge I will end up with, that is part of the experimentation.
-
You will probably have to rely on the loop wire for support, as PVC has a tendency to snap under tension. I would also paint it as the UV will cause the plastic to brittle over time.
+-RH
-
First test. Using two pairs of turns (I happened to have some #16 scrap wire of that length) feeding the resonant amplifier, I get a useable range of about 900-2200 kHz. Fairly high Q even with the small gauge wire, I have to re-tune each time I switch to a different MW channel.
Now to add some more turns. Or one more turn perhaps for all of MW? Not sure I have enough capacitance range to cover the entire MW band, however. A few more should get me down to DGPS, but I am not sure this will work well for decoding the entire band at once, as was my concern. I may be lucky to get a few channels. :)
-
Could you make multiple antennas on the same physical pipe covering a different part of the band, then combine the signals? If you need many, that wouldn't be feasible. But maybe several might?
-
Could you make multiple antennas on the same physical pipe covering a different part of the band, then combine the signals? If you need many, that wouldn't be feasible. But maybe several might?
I'm not sure how well a tapped resonant loop would work. Paging jFarley to the Electromagnetics Lounge.
Some more data:
Going up to 6 turns gives me a usable range of 350-700 kHz. Yeah. I just miss the DGPS band.
Going to 9 turns gives me a range of 250-530 kHz. In this case Murphy struck by my spool of wire being 3 ft too short to finish the last turn. Fortunately I found some scrap wire in the shed. This is a nice range in that it gives me DGPS plus the NAVTEX channels. Also most of the NDB band. Another turn would probably give me all of the NDB band but then I'd lose NAVTEX. Decisions, decisions.
The useable bandwidth in the DGPS band seems to be about 2 kHz, I'll try taking a screenshot later. I guess that implies a fairly decent Q. Especially for something thrown together with some #16 hookup wire.
What I have ended up doing, for now anyway, is supporting most of the weight of the PVC frame with a rope over a branch. The bottom pipe then rests on the ground, and I can adjust the azimuth of the loop. Works pretty well.
(http://i.imgur.com/QLgYU6S.jpg)
-
Now that is one awesome radio room you got there in your backyard Chris! :o
-
That's much more graceful than that 8' x 8' loop built of 2"x4"s of Dick Pierce's that made it in to Pop 'Comm back in the '90's. Kudos!
-
Just stuck my head in the door of the Electromagnetics Lounge, but the coffee wasn't ready yet. Will be back...
In the meantime, the most important thing I look for in a loop design is the Effective Height, which is a measure of its signal collecting ability. This is 2 * PI * N * A * Q / lambda, and if I have the following correct:
N = 9T
A ~ 9 M^2 For a loop with 10 foot sides
lambda = 1000M for 300 kHZ
then the He for the loop would be approximately 0.5 * Q Meters.
Knowing what the actual Q is would give a rough idea of what to expect from this loop in resonant mode.
I see that you are using the amp/supply from the HF loop. This uses back to back tuning diodes such as NTE618. These devices, IIRC, have a guaranteed minimum Q of ~ 100 at 1000 kHz. In a tuning diode, the Q (at a given bias) is generally a close to linear function of frequency. My recollections of trying this type of tuning with the ferrite loop seemed to validate this, and that the diode Qs seemd to be significantly lower at NDB frequencies. Even so, if the diode Qs were around 30, the the He would compute out to be around 15M, which would not be too shabby.
If you could actually measure the Q from a screenshot, that would be cool and helpful.
-
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.
-
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.
(http://i.imgur.com/3kBbW0M.png)
(http://i.imgur.com/78ZBQep.png)
-
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.
-
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
-
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.
-
Quit farting around, you know you want to roll a wullenweber.
http://www.premium-rx.org/ref/wullenweber.pdf
-
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
-
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
-
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.
-
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.
-
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.
-
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?
-
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.safetysign.com/images/source/medium-images/X4702-SRG.jpg)
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?
-
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?
-
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 (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 (https://www.law.cornell.edu/cfr/text/47/90.242)
+-RH
-
Two more photos:
The rotor. This is an old satellite dish rotor I used with the 250 MHz satcom helical antenna I built, which was destroyed on winter. The rotor used a DiSEqC interface, but that board died. So I removed it, and instead apply 12V DC on the coax cable directly to the motor. I have a box inside the shack with a DPDT switch which allows me to apply voltage of either polarity, to control the motor direction. I have no feedback for position. The motor drive does have an encoder and home switch, so if I was ambitious I could possible rig up something for position. I had to get a 45 degree PVC adapter to handle the weird angle of the rotor shaft. The plate the shaft is attached to rotates, so the pieces of PVC pipe and adapters align the long PVC pipe that attaches to the loop with it, so it can rotate. Yes, very Rube Goldberg, but it works! ;D
(http://i.imgur.com/vyQ0nXq.jpg)
This is the support for the antenna, a rope that goes up over a tree branch, and takes most of the weight. I don't think the rotor would be happy with all of it:
(http://i.imgur.com/bnV7Iuy.jpg)
-
OK, a third photo, the entire antenna, with a lawn chair for some perspective:
(http://i.imgur.com/N73bkS3.jpg)
-
OK... I've re-wound the loop AGAIN for resonant mode. It's going to stay that way. ;D
I've also made some changes to Amalgamated DGPS for a live SDR mode, where it directly talks to the SDR (no need for an SDR app) and decodes the entire DGPS band in real time. Barely seems to run on the old iMac, but it made it through the night. Now I've got a crude waterfall in the app so I can see where the resonant frequency is and adjust it. I don't have any position feedback on the loop orientation, so I have to look out the window. Which doesn't work when it is dark outside. Which is what it usually is when trying to DX DGPS stations. Maybe I need to add some flashing lights on the loop frame.
-
You have a lot more faith in PVC pipe than I do :P
+-RH
-
You have a lot more faith in PVC pipe than I do :P
+-RH
We have a possible squall line coming through the overnight hours tonight. So that will be the test ;D
It is supported at the top by a rope, in addition to the bottom support to the rotor (actually most of the weight is taken by the top support).
-
Maybe I need to add some flashing lights on the loop frame.
You should put a red light on top anyway, for safety.
To show the direction, put a couple of lasers pointing precisely parallel to it and a smoke generator to see the laser beams.
-
You should put a red light on top anyway, for safety.
Next thing you know, I'll get in trouble for peeling paint.
-
Next thing you know, I'll get in trouble for peeling paint.
To solve that, use red & white PVC in the correct positions, then you'll never have to paint.
-
OK... I've re-wound the loop AGAIN for resonant mode. It's going to stay that way. ;D
I've also made some changes to Amalgamated DGPS for a live SDR mode, where it directly talks to the SDR (no need for an SDR app) and decodes the entire DGPS band in real time. Barely seems to run on the old iMac, but it made it through the night. Now I've got a crude waterfall in the app so I can see where the resonant frequency is and adjust it. I don't have any position feedback on the loop orientation, so I have to look out the window. Which doesn't work when it is dark outside. Which is what it usually is when trying to DX DGPS stations. Maybe I need to add some flashing lights on the loop frame.
Sounds like a reasonable solution for the tuning. I use SDR Console which allows me to have a lower pane the width of the SDR span. I use this for tuning purposes only. I also display a zoom pane centered on the demod frequency. This is about 1 kHz wide and allows me to see barbaric CW IDs in a pileup.
-
One thing I find that is interesting is that ADGPS still decodes stations well outside of the resonant bandwidth. Not with as high of a decode rate, but still reasonably good.