The Small Loop RX Antenna Adventure Continues.

Slightly updated.

 This is another attempt to achieve a good receive antenna system for monitoring different frequencies in as many (ham) bands as possible.

Stage one was the experiment with the Mini-Whip described in recent posts.

The active Mini-Whip did provide some decent to good reception on frequencies ranging from 16kHz up to 18MHz.

Not bad for such a small antenna.

Now the experiments have started for a small passive wideband loop antenna. The main idea is not new (at least 60 years old), but there has been a resurgence of interest since SDRs with good sensitivity and with reception capability on VLF have started becoming available at reasonable prices.

Looking at the "YouLoop" antenna made for use with the Air-Spy SDR I decided to look into this. 

This is in essence a "Moebius Loop" type antenna, a design invented in the 1960s, as far as I know. A version is available at the well known auction site, so I took the lazy way and purchased one, just to see what all the fuss was about.

This is the initial result of a quick comparison of the Mini-Whip and the YouLoop:

First I want to point out that the YouLoop as purchased is not suitable for outdoor use - no waterproofing.

This means that the Mini-Whip is mounted outdoors in one of the least noisy places I could find in my garden, furthest from houses, mine or neighbours' houses, and the YouLoop was hung inside the house, in the noise field.

Because the Mini-Whip is active the signal level is much higher than from the small loop. However, whereas some signals were audible with the active Mini-Whip and too weak to be heard on the loop, others were swamped in noise on the Mini-Whip and weaker, but clear with the loop. 

I am aware that the loop might benefit from a sensitive preamplifier, especially on low frequencies and with low sensitivity receivers. This I need to test.

Signals on the loop were a bit weak and noisy on medium wave, and somewhat comparable in S/N to the Mini-Whip on 1.8 to 10MHz. Above this the loop did not increase the receiver's noise when connected, indicating either insufficient sensitivity of the receiver (possible) or simply a too low output of the loop. MW was full of rather strong signals with the Mini-Whip, and considerable weaker (and more noisy) signals came from the loop.

I suspect that a low noise preamplifier at the loop will improve this, especially if a long cable is used. Oh, darn! more projects ;)

Having seen the results I decided to make my own (weather proofed) version of the loop. I already had some hula-hoops (yes the children's toy ones) stored and found a few of them. The first test will be a loop of similar size to the original "YouLoop" (My YouLoop clone). Using two lengths of RG-58 soldered together crossing the shield and inner conductor at the top of the loop, feeding the inner conductors at the bottom to a 1:1 transformer (type 73 binocular ferrite core), and connecting the shields together at the bottom. Here is a description already made, so I will not repeat everything. I will report back when the test has been done

I will have to check whether or not a connection of the feed cable shield to the common shield is beneficial wrt noise performance. I suspect it will be.

I have two more (larger) hula-hoops, so I intend to test a larger (2.5x or so) version of the loop for getting higher signal strength.

The loop antennas do have another advantage over the Mini-Whip: They are directional and have a null, making it possible to reduce interference/noise by turning the loop. For now I intend to use both the Mini-Whip and loop antenna, and switch between them to get the best possible S/N on a given signal.

Using separate receive antennas is beginning to take shape, and with the addition of a distribution system,  preselector/filters/amplifiers a decent propagation monitoring system, such as FT8/WSPR and/or beacon/QRSS monitoring.

Ground connection for the Mini Whip. RX Test.

 I have a ground rod on the Mini Whip now. Not too far into the ground, about 60cm/2ft. Hammering a 2m (8.5ft) ground rod is tricky without the right tools ;)

The results are interesting:

A relatively detailed review was done today, by dialing slowly through the bands. Starting downwards from around 18MHz indicates that the sensitivity/SNR has improved reception on frequencies from about 1.8MHz up to 18.2MHz, maybe higher.

Medium wave is not bad, with signals on essentially every channel in the whole band, with a reasonable SNR, probably increased sensitivity.

NDB band appears noticable worse wrt SNR, possibly partly due to overload from MW stations in the evening, but noise level seems to have increased.

LW seems OK, not spectacular.

VLF (<100kHz) seems more sensitive, but with more noise. possibly due to overload of the RX. 

A different Mini-Whip with a real whip antenna (1 - 3m long) might improve sensitivity on VLF/NDB bands, but should probably have a low pass filter with a 3dB cut-off frequency of 530kHz. Because a thin whip mounted low is not too visible I might add this to my antenna farm, just to see how it works. If it works well I can make a permanent installation.

Because the Mini Whip is very broadband it may be an advantage to make some filters in any case, possibly as follows:

Low pass with 150kHz 3dB point, for the 136kHz amateur band and "VLF" reception.

Band pass with 150kHz high pass and 530kHz low pass, mainly for the NDB/marine bands and 472kHz amateur band.

MW band pass with 530kHz high pass and 1700kHz low pass.

MF band pass with 1700kHz high pass and 3MHz low pass, mainly for the 1.8MHz amateur band.

HF band with 3MHz high pass filter.

If I get all working nicely I may make a (set of) distribution networks, possibly with switches for the different receivers.

Yes, I like monitoring a lot of frequencies I do have some receivers, everything from purchased amateur equipment to portable transistor radio(s) with external antenna input(s), and, of course some SDRs, and some simple monitor receivers (e.g. a Pixie kit for 40m), and some that I may build. At some time I have been monitoring (audio or data) on 14 different frequencies simultaneously, part of it as monitoring propagation, and I expect this record to be beaten ;)

This is also part of my intention to make a contact on as many bands as possible with (at least partly) home made equipment.

Another Receiver on the Mini-Whip. VLF Reception.

 For a few days I was using my old FRG-100 with the Mini-Whip. The receiver appears to have a rather wide IF filter, meaning that I could hear a carrier all the way through zero-beat (part of the other sideband audible) Not good for listening to AM signals.

Also the lowest frequency I can receive with the FRG-100 is 130kHz. I would like to listen to lower frequencies.

Enter an old AOR AR-7030 I have. I think, according to the specification, the receiver covers 30kHz to 32MHz. Very nice, even if I wanted to test it on a lower frequency. Actually, the AOR frequency dial goes right down to 0kHz. Well not really. on the very lowest frequencies it is totally deaf (no surprise).

Tuning upwards with the Mini-Whip connected the noise starts coming up around 10 - 12kHz. Now it gets interesting. The lowest frequency signal I could receive (until now) is on 16kHz (origin unknown). This means that I should be able to receive the SAQ transmitter from Grimeton, Sweden, next time it has a transmission.

Further, the filters in the AR-7030 are clearly better than those in the FRG. Mounted in this RX are IF  filters with bandwidths from 2.1kHz up to 9.5kHz, in several steps, and also tone controls for bass/treble. There are even better filters in my transceivers, but those have a lower frequency limit of 30kHz or more.

Never the less, VLF signals from as far away as Grindavik, Iceland on 37.5kHz were heard in the day time. Strictly speaking not VLF, but definitely a nice reception. This is a data signal that I did not attempt to decode.

The station in Cutler, Maine (US) was not audible with this set-up, but then again, the antenna is not yet optimized for lowest local noise level. The weather and other activities have been in the way of connecting a ground connection to the antenna. and other steps.

Tuning up through the band, all the way up to 518kHz NAVTEX frequency I now come across a lot of signals. In particular I now hear more NDBs (non-directional beacons used for navigation) in the 300 - 500kHz range, along with other signals of unknown origin (to me at least)

One surprise was a signal on 505kHz:

OK0EMW in JN88ks running CW identification (with Locator) and some QRSS, too.

This must be a remnant of the activity that was allowed on experimental basis on the "old" 500kHz band. I was not aware that there was any activity there any more. Apparently the (QRSS) etc beacon was allowed there those years ago, and the permission has not been revoked.

There is much more to do regarding MF, LF and VLF reception. One thing is to test if I can really hear the SAQ transmission next time it will be active.

Another test should be using my IC-7600 for reception on LF/MF. With the much better (DSP IF) filters reception of many more NDBs and other signal should be possible. There is a lot of noise in that part of the band, so every improvement of the receive side will count. Also, a set-up with a low frequency SDR should be done. The first SDR experiment should be with the RTL-SDR in direct sampling mode, then maybe later I will get a more suitable SDR, also for those frequencies

For the lowest frequencies a test with a computer sound card should make reception up to 24kHz possible, maybe even up to 48kHz with 96kHz sampling.

Active Receive Antenna: The PA0RDT "Mini-Whip". First Experiments.

 A little while ago I purchased a Mini-Whip set (the outdoor unit and the bias-tee) from Chine. Quite inexpensive, and now when the weather is improving (some days, it is April, after all) I started an experimental set-up.

I was aware that I should avoid noise from entering the antenna via the outside of the coax, so the first step was using a common mode choke (CMC) at the antenna, with a low cost (green) EMI toroid from China.

The antenna is mounted on top of a 4m long telescopic fibreglass mast, intitially just attached using duct tape, placed at a relatively low noise point in the garden with more or less optimized distances to all surrounding houses. This is sufficient for the testing and can be changed later. The power supply is a battery, just to avoid noise entering directly the system via mains power.

The result was quite disappointing, but not entirely unexpected up to 7MHz there was a substantial noise floor, reducing the signal-to-noise ratio (SNR) when compared to a multiband dipole I have in the garden, and the noise level on medium and long wave (MW, LW) was killing all signals less than S9 Even then, and this was encouraging, more than half of the MW frequencies showed audible stations before sunset.

Next step: Adding a "SW" CMC just next to the bias-tee. This was using a FT 240-43 toroid core with 2x10 windings of RG-174 coax. Progress: The noise was reduced on SW, now the Shannon VolMet on 5505 was audible with a decent signal, not quite as good as with the dipole, but a definite improvement. MW was still quite noisy, and LW not good enough. Very few NDBs (Non-directional beacons, mostly situated at airports) were audible in the 300 - 500kHz band. signals on 472kHz were still drowning in noise.

Yet another step: A second LF CMC was added (one more of the green Chinese toroids) was added, making MW quite usable, though not perfect.Several NDBs popped out of the noise, and with better filters in a receiver there would probably be many more to find. The usable frequency range was extended to 400kHz up to at least the 30m amateur band at 10MHz. The Shannon VolMet now has a SNR comparable to the dipole. Good progress.

This is the state of affairs at the moment. There is still too much noise for my taste, but at least I can work on removing noise coming from my own house.

There is more to try:

1) adding a second (MF/SW) CMC at the antenna. This is not too difficult.

2) Adding a ground connection directly to the ground of the Mini-Whip PCB, "bypassing" the CMCs at the antenna.  This will take a bit longer because good weather is necessary if I want to solder outdoors. As an alternative the antenna could be dismounted and the soldering could be done indoors. Further, a 2m ground rod should be hammered down.

Still more to do with this receive antenna system, but it is a decent start

LW/MW Loop Renewed.

The wires were stuck in the 10/20 turns loop, so new hoops were cut.
The first one is a bit larger than the original one, and was wound with 5 turns. Resonant frequency got up to 2,1MHz, a little low for comfortably using it on the 160m band, so one turn was removed.
The 4 turn loop has self resonance on close to 2.5MHz, so I consider it acceptable for use on the 1.8MHz band.
It looks like an amplifier is needed to get better sensitivity.
The second hoop is intended for a single turn HF wideband loop, tests with transformer and  amplifier should be done this year, as I have a cheap LNA, officially with a low cut off of 5MHz, but I suspect it will work fine on the 80m amateur band, too. A later version will probably be tested with a 1MHz cut off LNA, which is on the way from China.
The 10/20 turn loop will probably be tried out with even lower frequency reception (<100kHz).

LW/MW Loop : Self Resonance Measurements

20181209 :
I added an improvised electrostatic shield on the multiturn loop. Simply some kitchen aluminium foil wrapped around the loop (yes, with the proper gap) and connected to the coax shield with a test cord with alligator clips.

This is a simple test, and shows a considerable improvement of noise performance for the loop, both on the LW, and also on the MW band.
NDBs (navigation beacons) started showing up in the 250 - 500kHz band, some noise disappeared from long wave broadcast stations, and the noise level on MW is considerably lower, even with the antenna still indoors. I expect the noise level to reduce further when the loop is placed outdoors, 10 - 20m from all noisy houses.

A new MW evening propagation test is planned for tonight.

A mechanically improved shield will be made. I have some copper tape, sold as snail repellant, that will be wound around the loop and connected to the coax shield.

The loop design is now beginning to take shape, and when a cheap LNA capable on low frequencies arrives in some weeks. I will test this and, if necessary, build it into the connection box of the loop, along with a bias-tee arrangement.

20181210 :
The MiniVNA Pro2 arrived this morning, and some measurements of the loop were made. Initially I could not get the Bluetooth system to work, so I used the USB port, and got it working.
With 10 turns the self resonant frequency of the loop turned out to be around 900kHz, somewhat consistent with the performance I have seen.
The test with 20 turns lowered the self resonance to about 300kHz, I estimate that to be due to a combination of the 4-fold inductance and a considerably increased stray capacitance between windings. This is also consistent with the performance I experienced when initially testing with 10 and 20 turns.

The electrostatic shield made very little difference to the self resonance, so it will be re-instated in the final version of the loop.

Next experiment : Reduce the number of turns of the loop to 5 or 6, and see how much the self resonance increases.

MW/LW Loop - transformed

I went to pick up RF transformers today. When inserted both noise and signals are attenuated, but it looks like the S/N is somewhat better.
The antenna was still located in the noise field of the shack, so I am not too surprised. 
Time to connect a longer cable (still indoors) to test the loop a bit away from the shack with all its computers and SMPSs.
The weaker signals may be due to a different impedance match of the receiver through the transformer to the loop.

Moving the loop further away from the shack noise, about 3 - 5m, offers a considerable reduction of the noise level on LW. The RTTY station on 138kHz is now essentially noise free, i.e. I estimate it could easily be demodulated Signal is about S6, and noise is down to about S1. Another RTTY signal on around 133kHz is now clearly audible, and one on about 147kHz - previously drowning in the noise - is now quite strong. That was not the case with the loop closer to the shack noise.
MW is not so much better in that respect, so maybe too many turns still create too much stray capacitance. MW is better, but a considerable noise level still exists on large parts of the band, mostly a harsh hum with many overtones, probably from SMPSs.
Is it possible that I should use fewer turns to optimize the performance on MW? One more thing must be tested before testing the loop outside, and about 12-17m from the noisy shack - and 10 - 20m away from other potentially noisy houses.
Enough experimentation for now. More in the week end. Electrostatic shielding should be tested to see if the S/N can be further improved.

Adventures in Wideband Receive Antennas for LF and MF, Part 1.

After seeing posts on Youtube about people making simple wideband LF/MF/SW antennas I decided to go make my own. This is just the beginning, and a first test.
Two videos are the origin of my design, which is still under development.
One describes a simple single wire loop of about 3m circumference, connected to a cheap Chinese LNA, covering 1-2000MHz (price about $10). Sort of overkill in relation to the frequency range, but it is a very simple circuit to make, even with some not too tricky mechanical work.
The second video describes the use of a hula hoop to support the antenna. The loop described here has several turns of single strand wire pushed through the hoop, and is supposed to work up to about 10 - 12MHz.
I decided to make at least two loop antennas, one covering LF and MF, and another covering the HF bands. This is the beginning of the experimental LF/MF loop antenna.
I have purchased some cheap hula hoops on the toy store, priced less than $10.
I already had some cheap old fashioned speaker wire, so how to get several windings of that into the hoop ?
First step is cutting the hoop, so you have free ends of the plastic tube available. The hoop is quite tense, so the moment it is cut, it expands the radius, and you end up with a half circle of tubing. That is actually not too bad when you start pushing wire into the hoop. the speaker wire went nicely through to the other end of the tube.
Next step is a bit more tricky, but with a bit of thinking it was not difficult, but it was some work. I bent the tube with the wire through it, so the ends almost met, distance about 3cm, then used duct tape (surprise ! ;) ) to hold the ends together, then taped the end of the wire to the piece going through the hoop.
Now comes some hard finger work : Pushing/pukking the wire through the hoop, about 2cm at a time. After about half an hour or so I had 10 windings of 2-wire speaker cord.
[picture here]
If connected correctly in series this amounts to 20 turns of wire, but at first the test was done on 225kHz with 10 turns connected to a piece of RG58 cabe and my FRG-100 receiver - (in the mid-afternoon, so not many MW stations audible)
The signal was a steady S9+5 on the meter, so it was time to try connecting the windings in series, creating the 20 turn loop.
The 20 turns showed a weaker signal. Time to think a bit. I will assume that the problem with the 20 turns is a higher stray capacitance between the wires, lowering the self resonance of the loop.
Back to 10 turns it was, and voila ! The signal was back up again.
Do you see a weak point with this construction - not mechanically, but electrically ?
The multiturn loop is connected directly to the cable, creating (at least) two problems :
- the loop is originally balanced, the cable connection with a coax cable creates an imbalance, making the loop more sensitive to noise.
- the direct connection further has a galvanic connection to other wiring, giving the noise more opportunities to enter the loop.
Those two problems can be solved by inserting a RF transformer between the loop and the cable. The balance problem may not be completely eliminated, but, at least, it will be reduced. A local ham has offered me a 1:1 transformer that should be capable of handling the bands the antenna is meant for. Nice for initial testing.
I do have some low frequency toroids cores that I will test when I have the proper instrumentation. I suspect that it may be a good idea to use some up-transformation of the impedance/voltage from the loop.
I would like to be able to use this antenna for 136kHz, 472kHz and 1.8Mhz, and, of course for the MW broadcast band. It is possible, however, that I may have to insert some filtering in order to avoud overload of the receiver by strong MW signals. This will be tested, and added if necessary.
Here is a question that some may ask : Why make this solution when a properly constructed tuned frame multi turn antenna is better (no doubt it will, technically) ?
Well, The first point is simplicity of construction. I find it hard to imagine a more simple construction of a LF/MF antenna. Simply, do not make things more complex than necessary.
Second, I wanted an antenna capable of being used for more than one frequency, simultaneously.
Third, I wanted to go through the process of improving the system as the design allows.
In the near future there are, at least, the following improvements :
- adding the transformer, and possibly using an up transformer.
- test the possible improvement with an electrostatic shield.
- test if an amplifier is necessary in order to improve the S/N ratio.
- test if filters are necessary, because I will use this antenna in an environment with transmitters on higher frequencies than the design of this antenna, and the possibility of needing a filter eliminating strong MW broadcast transmitters.
- building the mechanical construction supporting the base and feed point of the antenna

Until now the testing of the antenna has been inside the shack, which is a very noisy environment with computers, switching mode power supplies, mains noise etc, so some preliminary tests of the open construction outdoors may be necessary, especially to determine the need for an amplifier.

I look forward to do some more testing and finish the project, so expect a few more posts about this, and then phase II : HF active loop antenna.

I went to pick up RF transformers today. When inserted both noise and signals are attenuated, but it looks like the S/N is somewhat better.
The antenna is still located in the noise field of the shack, so I am not too surprised. 
Time to connect a longer cable (still indoors) to test the loop a bit away from the shack with all its computers and SMPSs.
The weaker signals may be due to a different impedance match of the receiver through the transformer to the loop.
Moving the loop further away from the shack noise, about 3 - 5m, offers a considerable reduction of the noise level on LW. The RTTY station on 138kHz is now essentially noise free, i.e. I estimate it could easily be demodulated Signal is about S6, and noise is down to about S1. Another RTTY signal on around 133kHz is now clearly audible. That was not the case with the loop closer to the shack noise.
MW is not so much better in that respect, so maybe too many turns still create too much stray capacitance. MW is better, but a considerable noise level still exists on large parts of the band, mostly a harsh hum with many overtones, probably from SMPSs.
Is it possible that I should use fewer turns to optimize the performance on MW? One more thing must be tested before testing the loop outside, and about 12-17m from the noisy shack.
Enough experimentation for now. More in the week end. Electrostatic shielding should be tested to see if the S/N can be further improved.