2018-12-11

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).

2018-12-10

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.


2018-12-07

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.

2018-12-06

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.

2018-11-17

Tropo on 2m

Yesterday we had some tropo propagation on 2m.
Stations from most of Northern Europe were received, as well as a Southern France station in Provence.
In the evening the best signal was EA2XR in IN83ki (about 1750km), though I did not have enough power to work that one, and I just saw it after the fact, anyway.
The PSK Reporter map of stations received looks good.

Update 0855Z : 0739Z R5WM in KO72qi ( Kursk - about 1550km)

2018-11-16

New Satellite With Amateur Radio Transponder Launched


The Es'hail 2 satellite has been launched and is going into geostationary orbit. This satellite has TV transponders, and something new for radio amateurs : a 2400MHz to 10GHz transponder in geostationary orbit.
The coverage should be all of Europe and Africa, to the East into India, and to the West a tiny bit of Brazil. I assume that some testing will be going on before the transponders go online, but this is an exciting time for radio amateurs interested in satellite communication.

For reception a 1m (3ft) dish with a PLL controlled LNB, as well as a receiver around 700MHz should be sufficient to hear the transponder on 10GHz, even if the frequency stability is probably not very good.
That would be my first step in amateur geostationary satellite communication.
Now I have to find a PLL-LNB.

2018-11-06

VP8 on a "Dead Band"

This afternoon provided a new DXCC for me, since I moved back to Denmark.
I was monitoring 10m FT8, and suddenly VP8LP (Falkland Islands)showed up on the screen, calling CQ. I did not even get the linear started
A single call from me was sufficient, and we exchanged -05/-09dB reports.
Now LW5DD is on ... starting the linear . LW5DD is long gone, but I called CQ for a while with no reply.
Earlier I could read ZS and 3B9 on 10m.
In any case, right now at the bottom of the solar cycle "10m is dead" .... No, not really.