Great idea. I'm going to put one of these together.
73 Tom

VE7RC <ve7rc@...> added the album Loop Launcher: My homemade loop launcher consisting of slingshot and cheap fishing reel. Used successfully to put a weight over 70-foot high evergreen tree branches.

VE7RC <ve7rc@...> added folder /Large Loop Modeling

Pat,
Interesting that you find yours works well at 20 feet high. This doesn't surprise me. Modeling shows how different the performance of loops can be at different heights. According to EZNEC, a higher fixed loop isn't necessarily better. The main lobe shape and maximum angle of radiation are very different depending on height. And unlike a good rotatable beam, with this type of fixed loop you want as omnidirectional as you can get. I notice in the model the directionality you mention. With a two wavelength loop at 160 metres results will likely differ from what you find.

In my case, the best direction is going to be in the SW through W direction. This is OK since I'd like to focus on the Pacific and Asia directions anyway. I figure I should be able to hear and work US/VE stations regardless. If I found Europe and Africa just weren't being received, I could go crazy and install a similar smaller loop in the other direction to favour the east. Also, I think modeling of this type of antenna only goes so far. Trying one for real will tell the tale.

Murray VE7RC

Frank,

Though Bob's youtube presentation is interesting, it is way overkill for my situation. The difference for my situation is I have lots of? tall, skinny evergreen trees. The problem with these trees is they have too few branches at lower heights. The branches they have start at about 50 feet high. But this is good in the sense that the branches and foliage don't get in the way of a slingshot. I have many times used a manual slingshot with fishing line reel attached to place a light fishing weight connected to thin line where I wanted it. It does take time, and patience, and repeated shots to get it right. You also must use a light weight and very thin line. Subsequently, once the weight reaches where you want, you pull up thicker and thicker rope over repeated pullups.?

Murray VE7RC

Murray
I have one for 160 m. I feed it with ladder line to 1 to 1 balun then coax to the inside of the house.
my favorite antenna. Mine is only have 20 feet and does well on 160 in the USA. On the upper bands it is very directional being fed from the corner to the opposite direction which is to the south east of the United States from the northwest. On 10 and 12. I think it acts like a rhombic .

The length and height sound good

With further consideration, I think the loop is more likely to be a rectangle as follows: two sides 165 feet, two sides 357 feet. I don't have enough space to make it a 261-foot square.

Murray VE7RC

Hi Mike,

Ah OK, that clarifies several things.

I think that it is important to include that information in more detail along with your design.

Your design decisions then make a lot more sense when this is known.

Regards,

Martin

Hi Martin,

I agree, the input capacitor of 1nF is too much in most cases.
I used this value with cross-parallel loops 0.7m diameter each, copper foil conductor on kids hula-hoops. The combined inductance is around 1uH. The QTH is a few km from a hill with many FM transmitters. And then it was used to monitor FT8 mostly on lower bands.
So yes, not a typical use case.

73, Mike AF7KR

Hi Bob,

I'm pleased to hear that you found my notes of interest, however the credit should really go to Owen Duffy.



Who was the person who originally inspired a lot of the investigations.

It was his concept of signal degradation



Which is not an 'official' unit of measurement, but one that I found useful as a guide to 'real world' performance.

One of the key factors in terms of what you can actually hear, is the receiver noise figure, and your receive site noise floor. In most cases modern amateur HF transceivers have a sufficiently low enough value of noise figure, that the main factor that limits what can be heard, will be the amount of unwanted noise present at the receive site, that ends up masking the wanted signals.

There are several factors that influence the amount of noise entering the receiver input. One is how much noise is inadvertently picked up and coupled in by the feed line, by mechanisms such as poor screening or unwanted common mode current, which we can usually do something about. The other is how much noise is picked up by the actual antenna, and is directly competing with the wanted signal, The only way we can improve this is to use an antenna that has some directivity that favours the wanted signal, and as much as possible rejects the unwanted noise.

The actual gain of an antenna is less important for receive purposes on the lower frequency bands, as it is the Signal to Noise Ratio that it produces that is the critical factor. On the LF bands the natural noise floor is very high, even in the quietist rural locations. This allows electrically small antennas such as the PA0RDT active E-Probe, to be able to produce good results on the VLF <100kHz bands.

On the HF and LF bands, there is a differing requirement between transmit antennas. Transmit antennas are generally required to be as efficient as possible, and have some gain in order to maximise the amount if power being radiated in the required direction. Whereas receive antennas just need to be able to produce the best possible Signal to Noise ratio. These are differing requirements, and there is not always reciprocity between the two. A good transmit antenna is not necessarily a good receive antenna.

However as we go up in frequency the level of natural noise decreases, and it is about 30dB lower at 30MHz in comparison to 1MHz. A receiver with a noise figure of about 13dB, using a typical dipole type antenna, will have an adequate level of performance, at 30 MHz. In fact in an urban environment, where the local noise floor is likely to be much higher, then a receiver with a much higher Noise Figure would still be perfectly adequate.

Once we get onto the low VHF bands and higher in frequency then things start to change. The natural noise floor is lower and tends to be predominantly extra terrestrial galactic noise, and receiver noise figures starts to become more relevant. Modern semiconductors can now achieve <1dB noise figures even on the microwave bands, and the only way to improve reception any further is once again to increase the directivity of the antenna, which usually equates to more gain in the desired direction. Because if this, there tends to be a lot more reciprocity between transmit and receive antennas, and a good transmit antenna will generally also be a good receive antenna. The antenna gain and directivity produce a narrow 'beam' in one direction, and this can both maximise the transmitted radiated power in the required direction, and produce additional receive gain in the required direction, in addition to rejection of unwanted noise and interference from other unwanted directions.

There is of course still the problem of terrestrial interference, but an antenna half power beam-width of just a few degrees at VHF / UHF / uWave frequencies, is a lot better at rejecting this than say a HF beam or worse still a wire antenna that may have a half power beam-width of several tens of degrees.

Once we start looking up to the heavens with our antennas, and leave terrestrial interference behind, then we really start pushing the boundaries. The natural noise floor becomes really low, as the cold dark vastness of space starts to approach temperatures of zero Kelvin. So for amateur Earth Moon Earth (EME) operation, there is an enormous technical challenge to be able to produce a receive system with a low enough Noise Figure, to be able to hear the very weak signals that are reflected off of the lunar surface, that just about make it all the way back to the earth. If you have a very big budget, like NASA, you can do even better with your antenna system, and receive weak signals from spacecraft outside of our solar system.

For the average hobbyist, even quite mediocre receivers are usually still good enough to hear just about everything ?a very expensive top of the range receiver can. The only main difference is the ability to withstand interference and overload from nearby very strong signals. Which?is usually only of concern to big gun contest operators, on good sites with very large antenna arrays. Something that most of us can only dream of.

Regards,

Martin - G8JNJ

On Tue, Feb 21, 2023 at 05:46 PM, vbifyz wrote:

The input filtering may need more work to handle strong nearby broadcast stations better.

Hi Mike,

A good project.

I'm a bit concerned about the value of the input capacitors which are shown as being 1nF, with a comment to reduce them to 100pF if there are no strong FM stations nearby.

Using 1nF capacitors a typical 1m diameter loop, having an inductance value around 2.5uH, will bring the loop to resonance at around 4MHz, which is not desirable if you wish to have good performance over a wide frequency range.

Ideally the value of the input capacitors (and resistors) should be chosen in conjunction with the actual value of loop inductance, in order to provide a suitable matched characteristic and impedance tracking across the whole of the desired frequency range.

Some further notes on this subject can be found here.



Good luck with your project.

Regards,

Martin

I already had schematic in my Files area, now the PCB layout is available. Licensed under Creative Commons, feel free to use as you wish (don't forget to credit me and LZ1AQ).
I did some IMD measurements, results are similar to LZ1AQ design (on which it is based): +33..35dBm IP3 and +70..80dBm IP2.
The current is a bit lower for the similar IP3, because the first stage can be biased independently.
The 2nd stage can be used on its own as a 50 Ohm preamp with some component changes.
The input filtering may need more work to handle strong nearby broadcast stations better.

73, Mike AF7KR

Alan,

Thanks for the time you spent on your extensive write up. Much appreciated.

To answer your questions/advices :

1. There is a steel reinforced concrete floor between the Baby Loop and the floor below. I do think that takes care of one of your concerns.
2. The PA will be 500 Watt, not 2Kw
3. It is impossible for anybody to touch the baby loop. The lowest part of the baby loop is 3.5 meter above the floor level of the roof.
4. At two side of the platform there is nothing.. It is at a corner of my apartment building.. The ground is 5 floor lower. roughly 15 meters.
5. The place the 1.5 meter high platform is build is 5 meters away from the patio wall with the neighbors. They will require to get first a ladder to get over the wall before entering my? ? ? ?
? ? patio. By doing this they would be unlawful entering my premisses.? But even then they can't touch the aerial because it is 3.5 meter high
6. I have no kids anymore at home. Both my kids are studying at university.

I think from a safety perspective I am ok. The manufacturer recommendation says 5 meter distance and I do comply to that.?
In addition 3000 of these baby loops have been sold and I have not been reading one incident with them.??
Lots of HAM radio amateurs are installing them in a far more compromised safety way. Like in-house or on a small balcony.?

I hope I have replied to your concerns. But I do want to thank you for the time you took to warn me. That is really appreciated.

73, Frans

开云体育

Not so much a propagation concern as keeping the near fields away from people. The voltage on the tuning capacitor will be of the order of many kilovolts? with a current of >10 Amps at around the 100W level. Any antenna is a transducer that converts electrical energy into electromagnetic energy. A resonant wire dipole can be very close? to 100 % efficient.? A good transmitting loop can also be efficient but the much smaller structure needs much higher E and H fields to radiate so the near field intensities are very much higher than a full sized antenna.

?A continuous screen over a roof would reduce the exposure to people underneath the loop. If it is a concrete and rebar structure there may be no connectivity between the rebar so there may be no screening for people a metre under the roof mounted loop.

Fitting a loop to a fence also implies that people on the other side of the fence can receive?? very high exposure without understanding the risks. You cannot protect children, babies or pets on the other side of the fence. So Cirro's? reply is easy to understand.

As? there are now? requirements to assess fields around transmit antennas it would be prudent to follow the recommendations. Personally I would want to keep at least 10m. away from a transmitting loop radiating 100W . Putting 2 KW into a loop puts over 4.5 times the current and voltage into the structure compared with 100 W operation. That would be enough to arc capacitors and generate huge RFI in many loops however the Cirro loops have exceptionally high power ratings with 14mm minimal capacitor plate spacings.But then the E and H fields are very high, don't worry about objects or lengths of cable- think about People and your humanitarian and legal position.

Wet trees are not that lossy, signals do not vanish when it rains even though a ground wave signal can go through hundreds of trees. Rainwater is quite pure and forms a very thin partial film on trees. Sap is a much greater attenuator potentially as it has a much larger cross section however it's not much of a problem to LF to VHF radio waves. People have used trees as transmitting antennas, not nearly as good as? 35m of metal mast but a lot cheaper. Trees have a very long history as supports for wire antennas, people don't complain about the tree getting warm but are happy to get the wire into the air.

Now that there is a requirement to consider the exposure of others in some Countries I would not ignore this need, a vexatious? group could cause a lot of trouble,? demonstrating compliance is a valuable defence and a wise precaution. Position matters.

73, Alan G8LCO.

I not put anything near the loop. I would only build something below the loop. A platform of 1.5 high where the 2 meter pool is standing on. And then the loop will sit on the pool.?
The loop will then be 3.5 meter high from the floor space of the patio.??