Re: Topband: Using shielded CAT5 data cable as feedline foractive antenna; benefits of multi-turn K9AY loop/SAL/etc?
Hello to everybody, My name is Chavdar Levkov LZ1AQ and I am new in this forum as a writer. As a reader I have followed it for a long time. I have some comments about the discussed topics. It is a long post so excuse me for that. 1. Wideband small loops are working almost in short circuit mode – they are loaded with amplifier with very low input resistance e.g. 6 ohm. In order to increase the short circuit current we must reduce the loop impedance preserving the loop area. Fat conductor or paralleling loops is the obvious solution. Crossed parallel connection of coplanar loops is the most effective way to do that. To reach the same low inductance you must use very fat conductor which is not practical. This method does not have any advantages when normal impedance matching is used or in the case of tuned loops. 2. The crossed small loops have the same radiation pattern as a single small loop with one very important exception – the limitation of 0.1 wavelength perimeter is no longer valid for crossed loop. This can be easily checked with NEC model. So we can use the small loop properties in much wider frequency range. 3. The effective height of the wideband loop (working in short circuit mode) is determined by the Area/L ratio. You will not find any difference in reception between crossed and single loops if this ratio is the same. 4. I do not recommend to use multi-turn loops – single turn loop has the highest efficiency. I do mean loops winded as inductor. I do not assume crossed coplanar loop connection as a multi-turn loop and I do not use this term to avoid misinterpretations. 5. The main problem in active wideband loops is that there is no impedance matching in its classical form. The induced current is very small and often the system is noise limited by the internal noise of the amplifier. Using crossed coplanar loops is a way to avoid this problem. My experience is that two crossed circular loops with diameter larger than 1 m with alum. tubing of 14 mm diam. have sufficient current and the noise floor is limited by the external noise (using common base amplifier as described in my article). At least up to 10 MHz. On higher frequencies, in quiet rural locations, the external noise might become sometimes very low and in this case the system becomes noise limited by the internal noise. 6. The closely spaced phased array I build with small active loops or dipoles is actually 2 element array. 3 antennas are used in order to have 4 directions. Its RDF is as any other 2 element array with active dipoles(verticals) and slightly higher RDF for loops. 7. There is a difference in polarization sensitivity between arrays with vertical loops and vertical dipoles. The loop array is highly directional for vertically polarized (VP) waves (better RDF than dipole array and is almost as 4-square for 2el array). This array has zero sensitivity in main direction for horizontally polarized (HP) waves but has two HP side lobes at +- 45 deg to the main direction. The dipole array has zero sensitivity for HP waves at all directions. It is a pure VP antenna. The take-off angle of the dipole array is lower compared the loop array. Practical consequences are that the loop array directivity is different compared to dipole array. Very often with one array I have high F/B ratio while with the other array it might be low and vice versa – it depends very much from the propagation. When we have nearly HP waves the directivity is minimal for both arrays. 8. The phased array with active loops has one advantage compared to active vertical dipole (or GP) array . In a small yard usually there are too many parasitic non-resonance conducting objects. A dipole array (with high input impedance amplifiers) often loses its directivity probably due to capacitive interactions with these close objects . The loops are much more stable. Both arrays are influenced from nearby resonant antennas and measures to de-tune TX antennas must be taken if the space between antennas is small. 9. For those who have an experience with small single loops it must be mentioned that the loop array is much more quiet compared to a single loop apart from directivity benefits. The reason is that a single vertically placed loop has in its pattern a lobe toward zenith which is eliminated in phased connection. 10. About CAT5,6 cable -You do not need to measure it - the specifications are widely available. As a balanced line it has certain advantages which will benefit the noise immunity of the receiving system. The only questionable parameter is the delay (which is important when building phased arrays). I have measured the difference in the signal delay between two 30 m cables from the same manufacturer. Using the same colored pairs, the difference is below 1 -2 ns. The accuracy was limited by my measuring equipment. Usual
Re: Topband: Using shielded CAT5 data cable as feedline foractive antenna; benefits of multi-turn K9AY loop/SAL/etc?
Sounds like you, Tom, and LZ1AQ are saying the same: in order for loops to be effective (low SNR and high signal levels), they must have large area and low reactance (inductance to be exact). Parallel loops or fat conduits increase the signal levels, while the CP configuration and other similar measures are aimed at lowering the loop's inductance. All this of course is paired with a designed-for-purpose amp that does match the low loop impedance. Rudy N2WQ There are a dozen ways to say the same thing, but the physical area of a loop (when it is very small) determines the maximum energy extracted. This is why small transmitting loops are all pretty much single turn and single conductor. The sensitivity and what configuration produces maximum sensitivity has a great deal to do with the load placed on the loop and how the loop matches the load. For example, if the loop has a high impedance amplifier or matching system terminating the loop, it might be more sensitive with the extra turns in series rather than parallel. Then we have things that are called loops and look like loops, but really function in a different mode than a small loop. All of the small unidirectional loops act like pairs of small verticals that are phased. This includes the EWE right through the flag or pennant. They ideally have uniform current, which is made uniform by the terminating resistance which terminates the wire in its surge impedance, but the vertical or sloped ends are what we want to act like the antenna. It is pretty risky to generalize across everything, but what it all boils down to is the multiple wires can be used to improve the matching or reduce the losses. Which is more effective depends on the exact antenna and the things we have terminating the antenna. I wouldn't count on a system of more series turns, more parallel turns, or a thicker conductor, offering improved S/N or performance without know the specific system, the external noise, and the internal noise. Pretty much everything "loop" I have played with gets into propagated noise without multiple wires or a thick element. The thickest element I have used was old flexible copper waveguide from a BD station, I think it was maybe four to six inches and oval. I've also used ribbon cable in small loops, but as a series connected group. For all of my directional loops, I never used more than a single turn because they all occupied enough area to get into external noise. If there was any magic in this, it would have been used 40 years ago. :) 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
