1
$\begingroup$

While researching this answer, I came across information that indicates that a many good Low Frequency/ Very Low Frequency antennas are loops, with high directionality and good overall characteristics. Can the same thing be easily done with HF signals, of having **some size of a loop with an appropriate amount of antenna coiled together? **

An example is this image from Wikipedia

enter image description here

$\endgroup$
  • $\begingroup$ Is the question "how to?" I read in the body, "can it be done?" $\endgroup$ – Phil Frost - W8II May 3 '14 at 16:16
2
$\begingroup$

Specifically, the antenna in your picture is a small loop. This is quite distinct from the resonant loop, where the circumference of the loop is equal to the wavelength. Resonant loops are essentially folded dipoles that happen to be round.

Yes, these antennas work at HF also. There are some reasons you may want to do so, but first let's examine the advantages of a small loop at ULF. Their applicability to HF is then apparent.

Firstly the obvious: it's small. When the wavelength is several hundreds of meters or more, a practical antenna is going to be electrically small.

If we just want small, why not a short dipole? This is certainly possible, and since the short dipole is the electrical dual of the small loop, the two should be very similar.

Because at ULF, ground wave is a significant mode of propagation, and ground wave works very much better in vertical polarization, ULF transmissions are most usually vertically polarized. While short dipoles and loops have the same pattern, the nulls point in different directions: for vertically polarized antennas a dipole's nulls are up and down, while for a small loop they are in two directions on the horizon. Thus, with a loop you can null noise, while you can't with a dipole. Thus, the "high directionality" of a loop, even though it's really not any better than a dipole when considered in three dimensions and free space.

At HF, size may be less of a concern. Also, since the usual propagation is skywave, where the ionosphere randomly rotates polarization, we are not restricted to vertically polarized antennas.

Beware, there are many popular small loop designs among amateurs, and many of them are of rather poor design because they unbalance the antenna, and consequently the feedline becomes as much of the antenna as the loop. There's some folk wisdom among amateurs that HF loops are somehow quieter, but the reason is that they are different, not magic.

$\endgroup$
  • $\begingroup$ Note the question was with respect to Ultra-low frequency, not Ultra High Frequency loops. Will edit my question to make that more apparent. $\endgroup$ – PearsonArtPhoto May 3 '14 at 15:24
  • $\begingroup$ @PearsonArtPhoto ah, yeah. I was wondering why it looked like there was a tuning capacitor on the top of that UHF loop... $\endgroup$ – Phil Frost - W8II May 3 '14 at 15:28
  • $\begingroup$ The best wire antenna I have ever used for 80/40/30 was a full wave square loop on 80 (full wavelength of 80 meters approximately). This is a horizontal square loop elevated to about 50 feet with the help of nicely positioned first trees in the corner positions. It was not a perfect square, more of a rhombus shape but that does not matter too much as long as it does not deviate too much from squareness (or roundness). Unfortunately, we had to take down one of the fir trees so I lost a key anchor for one corner. But, I consider these to be the best wire antennas for all HF bands. $\endgroup$ – K7PEH Nov 22 '15 at 21:58
-1
$\begingroup$

I have built a magnetic loop that tunes 15m - 40m and am quite happy with it. As stated they are not magic, but have the advantage of being small and rejecting nearby E-field noise. Small is important if you are unfortunate enough to live in an area that is not friendly towards antennas. The radiation resistance of a small loop is low, so to get decent results you have to keep the ohmic losses to an absolute minimum. This means high quality conductors and very low loss capacitors, such as vacuum variables.

Do a google search for 'helically loaded magnetic loop', this is the design I built. I am very happy with its performance and you will find a lot of others with similar experiences. It's not cheap to build but the results are worth it and it handles the full legal limit. The Q is extremely high so an additional benefit is you effectively have a narrow bandpass filter right out at your antenna. The noise floor is consistently several S units below that of my dipole. Don't skimp and use a cheap capacitor, you'll be disappointed :-)

$\endgroup$
  • $\begingroup$ That loops reject E-field noise is a common misconception. The loop has a higher field impedance, meaning it's more sensitive to E-field noise, than the equivalent short dipole at some distances: about 10% of wavelength to the far-field. See figure 1 at w8ji.com/magnetic_receiving_loops.htm. $\endgroup$ – Phil Frost - W8II May 8 '14 at 11:31
  • $\begingroup$ A measurement of the noise floor alone is also not a figure of merit. A loaded, small loop is by nature a less efficient antenna and will by consequence have a lower noise floor. It will also have lower signal. As an extreme example: my dummy load has an extremely low noise floor, far better than any antenna I've ever used. $\endgroup$ – Phil Frost - W8II May 8 '14 at 11:32
  • $\begingroup$ I guess it depends what you mean by 'near'. As you noted, what you said is only true 10% of the wavelength away from the antenna. I said it rejects nearby E field noise. The inverse square law is on the loop's side for noise farther away. The question was can a loop be built for HF, and I simply said yes it can, I actually did, and if properly constructed the performance is good. $\endgroup$ – ad7zj May 8 '14 at 17:23
  • $\begingroup$ Fair point on the noise floor, I should have mentioned SNR instead, but at this point I don't think I'll waste my breath. There are a lot of loop naysayers out there mostly because they've never actually built one, just read about someone who did, usually skimping on parts and getting poor performance as a result. $\endgroup$ – ad7zj May 8 '14 at 17:34
  • $\begingroup$ I didn't say loops are poor. I just said they aren't magic. I think if you edited your answer to be more fact supported by references, and less anecdote, it would be much better received. $\endgroup$ – Phil Frost - W8II May 8 '14 at 17:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.