Zaba, OH1ZAA / NN0Y
Issue 57 Six News, May 1998
Having attended and contributed (regarding antennas, though) to a recent IEEE / URSI Radio Science Conference, there would be many interesting things to report especially concerning Arctic propagation, but let’s first call for precision, as a lot of time is lost with unreliable data. It is a fact that many professionals base their research on data provided by others, or on assumed (traditional) models.
I am of the opinion that anyone (even theoreticians) working in the field of geophysics or wave propagation should do at least 1000 hours of obligatory exercises at the dial of a VHF-receiver. The output of the exercise should be a notebook full of observations. Not the least it would create the feel for reality and subsequent triggers for new studies.
The prerequisite for correct information is proper equipment. Many people use narrow band antennas for wide-band observations, and right there is a tremendous risk of wrong interpretation.
The following is a synthesis of a discussion that occurred in late 1997 on the VHF reflectors. This ‘Six News’ version is slightly adapted to be more precise with respect to terminology. The original wordplay with ‘fishbones’ is left out as fishbones are existing antennas. The difference with a log-periodic antenna is that both use a central feed line, but unlike the alternating connection polarity of the LP feed line, with the fishbone the feed line is a straight open wire line where the elements are connected with series capacitors. From a distance these antennas look identical, though their properties are quite different for the same element dimensions.
The alert VHF operator would like to track the rise of the Maximum Usable Frequency (MUF) over a wide bandwidth, without having to guess the direction of arrival of these signals.
Log-periodic antennas, the regular model, without breaks in the frequency range, eg 40-108 MHz are perfectly suited for this purpose. This range can be obtained with 9 - 15 elements. Gain is generally not of first concern; stacking takes care of that.
Side-by-side configurations will give excellent azimuthal resolution. Most important is the front-to-back ratio which should be better than 10 dB over the whole range.
Now to the yagis; here the headache starts. Most VHF-yagis are gain-optimised for a specific narrow range, generally 1-2% of the centre frequency. However, most serious observers will scan the QRGs up and down from the design centre, in order to seek after indicators of upcoming or ongoing conditions.
Warning: The F/B-ratio turns NEGATIVE slightly above the frequency of optimum performance (e.g. f > 52 MHz).
The reason for this is that all directors turn into reflectors at many subsequent higher frequencies and current amplitudes diminish to very low values in those elements. At this point the real reflector is already far from resonance, and poses just a slight ‘obstruction’ toward the original ‘back’ of the structure.
This takes only a few minutes to prove, when one has the ready *.ANT or *.YAG files, that are generally provided with simulation programs. Here are some spot frequency data for 9/11-ele M2 yagis. The indicated decibels are Front to Back figures. Negative figures mean that the reception is stronger on the back of the yagi.
9-ele M2 yagi
|
F/B |
45 MHz |
-0.5 dB |
50 MHz |
+20 dB |
51.85 MHz |
-0 dB |
52 MHz |
-1 dB |
53 MHz |
-6 dB |
55 MHz |
-10 dB |
59 MHz |
-7.5 dB |
11-ele M2 yagi
|
F/B |
45 MHz |
+2.5 dB |
46 MHz |
+4 dB |
50 MHz |
+17 dB |
52 MHz |
+6.5 dB |
53 MHz |
+2 dB |
53.165 MHz |
+0 dB |
53.25 MHz |
-18 dB |
53.3 MHz |
-16 dB |
53.5 MHz |
-10 dB |
54 MHz |
-9 dB |
56 MHz |
-8.2 dB |
| 58 MHz -8.5 dB |
Similar information could be given for the more common smaller models; however the following generalisations are applicable.
A yagi optimised at 50.1 MHz should have a F/B-ratio of + 15dB or better at the design frequency. Environmental effects usually keep F/B at less than +25dB, even if calculations promise more.
Down to 47 MHz the yagi may have more than + 6dB F/B and is useful to determine the direction of arrival (azimuth) of the signal. At lower QRGs the judgement gets difficult, especially as flipping the beam around brings in local or galactic noise sources with a different amplitude. Use is prohibitive at least for comparison of weak signals by ear. Below 45 MHz there is no F/B to speak of; if it gets negative, it’s just a dB or so. Depending on the matching network, reasonable RX-signal levels can be available though.
The up-direction is more tricky. Forward gain starts to drop rapidly as well as F/B-ratio. The effect is much more pronounced than when tuning to the lower frequencies. In the 51.5 - 53.5 MHz range there is generally a sharp resonance of the longest director, which results in excessive mismatch and low received signal levels. Somewhere there also the F/B-ratio dives first to zero and then to excessive reverse (negative, e.g. -20dB) values. Still further up, the F/B stabilises around -9dB, that is: forcing the main lobe to the back of the beam.
Depending on the matching network, some yagis provide a reasonable signal level despite the wrong frequency, so receiver sensitivity is not necessarily an immediate limit. But otherwise all is far from optimum due to the shattered pattern and the intake of cosmic noise.
The above generalisations were checked with a shorter yagi and most of it applies, except that with a short yagi and less directors the reversal of the F/B-ratio at the high frequency end is less pronounced, ie in the -3 to -7 dB range, which is plausible. Also, apart from the changes in F/B-ratio, there may be stronger side lobes exceeding the amplitude of the main or back-lobe. These lobes are generally not symmetrical around their peak response.
Providing reliable data; awareness of yagi behaviour vs frequency
Anyone distributing azimuth information should be aware of the preceding characteristics and make sure that existing records are corrected. The problems are here much simpler than in medicine, where we can have thousands of parallel parameters. Here circumstances at the site are nearly fixed with the given instant frequency.
The tricky parameters are up in the skies and up there are also the mysteries that we are chasing after. Join the adventure, dress your imaginary wings, and take off, on, up. Travelling this way is inexpensive, still it is not a virtual world up there. Myself, I often run out of fuel, but all my landings have been soft and gentle so far.
