Vertical and Above, Part IClick here to go back to the home page

by Peter Schleuss, HB9RUZ

After having seen, with a certain regularity, messages on the cluster like "rare DX _call even on a quarter wavelength 58 in JN..." or "worked him on first call with my HF vertical", I decided that it might be worth making this small contribution for the sake of these poor six-metre addicts living near Band I TV transmitters (yes, there are some big boys not slaughtered yet). Based on messages like those mentioned above one could easily guess that without having better equipment and under very good propagation conditions even an antenna with vertical polarisation could be used for a six-metre QSO.

May be that those six-metre `immigrants' with HF experience already have a broader view of the polarisation pros and cons. On the Internet one can find stories with exhaustive background dealing with the question of which polarity is better suited for contests and DX expeditions on various HF frequencies.

For me, having been an avid VHF and SHF DXer in my pre-six metres time, vertical polarisation was something only to be used when no other choice was possible (e.g. HT, mobile, repeater), the antennas being vertically polarised due to the characteristics of the hardware.

Within a radius of 80 miles around my QTH there are two strong band I/channel 2 TV transmitters. On average days both are audible by tropo and both of them boost the S-meter needle in a very uncomfortable way. Even when using yagis, there are broad sectors where ham stations can only be received when arriving with very high field strength.

In the year 1998 I decided to receive six-metre band signals with a vertically
polarised antenna. My hope was that in doing so, the level of interference generated by TV transmitters could be reduced to an acceptable level.

My wish was to find out how the field strength would behave as a function of the polarisation of the antenna on one hand and of the various 6m propagation modes with their specific patterns of polarisation on the other hand.

To accomplish this I decided to install an additional, vertically polarised yagi of the same type already used horizontally (a five-element Tonna modified according to PA2HJS). Fast changeover of polarisation was realised by means of a manual coaxial switch.

I did not keep written statistics but the experience gained during this summer showed that the vertical polarisation did not pose a disadvantage, at least not as far as Es propagation was concerned. Apart from some very rare cases where signals could only be received in horizontal polarisation, there was always a better S/N ratio with the vertically polarised yagi. This, as expected, was due to the reduced interference from TV video.

The ham signals were sometimes weaker than with horizontal polarisation and sometimes stronger. On average I could not find a big difference. I tried not to be biased in any way in favour of the vertical polarisation, not wishing to cheat myself concerning the further decisions to be made.

Now, at that point I had some discussions with HB9MFD who asked me why I didn't want to have the best of both worlds. He suggested that I put the same two yagis crossed on a common boom and put them on the top of the mast. In retrospect I think it was the risk of the time delay related to the extension of the mast by an insulated section and the construction of a polarity switch that made me refrain from such a solution at that moment.

It wasn't until much later that I found on the Internet that even on EME crossed yagis are successfully run without insulated mast sections.

I decided to go for pure vertical polarisation and to mount the yagi which had been previously used horizontally polarised in a vertical position. A stacking distance to the other vertical yagi of one wavelength was chosen. Thanks to the expertise and workmanship of my ham friends Berth, HB9DCE and Paul, HB9MFD we were able to reconfigure the arrangement of antennas within one day with a certain amount of sweat and swears but no blood and tears. Power splitting was done with pieces of quarter wavelength 75-ohm CATV cable found in the trash heap of a building site.

The only disadvantage in doing so was that I became practically deaf for the reception of tropo signals. Due to similar experiences on the two-metre band this was to be foreseen and did not deeply concern me. Six-metre tropo activity was very low then and even nowadays I rarely find references on the cluster to six-metre tropo QSOs covering 800km or more (claimed by some DX folk to be the minimum distance to count as DX on two metres).

It seems that tropo propagation is accompanied only by a small twist of the plane of polarisation, so that when receiving tropo with the opposite polarisation to that of your QSO partner's antenna you have to deal with a high degree of polarisation loss. This is the cause for the tropo deafness cited above. In all other modes of propagation met on six metres, com
ponents of radiation lying in planes of polarisation twisted in respect to the original one seem to be produced in a considerable amount. So considerable that the field strength of the arriving signals is in the same order of magnitude as to be found between stations using the same plane of polarisation.

I was willing to trade in the good tropo reception capability against a better S/N ratio on all other six-metre propagation modes that I had met so far (including Au and MS). The reason for the better S/N ratio when receiving vertically polarised is to be found in the fact that the interfering TV signals were made victim of the tropo deafness: hurrah.

The QSOs made within the last two years (100 countries in six continents) show that the usage of vertical polarisation in my special situation has its merits even with the propagation modes specific to the peak years of the solar cycle.

I think my next antenna reconfiguration will go in the direction of Paul, HB9MFD's propagation controller which he will describe in part II of this report.

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