Tuesday, March 7, 2017

Beverage Comparison: ARRL DX SSB

Due to circumstance I was unable to make a serious effort in this weekend's ARRL DX SSB contest. The major reason was (another) rotator wiring failure a day before the contest. It happened just as I was turning the yagis toward Europe right after working VP6EU on 40 meters Since this is not my favourite contest and the weather became bitterly cold I was unwilling to climb the tower and undertake the work. Instead I chose to tackle a slew of small chores I'd been putting off.

With the pressure off I decided to play on 80 meters Friday and Saturday nights to get a better understanding of how my new Beverage performs and to see who could hear my 150 watts to an inverted vee. I received an important education about not only my Beverage but also how well others hear. What I learned is important enough that it is worth another article, even coming so soon after the last one.

The method

It is no simple matter to compare the Beverage to alternatives. I have none. But other stations do, and it is them I must compare against. In this case it is easier to compare a receive antenna than a transmit antenna since we are both listening to the same stations.

It was quite easy to do. On 80 meters I would find a known big gun in the US northeast (reasonably close to my location) and sit on their frequency to listen for a while. If they listened split so did I with the second receiver in my FTdx5000. Conditions were poor so their rates were slow. I had to be somewhat patient, especially on Saturday evening when they had pretty well worked out the band.

When a weak European station called them I would pay close attention. The challenge was to copy the station better than the big gun. It is not unlike watching a quiz show on television where you try to outguess the contestants as they try to answer difficult questions. That can be quite a lot of fun, and so was this game I was playing.

What I found was that in most instances I could copy the really weak ones about as well as most of the big guns. Only rarely was I outgunned. In a surprising number of cases I could copy the DX better. This made me very happy since it means that my receive capability on 80 meters in that direction, but only in that direction, is comparable to the best contesters. Of course it is possible the operators were not as good as their antennas, however knowing who these folks are I highly doubt that!

On 160 meters my task was more difficult since activity was lower and rates worse. My sample of comparisons was far smaller than on 80 meters. I can only tentatively claim the same result on 160 meters.

What am I comparing against?

Perhaps the biggest unknown with my method of comparison is what antennas other stations are using for receiving on 80 and 160 meters. Even when I know they have separate receive arrays, whether those be pennants, flags, or more elaborate arrays such as Beverages and 8-circles, I don't know which they are using on any given contact. It may even be that they are sticking with transmit 4-squares on receive.

Since I can't know the answer to that question I have to assume that these operators, who are among the best around, would in each case choose the antenna best suited to getting the other guy into the log. In which case I am comparing against their best towards Europe. This is likely though not absolutely certain.

Off the main lobe

A directive antenna attenuates QRM and QRN, which is at least as important as its (relative) forward gain for copying weak signals. I had ample opportunity to test that as well during the contest. Of course I already knew perfectly well that the F/B, F/S, F/R or whatever metric you prefer had to be reasonably good or I would not have experienced such a favourable comparison. However that comparison alone is not the full story, nor should it be.

The test is to see how much signals and noise off the main lobe are attenuated. This can be precisely quantified by RDF and DMF figures used by W8JI and ON4UN, respectively. On the air a quantified measurement is not easy when the comparison signals weaken and strengthen in response to ionospheric changes, even if only Faraday location, and I don't know the precise direction of stations.

Instead I did a rough statistical assessment using as many signals as possible, thus reducing the impact of unknown factors in each comparison. DX stations were a little easier to assess since in many cases their bearings are known to within a few degrees. While I cannot easily measure the RDF or DMF of the Beverage there is the possibility of getting an approximate idea of what it might be by careful listening.

When there was QRM on frequency the benefit of the Beverage was greatly enhanced. Since in almost every instance the interfering station was in North America, and therefore far off the main lobe, the attenuation of QRM ranged from good to remarkable. The remarkable cases were most likely those that fit neatly into one of the nulls between the several minor lobes to the side and back of the antenna. For DX stations in South America, the Caribbean and Central America the results were the same, which is to be expected from my QTH.

The Beverage is a great QRM fighter. Calibrating the attenuation of signals required some quick work with the RX antenna switch and pre-amp (IPO) toggle switches on the FTdx5000. This is absolutelynecessary due to the unequal signal levels from the two antennas yet can never be perfectly accomplished. There is also the matter than my comparison antenna is an inverted vee which is horizontally polarized, while the Beverage is vertically polarized. Faraday rotation is a factor.

The worst case rejection of QRM was in the vicinity of -15 db and in the best cases was -30 db or better. Most times it fell somewhere between. I am very happy with the Beverage performance in this respect on 80 meters. On 160 meters it was more difficult to assess since the main lobe is broader and activity was less. Early indications are promising. I'll have to keep listening. I expect that a deep pile-up on a DX station will provide the best test conditions.

Higher bands

Beverages work on bands higher than 80 meters, and can work quite well. I tested mine on 40 meters and 20 meters during the contest with variable results. The main lobe becomes quite narrow on higher bands since it is long in terms of wavelength: 4λ on 40 meters and 8λ on 20 meters.

I couldn't see any benefit of the Beverage on 20 meters at all, even with the yagi fixed on north due to the faulty rotator, and therefore receiving Europe quite poorly. This may be because the 2.5 meter height of the Beverage is a substantial fraction of a wavelength. It would have to be lower to perform well since a Beverage works in part due to the lowered velocity factor caused by ground proximity.

On 40 meters it worked though with signal levels that were weaker than expected. Some European signals were received very well though not all benefitted from the Beverage. Performance was not consistent for some reason, perhaps the narrower main lobe. Once the yagis can again be turned I'll do another evaluation.

Proceeding with receive antennas

Before committing to building an array of Beverage antennas I would like to experiment with a couple of smaller receive antennas such as the K9AY or a type of flag or pennant. These are not difficult to build if there is no provision for direction switching. I would like to try them pointing southwest (US and South Pacific) and north (Asia).

I could of course build these in addition to more Beverages and have the luxury of doing extensive comparisons. That's a lot of effort that I really need to devote to other antenna farm projects this year. Perhaps the most profitable comparison will be a switchable directional antenna for 80 meters, whether a 4-square or a parasitic array. A transmit antenna with directivity is, after all, a type of receive antenna. Although these are typically not highly directive they have enough directivity to be effective, especially a properly adjusted 4-square.

Whatever path I do choose I will definitely have more receive capabilities on the low bands when fall rolls around later this year. My immediate need is better transmit antennas so that I can finally work the many stations I am now able to hear. It has been somewhat frustrating calling stations that cannot hear my puny signal on 80 meters.

SWR chart

Since my attempt to take a photograph of the analyzer display for the earlier article was so poor I decided to take one from inside the shack where I could control the light conditions. I'll close this article with that picture. You can see how nicely the SWR is from 1 through 15 MHz. A Beverage is truly a broadband antenna.

The periodicity you see is due to the termination resistor not equalling the antenna impedance; they differ around 10%. The impedance appears to be rising with frequency resulting in greater SWR swings.

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