Contesting on 160 With Receive Antennas

Contesting on top band with directional receive antennas has its challenges. Before delving into that subject I will first review the fundamentals of antenna reciprocity, and its close cousins: power, noise and antenna efficiency.

Reciprocity

Antennas for the most part are reciprocal: they work the same on transmit and receive. A non-directional antenna transmits and receives equally in all directions. A directional antenna favours one direction for both transmit and receive.

There are a couple of provisos to this fundamental rule:

• Efficiency: An inefficient antenna can receive as well as an efficient one but the inefficient one transmits a weaker signal in all directions (mimics lower transmitter power)
  
• Noise: Man made or atmospheric noise degrades reception (SNR, or signal to noise ratio) in all or some directions while having no effect on transmission

Efficiency and noise along with power and receiver sensitivity determine the effectiveness of communication with another station. For this discussion we'll set aside the communication aspect of the subject, despite its importance, so that we can focus on reciprocity.

Antenna efficiency is not typically an issue on the high HF and VHF+ bands, especially with horizontally polarized antennas above the roof or tree line. Verticals on the low bands can be efficient when in the clear and with lots of radials to minimize ground loss in the antenna's near field. 

Need for and use of directional receive antennas on 160 meters

Inefficient antennas are common on top band since horizontal antennas are necessarily low (with respect to wavelength) and verticals are usually short and have insufficient radials. The consequence is that you hear many stations that cannot hear you.

Let's assume that you have an efficient and effective antenna for 160 meters. You might also want to use high power to further the likelihood that others will hear you. Noise of all kinds is a problem for everyone on 160 meters. That efficiency and power guarantees that you will not hear many stations that copy you very well since their antennas are less efficient.

Few antennas on top band, even the full size efficient one, are directional. Verticals are omni-directional and horizontal antennas like inverted vees are nearly omni-directional, and what directionality they have is modest and usually not in any favoured direction. You just put up what you can.

For a contester like me this is a problem. My 160 meter antenna is efficient and omni-directional. When I turn on the amplifier I know that there will be many stations that will be frustrated by not getting my attention. Of course I don't know that they're frustrated, because I can't hear them, but I strongly suspect that I'm losing many potential QSOs.

There is way to overcome the reciprocity challenge and that is with a directional antenna. With a high RDF (receive directivity factor) more of those unheard stations will be heard. Unfortunately a directional and efficient antenna on 160 meters is rare because it requires two or more elements and lots of radials, and that entails a lot of land and expense.

The alternative is an omni-directional and efficient antenna for transmission paired with a directional receive antenna. The receive antenna can be small and inefficient since the atmospheric noise is high; that is, an inefficient antenna can improve the SNR while being sensitive enough to hear signals at the noise level. For the smallest and therefore most inefficient receive antennas a 10 db to 20 db pre-amp may be needed.

Contesting with a directional receive antenna is different

Using a directional receive antenna for regular DXing isn't complicated. If you're responding to a spot you already know the heading of the desired station. You choose your receive antenna and there it is. Occasionally there is skew path, long path or an anomaly with signal polarization that requires a different selection. 

When you CQ DX you have a general idea where to expect signals from and you choose that antenna direction. You know because of experience, paths that are in darkness, location of the terminator, population centres and other factors. For example, at our sunrise openings in winter the most likely directions are west (Oceania) and north (east Asia). It is a good idea to check northeast for Scandinavia and northern Russia.

In a contest signals can come from any direction. When you run with a big signal you will be called by stations in Europe, the Caribbean, South America and all over North America for most of the evening. Getting the direction right for every caller is not easy. This matters because speed counts in a contest. They might not even try twice if you fail to answer them on their first call. They also value their time.

The plot at right compares the azimuth patterns at 20° elevation of an 89 meter and a 156 meter long Beverage under the same ground and height conditions. As you can see the longer Beverage has greater directivity (and gain), with an RDF or about 10.5 db vs 8.5 db for the shorter Beverage. Side and rear rejection is poorer for the shorter Beverage. Top band DXers would prefer the antenna with the higher RDF. A contester sees it differently.

For a higher directivity (RDF) more receive antennas are needed to effectively cover all compass directions. In my Beverage system I am aiming for 8 directions with 4 reversible antennas -- I have 3 antennas and 6 directions at the moment. More directive arrays require more directions, such as 8-circle and 9-circle vertical arrays. A compact K9AY switched apex loop antenna can get by with 4 directions, at the price of a lower RDF.

The critical question for my contesting operating is: how many directions do I have to test until I can find or copy a signal that is at first weakly heard or entirely inaudible? Ideally I would like to not have to switch at all to save time and repeat requests. That is equivalent to omni-directional receive, which we already know doesn't solve the basic problem: copying signals that require an RDF boost from a directional antenna.

In last weekend's ARRL 160 Meter contest I regularly had to ask for repeats until I successfully found the right direction. My big signal attracts many smaller stations. When I operate QRP I rarely need to use a receive antenna.

On the plot above you can see that the F/S of my longer Beverages is high. The F/B is also quite good but not so extreme that I can't hear most stations off the back. For example, if I'm listening west I am more likely to hear a caller from Europe than from Florida to the south.

To minimize hunting I would often stick to the the NE-SW Beverage, pointing to Europe when conditions were favourable and southwest the rest of the time. It's a compromise that while not ideal places the side nulls where they do the least harm. When I heard nothing after a CQ the reverse direction was selected. I can thus hear south and west well enough to catch most callers. The nulls of the preferred N-S and W-E Beverages totally reject signals from too many callers. Once I fully copy the call sign of a particularly weak station I would select the Beverage in the correct direction. 

Later in the evening when west coast signals improved my strategy was to flip between west and south, which was good enough to receive most US callers, and then northeast for possible European callers, especially at their sunrise. With my rudimentary and allegedly temporary Beverage selector it was hard on the fingers after several hours of operating.

Unfortunately call signs are deceiving. There is no association between call signs and call districts in the US. Many was the time that I'd struggle with a weak W7 only to discover they were in Virginia. Switching from west to south would bring a barely audible signal up to S9. Some of them must have wondered why I seemed so deaf. A trick I latched onto was to space over the exchange field in N1MM Logger+ when I had a full call to see if their ARRL section was in the history file. That helped me narrow in on the correct direction.

With a shorter Beverage, an end-fire vertical array or a compact loop the F/S is quite poor. That would reduce the number of repeat requests since a weak caller is more likely to be copied on the first call than with a Beverage. The price is a lower RDF. It is an interesting question whether a high or low RDF receive antenna is more costly with respect to QSOs and rate. I don't know the answer.

There's more to it than a simple RDF figure since there is more than one RDF per antenna. The pattern is 3-dimensional and the azimuth pattern can vary a lot with the elevation angle. As repeatedly stated in ON4UN's Low-Band DXing, when you choose the length of a Beverage or the element phases of a vertical array you are also choosing the azimuth and elevation angle where the nulls appear, and at which frequencies. A knowledge of propagation from your QTH can help to make those choices. 

However, it isn't easy to figure that out and I have never tried. The elevation patterns of the Beverage (above left) and end-fire array (above right) are interesting, though not shown here. It is usually enough to pick antennas with approximately useful patterns and RDF. Obsessing over the details is rarely worth it. But do beware those nulls when your objective is contesting. The 3D plot of a short Beverage illustrates some of the complexity we must deal with.

Alternatives

The objective in a contest is to copy the call sign of the station quickly and accurately no matter their direction or signal strength. With that information you can select a direction or antenna to peak their signal for the remainder of the exchange. There are several alternative approaches, from simple to elaborate and expensive.

Directional transmit antenna: Reciprocity is restored when you transmit and receive on the same directional antenna, just as you do with a yagi on the higher HF bands. Unfortunately a directional and efficient transmit antenna on 160 meters is very large, very expensive and requires a large plot of land.

To cover all compass directions requires at least a 4-square or 3-element yagi like the K3LR array. I have the latter for 80 meters. Each antenna has 4 directions, with a broad main lobe and an RDF of 10 or 11 db for the former and about 9 db for the latter. Like all verticals, each element requires a large amount of wire for the radials (more under the yagi for equivalent efficiency) and about 1 acre per element on 160 meters.

A 2-element end fire covers two directions well and can be configured for bidirectional broadside. RDF is around 9 db for the former and probably no better than 8 db for the latter. I am seriously thinking about building this antenna by shunt feeding my two big towers. Since the land is farmed the radials must be removed in the spring and relaid in the fall.

Ergonomic switching: Fast switching among receive antennas and directions will find the best signal sooner and with fewer lost characters. One or multiple push buttons are probably ideal since it is almost idiot proof. Switches and knobs are slower and require more dexterity and finger strength. A touch screen or mouse can be as convenient as a push button.

Commercial products typically have knobs. Keep that in mind when you shop. That may not be so bad if contests are not your interest or priority. When DXing you won't be changing direction nearly as frequently. The home brewer can choose what works best for them.

Push buttons are what I plan for my Beverage selector, which is a project for the winter. The software is partly written, and the enclosure and design are in their final stages. I just have to put it all together. There is more to it since the selector includes direction and antenna selection for the other bands.

A design question I am presently contemplating is the button de-bounce algorithm to find the best trade off between switching speed and accuracy. Some experimentation will be required.

Whatever the ergonomics, as you spin through the directions you should revisit the ones you go past since QSB may be in play. Many times I've fastened onto the wrong direction because I quickly flipped past the correct direction during a signal strength dip. The call sign may be the hint you need to make the correct selection. Perhaps in the future we'll have automated selectors that scan all directions to find the one that delivers the best signal.

Diversity reception: If listening to one direction is good, listening to two directions is better. At least that's the idea. Diversity reception allows concurrent listening to two antennas. You do this by combining the signals before the receiver or with two receivers connected to different antennas. In the latter case you can combine the two channels (mono) or listen to one in each ear (stereo). This is getting easier to accomplish with current generation multi-slice SDR transceivers.

For example, you connect one receiver (or SDR slice) to an antenna pointing to a favoured direction, such as Europe, and use the other to select other directions. It's a little like operating SO2R. You can even use the second receiver to listen a different frequency. For this discussion we'll stick to one frequency and one signal.

Mixing two antennas (receiver audio or before the receiver) is a form of diversity reception favoured by DXers. Different antennas, pointing in the same direction or not, by being in a different location and having different polarization patterns can allow you to avoid many instances of QSB. Copy is better and you get through the QSO faster. For a contester it is better to focus on different stations and directions. Indeed, you can use diversity to disentangle multiple callers and choose the one you prefer to answer first.

Doing diversity well requires the antennas to have similar gain (audio amplitude). Receive antennas typically have negative gain so that signal levels are far below that of the transmit antenna. Long Beverages have higher gain than small vertical arrays and compact loops. Pre-amps or attenuators, whether in the antenna line or selected in the receiver, can help to equalize the levels. The RDFs may be identical but not the gain. You don't want the signal amplitudes to be far different or you'll defeat the purpose.

You should avoid mixing an omni-directional antenna (usually the transmit antenna) with a directional antenna. If you must listen to an omni-directional antenna it should only be done with two receivers and stereo. Otherwise the noise of the omni-directional antenna swamps the antenna with high RDF. I've seen hams make this mistake.

Low + high RDF receive antennas: Another strategy is to use both high and low RDF receive antennas. Start with the low RDF antenna since it will have less rejection of signals off the favoured direction. Once you identify the station you can switch to a high RDF antenna in the desired direction if it is needed to complete the QSO.

Of course an omni-directional antenna like a vertical used for transmit has a low RDF (0 db!) and that may be too low. If you have the capability to experiment it can be worthwhile. Do it in combination with diversity reception (see above).

My situation

The Beverage switch is designed to allow selection of only one direction. It is possible to increase the complexity of the switching matrix and run two transmission lines back to the shack but that is not my preference for diversity reception. I prefer to have a second receive system, perhaps a vertical array such as a receive 4-square or 4-direction end-fire, since it is less complex, allows for more varieties of diversity reception as discussed above and use of both in a multi-op contest.

One worry I have is the pre-amp that may be needed for small vertical arrays. The gain can be 10 db below that of the Beverages. This can be a problem in a contest when doing SO2R or multi-op. Many pre-amps perform poorly when there is a kilowatt transmitter in the vicinity. Many contesters have run into this difficulty. For the majority of top band operators this is not a concern since they never receive and transmit at the same time.

A second receive antenna system will be considered in my 2022 plan. It is not an immediate priority.