UPDATE: Looking for a comprehensive discussion or MF and LF receive antennas? Check out the talk given by Eric, NO3M / WG2XJM, at Hamcom 2016. The slides can be downloaded here.
When I gave my first talk on using separate receive (RX) antennas years ago, it was very interesting to see the large number of people who had no idea that separate antennas were being used by quite a few stations. The reason for that probably comes from the fact that only a handful of high-end rigs at the time (early 2000’s) had separate ports to connect a RX antenna. The compact rigs of the day, which were huge compared to the sub-compact rigs seen on the market today, were doing well to have a spare SO-239 and built in antenna A/B switch and even that was rare. A few years ago QST published an article on building an outboard RX antenna switch that allowed the user the switch an RX antenna automatically on the same port that connected the transmit (TX) antenna. This switching process utilized the rig’s PTT relay typically used to key an amplifier. It was at this point that that idea of separate RX antennas became mainstream, so much so that many manufacturers added separate RX ports to accommodate these antennas.
So what’s to be gained by using separate RX antennas? The biggest gain is improved signal-to-noise (S/N) ratio which involves how deeply into the noise a signal resides. The best way to improve S/N is by diminishing noise sources and that is typically accomplished using directional antennas. A lot of guys put entirely too much value on the idea of antenna gain, looking for the largest gain figures available in their antenna specifications. The most important values in an antenna specification should be front-to-back (F/B) followed by front-to-side (F/S). You can run an amplifier and/or preamp if you need more gain on TX and RX, respectively. The fact is most RX antennas have negative gain values, some in the -20 to -30 db range, while a good RX antenna has very good F/B (30 db is really good) and at least decent F/S (12-15 db is common). A lot of guys comment that they could never possibly hear a signal that low but in reality, good listening skills and a good pair of headphones dictate that the antenna level only need to exceed the receiver’s noise floor. One can test this concept by listening to a receiver without an antenna connected and then hooking up the antenna while listening. If there is an increase in the noise floor, the levels are probably fine. If one feels the need for more signal, a low noise preamp can be employed.
With any system that improves overall performance, there are always draw backs and there is never, ever a free lunch. For example, I mentioned that improved S/N is accomplished typically using external RX antennas. That suggests that these RX antennas exhibit a pattern which is used to null an offending signal. In the presence of a TX antenna, typically a vertical on the low bands, the RX antenna pattern can become skewed, in some cases losing all of its directivity. This is often characterized by an overall increase in noise and a lack of directivity when listening to known signals. I first experienced this problem using K9AY loops on 160-meters in the presence of my short, base loaded vertical. I was stubborn and it took my inability to hear the 3B9RF DXpedition before I fixed the problem by simply detuning the TX antenna while receiving. This concept of detuning can be daunting to a lot of people and RF switching, particularly at high power, is not for the faint of heart. A simple relay at the TX antenna feed point can switch in a network that resonates the antenna out of band. In some cases, depending on the physical length of the TX antenna, simply opening up the feed point or shorting it to ground can be all that is necessary. Relay control is accomplished using the rig’s PTT relay or a breakout box that is driven by the PTT relay. The relay should be scaled depending on the amount of power applied to the antenna, SWR, and voltages that are present on the TX antenna. My very first attempt at detuning resulted in a relay that was too small and I welded the relay contacts shut. Yes, it looked like an arc welder. I learned my lesson and began using the Gigavac G2-HAM vacuum relay, which is listed on the Gigavac ham discount program. No more problems and I started hearing well for the first time. I have observed in a few instances that some individuals try to cut corners by doing the detuning from inside the shack in order to avoid running control cables. More often than not this approach fails. For those who say that this approach works, its either denial or dumb luck due to some other aspect of their installation. Detune at the base of the TX antenna!
The need to detune the TX antenna can be minimized by giving a wide berth between TX and RX antennas. W8JI goes into a very extensive discussion of this concept on his website so I will forego the details. For most of us, we are limited to small chunks of property and detuning the TX antenna is the only option.
While some RX antennas might benefit from good ground conditions, like the ground-dependent K9AY loop, other antennas rely on poor ground conditions to achieve the necessary wave tilt to become sensitive to vertically-polarized signals. Most notable is the beverage antenna. The beverage antenna in its simplest form is just a straight, long wire, horizontally oriented and spaced several feet above the ground into the direction of interest. Typically the far end of the beverage is terminated through a resistor to a good ground rod and in some cases, more than one. The resistor value is determined experimentally and is dependent on the environmental conditions. Signals arriving from the far end of the antenna tend to have their wave front slowed due to lossy dielectric under the length of the antenna. This slowing results in a tilting of the wave which explains how a horizontal wire can be sensitive to a vertical-incidence wave. As the tilted wave front moves down the length of the antenna wire, the signal adds in-phase, resulting in a building voltage which is coupled to the receiver through an impedance matching transformer, typically a 9:1 transformation. Signal arriving from the feed point end of the antenna add similarly but are shunted to ground through the terminating resistor at the far end of the antenna. The result can be a very nice F/B, depending on the length of the antenna and care in termination as well as a tightening of the pattern which is analogous to a narrowing of the beam width. While beverages that are multiple wavelengths in physical length are often desired, portions of wavelengths can be useful, although the pattern tends to become more omnidirectional as the wire gets shorter. All of these factors contribute to S/N. As in the previous section where the importance of detuning the TX antenna was emphasized, locating a beverage over a radial field for a TX vertical, detuned or otherwise, can have devastating effects on the wave tilt and subsequently the sensitivity and pattern. It is recommended that interested readers carefully study W8JI’s website in addition to the ON4UN low band DX book, which are both considered important references for the low band operator.
Beverages come in a number of forms. At KB5NJD / WG2XIQ, beverages have been used on 80 and 160-meters in the past, both in the air and on the ground (commonly known as BOG’s, or “beverage on ground”). In fact, beverage antennas are responsible in the completion of 160-meter DXCC. In recent years, however, trees that supported and concealed beverage antennas 8-10 feet above the ground were damaged by ice storms and in most cases had to be removed. While many have reported good performance with BOG’s, relatively good ground conditions at this location tend to result in a significant loss of sensitivity and directivity. BOG users often find that due to the proximity to the ground, the dielectric effect allows the short wire to “look” much longer than it really is which is more or less a function of a decreased surge impedance, typically around 200 ohms for an average wire sitting on the ground. Running beverages down the sides of the street along the curb has been done by many and may be considered again here. Its easy enough to roll the wires up using a spool mounted on a drill motor. Of course, very low frequencies dictate very long wires in order to achieve a useful pattern so this concept bares more experimentation.
For low band amateur radio activities, the K9AY loop has historically been the most reliable, compact workhorse at KB5NJD. Sadly, that was not the case on 630-meters. Even making modifications to the termination resistance in order to improve the null depth in addition to changing the transformer material from type-43 to type-77, the performance seen on the ham bands was never achieved on 630-meters. Its difficult to achieve a good pattern with such small spacing on such a low frequency. Jay, W1VD, published an improvement to the K9AY loop on his website which includes increased element length, spacing and a variable termination resistance. Results from those using this design have been been very positive and I believe that this approach is what Jay uses on a regular basis to hear Europeans. The takeaway message here should be that the K9AY loop is a good choice for MF and LF reception if realistic physical size is considered and scaled accordingly. The very nature of the design has made it quite scalable.
Most recently at KB5NJD / WG2XIQ, the VE7SL multiturn resonant loop has been employed with very good results. This loop replaced the K9AY loops in the summer of 2014 and while this station has historically demanded a switchable cardioid pattern from the receive arrays, the performance of this antenna has been very good during late summer tests.
A rotator allows the antenna’s azimuth to be changed and even with the bidirectional response, performance has been really great. The loop itself is a parallel resonant circuit at a very high Q and uses an isolated feed which is accomplished with a single turn pick-up loop about 6 inches from the main loop turns. a 30 pF variable cap is used in parallel with about 250 pF silver mica padder caps and is adjusted with my MFJ antenna analyzer until the value of X is equal to zero, which occurs around 475 kHz. A W1VD preamp, based on the original design by W7IUV, utilizes a 2n5109 and offers very little noise insertion. Having built a number of poor-performing, noisy preamps, this preamp is very quiet. At zero level input (no antenna attached), there is no discernible difference in output level when the preamp is coupled to the receiver. This means the internal noise is very low. In fact, the noise figure specification is 2.5 db.
Because the loop is located in the near field of the transmit antenna, I used a 12-v relay to short across the resonating capacitor when I a transmitting. Doing so accomplishes two things: First, it prevents the high-Q loop from developing dangerous, damaging voltages which could not only destroy the capacitors but could also back feed the protection relay in the shack and ultimately damage the receiver. Secondly, a high-Q resonant circuit in the near field of the antenna would serve to sap power away from the antenna that should otherwise be radiated to distant locations. This problem is often seen in the broadcast service when resonant or near-resonant structures are erected in the vicinity of a transmitter site. The result is skewed pattern in directional arrays and a decrease in field strength overall. This problem is often corrected by “skirting” and detuning the structure, resulting it is disappearing electrically.
The most impressive system that I am aware of is the 630-meter 8-circle array at NO3M / WG2XJM located in western PA. Eric’s system encompasses 30 acres and is often reported to perform better than his long, phased beverage arrays that are also used on 630-meters. Still other space-conscious options that work well include the W7IUV rotatable flag, which is related to the family of terminated loops, much like the K9AY loop, while offering a significant improvement in performance and the EWE antenna which has been very well optimized by Mike, WA3TTS in west central PA.
While so much has been said about the use of directivity to improve S/N, it is important to mention that one should periodically listen with the transmit antenna because there is certainly benefit when the noise level is low. Because of generally poor performance with the K9AY loops in the winter on 2013/2014, the TX vertical was used almost exclusively during that time frame at WG2XIQ. When the band is quiet, which is often the case from mid- November into early January when the weather is generally stable here in North Texas, the noise floor on 630-meters can sound like 10 or 15 meters. These conditions are perfect for listening with the omnidirectional vertical. This arrangement is also useful living in the central US as it is very easy to be listening in the wrong direction when a signal may be arriving from another. Be aware of propagation and be an active participant in the operation of your station and you won’t miss much.