The Cizirf-Special Reception Antenna

The "cizirf-special" receiving antenna

A 40/80m receiving antenna optimized for short and medium distances

Patrick Destrem F6IRF

Introduction

I recently presented in an article in french dedicated to antennas for short distance on 40 and 80m bands, a concept of receiving antenna using 2 broadband dipoles placed at low height and fed with various phase relations(*). This antenna is not a DX antenna, but an antenna which vocation is to facilitate the "domestic"communications, which by these times of minimum solar index, are often more difficult to achieve than DX contacts. This is especially true on the 40m band. This antenna may also be interesting for those whom are affected by one or more sources of local noise, such as urban neon sign, transformer or power line, these disturbing signals arriving at low angles.

Following is the complete description of this antenna with the various available options. Although this antenna was initially designed with our “National contest” in mind, it may also be interesting for other applications.

(*) http://mangafight.free.fr/Antenne%20CDR%20part4.htm (you may try an automatic translator or just look at the pictures!)

NB: If you never used a separate receiving antenna, please note that a basic precaution consists in short-circuiting the entry of the receiver (for example by means of a coaxial relay) during transmission. The RF "collected" by the receiving antenna can reach dangerous power levels for the receiver. Even in the case of a transceiver having an "RX-ANT" input, it is recommended to make sure on the diagram that this input is disconnected, or shorted-circuited before reaching the first components of the receiving chain (it is not always the case...). In any case, the receiving antenna should be located as far as possible from the transmitting antenna.

The broadband dipole

Figure 1 the broadband dipole. As you can read it in the text, it is better for the dipole to be non resonant.

Why use a broadband dipole of 2 times 15m? The first reason is to maintain an acceptable SWR in the transmission line. Typically with this dipole, the SWR should hardly exceed 1.5 on 40 and 80m, whatever its height (from 1 to 10m) and the shape.

W8JI, in his article http://w8ji.com/small_vertical_arrays.htm explained the reasons why low-Q antennas should be preferred for this kind of receiving systems. I let to you refer to his article.

Figure 2 On top SWR versus wire-length (in meters) for the fig1 dipole. At the bottom Gain versus height (in meters) . In all the cases it is better to avoid resonance...

The length of wire (2x 15m) is imposed by the Gain-limit of -20dBi, below which the reception antennas are likely to require preamplification. The length of wire is not very critical. However, it is better to avoid half-wave resonance, as the SWR may reach about 4:1, due to the low radiation impedance versus the loading resistor. Taking into account tits low-Q, this antenna should not disturb too much the other antennas located at its vicinity (*) It is not either desirable to make the dipole too long, because with the length, the directivity increases to pass by a maximum at 2 times 5/8, dimension beyond which the secondary lobes become dominating. In short 2 times 15m seem a good compromise for 80 and 40m bands.

(*) the opposite may not be true: As mentioned by W8JI in the above mentioned article, a transmitting antenna may affect greatly the performances of your receiving antenna system. I did the experience myself: I noticed quite a difference in levels and noise between my East antenna and my west antenna… until I detuned my transmitting vertical!

Figure 3: Vertical gain and SWR of the broadband dipole on 40 and 80m versus height in meters for an average ground (NEC2). For a receiving antenna the gain is not a determining factor, but a minimum is required.

The antenna

Figure 4 the antenna consists of 2 broadband dipoles, such as those of figure 1.

The antenna consists in 2 of the fig1 broadband dipoles. Optimum spacing depends on what is desired. If one wishes an extreme NVIS antenna, a half-wave is a good spacing. If one prefers a directional antenna for the intermediate vertical angles, a more reduced spacing will be preferable. For use on 40 and 80m it is a matter of compromise, but around fifteen meters of spacing between dipoles, looks a pretty good choice (not very critical however).

Figure 5 vertical Gain and 45 degrees gain according to dipole spacing, for the two fig1 dipoles fed in phase at 7m AGL (NEC2).

Figure 6: Vertical diagram for a spacing of 0.2, 0.5, 0.6 and 0.8 wavelengths (MMANA). A half-wave (in red) is a good spacing for a NVIS antenna without low lobe on the horizon. However 0.6wl provides nearly 10dB more attenuation of the signals arriving at 40 degrees (in blue).

Feeding the dipoles

For the simple NVIS version (fig6), the simplest solution consists in mounting the dipoles in the same direction (in terms of phase) and using identical feeder lengths to a broadband matching device (25/50 ohms) such as a UNUN. For the “high angle” directional pattern option (fig 7 and 8) and if the goal is a particular distance (you may have a look at the relation vertical-angle/distance in this article – just look a the diagrams!) or direction, it is possible to use dissymmetrical lengths of cables to the matching device. If the dipoles are in the same direction (in terms of phase), it is the feeding line of the dipole toward the desired direction which should be made longer (see fig7). If the dipoles are mounted “opposite” (in terms of phase) it is the feeding line of the dipole opposite to the desired direction which should be made longer (*).

If you want to reverse the desired direction, I have described on my blog a simple solution, using a single coaxial relay and a UNUN. The article is in French, but the diagram is quite obvious… This simple solution (not tested up to now) assumes that the 2 dipoles are absolutely identical, and that the SWR of each one is low (you may need to adjust the loading resistors and/or the wire lengths). With this simple solution the main problem remains the mutual coupling between the 2 dipoles fed in quadrature (or around). However due to the low-Q of the dipoles, the degradation of the patterns compared to the simulation should be acceptable. Ideally an hybrid coupler should be use to combine the 2 antennas, but they are generally designed for one band (like the Comtek PVS-2). Anyway for receiving, a more flexible solution is provided by phasing boxes such as such as the MFJ-1025 or the DX-engineering NCC-1 (see tests). This said, even a very simple solution just using switched coaxial cable-lengths and a quickly made UNUN can sometimes provide very surprising results such as demonstrated in this 80m recording.

(*) For unphasing by more than 90 degrees, this solution requires less cable. Demonstration: starting with 2 dipoles A and B in opposite phase direction (like the 2 consecutive elements of a log-periodic antenna), adding 1/8wl to the feed line of the dipole A, will create a phase relation of 180+45 = 225 degrees (in other words -135 degrees), and the directivity will be A towards B (Our dipole A occupying the same situation as the reflector of a HB9CV). If the 2 dipoles are in the same direction (in terms of phase), it will be necessary to add 3/8wl in antenna B feed line to achieve the same result.

Simulated performances on 40 and 80m

The diagrams below were generated with MMANA (and verified with NEC2) using a spacing of 15 meters and a height of 7m. As can be seen on figure 3, there is some gain reduction by reducing the height, however the absolute gain it is not a determining factor for a receiving antenna and overall, the real performances should be close, even with a height as low as 1m.

Figure 7: Diagrams 40m according to the phase between the dipoles. All the horizontal diagrams are generated for 45 degrees elevation. For the 2 dipoles in phase the maximum Gain given by NEC2 is minus 2.85dBi and the SWR 1.42. For the 2 dipoles fed in quadrature the max gain is minus 3.69dBi and the SWR for each dipole is 1.4 and 1.47. For 15m spacing, the maximum F/B ratio is obtained for a phase-relation of about 110 degrees (green curve). If the low-lobes on the horizon are a problem, a phase relation going from 70 to 90 degrees will also provide excellent results.

Figure 8: The same configuration, but on 80m band. As you can see, the gain decreases notably when the phase difference is increased. The maximum gain given by NEC2 is -10.28dBi (SWR 1.44) for the dipoles in phase and of -15.65dBi (SWR 1.47/1.45) for 140 degrees phase relation. For 180 degrees the maximum gain goes down to -20.83dBi. For 120 degrees the diagram is excellent with no secondary lobe near the horizon.

As can be seen on fig7, the gain variation is minimal on the 40m band. At 45 degrees elevation, it is the same for the phased-dipoles and the anti-phased dipoles. It is the ideal case!

On the 80m band the gain varies more. However the antenna remains usable with good performances.

Of course, it goes without saying that continuing to turn around the phase-circle rotates the diagram by 180 degrees in azimuth.

Attention: It is not as simple as you may imagine: if you consider the two dipoles in quadrature, adding 180 degrees will provide the "mirror" diagram... it is not true any more if you start at 135 degrees (135+180 = 315), because in this case the phase-relation between dipoles will only be 45 degrees.

Performances on adjacent bands

The performances on the adjacent bands is interesting, as it gives us an idea of what could be done to improve the design (spacing and element lengths).

Figure 9 Diagrams on 30m. It is quite obvious that, if an NVIS antenna is the choice, a half-wave is a good spacing between dipoles ( 0.6wl is even slightly better). On the other hand as soon as the phase relation is changed, a back lobe appears rather low on the horizon. This goes against the desired goal. It can also be seen that the horizontal diagram (at 45 degrees) becomes resolutely oval. However the antenna remains usable for the defined purpose.

Figure 10 Diagrams on 160m. The antenna becomes difficult to use due to the insufficient gain.

Construction

I built the baluns on high-AL ferrite-tubes, initially intended for choking RG58 or similar cables. With three bifilar turns the SWR is in conformity with calculations (1.2 to 1.3 from 1Mhz to 30Mhz). The realization is far from being critical, even the “random choice" of resistors does not seem to affect the results at the considered frequencies.

Figure 11 The two baluns are rigorously identical. The resistors used for the tests are two resistances of 510 Ohms 1/4W assembled in parallel... I will replace by 3x 820 ohms /3W resistors when I get them, because these ones are likely to burn, the antenna being located in the close field of the transmitting antenna.

Figure 12 Checking of the assembly balun/resistor. SWR 1.2 to 1.3 of 1.2 up to 30MHz... Note my high-class network analyzer!

Figure 13 Mounting of the SO239 socket.

Figure 14 The assembly is made in a small electrical box. . Like the Balun, the dipole is constructed with a black and a white wire (of 0.5mm diameter- not critical ) to allow easy identification of the phase.

Figure 15 Here, the antenna is installed on a fiberglass mast supported by a tripod. The dipole apex is approximately 4m AGL and the ends approximately 2m AGL.

Tests with one dipole

With reference to my vertical, the results with only one dipole are already interesting. With the 2 antennas levels balanced on the background noise, the gain in SNR(*) is spectacular, frequently 10 to 15dB on the stations located at less than 500 kms. Only the most remote stations arriving stronger on the vertical. The improvement is such, that certain stations, completely inaudible on the vertical, appear above the noise when switching on the low dipole. Admittedly it is necessary to mitigate this result by the fact that the comparison is made with a vertical. Compared to a dipole not too high (< 0.5wl), it is likely that there will be no difference. On the other hand, on 40m, compared to high yagi or a dipole (>.0.5wl AGL) I think that the results will be similar to this 40m recording (the HB9 station is about 300kms away, TM8P is around 400kms away)

With this version using 2x15m wire lengths, the level is sufficient on 40 and 80m bands, not even requiring the use of the transceiver preamplifier. On 160m, it is necessary to use the preamp (for the IC756pro2, preamp position 2 which is approximately + 20dB gain). In spite of the very negative gain of the antenna on this band the results are also still spectacular (the station on this audio-clip is located about150kms away), the local stations emerging from he noise as by magic.

(*) Signal to Noise Ratio

Tests with the two phased dipoles

Making a point of testing a dedicated NVIS antenna, I initially installed the 2 dipoles (still 4m AGL) with 20m of spacing, 2 antennas being connected by equal cable lengths to the 2 ports of a "stackmatch" (to allow quick comparison between1 dipole versus 2). On 40m the first immediate result was a reduction of the background noise (created at home by 380 kV power line) by about 7 dB (compared to only one dipole). Of course on 80m the difference is tiny (approximately 2dB) the 2 antennas being too close for this purpose. On 40m monitoring close stations (100 to 300 kms), I noted more or less 20 dB improvement of the SNR compared to the vertical, for a part due to the reduction of the noise floor. The improvement of the SNR with the 2 phased dipoles is also very clear when compared to a single dipole.