40M PH Vert.


	40m phased verticals by Jelmer PA5R

	Introduction The PA6Z contest group is continuously working on upgrading the contest station. One very important factor obviously are the antenna systems used on the various HF bands. The saying "your best amp is your antenna" is very true. For the 40m band, it was time for another upgrade. Why a phased vertical system? Over the years, we've tried many different antenna systems on this band, ranging from simple dipoles and verticals to very large wire beams and rhombic antennas. But most of the latter have proved impractical for our situation. They are very time consuming to put up and break down, for us this is important because we don't have a fixed contest site. So after some thought we came up with the optimal solution for our situation: beverage antennas for RX, a dipole and a phased vertical system for TX. A two element system gives about 20 dB front to back ratio, and (more important for a TX antenna) about 3 dB of gain over a single vertical. The direction of the front lobe can be reversed 180 degrees by swapping the feed lines and with that, the feeding phase. From our geographical location, two main directions east and west are a good compromise when you can't put up a rotatable beam antenna. Because the forward lobe is pretty wide, most parts of the world can be worked by just these two directions.

	Some theory on phased verticals The principle of phased antennas is quite simple, see the figure below: Two elements are spaced one quarter wave length apart and fed 90 degrees out of phase, each with half the transmitter power. The wave arriving at the first element initially will be radiated in all directions, as will the wave that is 90 degree delayed and fed to the second element. The energy that is transmitted from the second element in the direction of the first element will cancel out, because it arrives there with 180 degree phase shift (-90 degrees from the feed and-90 degrees from the spacing distance). The energy that is radiated from the first element in the direction of the second element will add up because it is in phase (0 degrees from the feed and -90 degrees from the spacing distance). So in theory, no energy is transmitted in one direction, and double the energy is transmitted in the other direction. Knowing this, we started simulating the antenna, see the MMANA plots below. The model has two quarter wave vertical radiators with each having 36 radials at the base. The simulation shows … dB F/B ratio and … dB of gain. The maximum radiation peak is at …. Degrees elevation angle. Looks nice, so let's build it!

	Building the system or the radiators we use copper wire supported by Spiderbeam^TM glassfiber (non carbon!) telescopic rods of 12m each. These are placed 10m apart. Two times three guy wires are used to keep them vertical in our windy little country. The glassfiber rods rest on a small disc shaped plate milled out of Teflon, which sits on an aluminium plate for connection of the radials and coax connector. For details on the feeding point, see the picture below. Each vertical has 36 radials of about ¼ lambda (not critical). Needless to say, the more radials the better. Our measurements on single vertical radiators show about 38 + j0 Ohms feed point impedance with a tuned vertical with 36 radials. This would mean that there's only 2 Ohms of earth resistance (= earth loss), because an ideal vertical over perfect ground has a resistive part of 36 Ohms in it's impedance. That's why we use a minimum of 36 radials for all our vertical antennas. Pic of PH vert's

	Phasing the antennas T There are several ways of phasing the verticals with the correct phase delay, three of them will be discussed here: Phase line Reactive phasing network (PI, T or L networks) Hybrid coupler 1 Phase line The first and easiest option is to use a phasing line. When knowing the velocity factor (VF) of a coax or other transmission line, one can calculate the required length for any desired phase shift: L = VF300(degr. Phase shift/360) / f with l in meters and f in MHz. Most coaxial cables with PVC dielectricum have a VF of about 0.68, when the cable has some kind of air filled dielectricum, the value increases. VF for regular cables can be found on www.nogffzoekn.com Example: or a frequency of 7.050 MHz and a desired phase shift of 90 degrees, using a cable with a VF of 0.68, the required length is: L = 0.68300(90/360) / 7.05 = 7,23m This phasing method works fine, but only on one frequency. The null in the antenna pattern is very deep, but the bandwidth of the null is small, in theory only about … kc for a 20 dB F/B ratio. An advantage of this method is that is has virtually no loss, apart from the loss in the phasing line. Because it has no loss, it has very high power handling capability, only limited by the power handling of the transmission line used. 2 Reactive phasing network Another way of getting a phase shift is using a reactive network like a PI, T or L network, using lumped components. Some examples are shown below: When building such a circuit, using a circuit simulator and knowing your network theory is a must. Also, measurement equipment should be available like (at least) an LC meter, a signal source and oscilloscope or better, a network analyser. This is because these circuits depend on exact values of L's and C's, and making these require some experience in HF building and measuring practices. As an example, a … network for 90 degrees phase shift circuit for 50 Ohms is shown below. An advantage is the network takes very little space, and can be build in a water proof connectorised box for use in the field. The power handling of such a circuit depends on the components used. For example, the C's should be true RF types, like the tube or "door knob" types. The voltages across the capacitors is only … Volts at 1kW, but the current trough them can be massive. Don't use the high voltage disc ceramic types, for these cannot handle the large currents! The inductor can be constructed using air wound coils or adequate ferrite cores for the lower bands (see www.amidon.com for example). 3 Hybrid coupler Setting up and testing Setting up such a large antenna is almost a project by itself. The rhombic is 135m (400ft) long and 60m (180ft) wide. The four supporting masts have to be placed very accurate to ensure a pure symmetrical romboid shape of the antenna. We used a 50m measuring tape and worked out from the centre of the antenna. Starting with the front and back masts, the distance from the centre of the supporting masts were marked by a rod. We then would optically align it over the centre. Next, for the side masts the same principle, but you have to make sure this line is at an exact 90 degrees on the first one. All masts were fitted with an insulator and a small pulley. This way you can control each individual fixing point of the antenna for fine tuning. When the antenna is up, you will immediately see if it is symmetrical, only then the insulators and pulleys all run straight towards the centre of the antenna. The forces the masts will experience are quite large if you stretch the antenna so it is horizontal. Actually, we could never get it all straight, there still was a slight slag in the four sides. The antenna wants to pull the supporting masts inside, so extra guying in opposite direction (away from the centre of the antenna) is helpful. The wind load of all that wire is huge! Fig. 8: One of the side masts holding the rhombic up. From the first tests on I liked the performance of the antenna. The VSWR was below 1.5 : 1 from 3 to 30 MHz. Using the termination, the antenna was extremely low noise and had a constant low VSWR for every frequency: a true non-resonant antenna! When leaving out the termination and shorting the two legs together, the noise level went up a little. This is because the antenna is now receiving from two directions, so in theory the noise level should increase by 3dB. By removing the termination, the antenna becomes a resonant antenna, so the VSWR will not be low for the entire frequency range. I found that whenever one side of the rhombic is a multiple of ¼ lambda (odd or even!), the antenna resonates and shows a good VSWR. But outside these frequencies the VSWR increases. When you keep the coaxial cable short or use a balanced tuner, this actually is not a real problem. But I wonder what happens to the radiation pattern for frequencies were the antenna is not resonant. During the times we set up the antenna for testing, I was able to work nicely on it from 80m trough 10m. Best performance was on 20m and 15m, set up towards USA the signals were truly booming. On 10m the antenna simulations showed that the beam width becomes very narrow, so we were "illuminating" just a small part of the US. I wanted to make a comparison with a well known antenna, like a small triband beam or so. But unfortunately, this never happened so I cannot really say how good the rhombic is. On 40m and 80m the beamwidth becomes much wider and because of it's limited height most of the energy goes way to high for real DX work. But what does help is low noise floor because of the antennas good directivity. This enables you to hear very weak signals. One other thing we experienced during testing is the enormous build up of static electricity if the antenna is not grounded. Just before a big summer rainstorm, the noise level slowly went up to 9+10dB and I disconnected the antenna to save my transceiver. During the storm, the antenna plug was literally sparking, certainly destroying your rig if you leave it in! Contest use After setting up the antennas a couple of times in the field, it was time to use them in a contest. By now, we had two 320m rhombics, but only one termination. So we decided to set up one antenna to USA using the termination and one to Japan with the ends tight together. This would allow us to work into South America on the back of the antenna. The USA rhombic was at 18m high (54ft) and the JA/SA rhombic was at 12m (36ft). The antennas were to be used for DX on 40m, together with a vertical and a dipole. We could switch between the four antennas. During the night, the rhombics did a great job. Many signals were not even heard on the vertical, while perfectly readable on the rhombic. Even when DX stations were not in the direction of the beam, they could often be worked better on the rhombics that on the vertical because the noise level was so much lower. Because we were in a contest, there was no time for comparison of the rhombics with our 20m, 15m and 10m beams, something I really wanted to try. Maybe some other time then… Conclusions All in all, the rhombic is a very nice antenna to work with. It's a very low noise antenna with high directivity also on the lower bands. A comparable yagi or quad beam antenna (at least 3 to 10 elements from 80 trough 10m!) would be much more expensive to buy or much more difficult to construct. The two only serious drawbacks of the rhombic I think are it's size and the fixed beam heading. By using relays and extra open feeder, you can use it in two directions, but still this is not very practical when used for contesting. Another thing that does not help in a contest is the rhombics inherent bandwidth, making your station susceptable to interference caused by the transmitters working on the other bands. When you have enough square miles of land for 4 or more rhombics and work single band, I would go for it! For our contest group the rhombic antennas were used twice as 40m beam antennas. While performing great, we decided not to use them anymore because of their enormous consumption of square meters on the contest field and the amount of work needed for setting it up compared to the improvement in station performance. More interesting information on Rhombics can be found on the following links: http://www.mindspring.com/~cummings7/rhombic.html http://www.angelfire.com/tx5/wieser/rhombic.htm http://www.tpub.com/neets/book10/42o.htm http://www.fullwave.com/fsj/rhom1.htm