Giving the fact that I had enough spare time the last two weeks, inbetween the little Christmas trip I took back to my home town of Ploiesti and the New Year’s Eve party, I had the change to propery get my station back up on the air. Since I moved to the new location in June I didn’t really have a proper setup and the few times I got on the air was with some modest balcony antennas.
| Click to enlarge |
The image above is a Google Earth mock-up of my current antenna setup. The red one is the HF dipole, the blue one is the VHF/UHF vertical and the green lines are the two feedlines.
The HF antenna is a an off-center fed dipole (OCFD) 41.8 meters long; this makes it resonate – when properly installed – on 3.5MHz, but on all it’s harmonics also (7MHz, 10.5MHz, 14MHz, 17.5MHz, 21Mhz, 24.5Mhz, 28MHz etc). We usually feed dipoles at the center but due to the current/voltage distribution across the resonant wire, it will present a low (close to 50ohm) impedance only at the uneven harmonics (3.5Mhz, 10.5Mhz, 17.5Mhz, 24.5Mhz etc), while at the even harmonics (7MHz, 14Mhz, 21Mhz, 28Mhz etc) it will present a very large (theoretically infinite, practically a few thousand ohms) feed impedance, wich is very far from out 50ohm feedlines making the antenna unusable due to high feed mismatch.
Some witty guys discovered if you feed such an antenna not at the center but at certain points closer to one of the ends, it will present a higher impedance (maybe closer to 200ohms) but it will be constant troughout all the harmonics. Now, because the antenna is balanced and the coaxial feedline is not, we need a BalUn to adapt the feedline to the antenna, and because the antenna impedance at that point is about 200ohm and the feedline’s impedance is 50ohm, we will make this BalUn with a transformation ratio of 4:1. The best design for this type of BalUn is the Guanella 4:1 made on two separate toroids, and I’ve used two Amidon FT80-43 wich should be good for QRP and probably up to at least 50W (maybe even 100W).
At this point, we can say we have succesfully built an antenna that can pretty much work on most of the HF bands; here’s how the SWR looks on a simulation:
Not perfect, but in most of the bands good enough to be easily tuned by your transceiver’s internal tuner – or if you don’t have one, just back the power down some more and use it as is. The radiation pattern really depends on installation, but it should look very similar to a dipole on 80m and then spread in more and more lobes as you go to the next band.
The VHF/UHF dualband antenna is about 1m long and it’s built following VK2ZOI’s design. I have nevertheless used a thicker PVC tube (32mm instead of 25mm) to make it stronger, therefore I had to redesign the base choke: not it has only 6 turns instead of 9. I also had to add about 10mm in length to make it resonate a bit lower, as our VHF/UHF bands are smaller (144-146 and 430-440).
I used two runs of NextraCOM H1000N coax (one for HF and one for VHF/UHF), it’s basically the same as Belden H1000N, NextraCOM is a german company that makes professional comunications equipment so I trust them (and it was considerably cheaper than the Belden). Also, I have used a coax choke to keep stray RF off the coax braid: 6 turns of RG-316 on a stack of 4x Amidon FT80-43 works great. Since I use an SDR transceiver wich is connected to my PC, it is critical not to have any RF in the shack as it creates alot of problems with the computer: USB devices disconnect (even the transceiver), you get TX sound back in the speakers and the monitor starts displaying artifacts.
Another element of my recent installation is the shielded UTP cable run from my radio room up to the building, wich at the moment has no purpose but it will be used in the future to power and control a remote antenna switch, an antenna tuner and maybe even a rotator – you never know.
Testing this installation, last evening my station ran WSPR on 40m with just 500mW output and I got hundreds of spots all over the world, alot of them coming from North America – over 10.000 Km away.
Now the propagation seems to be very strong, so I’m off to 15m. Catch you on the air !