NVIS helical dipole antenna
Limited space in a temporarily rented house close to the heart of Bendigo in Victora, Australia forced the construction of a smallish 40m band HF antenna to go with the newly acquired Yaesu FT-710 HF transceiver.
Based on the work of Igor Grigorov VA3ZNW (see references at the bottom for a download link to a PDF document).
As a bit of context this antenna lives on a 3m tall PVC plastic pipe. The houses on all sides have metal roofs. There is a metal garden shed below (pictured) and an old Hills Hoist clothes lines just below. Even the fences here are made of metal sheets.
NVIS Near Vertical Incident Skywave propagation. It is used for military and paramilitary communications (and by old men playing ham radio). https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave
One of the key features of NVIS is the low height of the antenna above the ground. Essentially the radiation pattern becomes directed in an upward direction. Also low height is obviously desirable from a civic planning perspective since very few people can actually see the antenna at all. However, there is a downside to vertical skywave propagation - QSB. The critical frequency (below which reflection occurs) is proportional to the number of electrons per cubic meter in the ionosphere (maximum ionization density - caused primarily by UV radiation from the sun).
The basic antenna concept: 2 lots of 22 meter reels of enamaled copper wire from Jaycar Electronics (Bendigo Electronics) which just turned out to be the required 22 meters in length. (Ref 1: page 23).
Tuning the antenna with a NanoVNA. I can remember back in the day (Late 1970s) at RMIT Communications Engineering we had one of these guys (a Vector Network Analyzer). It cost multiple $10,000s. Mine is from China via Aliexpress for some $70.00. What's more there's a lovely bloke on youtube that explains how to use it. Oh how far we have come in 50 years - lol.
Here's how to use it:
Once the antenna tuned somewhere near the correct frequency (being 7.1 MHz) the windings were glued in place. This made it possible to slide the covers over without disturbing the tuning too much.
It should be mentioned at this point, that with the windings carefully spaced quite evenly, the antenna tuned at 8.1 MHz. This is bad. I then mistakenly moved the windings or rather spaced the windings closer to the outer ends. The resonant frequency then moved even higher. The reference document clearly states that one should move the windings closer to the center (oops). That worked quite well with the sweet spot dropping to 6.9 MHz. A bit more faffing about saw that frequency precisely where it was wanted.
Except for one thing, this was a good strategy. Unfortunately the covers dropped the sweet spot in frequency..... doh!
|As an aside and by way of explanation of center frequency choice: ITU region 3 (i.e. we here in Australia) have 7.2MHz to 7.3MHZ allocated to HF broadcasting which is mostly occupied by what seems to be Chinese language AM radio stations (This author doesn't speak and can't understand the content).|
Cementing some of the parts in place. Then some strain relieving of the coax's weight. As can be seen in the photo below, the T piece has been "machined" out allowing the pipe to fit all the way through. A rat tail file was pressed in to service to remove material. Then a large diameter blade drill chewed through the pipe wall to provide access to the copper windings on the smaller diameter coil former pipe.The whole arrangement provides pretty good mechanical strength without any glue (although some was added later to provide better waterproofing).
It's not quite possible to see the four ferrite beads around the coax. They just sit there with two cable ties, one under and one above them, which hold them in place. Four is probably over-kill, but they were left over from the UPS RF silencing project, so in to service they went.
At some point this piece of hackery will get a proper PL259 connector as part of a future attempt at making everything portable.
As noted above the resonant frequency dropped when the covers were placed over the windings. To overcome this 5 turns were removed from the ends. The antenna then tuned exacly at 7.1MHz. The bandwidth seems to be wide enough for the FT-710 with it's magic built in antenna tuner. Currently there is no external tuner here at the VK3PAB QTH.
Note the covers aren't glued in place yet. Maybe they never will be.
The inner pipe is 20mm while the cover is 25mm. I couldn't work out what the 20mm and 25mm referred to as far as the pipe specifications go. There is no dimension I could measure either internally or externally that corresponded to these numbers. For example the 25mm pipe has an external diameter of 33.5mm and an internal of 29mm.
Holman PVC mostly from Bunnings.
Antenna Mast Upgrade
As it turns out the PVP2512 diameter pipe flexes quite a lot. To try to overcome this a larger diameter was pressed in to service.
Help was needed from an adaptor but with slightly more support than just the standard pipe adaptor. BTW the ferrite beads, on the coax, are clearly visible here.
The adaptor had two parts made/modified in Shepparton in the brother's engineering workshop which is equipped with a lathe. Essentially the original adaptor at the top was bored out to match the external diameter of the smaller PVP2512 pipe.
Then the lower part, which is a pipe joiner, had its external diameter changed to match the internal diameter of the larger pipe. Finally the joiner was bored out so the smallet pipe could fit through. The result is a shim.
Cable tied in place with the new mast.
It seems Ham Radio folklore (or unscientific reviews) suggest the mast height should be "as low as possible".
Unfortunately the height here is not optimal according to reference 2 and 4. It's been working OK, but it really needs to be around 0.2 λ above the ground (8m) for transmit. Apparently the directivity should be OK (i.e. everthing gets blasted upwards), however it will be suffering some attenuation, although not as bad as initially thought.
Vertical radiation pattern of a horizontal half-wave dipole antenna 0.02, 0.06, 0.20, and 0.40 above farm-land soil at 5.39 MHz. Intensity axis shows antenna gain in decibels over an isotropic radiator (dBi).
Receive should be reasonably OK, as it seems something slightly less 0.1 λ is optimal (i.e. 4m or there abouts).
I also reckon all the metal sheeting around the place and the old Hills Hoist clothes line will be having some effect too. It's a wonder it works at all really.
Pretty sure this was the product used. Check the length (printed on top sticker - but length is not specified on the web product specifications) . The reel used here had precisely 22m of wire on it, so it's possible this image not the same as the diameter used..
HF Helical Antennas: Theory and Practice
By Igor Grigorov VA3ZNW
NEAR VERTICAL INCIDENCE SKYWAVE
The propagation mechanism, the impact of antenna, backscatter and solar flares
By dr. ing. B. A. Witvliet
Ministry of Economic Affairs & Climate
Radiocommunications Agency, Spectrum Management
- The www.antentop.org website.
- Near Vertical Incidence Skywave Propagation: Elevation Angles and Optimum Antenna Height for Horizontal Dipole Antennas
By Ben A. Witvliet, Erik van Maanen, George J. Petersen, Albert J. Westenberg, Mark J. Bentum, Cornelis H. Slump, and Roel Schiphorst
Center for Telecommunications and Information Technology, University of Twente,
7500 AE Enschede, The Netherlands
- VK3PAB (me)