Thanks to Derek's reminder on the reflector the other day that sporadic E season is underway, I decided I may as well put up a 6m antenna, particularly as this is the final antenna I need to be able to use all the frequencies available on my rigs. A yagi would have been nice, but with only 4-6 weeks of this years sporadic E season left, and with some concerns about adding yet more weight to my old rotator and single wall bracket, a simple fixed dipole was constructed as a temporary solution to get on to this band quickly.
A rummage through my parts bins indicated that I only had a few metres of RG8 coax available for this project, but I did have a roll of old but good quality 300 ohm ladder line and a beehive capacitor, so I decided to construct a folded dipole using what materials I had available.
Fig.1 illustrates this antenna. Being a folded dipole it promised wider band coverage than that of a conventional wire dipole, and when erected oriented north-south on the side of my house about 15ft above ground this was found to be the case. SWR ranged from 1.05 on the target frequency of 50.200 MHz (SSB calling frequency), up to 1.85 towards the top of the FM band segment.
During flat conditions 6m activity is low as I found during the first days listening. However, on day 2 there was a small sporadic E event centred in the Mediterranean which just reached the south of the UK. I heard IS0BSR calling CQ, but did not establish contact, perhaps not too surprising considering the antennas current position shielded from the direction of southern Europe by the house.
One big bonus of this antenna compared to a conventional wire dipole design is that dipole arm lengths do not usually require trimming to tune as SWR is set by adjusting the beehive capacitor. This fact came in very handy when the antenna was raised on to the house roof (around 22ft above ground) and configured as an inverted “V”. Following this move SWR rose to above 2, but rather than having to follow the laborious process of climbing up onto the roof, maybe several times, to re-tune the elements, which would have been necessary with a conventional wire dipole, the SWR was easily reset by adjusting the beehive capacitor which sits in a water proof box 2ft above ground level. SWR across the SSB segment of the band is now 1.65 to 1.81. If I was 10 years younger and a bit fitter I might have considered trimming the ladder line arms to restore the lower SWR's previously recorded, but performance-wise, with only 2m of RG8 coax used in the feedline, no significant power loss will result by running the antenna at the higher SWR levels so I'll leave things as they are.
I'm now looking forward to better sporadic E openings in the near future, and hopefully some good contacts will be made then. While the ladders were out I also took the opportunity to add a second wall bracket to the rotator support mast so maybe, perhaps for next years Es season, I'll think about upgrading this antenna to an HB9CV 2-element beam.
Stop press: The antenna has been up for three hours now, and running 30W (in deference to the beehive trimmer) I've managed to work three Spanish stations, and one station in southern Poland. Who said you need a beam to work DX on 6m.
Article written by John East G0OQX
The choice of HF antenna is vast so which shall I go for?
It's easy if you are wealthy with acres of real estate, multi-element horizontal beams mounted a quarter wavelength+ in the air over rotators. For the rest of us, the 99%, it's not so easy. I live in a typical semi with an area of around 35m x 10m with close neighbours so antenna options are a challenge.
Over the last eight months, since returning to the hobby after several decades absence, I must have explored many of the available HF antenna options Here's a simple summary of my initial conclusions.
For some strange reason, maybe because I saw this as a difficult challenge, I jumped in at the deep end and began my return to amateur radio by building a couple of magnetic loops.
A magnetic loop simply comprises a conducting loop 0.10-0.25 wavelengths in circumference with a 25-50mm gap across which a tuning capacitor is connected. The loop can be fed by several methods, the simplest of which is a coupling loop 20% of the main loop diameter, mounted close to, but not in electrical contact with the primary loop, and opposite the tuning capacitor.
The key requirement for an efficient magnetic loop antenna is that its resistance must be extremely low to display the high Q necessary to efficiently radiate RF energy. This consideration leads to the high cost and complexity often associated with this antenna. To keep resistance losses low the loop ideally needs to be constructed from relatively large diameter (5-20mm) copper tube, the tuning capacitor is usually motor driven and needs to be of the vacuum type, or at the very least the split stator variety, and all electrical connections need to be of very high quality.
As it turned out the route I chose, to evaluate the concept of a magnetic loop as simply and as cheaply as possible, worked rather well whilst still guaranteeing the very low resistance required. OK, this initial project didn't incorporate a motor driven capacitor for tuning, but I'm not sure one was needed as the following explains.
The first hurdle, to construct a 1m diameter circular loop using 15mm copper plumbing pipe, was much easier than I expected. A pipe bending spring was purchased from eBay for £7.50, and a 3m copper tube (cost £15) was bent into a near perfect circle using a wooden former mounted in a workmate.
Instead of connecting a variable capacitor across the loop gap a simple fixed capacitor was produced by using a gas blow torch to solder a 70cm length of RG214 coax across the loop gap. The target capacitance needed for this loop to resonate on 14.200MHz , as specified by an online loop design calculator, was 64pF. RG214 coax has a capacitance of around 100pF/m, so it was a fairly straightforward process to tune the loop by trimming the coax to the optimum length.
Having a fixed rather than a variable capacitor wasn't a big problem. The finished loop tuned from around 14.170MHz to 14.230MHz. If one decided to use this design for a permanent installation it could be tuned to cover most of the cw or data portions of 20m, and two such loops mounted side by side, with a suitable gap between them to prevent interaction, could cover a sizable portion of the ssb segment of this band whilst still taking up less space than a dipole. Also, the top of a 1m loop only needs to be mounted 2m above ground level, a significant saving on the 5m minimum height needed for a dipole.
So how did this loop perform. I now regret that my trials with this loop were somewhat limited because I was very keen to erect a multi-band antenna to get fully operational while designing and then building a multi-band loop incorporating a variable capacitor as a longer term project. On receive the loop was very promising, not least because it's sharp tuning characteristic rejected a lot of the QRM ever present in the urban environment. The wire antennas that I went on to build often had noise floors with s-meter readings of 5-7 on some bands. This loop had a noise floor of 1-2 which I'm sure would have given it great potential with weak dx signals. Comparing the loop to a temporary 20m ¼ wave vertical with 8 buried radials, most signal strengths were the same or similar.
On transmit, only a couple of QSO's were completed (the antenna wasn't up long) with 59's recorded both ways into Europe.
Overall, I rated this experimental loop quite highly. It was cheap (<£30), much easier to build with the necessary low resistance than I expected, and very low visibility for the neighbours, but on the downside my limited evaluation didn't promise much better performance than that of a ¼ wave vertical. Then again, mounting it higher up and on a rotator might have been a different story.
I've now purchased a lot of the bits needed for a permanent tuned version, and my multi-band antenna installation is complete, so all that's stopping me proceeding with a mk2 version is the biggest hurdle of all – motivation.
I erected ¼ wave verticals for 10m, 20m, and 40m. All were installed with eight buried ¼ wave radials, and much to my surprise worked well even though they were positioned close together (2-4m spacing).
Many QSO's were recorded, including quite a few with North and South America and the Caribbean on 40m, and I would have been happy to use them on a permanent basis. However, no VK/ZL contacts were made so there was certainly scope for improvement. I also thought the 40m vertical was too high for good neighbour relations, and wanted to be active on 80m/160m so my attention focused on to horizontal antennas. The 10m and 20m verticals have been retained as “back up” antennas and for comparison against newer antennas.
The successful version of this antenna that I finished up with was 32.6m long, 15m high at its highest point, and fed remotely at the bottom of the garden using an old LDG automatic atu mounted in a water proof box.
Attempts to feed this antenna with various baluns and ununs using a coax feed and an atu in the shack had proved universally disastrous. The SWR's were reasonable on a couple of bands but high on most. Losses in the 15m coax feed must have been high, and on receive the antenna sounded “dead”, often without the characteristic rise in background noise one expects to hear when an antenna is tuned to resonance. I learned a golden rule at this time.
NEVER FEED A MULTI-BAND ANTENNA WITH COAX UNLESS THE ATU IS AT THE FEEDPOINT.
This may not be a universal rule, for example a short length of very low loss coax may be tolerable, but as a rule of thumb I think it's a rule I will stick to in the future.
With the atu in the correct position, at the feed point, I was much happier with the performance of this antenna. It tuned up on all nine HF bands although several were 1.9 SWR. Comparing it with the 10m/20m/40m ¼ wave verticals that were all still up when it was erected, the inverted “L” was perhaps a little down compared to the 40m ¼ wave, but was generally similar in performance or sometimes better, compared to the 10m/20m verticals.
A minus with this antenna was the need for an extensive buried radial system.
My time with a remotely fed / remotely tuned inverted “L” was cut short when the outside atu stopped working. Faced with the prospect of spending several hundred pounds on another outdoor atu I decided on the cheaper option of a ladder line fed doublet, which would have the advantage of not needing an extensive radial system to maintain, and could be tuned by a manual atu in the shack without the worry of coax feeder losses.
The commonly employed 65ft x 65ft doublet was erected, fed with 38ft 6in ladder line. Each doublet half and ladder line half was made from one 103ft 6in length of insulated wire so waterproofing should never be an issue with this antenna as there are no joints anywhere. The ladder feed, spaced with pvc tubes giving a 40mm spacing and 450 ohm impedance, is fed directly into an MFJ941E manual tuner in the shack.
This antenna has only been up a few days, and I've already received a 58 report from North America on 40m, so performance seems just as good as that of the previous inverted “L” antenna. On receive, it is 2-3 S points up on the 10m ¼ wave vertical. Ten HF bands tune up easily too, so hopefully I can now spend more time in the shack and less time on the roof in the future.
So which antenna is the best?
To be honest, I'm not sure. It goes without saying, none of these antennas can match a beam yet they all give similar performance with which I'm more than happy. After all, it must get boring working VK/ZL every day. For all band coverage it has to be one of the horizontal wire options, and from a cost and maintenance point of view, I guess the doublet becomes the clear winner.
What's next to try?
Perhaps a quad or a loop might be worth looking at next, but I think I'll stop there for the time being and concentrate on operating.
Article written by John East G0OQX
Event Date: 19th April 2016 - 20.00pm ~ 22.00pm
This evenings presentation was hosted by John - G0OQX who very kindly brought in and shared his knowledge on his latest homebrew project, a dual band yagi antenna. He also agreed to submitting this article for the G4FUH Blog, many thanks John.
Having enjoyed my first foray in the RSGB March 2m contest and keen to include 70cm SSB in my repertoire, I decided to replace my ageing 2m 2 element HB9CV antenna with a higher performance home brew dual band 2m/70cm yagi.
I spent several weeks trawling the internet for yagi plans, and finally decided on one of the antenna designs describe on Martin Steyers' excellent site ( www.qsl.net/dk7zb ). The antenna I chose was 5 elements on 2m and 8 elements on 70cm (see Fig.1), fed with a single coax, to be driven by two transceivers via a duplexer. Not the best option for DX maybe, but a good compromise between low profile for use in a suburban environment, and a significant step up in gain from my current 2 element beam. Martin Steyer claims 8.5-9 dBd for this design. There are, of course, quite a few other websites offering yagi plans, but many seem to fall into one of two camps, either precise designs using components difficult to source, or with plans which were a bit ambiguous with respect to certain design features. Martin Steyers' site was selected because he gives a clear and comprehensive set of instructions specifying the critical dimensions, i.e. element lengths/diameters/spacings and feeding methods whilst leaving the detailed construction method up to the home brewer.
My main criteria for the design I chose was to have a robust construction capable of accurately achieving all the critical dimensions using the simple collection of tools I had available. A hand held drill, a workmate, a hacksaw, and some round files were all that were needed to fabricate all parts.
Space is limited in a blog post so I won't go into too much constructional detail here. Figs. 2 and 3 below are worth a thousand words and illustrate how the antenna was fabricated with the elements clamped by cable glands.
Other than the positioning of the holes in the sides of the abs box, none of these parts require close dimensional control because critical dimensions are set during final antenna assembly when the perspex element brackets are carefully aligned and clamped in position to the boom using mole grips. It is then easy to drill through each assembly and bolt it securely in position.
The 2m driven element is in two halves with a small gap between them. To hold this element rigidly a tight fitting plastic rod (or in my case an empty biro tube) was inserted through the gap and two self tappers, which also serve as coax attachment points, lock everything in place. Finally, the abs box was fixed to the boom with stainless self tappers, and the coax feed, soldered to copper washers was attached.
Tuning, with hindsight, was fairly straightforward, although fairly stressful at times due to my mistake in positioning the 40mm dia. 4 turn RF choke feeder coil approximately 150mm from the dipole. The plans clearly show it needs to be as close to the feed point as possible. See Fig.5 (Thanks Derek for pointing this out, and for the other tuning suggestions you made).
After correcting the above mistake the antenna was finished by waterproofing the abs entry holes with liquid rubber and mounted above a rotator on the roof. SWR's on both 2m and 70cm SSB calling frequencies were 1.10
Initial antenna performance is encouraging. Directivity, and by implication gain, is clearly a lot better than that of my old antenna. Only time will tell how durable this design is, but at least after a week on the roof nothing has fallen off yet!
Article Written by John East G0OQX
Event Date: 12th April 2016 - 20.00pm ~ 22.00pm
A demonstration of the processing power of the new Raspberry Pi 3 was presented by Kev - M6KNS at this evenings club meeting.
Kev set himself a personal challenge, that he wanted to share with club members, of tracking and decoding the SSTV Live Images that were being transmitted from the ISS using nothing but his new raspberry Pi 3, and a Bearcat VHF Scanner / Receiver.
The chosen Operating System was Ubuntu MATE and the software for decoding the SSTV transmissions was QSSTV, a free download from the internet.
A Baofeng UV5R HT was also used in the carpark to listen out for the distinct sound of SSTV signals as the ISS approached.
The Raspberry Pi was also used to track the ISS as it travelled towards the UK and just before it arrived across the west coast of Ireland, SSTV signals were detected by the software and our first image was being downloaded.
A short pause between pictures and a second image was on its way. While this was downloading, club members congregated in the car park to view the ISS as it passed overhead.
The evening was complimented by a talk / demo by Tom G4JRY about EME Moonbounce including audio recording samples from Howards (G4CCH) on-air activities.
Additional activities included discussions on a HF Radio Kit that Owen 2E0GBD is currently building and a 2m/70cm Yagi Antenna that John very kindly brought in to show members. These activities are attracting much interest with some members so will be discussed in further detail at future club meetings. Please keep a look out in the 'Whats On' area of the website.
Kev M6KNS also asked members to take part in his 'Ten Task Challenge'. A simple, fun list of activities to encourage members to be active on air and to help share and promote the new website.
For any member that didn't attend this evenings meeting that would like to take part, here's the download links. Please feel free to print it out and take part in this club activity.
Dont forget, it's all for a bit of fun so come on folks, find that animated CW Key within this website, get pressing that PTT Switch on your rigs to complete the first couple of challenges and prepare to give a few minutes talk at the club about how you got interested in Amateur Radio!
**Please submit your forms by the end of the club meeting 10th May 2016 to Kev M6KNS or Owen 2E0GBD.
Event Date: 31st October - 1st November 2015
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