Tag Archives: multiband

40m compact fan dipole for 40, 30 and 20m bands

One of my aerials has just come down in the wind, a 40m compact dipole arranged as an inverted V with the ends coming down as far as the 6 feet fence height.

My garden is about 15m wide (actually it’s 51 feet wide, so a whisker over) however it’s too small to fit in full-sized flat-top dipole but an inverted V works well. Whilst you are at it, why not add in elements for 30m and 20m and have three bands on one feeder?

40-30-20m-fan-dipole

Width of this antenna is 15m. You can make it smaller to suit.

I have designed this antenna to be a flat top with droopy legs. The centre will be held up with a very sturdy aluminium scaffold pole with a 4.6m sailboard mast sleeved over the top. The aluminium mast will cross-bolt to an already installed steel scaffold bar already concreted in the ground. Bottom line is that I should achieve around 10m in height (30 feet or so).

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Working 15m band on a 40m vertical antenna

Note: This article discusses the merits of a 3/4 (three quarter wave) vertical -vs- a 1/4 (quarter wave) antenna.

You can build a 40m vertical quarter wave antenna and ground mount it with 16 x 4m radials and operate it at the third harmonic; 21MHz.

Actually, all my experimentation has shown that if you multiply the quarter wave resonance by 3.03, you’ll have the next available usable band. In this case, if you tune a 40m vertical to 7.00Mhz, you’ll have the whole of the 15m band to play with with a centre-point of 21.300Mhz. Oh, and you’ll still have the whole of 40m band under 1.3:1.

Now here’s the controversy:

Most people who read antenna publications or the ARRL handbook believe that if you actually make this antenna, you’re creating a cloud-burner on 15m.

Technically correct (sort of) – but for DX, wrong.

On the surface, the 10m long 40m vertical that’s used on 21.225MHz does indeed look like a cloud burner. Here it is. 15m band in green -vs- a pure quarter-wave in red).

quarter-vs-three-quarter-wavelength-vertical-antenna-plot

(click to expand quarter-wave in red, three-quarter wavelength in green)

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How to build a proper Skyloop (Delta loop construction)

Well folks, I’ve been continuing my experiments with my two sky loops (closed loops of wire held above the ground at three or more points) and comparing them against various verticals. Sometimes the verticals win, other times the loops do. I’ve had 8m verticals and longer too. Today, I’ve put the 12m vertical back up (http://www.m0mcx.co.uk/?p=102) fed with my trusty SG230 coupler. I may add that anyone interested in fast antenna experiments should get one of these. A tremendous piece of kit. Matches anything.

SkyloopApart from the noise factor (loops are quiet), the vertical seems to make a better impact on transmit more often than not for DX, unless I’m into NVIS or near NVIS (500 miles or so). Even then, up to 1,500 miles, the jury is out – either antenna can win. Beyond 1,500 miles the vertical seems to do a better job most of the time – not always, however my loops are very low to the ground, maximum height of 7 meters. On 40m, I would say the vertical is a better antenna to have almost all of the time unless you need NVIS for local copying – and with the sun spot cycle as it is, you won’t get this for a while yet!

I used to have the opinion that loops were more often than not, cloud warmers. However let’s face it, so are almost all low-to-the-ground HF antennas. My interest was tickled recently when VE7HA mailed me an MMANA file of a very high (80- foot) 300 foot circumference skyloop that he had built with three very large trees. He claimed great contest results with this loop. I had a look at the plots. Yes, he was right.. Check this out, 10.4 dbi gain at 5 degrees to the horizon on 10m? On 15m band, we’re looking at 12.5 dbi at 7 degrees to horizon! On 20, it’s as good with 10.5 dbi gain at less than 10 degrees.

Skyloop Far Field PlotNow.. This is a very different animal to the Skyloops that most people build. If you want a pile-up generator, this is the animal to have. It fundamentally, takes all your RF energy and pancakes it low to the ground in various star shapes. Very Of course, you will sometimes null out the station you want but you could get really clever and change feedpoints with relays. But ignoring the clever engineering, this has changed my mind as to the effectiveness of a high skyloop.

Be prepared to get into some matching territory though, ideally an automatic coupler at the fedpoint with open wire feeder, or perhaps an olde-fashioned tuner in the shack!

Some day I will build one, I just don’t have the trees :(

(Years later: I did build one. They’re amazing).

73

Callum.

Antenna -v- High Wind

Movie IconWas I QSB this afternoon on PSK31? Probably!

(Can’t get video to play? Right-click and save it to disk first! Don’t ask :)

In the main, we’ve all heard that wind normally beats antennas when it comes to staying up. In case you haven’t ever seen a cheap fishing rod blank in high winds, have a look at this. I call it a 10m vertical, in fact this one is only 8m. The matching is done at the feedpoint with an SG230 and 9 raised radials. It’s a temporary antenna. I’m just testing it out for fun. The 12m beats this by a fair margin on the 40m band.Anyway, enjoy the little film!

By the way, don’t be fooled by the helically wound look. It is hardly that. Here’s a real helically wound vertical from a couple of years ago – and this one does tune on the 40m band without help from an SG230. It is directly fed with coax:

73

Callum.