Antennas

Under the hood, “Fan Dipoles”

10m and 15m nested dipole

10m and 15m nested dipole

Last year, with the assistance of Stu, M0NYP, I built a fan dipole out of D10 military telecoms wire for 80m, 40m and 20m for club field days. There was a fair amount of gap between each element, probably about 5 to 7 degrees becasue that’s what I thought you should do. For those people who came to either the Avoncroft Mills on the Air day, or the SSB Field Day, will recall the antenna. However, we lacked 15m and 10m though which I found a disappointment.

To become “all band”, I considered adding more elements to the Field Day antenna but experience has taught me that multi-band wire dipoles have a tendency to get tangled in the field. Adding more elements would probably just mean more tangles = less fun. In a Field Day situation, that’s a frustrating day out.

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How to make a loop antenna for 40m and 80m bands

SkyloopI’ve been an addict of full wave (and partial wave) loops since realising many years ago that in comparison to a dipole, you get more bang for your buck if you build a loop – certainly you get more copper in the air – and loops are resonant on EVERY harmonic so a 40m loop will be resonant on 20, 15m and 10m. A multi-band antenna for peanuts. They will receive better too, so for a housing estate, these are mandatory.

When I first started out with this hobby, I had a half-sized G5RV and I genuinely thought that I’d never get onto the 80 meter band. Within 18 months, I had worked out that you can build a loop of a wavelength in circumference (give or take a percent or so) and feed it directly with coax (and a 4:1 balun). Even better was the idea of putting loading coils in each corner of a square loop and you could lower the resonant frequency by a substantial amount.

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Footnote: 40m and 80m skyloops for your back garden

You may have a smallish back garden like me; 10m deep and 14m wide. Your 40m loop (that’s really 40m [or so] all the way around) will resonate on the 40, 20, 15 and 10m bands. Mine is only about 20 feet off the ground, around gutter height. Its not quite square but the far side and the near side are completely different lengths to make it nearly a triangle. Not quite. I have coax to the feedpoint and a cheap 4:1 balun there. With some trimming on a sunny day, you can make it work on all the amateuir bands – and I even used it on 2m once!

My 80m loop was (and still is) square at about 15m per side which is actually too small for the 80m band. My 80m loop is actually 60m all the way around. To make it “bigger”, I added 4 x loading coils into each corner. These coils are 2 inch in diameter, 6 inches long and approximately 30 turns on each coil. With some farting about, you can easily make it tune either the CW or the SSB portion of the band. If you have a tuner, you’ll dial that out easily anyway, particularly at lower power: 400 watts and under. The 80m loop happens to work on 30m band too. Just a fluke. Great for digital.

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40m Vertical Parasitic Array Antenna Part III

Vertical Array 40 meters

We finally built the 40m array on Saturday and conducted a full test on Saturday night, in preparation for CQWW this weekend coming (24th/25th Oct 2009). The good news is that signals bearly audiable on 40m on our standard Mega-Loop came up a few db on the array, to around 5 and 7.

The front to back ratio could be higher. Germans were crawling all over us, working Worked All Germany contest which was a good test, but it did hamper our out-and-out gain tests to US. Many of them probably switching off and saving their energy for next weekend. The modelling we went for gave us maximum gain at 10 degrees take-off. We could have gone for slightly less low-angle gain and instead aimed for a very high front-to-back ratio dialing in up to -15dB off the back. As it is, this current antenna only gives us around -6 db gain off the back. There are some benefits though, like working VK “off the back of the beam”, which I’ve never said before on 40m. Great fun.

Essentially, we have the gain of a non-steerable 4-square array so we should have fun into the top end of South America as well as all of North America, right up to Vancouver and Alaska.The proof of the pudding though, is this weekend and needing 40m to work all through the night, from 7 at night through to 7 in the morning – possibly more. This antenna needs to give us 12 hours out of each 24 hour slot, a must for a Multi-Two entry. Station #2 will have 160m, 80m and 20m to play with all night.

Remember to dial your logging program to update GetScores.org. A live scoreboard is hilarious fun for teams and will keep you on your toes all weekend.

Good luck and have a blast.

C.

40m 3 Element Vertical Yagi Antenna – Part II

We’ve built some large antennas before but never this big; a 3 element 40m vertical array with raised radials. We made it a raised radial system for a) a quick match to 50 ohms and b) it needs to be a “field” system. We can’t permanently leave our antennas in a public park.

First, you need to get into the scaling to believe it: Take a 6 meter scaffold pole of 48mm diameter (21 feet x 2 inch). Stick it upright on the ground and sleeve inside it, an inch and a half (30mm?) 4 meter (13 feet) pole. On top of this, sleeve a 10 meter (30 foot) fishing pole blank.

Sleeved Scaffold Pole AntennaYou will now have a structure that is effectively 20 meters tall (65 feet). Now then, the fishing pole blank will become the vertical part of an antenna which happens to be a quarter wave for 40 meter band. Being a raised antenna, we need radials and since we’re closer to the ground than a wavelength, we need a more than the traditional two radials to counteract the ground losses. We decided that 8 x radials will be about as good as 60 or so regular ground mounted radials. Do we have the maths right? We think so.

The radials have been modelled at 10 meters length each since they are essentially part of the circuit and will have currently on them, hence the quarter wave dimension. They slope to the ground at approximately 45 degrees. We need to attach some paracord to the ends of the radials and extend them down a further 7 meters before we finally hit the ground. Imagine how far away you are now from the original scaffold pole? I can tell you, it’s 13 meters (42 feet). The diameter of just one of these then spans 26 meters (84 feet) and we have three laced together at a spacing of 10 meters each (three element array).

40m Radial System Today, we did all the hard engineering and measured out all the bits and pieces, ready for a trial the week before CQWW. James and I laughed at the thought of how big this monster really is – and then wondered if it actually fit inside the park so as not to distrupt the walklers? Thank goodness we checked. For those of you unlucky enough to have been to our Scout Hut, you will know that when leaving our front doors, you will notice an oak tree in the distance that houses one corner of our mega-loop. James and I measured from the grass outside the doors to the last radial and we were only about 5 meters from the oak tree. Bloody hell!

We’ve had to re-engineer where we had planned to fit this monster into the park since it has a total wing-span of 52 meters (170 feet).
A picture tells a thousand words, so feel free to check the pics.

40m 3 Element Vertical Yagi Antenna

We’re putting together a Multi-Two entry for CQWW this year and it’s pretty clear from our experience with CQWPX that we not only need gain to the US on 40m but we need excellent front-to-back ratio. A new antenna was required.
A two element yagi was considered but we don’t have a tower for such a beast. We did though have various 10m fishing rod blanks and a load of aluminium scaffolding tubes. With some analysis, we feel we can build a high gain array utilising Yagi’s principles of a driven element in the middle and a reflector and a director element front and back.

Original modelling was conducted with MMANA however, the team has recently started to convert to modellilng with NEC2. James’s NEC model confirmed my 5db gain using MMANA at 10 degree take-off angle.

40m Three Element Vertical      3d Far Field Vertical Yagi     40m Far Field

The feedpoint for each element will be at 9 meters above ground using 6 meter scaffold pole sleeved with a 4 meter inch and a half steel pole. The 10m Sky Blue Leisure flag poles sleeve to the inch and a half steel poles. I’ll take some pictures tomorrow of the build.
Make no mistake, these will be monsters with 8 raised radials per element. Today, I made the insulated guying blocks for the radials. This is going to wipe the floor!

Watch this space.

MegaLoop Antenna

XXT Meage LoopI’m currently writing an article outlining the characteristics of the Mega Loop antenna we use for Dorridge Scouts. If you would like a preview of this white-paper, please let me know.

The XXT Mega Loop is the name given for the application of a horizontally mounted delta-loop that has known gain dependant on wavelength size and height above ground.

At quarter wavelength above ground and above, Mega-Loops start exhibiting gain that is proportional to its wavelength and height. A whole series of articles is currently being written by the author, however for the time being, these few articles may help you determine how we operate the Mega-Loop at Dorridge Scout Group HQ.

Firing supports over trees:

The 160m Loop

Transmission Lines

I have an article brewing in me which is all about matching a low dipole for 160m to regular 50 ohm transmitter. The dipole I had in mind will be about 15 meters off the deck. My impedance will be about 20 ohms, maybe 25 if I can get it a bit higher. Basically, this is unnaceptable to me since I like to match things nicely.

Chris (G0EYO) sent me this link http://www.qsl.net/w4sat/qtrwavtr.htm which tells me that I need to insert a quarter-wave piece of coax that has an impedance of 35 ohms. Well, I don’t know about you, I don’t often have 35 ohm coax lying around, however a pair of 75 ohm quarter wave stubs will reduce the impedance of 75 ohms to 37.5 ohms. Near enough? I think so.

Thanks Chris, I think I worked it out now.

C.

Mega-Loop-Update

Not much to report this month and I’m only typing because I’ve currently got man-flu and feel like sh*t warmed up.

Hyper Mega Loop AntennaThe Mega-Loop is working superbly at the Scout hut, although I’m trying to find something better for 160m. The impedance is already slightly low on top band and with a 4:1 balun in line at the feedpoint, it seems throws the SWR out by quite a bit. The ACOM 2000 seems to handle such a mis-match, in fact, it only trips out with a return power equating to around 575 watts. Technically, does this means that I can fire 1,000 watts up the coax and have nearly 600 watts come back to me? I don’t know – and certainly not above 1,850 anyway (UK band plans & license conditions etc). I’ll need to check.

Here’s some fun. The picture shown is the design for a Hyper-Mega-Loop, this is twice the size of our current Mega-Loop at 340 meters in length sitting at 18 meters height with a design sag of 5 meters in the middle of each leg. Using MMANA, it seems to still develop amazing potential for 40m band. How about 8dBi at 10 degrees take off for the US? I wish I hadn’t thought this up, I’m tempted to build it. Yes, the RF will fire just North of West at around 285 degrees.

Last Monday night, Tim and James helped me to demonstrate amateur radio to the Beavers. We did a standard JOTA style 90 minutes with little ones starting with Morse code, continuing to QSL designs and passing greeting messages on 80m. Thanks to Chris, G0MLY who was was an excellent victim in managing the little ones pass their greetings message. Also thanks to the many others who helped including our friends, Barry (G0DGQ) and Chris (G0EYO).

Callum.

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 (https://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.

Andrew Grid Dish 2.4 GHz

Last year, I became inquisitive and managed to take apart one of my Andrew Grid Dishes. They’re rather well made so it takes some patience and some sharp Stanley blades – along with an excellent first aid box! I nearly cut my finger off. Not the first time. My original intention was to replace the feeder directly with heliax to trim down the losses but looking at how they assemble these in the factory, I needn’t have worried.

You will see from the pictures (see link below) that these dishes are well made. I wonder what the goo that completely blocks up the tube that supports the dipole centre? It’s clearly some sort of water seal and I must say it’s completely over the top. It took a heck of a struggle to get the coax out.

These dishes are shipped in two halves along with the coaxial dipole mounting centre that bolts to the grid from behind. It’s possible to get the polarisation incorrect from the beginning. Notice in the picture that this one is vertically polarised. To change polarisation, you DO NOT change the plane of the dipole feepoint, instead you must rotate the dish through 90 degrees. Note that all fasteners are imperial dimensions, not metric. Don’t start losing nuts and bolts if you live in a metric country.

With a 23dbi gain and an 8 degree lobe of RF pointing in the appropriate direction, this is a serious bit of kit but please use low-loss coax; LMR-400, Westflex or if you must, RG213. Don’t use RG58, this is actually very HIGH-loss coax and shouldn’t be sold as low-loss at all. The losses at 2.4GHz are tremendous and you could easily see most of your signal wasted in the coax. The dish is terminated in an N-Type connector on a flying lead with RG-213 type coax. Point-to-point, you probably can’t do better unless you start to spend astonishing amounts of cash.

James and I will try and give a pair of these a test in the summer from Monument hill to the Scout hut – about 10 miles. Apparently a breeze. The world record is for an un-amplified signal is something like 150 miles!

Callum.

Shakespeare Marine 5300 HF SSB Antenna update

I was very excited about taking delivery of my 2-piece 5300 Shakespeare Marine SSB HF Antenna for my narrowboat this week.

IMG_8483Close inspection showed that the antenna is a 2-piece 28’6″ (8.5 meter or thereabouts) white fibreglass hollow pole with a 2 foot aluminium heavy-duty sleeve at the base for mounting purposes. This main lower section (of around 17 feet or so) has three elements running the full length embedded inside the fibreglass at time of manufacture from the side feed to the top, in 120 degree arc segments. A heavy-duty male screw fitting at the top, electrically connects to the top section’s female thread.

Since the wind had died down this morning, it was an ideal opportunity to take down the 40 foot vertical and replace it with the 5300 as a test. I inserted one of my light duty aluminium poles up inside the aluminium housing approximately 12 inches in depth to where the fibreglass stopped inside the sleeve (from the other end) and I raised it to the vertical by walking up the antenna from the pointy end until it was raised vertical. I then lowered it using the same technique.

 

Lowering it at around 45 degrees, the fibreglass groaned, cracked at the sleeve point and fell to the ground.

Clearly, there’s a flaw just above the aluminium sleeve. A one-in-a-million manufacturing defect.

I’ve written to shakespeare and await their reply.

[Later] On Monday morning, I was emailed by their UK sales department explaining that another 5300 would be shipped immediately. This is the first time one of these has ever broken in the history of the 5300 antenna production and clearly this is a freak. Good news Shakespeare, thanks.

Callum.

80m Vertical – High voltage elevated radials

RF Weld - Click to zoom

RF Weld – Click to zoom

The M0XXT Double Xray Firm had one of our outings again last weekend with myself, Tim (M0URX) and James (YOMsoft author). We built our biggest vertical yet, a loaded quarter-wave for 80 meters. Click the picture to compare Tim and I against its size. Awesome! 21 foot (6 meter) scaffold pole with 40 feet (12 meters) of SpiderBeam pole above. We’re getting an 18 meter version of this which should be great fun.

For speed of assembly on the day, we used 4 very thin strands of insulated copper wire out of a three-pair (6 way) telephone reel for the elevated radials. They’re very thin but would do the job of allowing us to raise this baby up in the air 6 meters.

The radials ended up at ground level, wrapped around galvanised tent pegs. Without realising it, we were trusting the insulation coating of the wire itself to keep the radials from being grounded. Actually, we never really thought about the consequences! Anyway, a 100w squirt proved most efficient and we achieved a 2:1 SWR curve from 3.625 to 3.750. We needed to shorten it a few more inches, however before we took it down again, I thought it prudent to fire 400w up it to make sure nothing broke down. Thump; I stuck a good carrier from the ACOM 2000 down the pipe and “pop”. The SWR pinged up to over 3:1. What had happened?

The insulation on every single tent peg had broken down in an instant and melted through to convert our careful elevated radial system into a grounded system in a flash.

20 meter quarter-wave vertical antenna experiment

Not content with attempting to understand and build 40 and 80 meter verticals, I’ve decided to build a 20 meter version tonight. This has 4 x sloping radials at 45 degrees with a feedpojnt up at about 7 meters in the air. On receive, it’s actually pretty good; only for extremely weak or barely readable signals is it beaten by the 40m loop or my attic mounted half-sized G5RV.

For the record, it performed straight out the box with dimensions of a whisker under 5 meters for both radiator and radials.

Picture below shows typical far-field plot for a quarter-wave vertical.

vertical-gain-at-30-degrees

Dedicated 80m Vertical

80 meter loaded vertical

I didn’t document fully last weekend that my 40m vertical experiment gave me a rather good match on 3.8Mhz. This gave me the idea of building a better 80m vertical this weekend by loading up both the vertical – and the radials. As I type this as I discover that first time up, I’ve achieved a resonance (of sorts) at 3.456 Mhz however, I’m not getting 50 ohms. The SWR is at best just under 2:1. I have reduced the legs of the radials a bit because of the loading coils (which all have 68 turns on my favourite 40mm plumbers pipe). I shall now adjust only the vertical coil. I know I should in theory adjust all the coils to keep them in unison however, the actual length of the radials is not precisely known; only that they are ‘about’ 9 meters plus a loading coil – and in any case I can just shorten the radials for fine tuning.

Here’s the resonant coil adjustment chart for the vertical coil:

  • 68 turns: 3.456 Mhz
  • 67 turns: 3.470 Mhz
  • 63 turns: 3.535 Mhz
  • 59 turns: 3.600 Mhz
  • 55 turns: 3.690 Mhz (1:1 SWR dead)
  • 54 turns: 3.691 Mhz (I tightened up the radials which had an adverse effect so that the resonant frequency hardly moved)
  • 52 turns: 3.740 Mhz (SWR rising a whisker now)

I finished on 53 turns in the end. I’ve started keeping a couple of turns or more spare, never cutting off all the ‘dead ones’ as I take them off so I can wind them back on if I overshoot. My ‘best’ spot frequency is now 3.715 at 1:1 SWR with a 2:1 SWR curve starting at 3.6 and going all the way beyond 3.8. In fact, 3.8 has an SWR less than 1.5:1.

Coax Loss Comparison

I can never find the comparison charts between RG213 and Henry Westlake’s Westflex W-103. At last, I’ll have this logged forever now. Per 100 meters:

RG213/URM67(Mil spec)
Westflex 103
 100 MHz                      7 dB 5.0 dB
 144 MHz                    8.5 dB ?? dB
 200 MHz                     10 dB 7.5 dB
 300 MHz                     13 dB ?? dB
 400 MHz                     15 dB 11.0 dB
 1000 MHz                   27 dB 18.0 dB