Successful Morning of 2m SSB DX – 12 Nov 2010

Well, it's taken me awhile to post this (now in the last week before my wedding), but here it is.

Last Friday morning I headed up Red Hill again and had a great morning with my FT-817ND feeding my WA5VJB Cheap Yagi. Good contacts into both NSW and VIC were made on a maximum of 4W (more on that below). Here's the log:

Call – Recv QS – Sent QS – Notes

VK1CJ (John) – 55 – 57 – Canberra
VK1BG (Ian) – 58 – 59 – Canberra
VK3BJM (Barry) – 51 – 41 – Victoria
VK1DA/m (Andrew) – 51 > 59 – 51 – Barton Hwy, NSW/ACT (signal improved in ACT)
VK1DR (James) – 59 – 59 – Mawson, ACT (Just down Red Hill from me)
VK3II (Jim) – 51 – 51 – Victoria (near Western Point Bay, but I couldn't find this on Google Maps), significant QSB
VK2BX (Brian) – 51 – 51 – Between Kiama and Nowra

So, not a bad turn result for an hours play time. Great to see James (VK1DR) up and active by simply mounting his beam on a tripod in the garage – good effort and adhering to the principle of, "the best setup is the one you actually have and use"!

Starting to notice a pattern with the QSB (which all the regulars no doubt are well aware of – but new to me) with peak times being around 8:30am and 9am. Look forward to further observing this in the future.

Also observed the benefit of being able to peak a beam in my attempt to pick up Brian (VK2BX). At first he was very faint (off the back of my beam) but I could hear him making a contact down to Jim (VK3II). So I played around with my beam and peaked him and then attempted a contact. Great stuff!

On to my output power. First up, I'm currently experimenting with a fairly lightweight setup by using the smallest practical Sealed Lead Acid (SLA) battery – practical regarding run time. This means, I've gone with a 1.2Ah battery – however I do also have a 4Ah one on standby. The reason I do this is in the interest of a truly portable setup. My ultimately goal is build a nice wooden case for the FT-817 with associated cables, speaker and battery. And ideally, it'd be great to take this with me when I go to Tassie and the like on the plane. Further though, the easier it is to pack up the car for the morning sessions the better – and no one enjoys lugging around big gel batteries so I'll save that for field days (where I need to operate for many hours).

Anyway, as a result of using the little guy, it means at full charge I put out 4W peak – according to my bench test at home with RF meter, dummy load, and using FM as the mode. (Now, I'll quickly acknowledge this is not a correct determinant of PEP, however it's good enough for my purposes.) In the FT-817 specs it does explicitly say that the full power of 5W is only when powered with 13.8V DC, so that's no surprise (and not going to be obtained with 12V batteries – not even the gel cell).

However, the key thing here is the 4W is only at full charge. With only 1.2Ah it means by about half way into my session I'm down to about 3.75W and that's when the low power (output) indicator starts flashing on the FT-817 – signalling less than 5W output (if it's solid, it then signals 2.5W output).  At first I thought this meant I was indeed down at 2.5W, so when I got back home on Friday afternoon I hooked up the same battery to see what it was putting out.

To my pleasant surprise I was still putting out a whopping 3.5W (on FM) – which was great considering I expected 2.5W. Further, this means I made the contact to Jim (VK3II) on 3.5W as the indicator had started flashing by then. And as Ian (VK1BG) said, that's pretty good watts per kilometer. 🙂

Well, I still need to do a run with the FT-817ND and my power supply at 13.8V to confirm the 5W, but for now that'll do.

Oh! And the Multi Purpose Holders worked a treat. They make the whole adventure so much quicker and easier (especially when dressed for the office). I recommend them!!

Photos:
1 – The setup, and you can see the 1.2Ah (and the first use of an external speaker with FT-817)
2 – The beam pointing South West for VK3.

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2m SSB DX Tomorrow Morning

Tomorrow morning I will be once again up on Red Hill for some 2m SSB DX. There's been a lot of rain lately, so it's not been an option, but the sun was out this afternoon and is meant to make a repeat appearance tomorrow morning. So, if you hear VK1FOTO calling, please say hi! 🙂

I'll be QRP again at about 4W from my FT-817ND into my 4el WA5VJB Cheap Yagi.

Also, due to the rain I've not been able to really try my new mast carrying system. That sounds more impressive than it is, but have a look at the pictures to see.

Basically I went and grabbed some "Multi Purpose Holders" for my roof racks. This way it takes about 30 seconds to load a mast on my roof, and to take it back off. This compares to the 10-20minutes it used to take when I was lashing it on with rope! The benefits of this is that to pack the car the night before now takes about 10 minutes; to setup and pull down once in position will only take about 15 minutes each – meaning more time on the air, and less of a rushed pack up to get back to work.

Hope it all works! 🙂

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Is a SWR(50)1:1 Dipole Possible?

One of the first antennas I ever built was a 1/2 wave dipole for 2m (FM segment). I chose this as it seemed like the simplest antenna to build and get going. And indeed it was.

The idea is simple and execution is simple:
1) Figure out what half the wavelength for the band you want is;
2) Figure out what half of that is, as that’s the length of each of the two legs you require;
3) Support the two legs a bit apart, connecting the coax (via a balun – or without, although not the recommended) to the two legs – centre conductor to one leg, braid to the other leg;
4) Connect to a transceiver via a SWR meter and then trim until you get as close as possible to an SWR of 1:1.

Easy, and generally you end up at an SWR at 1:1 or close.

But lately I’ve found that to model a dipole in 4nec2 and obtain an SWR of 1:1 is impossible! There, I’ve said it. It’s impossible to get a 1:1 SWR for a dipole in 4nec2. I’ve run it through the optimiser and optimised on pretty much every parameter I could think of.

In the end, the best SWR I can get is ~1.4:1.

So if that’s not enough to get an F call wondering, what is! Well from there I’ve been reading and reading and reading, and have found some interesting things.

First, there is a slight extra bit one must do to figure out the length of a half wave dipole. The formula is:

l = ((c / f) / 2) * k

Where:
l – length in meters of 1/2wl dipole
c – speed of light: 299.8
f – frequency of desired band
k – ‘k factor’: normally in the range of 0.9 to 0.98

What!? ‘K factor’, what’s that doing in there. Ahhh! Well, that’s the additional bit. The K factor is used (varies) to accommodate for the diameter of the conductor/wire. This is interesting…

Fred Swainston in the Radio Theory Handbook (section 21.8) further describes this as:

“At the open circuit end of an antenna, a small amount of capacitance coupling to the environment exists. This capacitance allows a small amount of current to flow from the end of the antenna, thereby reducing the impedance at the end of the antenna from a theoretical infinite. This is termed end effect and will cause the impedance of a centre-fed dipole to be reduced. It is a contributing factor to the k factor when calculating the physical length of a dipole.”

Ahh, so that explains why I always seem to have to trim so much off my dipoles before I arrive at a reasonable SWR – though normally near 1:1. But still, how come I can get an SWR of 1:1 but I still can’t model that.

Well, that comes to the next bit I found. What is the typical impedance of a model dipole?? Well, it turns out things are not as simple as I thought and that 1/2 wave dipoles actually have a typical impedance of ~73 Ohm. That’s right, a better match for 75 Ohm than 50 Ohm. So this means we should normally at best be expecting an SWR of:

73 / 50 = 1.46:1

Further, as supporting evidence, that explains why the impedance of a folded dipole is a multiple and has a typical impedance of ~300 Ohm (4 x 73 = 292). As the formation of a folded dipole is said to result in an impedance transformation of 4:1 – and hence why you use a 4:1 balun with a folded dipole.

Alright, so there’s two additional bits of detail:
– k factor to accommodation for wire thickness and capacitive coupling to the environment;
– typical impedance of a dipole is ~73 Ohm and thereby typical SWR would be 1.46:1

That then tells me that my 4nec2 models are probably fine, but it doesn’t explain how in the real world I get a SWR of close to 1:1 for my dipoles.

Further reading though tells me that environment can have a big impact and one of which is the ground. Fred Swainston again comes to the rescue (section 2.16) when considering the ground with this statement:

“A horizontal half-wave antenna should be no less than 0.15 of a wavelength above the ground otherwise the feed point resistance will be less than 40 Ohms.”

So in theory if I started at 73 Ohm in free space and then headed down towards 40 Ohm when I added a ground I should be able to reach an SWR of 1:1 surely. Well, unfortunately I’ve still not be able to model this, however ground is not the only thing to consider. Another great one I looked into was transmission line loss and saw how that could also have an impact. So bit by bit, I could see these things adding up and lowering the impedance and thereby correcting the SWR.

In the end then maybe when many factors of the real world are combined – oh, how about conductivity of the materials used – then I guess an SWR of 1:1 is eventually achieved.

BUT, at what cost. I also noticed that when adding in the ground to my model and having it at a height of 0.15wl that the radiated efficiency plummeted to 32%! So I do wonder how _efficient_ my 1:1 dipoles are…

Couple of bits of online reading you might like:
Wikipedia
VK1ODs Optimised Dipole Efforts:

Also, included below is a 4nec2 model I’ve made with lots of symbols so that you can ‘configure’ and optimise it to your hearts content. As well as a couple of comparison SWR graphs for the dipole with real ground, and in free space.

Enjoy!

Details of graphs:
1. Dipole in freespace;
2. Dipole with model ‘real ground’.

DipolefreespaceDipoleground

VK3DIP, AR Magazines and YagiCAD

Back in 2008 I came across an article in the AR Magazine that looked like it described the first yagi I'd be able to build. It is due to that first article that I now have a bench drill and have at least one yagi completed – more planned, and half a one in progress. The article I reference is that by VK3DIP (Paul McMahon) and is titled "Simple wideband Yagis for 2 m and 70 cm" – AR Magazine, September 2008.

I actually have about 1/2 of the 2m version built – boom drilled, directors all installed, and all other elements ready for mounting – however not long after starting I met my soon to be wife and then moved house and have not got back to it. But I will one day, and I look forward to it.

Paul has actually also posted up further photos for these beams here, and I've also seen the installed version that VK1GH (Gilbert Hughes) built and he seemed happy with it – reporting it was performing to spec. Also of interest is that Paul says that for some of the design of this he used a program called YagiCAD.

YagiCAD used to be a piece of Shareware the Paul wrote, but now he provides it as freeware. I'm keen when I get a chance to download it and have a play (as it sounds like it has some nice simple functions (maybe reminiscent of YagiMax). But for now all I can do is to provide you a link to it.

YagiCAD aside, I'm hoping sometime soon – time again permitting, getting ready for a wedding sure keeps you busy – I will model these two beams up in NEC2 just for a bit of fun. Originally I hoped to post something up with the models etc., but appears not yet.

Instead, what drove me to post this is that I've recently noticed that Paul is a somewhat regular contributor to AR Magazine. In the most recent release (October 2010) we see Part 2 of his article on "Build[ing] a generic PC interface". This article caught my attention so I scrummaged back to Part 1 to have a read. While reading this I noticed he made reference to a previous article that he did back in June 2009 titled "A simple sensitive power meter". This reference actually caused me (while sick) to rummage through my stuff and collate all my AR Magazines and do a stock take. As Murphy would have it, June 2009 was the last one I could find.

But the article was worth it, as on the side I've been reading about setting up an antenna range. Currently it seems not too hard to establish a real basic one, but the hardest bit is probably getting a suitable power meter. Well this article introduced me to the AD8307 which seems to be the cornerstone of a lot of home-brew RF meters – considered as accurate as a lot of the very expensive ones. So whether I use Paul's design in the future, or I search for the AD8307 and find another design I like (and there are many) at least I know what the starting point is.

Anyway, I just thought it was interesting to see how a few articles from one person in one magazine can really get someone else thinking. Plus I think Pauls articles are great and recommend people having a read. I've scanned and included the covers below.

Ar_covers_2008-09Ar_covers_2009-06Ar_covers_2010-09

Books Have Arrived / Study Plans

Ahhh, so to my delight the speedy WIA bookshop had the books to me in a very short amount of time – first day back at work and they arrived at my desk.

In the end I grabbed two books:
Radio Theory Handbook, Fred Swainston

Now admittedly I grabbed the first book based on the decision to make the cost of postage worthwhile – so I didn't really need it. However, I DID want a book with more information and focus on antennas for the _higher_ frequencies. Specifically, I wanted more of the design theory around Yagis and the like and this book looked like it would have it. (The only other antenna book I have is the ARRL Antenna Book and it seems to focus more on HF and wire antennas.)

This book has worked out well as it does indeed provide additional information on the items I wanted. Further it does so in the form of good practical build projects as well as theory. However, I must admit I still want more of the theory around Yagis so may have to search for more. But the book is only 144 pages so you can't expect too much depth when it's covering everything from dipoles to dishes; but it definitely does add to the ARRL Antenna Book – also by providing more information on dip meters.

But the real book in this order was the Radio Theory Handbook. My intention (see previous post) was to use this in conjunction with the ARRL handbook and other references to do up a study guide against the AOCP syllabus to use for my license upgrade. BUT, to my pleasurable astonishment, this book has already done this. In one of it's many appendices, it has a copy of each of the syllabuses (both Standard and Advanced) and references to each section in the book – as it does state (in it's How to Use section) that there is more in the book than you need to know for the exams. Further, each chapter in the book has example multiple choice questions for you to validate your understanding. Finally, it now has a CD that has 100 exam questions (with five multiple choice options, rather than the exam style of only four – to make it harder) and some software to randomly select 50 for you each time – although I've not run it yet. Oh, and one more thing, it seems to have a very well thought out progression of chapters. Fantastic!

I've not really got stuck into reading it yet, but the bits I have I've been impressed with. It feels a lot more accessible than the ARRL handbook, and I truly think it will be the book of choice to try and pass my exam with. I can wait until I can get studying in earnest.

But really the back cover of the book sums it up best. So let me repeat some of it here:

"This book has been written as a study guide to cover the Australian Communications and Media Authority syllabus for Australian Amateur Operator Certificate of Proficiency. The book contains the theory necessary to pass the examinations to become an amateur radio operator. 

The material presented in this book has been written to be concise and easy to understand with the view to provide the reader with a holistic understanding of radio and electronics."

Anyway, all is now looking good and I'll post up more information in the coming months about these books I think. 🙂

AntennasRadiotheory

VK1 2m FM Lunchtime Net on this Thursday

Hey Everyone,

Well, I'm pretty much over my flu (well, enough to be back at work with a box of tissues) and as a result I'm happy to report I'll be running the 2m FM Simplex lunchtime net this Thursday. So, once again dig out your gear and see what you can hear.

Hope to have a great turn out, as it may be the last one until December.

Cheers,

VK1JA’s 2m/70cm Beam

VK1JA (Jayson) has also been playing around with 4nec2 recently attempting to come up with a dual band beam for 2m and 70cm. He has asked if I could share around his design and would be interested to hear peoples feedback. Specifically, Jayson says:
 
"The 2m/70cm dual band yagi is a bit of a compromise and I did so many hours on the design trying to get it to work on both bands with ~50Ohms at low swr and a half decent pattern.  The big compromise is with the 70cm band, very wide focus on the front of the antenna with a lot of power radiating at a very steep angle L  If you have any ideas how to lower the radiation pattern on the 70cm band without compromising the 2m band please let me know !!"
 
So here you have it, let us know your thoughts.
 

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