Posted on June 12, 2017
I’m very pleased that my “Mod-Bob” antenna ended up being one that used “tuned feeders”. Well – therein lies the rub. I don’t really, really understand this antenna – because it seems to look like three phased verticals like a Bobtail Curtain – but does so on 30M as far as EZNec is concerned. But the feed is not at all a classic Bobtail feed – because I am not feeding against ground radials – which would be a voltage feed. Instead, my feed looks more like a dipole current feed.
Here are some simple, yet very easy to understand references to look at for a 1/2 wl dipole feed:
and a 1/4 wl vertical feed:
I’ve heard conflicting opinions regarding the efficiency of an “L” tuner circuit vs. the more common “T” circuit, but the Palstar BT-1500A is designed for open line feeds to the antenna. It has several switches and settings to allow for high and low impedance matching – and seems like I will be able to learn a lot from this antenna and project. The manual is very good – its an education in itself, but with my very trusty and well used Rig Experts AA-30 antenna analyzer and “field laptop” (an old Dell “beater” laptop that can go out in the yard with me), I will be able to try different settings on the tuner and measure what happens with the AA-30. I’ll learn right away if I need to run a 12v line out to this remotely installed tuner – or if its “default” switched circuits are good on all low bands.
I do know that the antenna is 50 ohms on 160M. What is interested is each “side” of the feed is 117 feet long. This is almost 1/2 wl on 160M – which is pretty cool for a 160M antenna that fits in a small suburban back yard. So, for starters, the impedance is LOW on 160M.
I know for a facty that on other bands the impedance will swing high – I suspect that on 80M it is especially – because the KAT-500 does not like the SWR much at all and I’m restricted to 200 watts or so there. Otherwise – on 160, 40 and 30M its fine.
The AA-30 will give the full story – and then, by switching in combinations at the antenna where the BT-1500A will be mounted – I can really get down to business and document this antenna from a feed point perspective.
The actual measurements plus EZnec plots will be useful – I like “data visualization” because its a way to take a complex set of variables and produce something a non EE person like myself can understand.
I build data warehouses for a living, and love to get to the “art of understanding” with “visualized data”.
Posted on June 12, 2017
On Friday, I sold my car and now we are a one car family. I’ve wanted to do this for many years, and the whole hub bub regarding the Paris Accord made me finally do it with conviction.
I also had an epiphany after toiling over the nasty political climate in the US. I kept reminding myself what I was taught in the Episcopal Church and something my father said. The Church taught me “Do unto others as you would have them do unto you”. My father was much more succinct, he said “If you don’t have anything nice to say, don’t say it at all”.
My epiphany was that no politician will ever save the world. In fact the truth is that people must contain politicians from ruining the world. Taking sides on social media is downright idiotic. So, as I mentioned in my previous blog, I will be very careful to try to channel all of my energy to positive thoughts, words and most importantly, deeds.
Imagine if every one on the planet consumed less, wasted less and walked just a bit more softly.
This no “Kumbaya moment” since I’m saving for retirement, I already know that these actions can help save big money. My grand parents lived this way when they immigrated and went through the Great Depression. Modern convenience (with massive waste) was an invention that happened once the US economy recovered. There is no amount of money or no political organization that can come close to the power of the people. And we don’t need to organize, this is something each of us can do.
I will blog about the ways that I plan to contribute to society and the earth and let you know what happens. Becoming a one car family is already an incredibly freeing thing. Not only do I not miss my car, my mind is now open to being more creative in how I get around.
I’ll only blog about things I actually do in this regard, but the thing I like about what Mahatma Gandhi said is that it takes the Christian “Do unto others” idea or the Buddhist idea of “Do nothing extra” and steps it up a notch. It transforms this to a question of what can I DO.
Going from a two to a one car family is a pretty good start and now I’m thinking creatively on what I can do next.
Posted on June 11, 2017
40M Phased Array verticals at KY6R. There are 44 buried radials that connect between each vertical plus chicken wire ground screen not connected to anything. The chicken wire helped hold the radial wires on the ground – and then the grass grew over the wires. This ground system seems to work exceptionally well. The creek bed is just below these verticals to the right.
One of the reasons why I deep-sixed my old blog is that I had some posts that were getting really out of date – and I mean a LOT of them. Back in the day, I was very much into Cabella’s Crappie Pole Moxon’s and a Christman fed 2 element 40M vertical array. With more experience (and new products coming on the market), I have found some better antennas and approaches. I’ll get to the Moxon antenna in a subsequent post – but lets take a look at the Christman phased array “replacement”- the DX Engineering DVA phased vertical array system.
One of the things that I have come to love is the DX Engineering DVA-40-P 40M phased array system. DX Engineering will sell you the entire kit – and its kind of expensive, but luckily, I had the aluminum for the two verticals in the array. I purchased the DVA-40-P controller gear – which consists of a switch and a remote control box – and all I can say is – these guys really know how to build some serious gear – and the outside switch has the absolute best gasketed weather proof case I have ever seen. Its also UV protected – which believe me – is super important. I was feeling lazy, so I paid them to build the phasing cables – you can save good money if you do it yourself.
In the old blog I sang praises of using an Array Solutions Stackmatch II switch and cutting the 3 antenna cables (2 feeds and one delay) using the VA7ST Christman calculator available online. The approach is well documented in the ON4UN Lowband DX-ing book, but I now think the DX Engineering approach is far superior. Its the same thing electrically – but mechanically, I feel like the DX Engineering approach is better.
I can say without hesitation, just go to the DX Engineering web site and purchase their gear. Yes, it will cost about $200 more total, but believe me – it is well worth it. One reason is the durability and weather proof part – the other is you only need two cables from their phasing switch, and no set of crazy coax T and L connectors. Its a much more “elegant” solution – and that means a lot when you want something to work over several winters, and heck – even the heat of summer.
The delay line is essentially replaced by a toroidal transformer. These units would be very easy to repair in the field if need be.
What is especially nice is having two simple feed cables and no delay line. Its VERY easy to cut the cables incorrectly, and if you do – your phased array will be a very disappointing exercise in futility. With the DXE boxes and cables – the only thing I had to do as build the two verticals (I pop rivet and use Alumox on all of the top sections and then use a bolt at the first section so I can slide the element up and down and tune each element per DXE’s very well written instructions), and using my Rig Experts AA-30 antenna analyzer. In fact, it took maybe 1/2 hour per vertical to tune it to the frequency that the DVA manual suggested so the center frequency would be at 7.150 mhz.
I even purchased a second unit – so if one blows up, I can actually fix it myself. I will have no downtime this way, and believe me – when Bouvet hits the airwaves in just months – you better have 40M in your quiver of antennas! Conditions on the high bands have been so close to bottom of cycle conditions – I am thinking that the 3Y0Z propagation predictions are actually a bit optimistic. I hope I am wrong, but 20M is EXACTLY like it was at the bottom of the last cycle – there are maybe 3 or 4 stations on the air – and they are just US stations – no DX. They are all weak. Yes – there is an occasional spike – but this downward trend is real and somewhat “alarming”.
During the last bottom of the cycle – if I did not have a 2 element phased array on 40M, I would have almost just shut down the station for 2 or 3 years.
For two years I had the N6BT DXU-32 and had to take it down so I could get serious about 160M, while retaining something decent on 20 and 40M.
The DXE DVA-40-P is not quite as good as the 2 element “shorty” 40M yagi I had up at 50′, but its not too far behind. The difference between the two is when you will get in the log – with the phased array, you will be in line right after all of the full sized and shorty 40 yagi owners – and you will be before the high dipoles and the 41′ verticals and low dipole and inverted vee DX-ers.
Here’s something to consider – 2 phased verticals close to ground is childs play to maintain – even a high dipole (with squirrels eating through my support rope) was trickier. And the DXU-32 on a tower – that was a massive undertaking compared to the phased verticals. Also – if you have an HOA or CC&R – you might get away with two aluminum or wire verticals in your back yard.
Again – if you want to work serious DX and rare ATNO’s in the next 4 years or so – you better have at least a 2 element phased array – and the DXE DVA-40-P is the easiest way to get serious fire power on a small lot!
Posted on June 11, 2017
This is the most amazingly comprehensive book in ham radio – and its focus on the Low Bands shows just how much there is to the Low Bands, and why I have ended up in this space with this new blog. I contacted the ARRL to see if they will put out a 6th Edition – and they said there were no plans. I knew that John, ON4UN was retiring after this 5th Edition. I HIGHLY recommend purchasing this book. Mine is so dog eared that I purchased a second so that when the day comes that my well worn one gives up the ghost – I will have a fresh new on on hand.
This book is not a Low Band book per se, but its the best book on propagation – where the topic (which can be quite complex) is described in a very easy to understand way – and has just the right illustrations that match the text.
In fact, later today I’ll have a nice glass of wine and just read select articles from these two books. I usually learn something new every time I open them.
Posted on June 11, 2017
I have found several other “kindred spirits” who have worked on variations on the Bobtail Curtain – that have some things in common with my “Mod Bob”. This one came up with a Google search on Bobtail Feed, and this link will give you hours of joy looking at creative antenna ideas – especially the Low Band Antennas:
What is most interesting is that unlike the higher bands – 20M on up, there are quite a few people out there who have experimented more on the low bands with all kinds of interesting ideas. I am sure its because if you want an antenna that works “reasonably well” as a DX antenna on the bands 160 – 30M, its quite a challenge. But that – just like making any QSO on 160M – is what drives me. The day that the challenge is gone is the day I will hang up my key. Luckily, I don’t see that happening, because just trying to understand 160M propagation could be a life time journey, and I will never work them all on 160M – but will try to keep my “count” climbing. For me – that’s far more interesting than the DXCC Challenge.
I think this is called an NRY antenna, but it does look like a “Lazy H” antenna. In any case, the dimensions can be modeled, and you can even try variations, especially looking at height above ground. One antenna that works well with the bottom at least 8′ above ground is the Bruce Array, and Rudy, N6LF has written the best articles about this fine antenna:
OK – so I have wandered a bit away from the Bobtail Curtain feed – but that’s precisely why this is so much fun – grab a nice beer, sip slowly, and you can get lost tracking down these wonderful antenna ideas. Since its 4 AM PST right now – I am listening to very low noise but no signals on 160M, drinking a nice cup of Sumatra coffee, and blogging. Beer will have to wait!
Here is an interesting “Inverted Bobtail” which can be found at:
Finally, here is a hybrid idea:
I remember seeing an article in QST that was written by Hal, N4GG where he created a “fan dipole” version of this. I think he said its usable, but not a great DX antenna. Its a good place to start as far as trying to experiment with Bobtail Like antennas and feeds.
What is so interesting to me is that there are just enough variables in trying to create a resonant (or perhaps a non resonant) antenna for one or all of the low bands. There is the actual dimensions of the antenna and where you might put such an antenna on your property. If you have minimal space like I do – the equation can be quite tricky:
I have a narrow strip of land that is right next to a creek that wraps around two sides of my house. Its a very pleasant and calming environment, and with my new landscaping that I worked on with my son Trevor, its a great place for everything (except being able to put lots of antennas up). But I took that challenge and still made Honor Roll and am closing in on 160M DXCC.
Two of three elements of an antenna that is 41 feet high and 70 feet wide – not bad for a small suburban and wooded lot.
Two of the three elements of my Mod Bob antenna. The creek is just to the right – so you can see that my space is tight. The wires on the left go into an Array Solutions Rat Pack remote antenna switch. I have my Mod Bob and DXE 40M phased array plus 2 element 20M yagi attached to the Rat Pack and thus have only one (transmitting) coax line coming into the shack. I also have a coax line coming in from the Wellbrook ALA1350LNP receive antenna, plus several control lines.
I will be replacing the 1:1 current choke with this:
And will have it in a very nice weatherproof utility box. I’m doing this because right now my Elecraft KAT-500 does not like the Mod Bob on 80M. It will be a little bit of a hassle to go out and tune manually for each band – but this might get me to try to build a remote control circuit using stepper or other synchronous motors and also remote switching for the two relays:
The relays switch in Hi Z or Low Z double L circuits.
OK – hopefully this blog post gives you some ideas – just thinking and reading and drinking coffee is fun. I just spun the dial on 160M – and hear ZL2OK quite well on 160M SSB.
Not bad for being a couple weeks away from the Summer Solstice – right at our sunrise. He was a good S7 here. The noise is much lower than it has been – which gives me hope, and ADXO says a team will be on T8 soon – which I need on 160M.
I sure love the Low Bands!
Posted on June 10, 2017
Its somewhat rare that I keep an antenna up more than 2 years, but this one will no doubt be an exception. This antenna “sort of” looks like a Bobtail curtain, and at element spacing of 35′, it could be a 20M Bobtail. The only problem is that its not fed against ground, but fed as a dipole like feed at the bottom of the center. Think “tuned feeders” I guess (?) There is a ground screen underneath – so I have that in the model. ON4UN in his book “Lowband DX-ing” recommends a ground screen under any vertical type antenna. The verticals are not 1/4 wl on 20M, but would actually be 5/8 wl elements on 20M. That might even be OK, except my 2 element yagi up only 33′ is a better antenna. So, what the heck is this “Mod – Bob”?
Its easily the best “compromise”, non-resonant low band antenna I have ever built. It works quite well on 160M – 30M, and that really is a feat to achieve. Here is the crux of why its so good – rather than look like a low dipole on any of the low bands – it instead looks like phased verticals on all of the low bands. The horizontal wires look a lot more like phasing lines – like in a half square, bobtail or bruce array instead of like a dipole. Sure – a single vertical will give you a decent omni directional fairly low angle pattern – but with a lot of loss in the radial field – and this Mod-Bob antenna is at worst – unity on 160M – and gain on all of the higher bands. On 160M, its better than an Inverted L – or shortened vertical because rather than have loss – it actually has a tiny bit of gain – with a nice low takeoff angle and less horizontal radiation than an Inverted L. On 30M it really shines:
In fact – that pattern, with the tell tale “ears” looks like a 30M Bobtail. Its has the same pattern and gain, so while it doesn’t walk like a Duck, its a Duck (er, Bobtail). Its as good as a 1/4 wl dipole up > 50′, so that’s pretty good. A dipole up at least 1/2 wl would normally model at 8 dBi as well – so I’m actually quite happy about this with this antenna.
This antenna came about because I had a 60′ Top Band vertical, and in a windstorm this past winter, the antenna bent over rendering usable but scary looking. I decided to just take materials I had – or that I could purchase from the VK0EK DX-pedition (think Spiderbeam aluminum push up masts), and I just put the biggest antenna up that I could.
Very surprisingly, its SWR is exactly where I want it to be on 160M – in or around 1.822.5 mhz – so it is actually a resonant antenna. It does not model well SWR wise in EZNec – and I have no idea why. It works very well on 40 and 30 with a tuner, but it has some problems with 80M and the KPA-500 folds back to only allow a couple hundred watts.
One thing I’ve just ordered is a balanced Palstar tuner – the BT-1500A. It has two switches that activate relays and switch the circuits to be either high or low impedance. This should help make the antenna on all bands – but specifically 80M, which I think will prove to be an important band during the upcoming Bouvet DX-pedition. If I were feeding this antenna against ground – then I am sure 160 and 80M would be very low impedance around 12 ohms – but this is a different beast. 80M must be either higher or lower than the other bands impedance wise – and far enough away from what the KAT-500 can match. There are no matching issues on 160, 40 and 30M.
Its a double L tuner for balanced line – and so it seems to be perfect for my needs – even if it only gets me on 80M. I will house it in a weatherproof box out back and manually change settings using my AA-30 antenna analyzer. Maybe in the future I will remote it and use motors to turn the shafts. Maybe an Arduino board can be hooked up so it becomes an automatic remote antenna tuner? C’est possible.
I’ve really committed myself to the low bands – besides this antenna, I have the fantastic DX Engineering PV-40-P 2 element phased vertical array on 40M, and I also have a 2 element yagi on 20M – but doubt it will get that much use.
So – that’s why my new blog is very seriously oriented toward the Low Bands – its a place where a lot of experimentation can be done and on 160M especially – there is a lot we do not even understand yet – propagation wise.
Posted on June 10, 2017
Complex Ionospheric Propagation Modes
I found this image on a wonderful site regarding the many facets of ionospheric propagation:
There is also a very interesting PDF available for download here:
This paper confirms something that I have experienced on 160M, which is “ducted mode” propagation. In the diagram above, this is the blue line, and basically, rather than having your signal bounce between the ground (or water) and the ionosphere, it gets “tunneled” within a channel in the ionosphere, resulting in very low signal loss – as opposed to your signal buncing off the ground. This is where a station very far away comes in amazingly strong. I have experienced this with VK0EK, FT5ZM and a V51 station. Here is what the author of what this paper says:
“You may not realize it, but a considerable number of DX openings on Topband over distances
greater than 4,000 kilometers may owe their occurrence to a phenomenon known as signal
Its impossible to really know if I experienced this form of propagation, but I do remember having a hand held 2 meter radio standing in my back yard on Mt. Davidson in San Francisco, and talking to someone on a simplex frequency in San Juan Bautista – where he sounded like a local. Sure – it wasn’t HF – but it was a form of ducting – due to an inversion.
Since 160M is just above the AM radio band, and because when I was a kid I had built a crystal receiver to listen to the local AM radio station – I’ve always had a romance with this part of the radio spectrum. In fact, before I became a ham I was an SWL and did a lot of BCB DX-ing. I recently bought a used Palstar R30CC:
and Wellbrook ALA1530LNP loop receiving antenna:
and this combination makes it fun to see what I can hear at night. Its a “place holder” while I wait for DX to pop up on Top Band.
So – now you know the reason why I started new blog with the name that I am using – these complex propagation modes that enable very long distance DX-ing on 160M are simply magic – and something that while you can’t count on, when it does happen – its way more memorable than simply pointing your yagi and throwing full steam (on 20M and above) at the DX and almost taking it for granted. Like shooting fish in a barrel?
Top Band DX-ing is easily the most challenging and rewarding band – and the one place in this wonderful hobby where I don’t see an end to the fun. Its my new “chapter” – my new pursuit.
I also plan on concentrating on all bands from 160 – 30M, and my station has become an (almost) 100% dedicated low band station. This happens to fit where I am at with my goals and also where the sunspot cycle is heading (we’re already there me thinks). As you remember, 40M is my favorite “every day” band. It offers great DX year round and sunspot cycle round . . .
My NEW DXE 40M phased vertical array will be featured soon. Stay tuned.