The “Mod Bob” Antenna

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.

 

Complex Ionospheric Propagation And Chordal Hopping

Complex Ionospheric Propagation Modes

I found this image on a wonderful site regarding the many facets of ionospheric propagation:

http://www.sws.bom.gov.au/Educational/5/2/2

There is also a very interesting PDF available for download here:

http://solar.spacew.com/cq/cqmar98.pdf

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
ducting.”

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.

The Top Band Chordal Hopper

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I had a blog that ran for years called “The Armchair DX-er“, and it basically ran its course. The idea behind that blog was mostly my ham radio road to DXCC Honor Roll and also my personal involvement with the 2016 VK0EK Heard Island DX-pedition, but that chapter of my life is done, and that blog had years worth of “cruft” that I did not want to clean up. At each milestone, I created a presentation that I usually gave at Pacificon or the International DX Convention in Visalia, and those can be downloaded here:

KY6R Antenna Presentations

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KY6R QSL showing a Hop Cone pinging around the ionosphere – shooting through the Golden Gate on its way to Orinda. This could have been FT5ZM or VK0EK, both of which I worked on Top Band – my all time personal best DX QSO’s ever.

I’m not a pack rat, and after a divorce in 2005, I tripled down on “traveling lite”, and its become a really great way of living. I only keep the minimal number of things that I need to live, plus the minimal amount of gear for my hobbies – which includes ham radio, photography and cycling. I have also become a lot more frugal because I will be retiring in about 9 years, maybe less. I’ve been a database software engineer for 37 years, and actually love what I do more than ever. I can create software faster than I ever have, I love to mentor the younger staff, and Credit Karma in San Francisco is truly the best company I have ever worked for. I’ve only been there about 3 months, but its a very successful company that has a big heart – proving you can make money and be socially responsible.

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I sold my 2005 BMW X3 yesterday because I have been riding my bicycle to work for at least two years and it has been sitting in my driveway hardly ever used. It is a big deal because I purchased the car during my divorce, and now have been re-married to my wonderful wife Kat for 10 years – who I am very compatible with.

Part of selling my car is also political – after having a really hard time trying to understand how in the world we could ever end up where America is politically, I realize that there is a big separation between all of the hub bub media wise, the crazy politics that go on at the federal, state and local level, and that my life is not ganged to any politician directly. I made a list in my last blog which reminded me that I have the power to contribute to changing the world in a positive way – and that if each and every person practiced the art of “walking lightly” on this planet – it might last longer. Selling my car means we are now a one car family – and that is the best statement I can make. We also use as few resources as possible – less energy and water consumption, I ride my bike to BART and take mass transit to work – and the list I made in my last blog really surprised me. I was actually surprised at just how much power I have to contribute to the world in a positive way. “Be the change you want to happen in the world” has never felt so good and meant so much – and this is my reaction to the worst politics I have ever experienced in my life. But I can fight back with positive actions instead of jumping into the “pissing match”.

I got off of Facebook and almost left Twitter because I found the political landscape of this past year in the USA to be overwhelming and way over the top on social media, and frankly, depressing. The very good news is that I have worked through all of that and feel I can “reset” my blogging and avoid all of that unnecessary noise. I have lived through good and bad administrations, and I still believe that American People will always watch the government like a hawk and let their voices known. Democracy can be ugly though – and I guess I accept this because its far better to be messy than have an absolute dictatorship. Anyway, I have moved past my issues and am ready to get back to accentuating the positive things in life – which I feel I have a bounty of – wonderful wife, two great kids (young men actually), fantastic job, fun hobbies and decent health.

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These are the two ham radio DX-ing awards that I cherish the most – DXCC Honor Roll and DX-pedition of the year. Nothing will come close, except perhaps DXCC on 160M. We will see – these two are significant and I feel that having just received the DX-pedition of the Year that I have closed the door on a real “era” in my ham radio hobby. As they say, when one door closes, another opens – and I have always been forward thinking and looking.

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My current love and obsession ham radio wise is Top Band and all of the Low Bands. They offer the biggest challenge, and because I have accomplished what I have – the next “step up” in the DXCC Ladder is most certainly Top Band DX-ing. Even Top of Honor Roll seems to pale in comparison. I also love “living history”, and since 160M is just above the AM Broadcast band, there is a real sense of history that I feel when I make a CW QSO on Top Band. Each and every QSO is more special there than on any other band.

SO – this new blog will be similar to my old blog – but moving past where I was with the Armchair DX-er and looking forward down some new avenues.