Son of Mod Bob: Step 1 and Results

This worked pretty well – I was able to take down the center support, pull on the two top wires, solder them together and then let it “snap up”, which it did. This weekend I’ll lower both elements and replace the spliced wire. Its easy enough to do – just telescope them down. I then used the BT1500A as a base fed tuner – its not the right tuner – I only need a simple L – C tank circuit, but it works anyway. I’m on 160M still, which is all that matters.

OBSERVATION: The Mod Bob was a quieter antenna – which I could use on RX. The noise level is a little higher tonight anyway so I will test this again when the DXE 40M array is behaving like it normally does – which is almost zero noise. Its 2 S units higher – S3 and the Inverted U is S7, which ends up being a difference of 25 dB more noise. The Inverted U sure sounds like a vertical! The Mod Bob was quieter, by a couple S units. I was heard in Milwaukie at 9 dB SNR, but could not make the 9Y4 contact last night – which would have been #89 on Top Band. I did work ZF9CW and could hear him just fine on the Inverted U. It was a very easy QSO – so I am glad that I am getting into the Caribbean this well. Conditions last night and this morning are plagued by some solar noise for sure.

The BT1500A does work (default mode with no power on the unit) with the Inverted U – but I think the tank circuit is the better choice. Some things I will try:




NEXT STEPS: I’ll play with this until Saturday, when I get everything set. In the meantime, I think I will get the u.RAT set up in a way that will make it easy to deploy and tweak at the base of the antenna. I will also run the new control line since I can catch the last rays of the sun, which is ll I need since most of the run goes under the house in our basement.

A little bit of prep over the next couple of nights means smooth sailing Saturday.



Steps to the Son of Mod Bob . . .

I have three aluminum elements up for the Mod Bob and will take down the center element:

I already have a coax cable running from the Array Solutions switch you see to the left that goes all the way to that far element.

The outside elements have the wire attached and it comes down to the center via a cool resin plastic (DXE) guy ring. I have figured out how I can solder the two wires using what is essentially a “bow and arrow” technique:

The two outer elements already are “leaning in” a bit from when I made the wires taught and brought them down to the center balanced feed. I can get on my tall ladder and pull and clip the wires in a way that keeps the bend on those elements. I will solder them and let the two elements “snap back” to where they want to be – which is straight up and down. If there is a little center sag – so be it. If there is more than I’d like, all I have to do is loosen up the clamps on one of the elements and just twist the element around. At the top of each element I have three wire ties for strain relief, and the wire has a very secure lug ring which is crimped, soldered and covered in Alumox – so its very strong and will stay electrically and mechanically connected.

Worst case – I can just loosen one element and just lower it until I can connect the two wires and solder and then add liquid electrical tape. So, the steps:

  1. Take down center support and get the two wires connected and soldered. The BT1500A and its box needs to be set aside – I will re-use the box and sell the BT1500A
  2. Run a new long control cable out to the far element
  3. Set the u.RAT remote tuner at the base in the box and wire up the control line
  4. In the future – the old control line that goes to the (now removed) center element will be used for the other element – where it will switch the grounded loop line in and out just by supplying 12v that is switched from the shack. Since my main goal and priority is 160M, I will only worry about that first. I might just use the Inverted U as the 160M antenna – I might not run a
    parallel line that is an Inverted L. Here is the difference:
    I think trying to add the Inverted L wire in parallel is a negative because the pattern is not much better and the added weight and complexity is simply not worth such little improvement
  5. Test the u.RAT on all bands and tweak where need be. I will see if maybe I can have a single inductor installed where the only moving part that I need is the variable capacitor. That would be superb if I can pull it off. This is because I don’t like how long it takes to move the roller inductor – it is always slower than the variable cap. I also would like it even more if I could use the Comet vacuum variable capacitor instead of the air variable cap. In fact, if I could just use a variable capacitor and a fixed inductor, I might just use an analog rotary encoder and then I could sell the BT1500A and the extra KPOD that I have. Then my control box would be two switches and a rotary encoder. The remote is a single stepper motor on one side and a relay on the other. Very simple and very reliable.
  6. Re-run the center control line to the northernmost element in preparation for adding the relay to switch in the grounded loop wire. Until I actually have the relay in place, I will use an alligator jumper when I am testing tuning on 80M. Because John says that I should expect 70 ohms on 80M:
    I think this might bode well for just having to use a variable capacitor and a fixed inductor

I can’t package the u.RAT up until I perform these experiments. This will no doubt take the longest amount of time, and it means it will be a full day job. I think impedance on all four bands will be in the range – mainly because of the grounded loops 70 ohms. I know with the Mod Bob – 80M was much higher impedance than the other bands, and that’s why the Palstar 1500A was required. I’m quite sure this solution all the way around will be better and easier too.


Benefits of Son of Mod Bob

There are several things that will be big benefits from Son of Mod Bob:

  1. I will remove the center element which is way to close to my 40M vertical array. This will eliminate the distorted 40M pattern
  2. I will no longer turn on the Trane HVAC when I transmit at full power
  3. I get three antennas in one, a half square on 40, Inverted L on 160 and grounded loop on 80. 30 will be great on all three of these configurations
  4. I will have an automated remote antenna tuner. I never got around to mounting stepper motors on my Palstar BT1500A, which is a good thing. My uRAT prototype becomes my remote tuner
  5. I can sell the BT1500A
  6. Almost all of the wiring is already in place, meaning this is a one day project. I have 96 buried radials and tie points and even several wires that can be used as a solid grounded loop connection (instead of counting on buried radials that could be compromised over the years. I do have gophers under the radials

Its always good to assess the pros and cons of a project. There are absolutely Zero down sides – which is always nice – when you do a benefit analysis sometimes there are trade offs.

Pacificon and this blog have sparked this next phase of my low band antenna pursuit. The Mod Bob was up about 10 months, and I learned a lot, and now I take the next step.

A little creativity goes a long way!

Son of Mod Bob!

Son of Mod Bob!

Page 14-7 from the ARRL ON4UN Lowband DX-ing book

Chapter 14 in the ON4UN Lowband DX-ing book is a gem. John shows how to take a 40M half square and feed it just like the grounded loop – and where it then is a close array on 80M with gain. The dimensions in the book call for 72′ wide and 34′ tall. I might drop the height down to 34′ – maybe 35′ since I only have 70′ max width to play with. I doubt it really matters.

The feed and switching is like what I mentioned in one of the last posts:

if I add an Inverted L in parallel with the element on the left and the top wire, I get the best 160M performance where the wires fit in my back yard:

I will have to build a fairly simple switching scheme . . 

There is gain on 80M now – “close enough” to what John documents in his FB book . . . 

On 40M, I would switch the bottom wire out and its a half square . . . 

On 30M its close – but the 40M half square is kind to this band

So, besides my u.RAT remote L-C tuner, I would have an Inverted L parallel to the half square – and would need a high power relay to switch from one to the other. I would also need another relay to switch the right side to be a half square (un-ground the grounded loop), or ground the loop.

Because I have been building maker stuff, I’m more than happy to add  couple relays and the u.RAT and make this project sing!


VE2CV Grounded Loop vs. Inverted L

I would guess that the Inverted L and Grounded Loop would be about the same on 160M.

It has more gain and a better take off angle on 80M

It has about the same gain as the Inverted L, but better take off angle. Its a moot point though since I have 40M already covered

Same on 30M

I am not showing the Inverted U in the plots because the only band where the Inverted U looks good is on 40M – where it shows as a half square – which is 4.1 dBi gain. This is actually useful, because the plot of the broadside gain for the DX Engineering DV-40-P is only 1 dBi or so.

So I think I am going to use my Mod Bob end supports and convert it to a switchable grounded loop and Inverted U. This combination gives at least as good – or better – gain and take off angle than the Inverted L.

Since DX-ing has been abysmally boring lately – I’m glad I now have an interesting antenna project – and about 1 month before it starts raining in Northern California.

The Grounded Loop Antenna (Thanks Jan, OM2XW!)

The feed is a simple L-C circuit, left, and the antenna can be switched between being an Inverted U or Grounded Loop, right

Jan, OM2XW, wrote and reminded me of an antenna that is in the ON4UN Lowband Book, and one I have never tried. AA7OK has a VERY nice write up with great references at his web site – As I understand it – Jack Belrose, VE2CV originally proposed this antenna some years ago. I will look at the ON4UN book again when I get home tonight.

Thanks to Jan, I am now seriously considering an antenna that will very easily replace the Mod Bob, and the only thing I need to do is take down the center wire support (a big plus), and move the feed and matching unit farther away from the house and my Array Solutions switch (another big plus). The first plus is that it become s a simpler configuration, and more importantly, the center elements and feed of the Mod Bob are too close to the house, and when I transmit at full steam, the Trane heater turns on! In the past, when I had the feed for my single vertical, I had no such interference.

I will do some modeling and also check it against the Inverted L, which is even simpler yet, but one thing for sure – I will be replacing the Palstar BT1500A with my “u.RAT” remote tuner at the base. I have a Comet vacuum variable capacitor, and might even try experimenting with that.

The KPOD and the Raspberry Pi are in the shack . . .

An additional wire would be used to switch the far end for the Inverted U / Grounded Loop. The best thing is – I have everything in place, and this will be a very nice weekend project. I can even get the control box and u.RAT ready – knowing that the actual components at the base of the vertical with the feed could be any combination of Land C and air variable vs. vacuum variable.



Inverted L vs. Inverted U vs. Mod Bob

Inverted U on 160, 80 and 30 and half square on 40M

Just for laughs, lets compare three antennas that I could have – against what I have now – the “Mod Bob”.

An Inverted L would be MUCH better than the Mod Bob. Its something I need to consider – because I could use the u.RAT to tune it.

On 80M, the Mod Bob seems to have the best take off angle by far, but the gain (loss) looks pretty bad.

On 40M, the inverted U is a half square and it is best out of the three

I had my friend Andreas, N6NU, model the Mod Bob vs. the Inverted L, and on 160M and 80M, the Inverted L is a clear winner.

His models are a little different than mine, but I needed to have this “peer review”.

The Mod Bob seemed to perform better than the models suggest, which show the Mod Bob as a cloud warmer. However, not all DX angles are low, especially during sunrise and sometimes at sunset.

I wish I could do an A-B test, but I can’t.

I think it’s time to convert the Mod Bob to an Inverted L, because 160M is the one band I really need, and both his model and mine match and tell the same story. I will use the u.RAT to control the tuning now that it will be an unbalanced vertical feed.

I do remember that when I had a Cushcraft MA160v that I feed it with just one variable capacitor and that’s all it took to tune it on 160M. I did this to avoid tuning the stinger at the top.

 Ok, lesson to remember – ask a friend to peer review your antenna modeling!