Inverted U Matched on 160 and 80M WIth Low Pass (??) Circuit

This low pass (??) circuit lets me get on 160M CW and 75M SSB with very low SWR and no tweaking. I would need to tune when going down to the CW portion of 80M. The Variable Capacitor is a Palstar C600 and the Roller Inductor is a Palstar RI-28. I tried quite a few different L circuits (all of the 8 documented in the ON4UN book that has either two caps and an inductor one cap and one inductor and two caps, but this configuration seemed to work the best.

Right now I am just feeding in the lower left hand corner an its an Inverted U. As you can see – the total length of the “vertical” is actually 148′ – so its longer than a quarter wave on 160M, but not by much. This means its also about 1/2 wave on 80M, and I was surprised that 160M had a very low SWR – 1.06:1 and on 80M – with no tweaking of the components it is 1.5:1, and both are well within the Exper SPE 1.3 K amplifier, which was my design goal.

It would be nice to tune 40 and 30 as well – but just getting 160 and 80M was the absolute must. 40 and 30 are just “nice to haves”.

I don’t really know what is exactly going on – but am happy with the results. Because my 96 buried radials are mostly way too short – with only about 20 that are 50′ long, maybe the inductor tunes them since they are all kinds of crazy sizes and while they do fan out in most directions, its a crazy pattern. Maybe the capacitor on the vertical element takes care that it is a bit longer than a 1/4 wl on 160M and bit longer than a half wl on 80M.

If a reader can explain this – I’d appreciate it. I did not find anything regarding this in the ON4UN Low Band DX-ing book.

Unless someone warns me that what I am doing is not such a good idea, I will go with the u.RAT – because it is needed to cover the CW and SSB portions of the bands. I will try the grounded loop with this tuner – the Palstar BT1500A did not like it at all.

Because I have a great 40M array – I don’t have to worry much about matching this Inverted U to 40 or 30 – I found out that the 40M array works great on 30M and the SWR is already low. But still – it would be really nice if I could.


The Pumpkin Tree

The Chinese Pistachio tree is looking like a neon pumpkin these days, but the leaves seem to be falling quicker than last year.

All the colors of fall in one tree . . .

Orange on the sunny side, yellow on the shady side

I never get tired of this – like the ionosphere, its one of the magical things in nature.

High Pass T Circuit

High Pass “T” Coupler Circuit from

Trying to get 4 bands 160 – 30M (and maybe even 20M as a bonus) is tricky with a 2 component L circuit, so I will follow Larry, W0QE’s lead and start with a High Pass “T” circuit – which gives great range at the sacrifice of a little loss. I plan on trying to find one position on the roller inductor and tune using the KPOD just the two variable capacitors:

I’ll move the two capacitors outwards which will give enough room to put the roller inductor in the middle. 

I will first try to use a fixed inductor in between the caps using this slight variation on the circuit:

The fixed inductor might work out instead of a roller inductor . . .

When I tested the use of three components, I did so as a balanced pi circuit. I have a very good feeling that by changing the wiring and making a High Pass T for the Inverted U that I will be able to match all bands 160 – 30M, and using one fixed inductor

I should have the control wire run and the top wire replaced and the u.RAT tested by hand later today after work. I’ll test various components on the u.RAT board. I will go with one of several possibilities – and use clip leads to test a variant of a 2 component L – C circuit and then the 3 component High Pass T circuit – if I have trouble matching 160 – 30M (higher bands being a bonus).

Speaking of higher bands, are there any higher bands that would work using the Inverted U?

Well, sort of – 20 and 17M look great on the elevation angle, but their azimuthal patterns are pretty weird, not even worth thinking too much about.

Here’s my test plan:

  1. Try just one variable capacitor series feed (maybe inductor as well, but I fully expect a capacitor in series will match some bands – but not all)
  2. Try variations on the 2 component L – C
  3. Try the T with a fixed inductor

The u.RAT board will either end up driving one L and C or two C’s in the circuit. If I find that the fixed coil needs several taps to make this work, then I could use the one last spare wire in the control cable to add in a high powered relay and switch in maybe 2 different taps. I would change the KPOD code so that one of the function buttons changes the tap by applying power to the relay. It will have a default setting and a push of the button toggles 12v to that relay – it would mean one GPIO pin – so its really easy to do. I am very confident that one of these methods will work and work well. I can also look into the many relay driver boards that would allow a more exotic set of switchable taps – but I might exceed the number of wires needed in the control cable. I’m hoping I don’t have to go this route at all actually. Just having two steppers doing all the work would be what I really want – so I can get this thing on the air ASAP.


Grounded Loop: Step 2 – Test Adding the Ground to the Inverted U

I added the ground to the Inverted U, and while the tuner liked it – in the shack with high power, the amplifier faulted. I know the tuner is the wrong tuner, and the values of L and C were both very low.

I don’t really know why, but I will experiment more using a different tuner. For tonight and tomorrow I’m back to the Inverted U.

Low Band Grounded Loop

The ARRL told me that they do not plan on continuing to update this book with new revisions. If you don’t already have it – you really need to order yours now – because it is already a classic. 

More than ever, the combination of Low Band Antenna Experimenting and Maker projects is the cats meow in Ham Radio. I don’t even care so much about DXCC – except to use it as a marker for progressing with my “education” and experience while learning along the way. The award is like the road we drive on – its a useful marker for where we are – but looking at the scenery along the journey is far more interesting and exciting! I am having more fun than ever these days.  Last summer low band RX antenna experimenting started it off – and the u.RAT and now “post Mod Bob” Son of Mod Bob is my next exciting chapter in my journey.

Instead of calling it “Son of Mod Bob” (which was part of “semi-intentional” Halloween theme I suppose), lets just call it a Low Band Grounded Loop. WHen I do add the relay to open the circuit and turn it into a 40M Half Square, we will just consider that that is part of the Grounded Loop.

Jan, OM2JW emailed me this week and reminded me of an antenna that I had read in the ON4UN Low Band DX-ing Book a while back, and while I have tried so many antennas, switching circuits and tuners in this book, the Grounded Loop is the one I haven’t tried. I have tried the 40M Half Square and various permutations of the Bobtail Curtain. I settled on phased verticals for 40M and have wired them using the Christman Method documented in the ON4UN book, and more recently took the easier and more robust approach and use a DX Engineering DV-40-P phasing solution. For a short while, I had a 40M “shorty 40” yagi up 50′, but in my tiny antenna space – I had to chose tower vs. better low band antenna. I chose the latter and put up a 2 element 20M home brewed yagi since we are rapidly approaching the bottom of the cycle and also based on my DXCC last 14 QSO needs. (2 towards HR #1 and 12 on 160M for DXCC Top Band and 9BDXCC).

The ON4UN Lowband DX-ing book only mentions that this “grounded loop” can be used on 40 and 80M, but Jan, OM2JW said that he has used the same antenna on 160M with an LC circuit. And that makes sense because the loop is 222 feet, which is between a quarter wave antenna and a half wave antenna – its closest to a 3/8 wl, although I’m going to guess that the radiating portion of the antenna is actually just what is above ground (152′) which is closer to a quarter wave vertical. In fact, I’ll bet it is the best “low” equivalent of a full size 1/4 wl vertical for Top Band that results in an impedance that is easily tunable, and which might also be far more reasonable than a short top hatted vertical – which I know from experience results is ridiculously high voltage at the base of the antenna – and which can over power most components you can put there. I know from the arcs and sparks I have experienced when experimenting with very short 160M antennas. I’m also guessing that the ground wire acts as a part of the impedance matching circuit more than anything, plus it also acts as a counterpoise or reflector. I think this ends up meaning that instead of a very low impedance vertical – this antenna is higher impedance and is even within the “easy” range of a tuner – which is exactly what I need since I have had some circuits that could tune one band but not the other. 160 and 80M can be a challenge especially. Pretty cool design – actually!

I must admit – when I saw this antenna years ago – I guess I did not like the idea that any part of the antenna was buried, but Jan assured me that this is actually a plus, not a minus by letting me know how well he has done on Top Band.

Assuming that only the part above ground is what counts length wise – this antenna is at least close to:

  • 1/4 wl on 160M
  • 1/2 wl on 80M
  • 1 wl on 40M
  • > 1 wl on 30M

Jan says the antenna even works fairly well on 20M, so I will need to compare to my 2 element home brewed yagi

Last night and early this morning I did enough testing to get a good idea of how an Inverted U works – its similar to an Inverted L, and is easy to tune on Top Band, even with the “wrong” kind of tuner. Tonight I will add a few jumpers and get the Grounded Loop wired up.

I am starting to believe that the matching circuit might end up as easy as a single large Palstar C600 variable capacitor (or that Comet vacuum variable capacitor I have under the house) – because the antenna is longer than a quarter wl on every band that I intend to use it on. There – that’s my hypothesis – lets see how naive that ends up being!

I just double checked EZNec, and the Grounded Loop actually looks better on all bands 160 – 30M than the Inverted U. So, I have a strong feeling that the Grounded Loop will end up being the best low band antenna that I have put up on my small lot.

I am really glad that I tried the Mod Bob – but its funny how the one antenna that I avoided in the past – because I had qualms about the “grounded” part might prove to be exactly what I needed, and a really elegant solution for a small patch of land 10′ x 70′ that I have for a Low Band antenna.


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.