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.

 

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