The sounds of DX

Autumnal equinox, DX fans. Symmetrical arrangement of the sun, north versus south of the equator. Happens twice a year. Good time for DX.

To borrow from Robert Duvall as Colonel Kilgore from Apocalypse Now and his quote,

"I love the sound of signals coming in long-path or with polar flutter in the morning. It reminds me of...DX IS"

So back to my dilemma of achieving low angle radiation in the direction of SE Asia, I am experimenting with a Bobtail antenna. It's an arrangement of wire that acts as if there are three vertical quarter-wave antennas in phase. Radiation direction maximum is broadside, at low angles (19 degrees).

MFJ has an explanation on their website with some interesting guidance on additional performance enhancements, although the gain claims are (in my opinion) exaggerated.

K3KY has an excellent discussion around the design and considerations for ground.

N8FSC, on his website, has an excellent description of the antenna:

The antenna system of Fig 62 uses the principles of cophased verticals to produce a broadside, bi-directional pattern providing approximately 5.8 dB of gain over a single element. The antenna performs as three in-phase top-fed vertical radiators approximately 1/4 wavelength in height and spaced approximately 1/2 wavelength. It is most effective for low angel signals and makes an excellent long-distance antenna for either 3.5 or 7 MHz.

The three vertical sections are the actual radiating components, but only the center element is fed directly. The two horizontal parts A, act as phasing lines and contribute very little to the radiation pattern. Because the current in the center element must be divided between the end sections, the current distribution approaches a binomial 1:2:1 ration.


The vertical elements should be as vertical as possible. The height for the horizontal portion should be slightly greater than B, as shown in Fig 62. The tuning network is resonant at the operating frequency. The L/C ration should be fairly low to provide good loading characteristics. As a starting point, a maximum capacitor value of 75 to 150 pF is recommended, and the inductor value is determined by C and the operating frequency. The network is first turned to resonance and then the tap point is adjusted for the best match. A slight readjustment of C may be necessary. A link coil consisting of a few turns can also be used to feed the antenna.

In the picture above, the antenna is fed at the base of the center vertical section. It presents a high impedance...well above 50 ohms. This can be addressed with the components as shown, or a remote coupler like those made by CG corporation or SGC. I already have the CG-3000 on my 43 foot vertical, but don't want to move it over to the Bobtail AND lay down another radial field.

Fortunately, there is another option: the Bobtail can be fed at the top corner of one of the end vertical elements. The impedance here is on the order of 120 ohms with some reactance, easily matched with a tuner in the shack. With low-loss coax like RG-213, even longer runs (like 200 feet of coax in my case), the calculated losses due to SWR on the line are 2.2 dB (I measured my SWR as 2:7 to 1 on my instance). This means 100 watts into the system results in 60+ watts provided to the antenna.

I picked up one of these from K4TR, since my free time is very limited for building antennas. I wanted to experiment on 20, since my trusty Hex beam is stowed in anticipation of the upcoming move. If things work well, I would like to try something like this on 40 and 80. Also, this would fill in the radiation gaps at very low angles on the 20 meter band at the new QTH.

The antenna arrived last Saturday, and I put it up quickly on Saturday night with the tails hanging 6 feet off the ground. SWR as I mentioned above for the corner feed was 2.7 to 1. Not ideal, but I am experimenting here.

So how did it do in the first 24 hours? I pointed it WSW/ENE so that the broadside was towards SE Asia and S Africa. Here's the email I fired off to K4TR on results. Beside the fact that his product is sturdy and super-stealthy, I was pleasantly surprised:


0745 local, UA9MC booming in 589. Can’t hear the US stations that he is in QSO with, but I work him on first call. Nice. Looking at the log I see very few QSO with UA9/UA0 except on the hex beam. And normally after sunset my time. I only see one or two QSO’s with UA9/UA0 at this time of day. One in January of this year on the vertical, noting a very strong UA9. Granted, I am normally not on the air in the morning but if I am…I would always try to work Asia.

0750 local, I can just barely hear FS/W6IZT (as it should be – right direction but short skip), LZ1FM heard 599

Unable to hear UK8WFF as spotted, he is spotted by EU only

0810 local, P41USA worked 599+ (short skip, but correct direction SSW). Got a 599 back from him as well. I can barely hear some EU working him

1428 local, worked KH6MB 559 exchanged both ways (interesting broadside to the pattern, although weak. I was able to hear him because the ANTENNA is very quiet compared to my vertical)

1550 local, heard 5N/LZ1QK, UA6CVE 339 (as expected)

1551 local, heard LU5BE, 459; WG7E 569, Z30GV 559

1556 local, worked 9A4WY sent 569 rcvd 559

2020 local, heard LU6CAT 579 (hardly ever heard on the hex)

2022 local, heard JT1BE 579 YESSSS! This is why I bought the antenna!!! Louder than I heard him on my hex beam last fall!

2023 local, heard DS5USH 549 YESSS! The thing works! Have not heard him on my hex beam


Since lat Sunday, I have repeatedly heard JT1BE. I have not been able to work him, but maybe I need more power. Conditions not all that great, but I am hearing him consistently.

CONCLUSIONS:

• The design works. Although quickly discarded for 20 meters by most folks, I have a specific need and this can fill that need.
• On 40, this could be a real addition for getting into SE Asia. The 43 footer is great on that band, but omni-directional.
• On 80, if I can get the antenna up 64 feet high (should be do-able at the new QTH), I need to find 256 feet of straight run (a challenge). But this would be a killer improvement on 80.
• I recognize the reductions in performance provided by corner-feeding the Bobtail. Perhaps I will try feeding the bottom of the center leg with a remote tuner. This will require ground screen of some sort. It's something I will look into after the move is complete.
  

73 and good DX!