Sunday, July 28, 2013

Final Modifications to My 20 Meter Extended Double Zepp


Nice sunrise, eh? But, wait, what is that funny wire with holes in it at the top of the photo? Well, actually, that is the feed line from my VK2ABQ that sits on a mast at the side of the house, but before I go there in a follow-on post, allow me to digress and return to my EDZ modification that I mentioned previously. As you'll see, the two antennas, though not literally linked to one another, are connected virtually by the new "electrician's nightmare" I have created.

Recall that before I tackle the craziness of the dual element 10 meter EDZ, I wanted to complete a mod to the existing 20 meter EDZ I have in the air, which consists of two parts: 1) add series capacitors to each leg to bring down the SWR curve across the bands (40 to 10 meters), and 2) ditch the coax from the EDZ tuning stub so that I have window line all the way to the station.

One of the caps, made from RG-8/U coax and sealed 8 ways to Sunday, is pictured to the right in place at about 24' in from the far end of one leg of the EDZ. I used 4nec2 to model the effects of placing the caps at different (but equal) locations along the wire legs and as in all things multi-band one must choose among a number of compromise scenarios. For this compromise, I optimized the pattern for 17 meters, since it came out with 10.77 dBi, higher than I could get on 20 meters. The caps essentially turned the antenna into a 17 meter EDZ and I lost about a dB and a half on 20 meters (but gained some beam width). 





As I often do with antenna mods, I abandon the scientific method temporarily and changed multiple variables at once. In this case, I added the caps and the new feed line before taking any measurements in the interest of reducing my on-the-roof time. Thus, I reduced my chance of falling and was able to beat the sun, which was sneaking out from the morning cloud cover. In this case it all worked out anyway, so no harm done in believing in magic, this time.

Turning to the problem of how to bring the window line into the house, I had a couple of ideas, but the one I landed on was to drill a couple of 8 mm holes through the concrete wall and insert two 1/4" steel threaded rods, encased in clear vinyl tubing for insulation. Easier said than done. I have an awesome hammer drill, which usually makes these holes like the proverbial hot knife through butter, but as luck would have it on both holes I hit either rock or re-bar and the holes ended up not being quite straight through. 
The outside view of the pass-thrus with antenna selector switch between

Well, after driving through the rods, patching up and repainting the broken stucco (by coincidence I did this work on a day the XYL was away, hi), and bolting them in place, I had a pass-thru for the wire. This task also required moving around the station a bit so that the tuner would lie more or less directly below the pass-thrus.
Inside view of pass-thrus with 4:1 balun attached

I next strolled down to the radio shack down the driveway to fetch my recently acquired Johnson Matchbox and put it in-line. Now the measurements could begin starting with a sweep of the bands using my MFJ-259, straight up, that is without tuner or balun. Results were, let's say, mixed. Next, I tried with the Matchbox in-line. Better, but before I could finish the sweep the Tuning knob on the tuner stripped. OK, set aside the Matchbox, repair it another day. Back down to the shack to fetch my trusty "Blue Tuner", made by G3VKM and so named for it's light blue hammer coat finish. It's an L-C setup made for an end-fed antenna, but it often does the trick.


 Now we were getting somewhere! I was able to tune up every band from 40 through 10, though 30 and 15 seemed dicey (5:1 and 4:1 after tuning). The interesting part about the sweep using the G3VKM was that I hardly touched the capacitor adjustment, just the inductor needed much fiddling and very little at that. So, nearly all the reactance in the antenna is apparently capacitive.

Finally, I turned all the testing over to the venerable K2 ATU, which does a fine job with just about anything I can throw at it. And after that I dug out a 4:1 voltage balun, put it in-line, and then into the K2 ATU. Things were looking up. All these measurements get scribbled down on scraps of paper and you can see the end result above. Even though the MFJ showed 80-30 meters off the chart, the K2 brought them down to 1:1 SWR. Those bands are likely very low impedance, hence even higher SWR courtesy of the balun. I can live without those, actually. The remainder of the bands tuned up very nicely, no strange buzzing in the computer speakers or locking up of the K2, etc., so job well done. 


Finally, I wanted to put in a little lightning protection on that feed-line and my partial solution for that is pictured above. The wider feed-line (450 ohm) comes from the EDZ into the middle of that honkin' big DPDT electrical switch. Out the top the line runs to the station, the bottom connection to a ground wire, which runs another 15 feet to a ground rod.

WA9JBR was my first contact out after everything was hooked up on BPSK31, 20 meters. The antenna seems to be working well, though I haven't yet tried all the bands it can manage. It certainly, now, can manage a few more bands than when it was a straight EDZ. Even at the highest SWR, on 15 meters of 7:1, that 40 feet of line is only going to induce .415 dB of signal loss. I can't argue with that. When/if I make some 600 ohm open line the losses will be insignificant. Let me just close by adding a few other, often overlooked, advantages to window line vs. coax besides low loss in the face of high SWR: lower cost (compared to lower loss coax), much lighter weight, and damn easy to splice! 

cul es vy 73,  - Casey, TI2/NA7U

1 comment:

  1. Very interesting antenna Casey and it was a good idea to run the threaded rod when your XYL was out and about. Do give us more reports on the antenna works once you have a chance to give it a full workout.

    ReplyDelete

Thanks very much for your comment! 73, Casey

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