Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Aug 22, 2018.
Once you decide on a particular length, it is no longer a random length.
Ok, that moves the simulation closer to the measured results.
I measured the Vf, but not the Zo. I suspended a 40ft piece of WireMan 552 vertically, and used the AA-600 to determine its electrical length. Vf came out 0.92.
I am using the data provided by Dan in this recent post.
What a waste of space. Doesn't answer my very specific question.
Because I don't want to be a knob twiddler! Because I dont like twinlead or ladderline, except outdoors.
Note that in that thread, the table lists two nominal Z0 values of both 380 and 400. That's a considerable range of uncertainty that may result in some variation in the required length to match the impedance.
I have a very specific requirement for a coax-fed, symmetric (no OCFs) multi-band antenna that can be supported on a single 40ft support as an inverted-V, whose legs lean to one side of the support mast. I will feed it (most of the time) from an SB220 (PiNet output), so I would like it to work without a tuner, especially on 40m and 20m. Casual use on 75m, 17m, 10m and 6m with a 100W rig and a tuner would be a plus.
I have previously built two different ZS8BKW antennas. The dimensions came from various published articles. They worked really well on 40m and 20m, but they missed the mark on the other bands. I assumed that some of the reason for that is that I deployed them as an invertedV, at a different height than the originals.
This time, I knew the geometry of how they would be erected with some precision. I knew the characteristics of the LadderLine I am using. I also had seen Cecil's (W5DXP) trick of adding a capacitor in series with the coax feed line to improve the SWR on 75m.
I also have AC6LA's AutoEZ, so I used it to optimize the length of the LadderLine (used as an impedance matching device, not to feed the antenna) and the lengths of the antenna wires. All other aspects of the antenna design remaining constant (Apex Height, Apex angle, Out-of-plane angle, LadderLine Velocity Factor, LadderLine Impedance, value of the series capacitor), I was using the optimizer to find the optimum Wire leg lengths and LadderLine length to get the SWR50 as low as possible on several HF bands simultaneously. I got a solution, built it, and measured it...
Basically, I am repeating the process that Brian Austin did when he invented the ZS6BKW antenna, although at the time he did it, his computer modeling was adhoc. He didn't have (or use NEC). He did essentially the same thing: find an overall length, and the length of the matching section after assuming height, VF and Zo.
My motivation: learn how to use the optimizer in AutoEZ, and come away with a working antenna specific to my explicit needs.
Along the way, I learned to guide the optimization process so that the solution favors my favorite frequencies, with more weight being given to 40m and 20m. I made a separate optimization on 75m, to determine the best value for the series capacitor.
I have a standing 4" o.d. 40ft 6" aluminum pole in my backyard, as shown on my QRZ page. It normally holds up one corner of my 160m Horiz Delta Loop. For testing the ZS6BKW antenna, I lowered the loop, got it out of the way, and used the pulley at the top of the pole to pull up the prototype ZS6BKW antenna, as shown here:
click for larger.
Here are the measured SWR plots of the actual antenna:
Note that 75m will need some help from a tuner. I will be feeding the antenna with about 72ft of foam dielectric RG8 (dont have the number here), so I am not very worried about coax losses. The SWR is good enough on the other bands (with the exception of 28.5MHz) that this will work for me.
The final overall wire length used in both the model and the actual antenna is 92.8ft. Per the suggestion in post#3, I re-ran the model with a Ladder-line length of 40.66ft, and here is the modeled SWR for 40m. Compare that to the actual plot, above.
Hope this shows the power of modeling to some of the skeptics.
The take away for me: I'll continue to model every antenna I build. I usually am very methodical, and make the model reflect the reality as closely as possible (for example, the inclusion of the 40ft metal pole in the model does not effect the results). The Optimizer in AutoEz is killer. I used to use it in Mmana, and missed it greatly in naked EzNec.
Take away for others: The configuration of the antenna effects the optimum lengths L and T for just my configuration; those lengths may not be optimum for yours.
This shows the parameters used in the model. The 2ft RG8 coax M is to account for the jumper between the AA-600 and the test antenna.
Sometimes saying less is more.
BTW. Interesting QRZ page. That said, I live in Manhattan Beach, near El Camino Community College. It has never offered an engineering degree. Half, quarter or otherwise.
Brian - K6BRN
Here's another little trick that will cure your 10m problem. Add a ~1 foot parallel 300 ohm ladder line stub ~5 feet up from the coax/ladder-line junction. That will move the 10m resonant frequency down a bit without appreciably affecting the other frequencies. Like the series capacitor has less effect as frequency is increased, a parallel capacitance has less effect as frequency is decreased. And of course, the parallel stub can be replaced by a very small capacitor. I used a stub because at approximately one foot long, it is self-supporting.
Fair enough. You would only have 1 knob though. Put it outdoors where you intend to put the birds nest. Once you learn how to use the thing band changes are just as fast as what you're trying to do now. I actuall do this so Im not just blowing smoke. Once you've marked the coil tap locations and cap setting for each band it's easy.
You will have the ability to get a perfect match on all bands. Your outdoor only ladder line can be whatever length you need to reach the tuner without having to deal with a specific length and the antenna can be as long as needed or as long as your property will allow.
The limiting factor on the balanced L is the capacitor you use. If the voltage rating is high enough you don't have to worry about what ladder line lengths to avoid. It will handle almost anything. You'll never go back to a store bought tuner for balanced line again.
I'm also a big fan of modeling, and it's very satisfying to see everything work just as the laws of physics predict.
I just put up an inverted V doublet with homebrew ladder line, also on a 40' pole (the ends are 20' high, and the length is 70', the limits of my small yard.) I modeled the antenna in CocoaNEC, and wrote my own code to model the ladder line across all bands. I even measured and modeled the effect of the 1:1 balun---it's bifilar #14 THHN, with a Z0 of 90-100Ω, which rotates 60-j25Ω into 55Ω resistive.
When I erected it, I saw impedances that matched the model, and the effect of the balun was bang-on what I predicted; I was able to trim the line as expected to be tuner-free on 20m. However, the exact length was easily a few percentage points off, and in the end the exact tuning was ad-hoc. I measured the impedance with every few inches I trimmed and everything fell into place perfectly, but I still had to trim inch-by-inch.