AIM 4170C to horizontal loop fed by ladder line?
On the recommendation of this forum i have bought myself an AIM 4170C antenna analyser. looks a nice bit of kit I am new to amateur radio and have an immediate question. At the moment i have a 25 feet high horizontal quad loop of about 490 feet in length. It feeds off a corner into my bungalow through an upper window, via 450 ohm (I think...) one inch wide ladder line. This goes straight to the two screw terminals on the back of my Yaesu FC-902 ATU, and the ATU then goes to my FT-902DM transceiver. The analyser has a SO-239 socket on it for connection to the DTU. How should I be connecting it to my antenna system? I think I may need something like a 4 ; 1 balun at the house end of the ladder line, then connect the analyser via a short piece of co-ax to the balun, which in turn is connected to either "side" of the loop antenna ladder line? Or have I got this wrong? Is there an easy way to connect the ladder line so it terminates in a PL-259 plug, or is this an incorrect way of measuring things? I wiah to use the loop on HF multi band and in particular see if it can be optimised for either 160 metres or for PSK31 digi modes on either 20 metres or 17 metres. At the moment the AIM 4170C is probably 100 times more complicated than I am comfortable with, but I thought buying something really basic may be false economy in the longer term, as I feel I could get quite into this hobby Thanks.
I also bought that analyzer recently and I also want to measure ladder line loss using it! But I went a bit far ahead - I've took a piece of ladder line, about 6 meters long, connected it to analyzer via short (70cm) RG58 cable and run several sweeps, with ladder line far end open, closed, terminated with 50 or 450 ohm resistor. I got a lot of nicely drawn curves, which appear to deliver nothing, since AIM refers to 50ohm as base resistance, so it need to changed to 450 ohm somehow.
Regarding your question, since you're measuring complete antenna system, measure it in way it is connected to transmitter, since you want to determine transmitter-antenna matching conditions, aren't you?
I don't think your antenna setup is 50 ohms on ANY frequency.
I have put up several horizontal loops but used a quarterwave of 75 ohm line to transform Z down to 50 ohms. (this ONLY works on one frequency) where the loop is one wavelength in circumference on a single band ! (they worked fine without any tuner).
You could hook up your analyzer to the radio end of the tuner but that only tells you the tuner is adjusted and nothing about the actual antenna radiator !
I have resisted using balanced line feeders until I built an experimental DZ/XDZ antenna for some bidirectional gain on 30/20M last fall
.I Had to buy the new MFJ balanced line tuner for it. The directional wattmeter in this tuner measures the SWR on the 50 ohm unbalanced side so you never actually know or worry about the SWR on the balanced feeders to the antenna radiator !
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I also have the need to measure the impedance of balanced line loads. I made a coil of RG178 on a coil form (neatly wound). With a variable cap connected from the braid at one end of the coil to the braid at the other end, I tune this parallel circuit to resonance at the freq where I am measuring the impedance. This creates a high choking impedance balun where at resonance the Z can be about 75K because the RG178 on the form has a Q of over 200. Knowing the electrical length of the coax on the coil, I can measure the Z at the input end of a balanced line antenna system with an unbalance impedance measuring instrument. In my case the electrical length of the coax is about 4.7 meters so I can rotate around the Smith Chart with this length converted to wavelengths for the freq of measurement. I never had a reason to measure the loss of open wire or other balanced lines. If the line is in good shape, the specs are well known and in the handbooks. 73, Pete
I do a lot of measurements of balanced line fed antenna with my AIM 4170C. The stock recommendation from Array Solutions and others is to use a 1:1 balun at the analyzer and do a custom calibration using the standard loads (short, open, and a known pure resistance - often 50 ohms). The custom calibration is supposed to factor out any impedance shifts caused by the balun. My tests with a couple of well-regarded baluns (DXEngineering designed by W8JI and Balun Designs) have shown that these baluns often transform the impedance to very low values on the input when the impedance on the output is high. This makes sense because they are just low impedance (~50 ohm) coax wrapped around cores. The length of coax in the balun transforms impedance when the load isn't 50 ohms resistive.
However, I've found that I can get good readings of balanced line antenna systems without the balun, if I am careful not to ground any of the measuring equipment and use short leads. Specifically, I use a laptop on batteries and power the AIM with a battery pack or 9V battery. I use two adapters on the back of the AIM 4170C - a PL-259 to BNC adapter and a BNC to binding posts. The balanced line attaches to the binding posts. I did a custom calibration of the adapter combination. This setup has worked very well for me in determining the best feed line lengths to use for minimum SWR (50 ohms) on various bands.
That said, I still use a 1:1 current balun between my balanced line and my antenna tuner after I get my system tuned. I then use the AIM 4170C with the balun to plot the impedances that will be seen by the tuner (or directly by the transceiver if the SWR(50) is low).
I would recommend that you add a 1:1 current balun between your tuner and the balanced line. This should help minimize the common mode currents that can cause RFI and high noise levels on receive. Don't be fooled by the idea that 450 ohm ladder line needs a 9:1 balun to give you 50 ohms. The impedance at the shack end of the ladder line is almost never 450 ohms or even near it.
I think you will find the AIM 4170C is a good investment and will serve you well for many years. It just takes a while to learn how to use it to best advantage. There is a forum devoted to the AIM and similar analyzers. I believe Array Solutions has a link to it. If you can't find it, email me at my call sign at arrl.net and I'll send you the link.
If the 1:1 coax wound current balun you are using is transforming hi Z's to low Z's its because the electrical length of coax is a quarter wave or odd multiples of quarter waves long. It is not just because the balun is wound with 50 ohm coax. My single frequency parallel resonant balun uses 50 ohm coax and is close to a quarter wave long on 20 meters (elect length is 4.7 M). The Hi Z's I encounter are transformed down so low on 20 meters that my old impedance instruments can not read the Z. If I want to run tests at Hi Z's on 20 meters I will have to reduce the electrical length of my coax coil or go the other way and add some more cable on the impedance instrument side of the coil. I have no easy way to calibrate (O-S-L) my set up so I use 50 ohm cable all the way to the measurement point ( I just have to know the electrical length and rotate around the chart). Even if you can calibrate out the transmission line I bet it is still tough to get an accurate measurement at at Hi Zs thru a typical 1:1 balun when the choking impedance is only 1000 to 2000 ohms plus the network analyzer is designed around 50 ohm directional couplers (internal to the instrument). 73 and have fun tinkering and learning. Pete
I agree that the most dramatic impedance transformations of mostly resistive impedances are with a quarter wavelength of coax (e.g. 2000 ohms is SWR(50) = 40:1 yielding 50/40 = 1.25 ohms at the other end of the coax). However, anytime the electrical length not an integral multiple of half wavelength, there will be some transformation. When the SWR on the line is high (as it often is for the coax in the balun with a multi-band doublet), the transformations are amplified. I'll try to remember to post some example graphs a little later from the AIM 4170C, with and without the balun, to illustrate this.
The 4170C does have the ability to calculate the impedance at the load when measuring at the transmitter end of a feed line, if you calibrate the feed line with the analyzer first. I think it will let you do it by just entering the length and velocity factor, too, but I can't remember for sure.
The Smith Chart works less well for ladder line than coax, in my experience. I think it's because the impedance is not uniform along the ladder line. Some of the dielectric includes air (the windowed part) and some is solid.
I strive to find balanced line lengths that produce low enough SWR(50) < 10:1 on the bands of interest to allow using my K3's internal tuner. Currently, I have a 67' doublet that achieves this on 40, 30, and 20 meters with one feed line length and on 17, 15, and 10 meters by adding 7.5'. I can add or subtract the 7.5' extension in less than 2 minutes. Similarly, my perpendicular 34' doublet covers all 6 bands from 30 - 10 meters with just two feed line lengths.
I promised to post some AIM 4170C graphs of the balun impedance transformation. All of the graphs are for a 34' rotatable dipole fed with 60' of 300 ohm ladder line. The first one is without a balun:
The SWR is in red, the resistive part of the impedance (R) is in orange, and the reactive part of the impedance (X) is in green. Notice that R never drops below 50 ohms (minimum of 50.6 ohms at about 24.6) MHz and hits a maximum of just under 1500 ohms around 11.670 MHz.
When a high quality balun (DXEngineering BAL050-H10-AT, a W8JI design) is added, the plots look like this:
Notice that R remains below 100 ohms on all frequencies above 10.2 MHz and is below 10 ohms across about half the spectrum (minimum is 2.9 ohms around 28 MHz). The plots for the Balun Designs 1115t are similar:
The 1115t does generally provide higher values of R (minimum of 4.4 ohms at 28.4 MHz, over 200 ohms at 14.75 MHz) than the BAL050-H10-AT, but at the expense of more X yielding slightly higher SWR(50) values over most of the spectrum.
Tuners tend to be lossy matching low R values. I imagine a 4:1 balun would transform the R part of the impedance even lower, making it very difficult for the tuner.
In theory, the 50 ohm coax in the baluns should not change the SWR(50), other than lowering it if there are significant losses. However, both baluns actually raise the SWR(50) over most of the spectrum. Here are the SWR plots for the 3 configurations on one graph:
The lower orange plot is with no balun, the red plot is with the DXEngineering balun, and the upper orange plot is with the Balun Designs balun. Notice that all three show SWR minimums at the same frequencies and their values match closely anytime the SWR is less than 5:1.
These measurements, and similar ones with other, multi-band doublets have led me to choose feed line lengths that provide relatively low SWR(50) on the balun output. The goal is to avoid very low impedances (R < 10 ohms) to the tuner. Generally, there are lengths that will do this on multiple bands. I have been able to cover 6 bands with just a couple of feed line lengths. For example, the 60' ladder line used in the system plotted above with the Balun Designs 1115t provides SWR < 5:1 on 20, 17, and 12 meters. By subtracting 8', I get low SWR on 30, 15, and 10 meters.
One other thought: there has been convincing evidence from well-respected sources (e.g. W7EL & W8JI) that putting a balun at the input of a tuner subjects it to no less common mode current stress than putting it on the output. I believe that. However, it does seem baluns on the output of a tuner often transform the impedance from a value the tuner can handle easily to a very low impedance that will strain the tuner. I don't advocate moving the balun to the tuner input. That requires floating the tuner above ground creating problems that VK1OD has documented. I do think it is worthwhile to try to find feed line lengths that avoid a low impedance on tuner output / balun input junction.
Just to be clear: that is true if the tuner is unbalanced - it's not true if the tuner is balanced.
Originally Posted by AD4J
Agreed. With a balanced tuner, the only reasonable place for the balun is on the input. Thanks, Steve.
Originally Posted by G3TXQ