Or bandwidth of course… Let me start be saying that I’m really not a very good technician. I passed the amateur radio tests mainly on logical rather than technical thinking. I’m no elmer by any means, I’m just trying to understand this stuff because I find it very interesting.
For some time I’ve been wondering how efficiently the HWEF auto transformer (1:49 UNUN) works from 3.5 to 28 MHz. To be honest, I had my doubts it did. I wanted to find out more about my probably-not-so-broadband-43-material-based autotransformers so I did some measuring on my homebrew transformers. I’ve recently also measured Steve’s (N4LQ) design using three stacked FT240-52 (a.k.a. FT240-K) toroids.
To start off, I connected my Palstar DL1500 dummy load. The screenshot on the right shows X (imaginary resistance) nicely around (a little under) 0 Ohms and R almost equal to |Z|. I connected the dummy with a 1.5 ft RG213 patch cable with PL256 connectors. Probably the cable adds some capacity, that would explain the imaginary resistance (2 to 7 Ohms from 3.5 – 32 MHz).
The R (real resistance) is around 60 Ohms at 3.5 MHz and slopes a little under 50 Ohms at 32 MHz. It might have to do with the use ‘n’ abuse of the dummy, I was used with an Acom 1011 amp, maybe the high power carbon resistor got a bit too hot now and the. Even though it runs a little out of specs, all seems normal.
As you can see, these numbers result in a nice low SWR from 3.5 to 32 MHz, better than 1:1.2:To test my transformers, I connected a carbon composite (the brown non inductive/capacitive type) resistor of 1k5 and 1k Ohms in series from the transformers antenna feedpoint to ground and did a scan with my RigExpert AA-54 analyser.
I did similar measurements on four autotransformers terminated with the ideal half wave; 2k5 Ohm carbon composite resistor. The transformer was connected directly to the RigExpert AA-54 using a PL256 connetor with a minimal length of RG58. The transformers were measured from 3.5 – 32 MHz. I measured:
- Single FT240-43 with 2:14 ratio
- Double FT240-43 (stacked) with 2:14 ratio
- Double FT240-43 (stacked|) with 3:21 ratio
- Triple FT240-52 (stacked) with 2:14 ratio (N4LQ design)
As you can see, the R (true resistance) graph is relatively flat and around 50 Ohms (good!). The X (imaginary resistance) is relatively low varying from 0 – 25 Ohms with the optimum (X=0) a little over 14 MHz. The ideal transformer should show minimum X=0 and because of the 1:49 impedance transformation a little over 50 Ohms (2500/49=51,02). This would result in a 0 degree phase (red line) en optimal SWR (blue line). Looking at the SWR graph you can draw the conclusion this tranformer really is broadband with an optimum from 10 – 14 MHz. Will it work on other bands? Yes, but on higher or lower frequencies it will be less efficiency. In practice, this would make a nice 40m – 10m transformer with the optimum on 20m, working relatively well 40m and you will probably get 10m as a bonus. When 10m is open, mostly you can work dx with a nitting needle so a little less efficiency is not a problem. Oh, and yes, I reckon it will work on 80m as well, it’s very difficult to make a non radiating antenna HI.
Double FT240-43 with 2:14 turn ratio
(click on the pictures for close up)
Interesting to see the different curves. Because of the stacked toroids, this transformer has double the input and output impedance. X=0 between 7 – 10 MHz but R is around 40 Ohms so the SWR will be better on low frequencies. While this is not a real antenne, the question is; where does it perform best considering what it’s supposed to do. It’s supposed to show R=51 and X=0. Looking at the X, you’d say the optimum is between 7-10 MHz, looking at the X you’d say the optimum frequency is a little lower. I think it’s fair to say this transformer seems optimal for 40 and will work on 80m and 20m as well. As the X goes up from 14 MHz and up, it’s probably not the best choice if you want 15m and 10m. Note that this transformer resembles the transformer I currently have in use; it’s a single FT240-43 with a single (smaller) FT140-43 inside.
Again totally different curves. X a little over 0 at 3.5 MHz where R is around 50 Ohms. No doubt this transformer has it’s optimum on 80m. The phase and SWR graphs confirm this; phase is lowest at 3.5 MHz and that’s where the optimum SWR is as well obviously. This transformer is suited most for 80m, but will work on 40m and probably on 160m as well. Let’s say not suited for 14 MHz and higher frequencies.
Triple FT240-52 with 2:14 turn ratio (Steve Ellington N4LQ)
(click on the pictures for close up)
I was very curious about this design. I read about this transformer design in a Facebook group called “End Fed Half Wave Antennas”. As I was also looking at the 52 material rather than the 43 material for my 10/15 vertical endfed, it caught my attention. In the opinion of many, it is the optimal broadband tranformer for 80m – 10m. My measurements however seem to indicate this transformer has a limited working range looking at the reactance that’s introduced below and above the optimum.
The primary and secundary inductance measure 4.3 uH and 156.6 uH (will do similar measurements for my own transformers). Looking at the X, the optimum (close to 0) is a little under 10 MHz and rising quick on both lower and higher frequencies. The R of 50 Ohms a little under 10 MHz seems to confirm the optimum as well as the phase and SWR graph. The ideal transformer would show graphs that look like the dummyload graphs (no imaginary resistance / reactance, just real resistance). It seems like the single FT240-43 with 2:14 ratio still seems to have the highest working range looking at the reactance of the system. I recently learned that the 43 material results in lower Q so more bandwidth but also more core loss. The 52 material has higher Q so bandwidth will be less but efficiency will be higher. The optimum of this transformer is a little under 10 MHz. I’m sure it will work fine on 40m and it might as well do so on 20m but I seriously doubt it works well on lower and higher frequencies. Using this transformer involves all kinds of tricks to get the SWR down on 80m (using a loading coil at 2 meters from the feedpoint) and higher bands (using a capacitor in the middle of the wire). I don’t know… I believe the SWR will be fine on all bands, but that doesn’t make it a good radiator per se.
I do notice a lot of fellow hams are looking at SWR only when testing their antennas. Very bad antennas can have low SWR for all kinds of reasons. Of course you can compensate the transformer’s inductive or capacitive behavior with caps and coils, but to me that sounds like a snake oil solution. A low SWR does not mean the antenna works, it means the sytem (including feedline and transormer) dissipates the power you put into the input. Whether that’s a good thing is totally depending on whether it transforms electrical energy into magnetic energy. An SWR of 1:1 is not a garantuee for that even though many people seem to think so.
I will measure inductance of all the transformers I have lying around in the shack. I’m curious if the measured inductance is close to their calculated values. I’m using the calculators from http://www.kitsandparts.com e.g. Kits & Parts FT240-43 or Kits & Parts FT240-52. Note that by stacking two cores, the inductance will be double and doubling the number of turns will square the inductance.
Two turns on a single FT240-42 toroid has a calculated inductance of 4.3 uH, so two turns on two stacked toroids will be around 8.6 uH. When you use the calculator with freq. of 3.5 MHz and inductance of 8.6 uH (all other fields blanked out) you will find that you would need 2.8 turns on a single FT240-43. The typical impedance on this frequency is 189 Ohms. For 40m, the 8.6 uH will present an impedance of 378 Ohms. While the transformer has to transfer energy rather than dissipate it, the typical impedance of the primary winding should not be 50 Ohms but higher so I’ve been told by my elmer Egbert (PA0EJH) and some others. According to the Elmers the optimum impedance for power tranformation, would be around 200 Ohms so in theory the 2:14 ratio transformer two FT240-43’s should be optimal for a frequency in between 3.5 and 7 MHz. It’s not exactly what I measured, but it is fairly close.
Now let’s calculate the inductance of 3 turns on a single FT240-43 (9.7 uH) and double that because of double toroids (19.4). When I calculate the impedance for 80m using the same online calculator I find 427 Ohms at 3.5 MHz indicating the transformer will probably work on 160m as well. More about this in another article, I’m getting closer and closer to the truth HI. I did some calculating on the N4LQ design (triple FT240-52 with 2:14 ratio). The inductance for 2 turns on a single FT240-52 is 1.32 uH so 2.96 for the three toroids. The calculated impedance of 3.96 uH on 3.5 MHz is 87.1, too low according to the elmers. On 7 MHz it’s 174 MHz, close to the elmer’s optimum. This seems to confirm the N4LQ design’s optimum is over 7 MHz (the XR measurements above confirm this).
This story is continued in my article HWEF: what toroid(s) to use?