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 thinking rather than technical knowledge. There… I’ve said it. I’m no elmer by any means, actually the more I learn, the more I realise there’s much much more to learn.
Having said that, I want to share some thoughts with you.
For some time I’ve been wondering how efficiently a HWEF transformer performs from 3.5 to 28 MHz. Is it really possible to make a broadband transformer that efficienty works from 3.5 MHz to 28.5 MHz? Some people claim it is because they get good VSWR on all bands. In reality, it’s not that simple.
I decided to do some measuring on the transformers I made in the past. I’ve also recently made the N4LQ (Steve Ellington) design using three stacked FT240-52 toroids with a 2:14 ratio.
To start things off, I connected my Palstar DL1500 dummy load to the RigExpert AA-54. The screenshot on the right shows X (imaginary resistance) nicely around 0 Ohms and R almost equal to |Z|. I connected the dummy with a 1.5 ft RG213 patch cable with PL256 connectors that might introduce some imaginary resistance.
As you can see we got a nice low VSWR from 3.5 to 32 MHz, better than 1:1.2. To test the 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. Of course an antenna wire behaves differently than a resistor so the measurements isolate a lot of “real life factors”. Nevertheless, it is interesting to see how good (or bad) a transformer performs (actually, transforms) an ideal load of 2500 Ohms to 50 Ohms because that’s a first requirement I reckon. Alle 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 (Ellington design)
Single FT240-43 with 2:14 turn ratio
(click on the pictures for close up)
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. 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 you might expect a higher VSWR.
Using the Calculate ferrite cored inductor (from Al) page from Owen Duffy learns us that even though the insertion VSWR is quiet good, the efficiency of this transformer is not. The losses are substantial: 2.3 dB on 80m (41%), 1.9 dB on 40m (36%), 1.6 dB on 20m (31%) and 15m and 1.5 dB on 10m (29%). This transformer will do reasonably well on 40 – 10 meters (optimum on 20m) but will turn 30% of your power into heat. This type of transformer was used in the 20 meter long and the 12 meter long shortened HWEF’s that started the hype back in 2011/2012. For very little money you can make yourself a practical antenna that has reasonable performance on 40/20/10 meters. For holidays, I would still use it, maybe with the bigger FT240-43 toroid or with a 3:21 ratio for a little better efficiency and better performance on lower frequencies.
Double FT240-43 with 2:14 turn ratio
(click on the pictures for close up)
Because of the stacked toroids, the input and output impedance is doubled so this transformer will work a little better on lower frequencies. X = 0 around 9 MHz and looking at the VSWR graph the optimum insertion VSWR is around 7 MHz. All graphs are less flat than the previous ones so you a sensible bandwidth would be between 3.5 – 10 MHz. Will it work on 14 MHz? Yes but VSWR will be higher but this probably can be compensated with a capacitor on the input.
I’ve used Owen Duffy’s calculator again with double Al value (2150 instead of 1075). While the shape of the toroid configuration is different, it’s not entirely accurate but it gives a good ballpark figure. The calculated loss in this transformer is 0.8 dB on 80m (17%), 0.7 dB on 40m (15%) and 0.6 dB on 20, 15 and 10 meters. It’s better than the single core transformer and in practice, it can handle around 400 Watts PEP (SSB phone). I’ve used a similar configuration with my Acom 1011 amplifier up to 600 Watts and never had any problems. This transformer resembles the transformer I currently use; a single FT240-43 with a single (smaller) FT140-43 inside and 2:14 ratio.
Double FT240-43 with 3:21 turn ratio
(click on the pictures for close up)
Because of the higher inductance, the insertion VSWR is better on lower frequencies. Again the graphs are less flat than the previous ones. X is a little over 0 at 3.5 MHz where R is around 50 Ohms and that’s where the VSWR is best. I’ve used this transformer succesfully on 80m and 40m with 400 Watts PEP (SSB phone). Note that it will probably work on 160m as well. Probably not so good for 14 MHz and higher frequencies. The calculated loss is 0.3 dB (8%) on 80 – 15 meters and theoretically 0.2 dB on 10m (probably needs a lot of compensation to get the VSWR down). A good transformer with good efficiency for 80 and 40 meters that will probably work efficiently on 160m as well.
Triple FT240-52 with 2:14 turn ratio (N4LQ Ellington design)
(click on the pictures for close up)
This is the Steve Ellington design that has many followers in the the Facebook group “End Fed Half Wave Antennas”. Looking at the X ( = 0) and R ( = 50), the optimum is around 9 MHz. Insertion VSWR on 40m and 20m is acceptable but not so on 80m and 15/10 meters. In practice, many users obtain good VSWR on all bands from 80 – 10 meters but the system requires a “compensation coil” for 28 MHz at 2 meters from the feedpoint and a capacitor in the middle of the aerial to get the resonance frequency on 80m down. The 52 material has significantly lower losses compared to the 43 material: 0.1 dB (1%) on 80m, 0.2 dB (5%) on 40m, 0.4 dB (9%) on 20m, 15m and 10m. Note that I’ve simply tripled the Al value of this core assembly (so used 990 instead of 330). Again this is not entirely accurate but give a good ballpark figure. The calculations predict a very efficient tranformer, but the high insertion VSWR worries me. The 52 material has lower permeability and higher Q so losses will be lower but bandwidth is limited. It seems that the inductance is too low for 80m and even a little low for 40m. Of course it is possible to compensate a transformer’s inductive or capacitive behavior with caps and coils to compensate for the imaginary part but I don’t believe that will improve the performance, it only allows the system (feedline, transformer, aerial wire) to dissipate the power. 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 tend to think so. In practice, VSWR with a wire connected is good from 80 – 10 meters and users of this transformer in the mentioned Facebook group confirm it will handle 1 kW PEP (SSB phone) without a problem.
I will measure and do calculations on more transformers in the future. Because I’m using a dedicated aerial for 80 and 40 meters and one (probably vertical) dedicated for 20, 15 and 10 meters I have to luxury that I don’t need full bandwidth. For 80/40 I’m leaning towards my old transformer that has two stacked FT240-42 cores with 3:21 ratio. The calculated loss of 0.6 dB is reasonable and it has enough magnetising inductance for those frequencies. For 20/15/10 I’m considering two stacked FT240-52’s or the smaller FT140-52’s with 3:21 ratio OR the assembly made with 2 x FT240-52’s with 4 x FT82-52 inside (total Al value 1170) with 2:14 ratio. All these configurations will have a calculated loss of around 0.3 dB (7%).
I’ll keep you updated!\
73 de PA3HHO (Pleun)
PA3HHO's 