unun

[Crowling]

Im building a 49:1 unun using 14 gauge copper magnet wire. Fairite cores from mouser with fairite p# 5943003801 and mouser p# 623-5943003801. Im using 1 capacitor value 100pf@20kv. My question is can i stack more than 3 cores ?? I see video's using 3 but never more than 3. Can i stack 4 or 5 and use the same winding configuration? Thanks Craig KY4PI

 

[AC9KH]

You can stack up as many cores as you want. Typically three will handle a kilowatt on 80m without saturating the cores using a 43 mix. So use a little common sense as to what you actually need. I personally recommend against using the bifilar primary winding that a lot of the tutorials show, especially if you're going to use an amplifier. There's many thousands of volts on the secondary if you're using power and keeping the primary and secondary windings isolated will prevent arc-thru on that bifilar winding. I'd also use a three-turn primary instead of two-turn like many tutorials show. With a 21-turn secondary it's easier to add or subtract a winding on the secondary to get the desired SWR match. One less or one extra winding gives you a finer adjustment vs the "classic" 2/14 turn transformer, and also makes better use of the magnetic properties of the core.

 

[Crowling]

You can stack up as many cores as you want. Typically three will handle a kilowatt on 80m without saturating the cores using a 43 mix. So use a little common sense as to what you actually need. I personally recommend against using the bifilar primary winding that a lot of the tutorials show, especially if you're going to use an amplifier. There's many thousands of volts on the secondary if you're using power and keeping the primary and secondary windings isolated will prevent arc-thru on that bifilar winding. I'd also use a three-turn primary instead of two-turn like many tutorials show. With a 21-turn secondary it's easier to add or subtract a winding on the secondary to get the desired SWR match. One less or one extra winding gives you a finer adjustment vs the "classic" 2/14 turn transformer, and also makes better use of the magnetic properties of the core.

Thank you Mr. Olson for the reply. I asked because a 4 core adds more copper to the equation and was unsure if that changed the formula. I found using 3 primary windings is a must. I could not get a 2 primary to work.Even on a single core. I’m using a 2450 ohm 3 watt resistor as a dummy load for testing. I then test with resistor at the end of the feedline run. Then with antenna connected to transformer slightly above ground. No matter which configuration of the 49:1 I build it never works like the videos online !!!! I’ve used the mfj-259, a Rigexpert AA-35 and have a nano vna-4h arriving this week. Swr’s are way above what the online builders achieve. Would it be ok to attach photos here? I’m new here. Sincerely Craig KY4PI

Thank you Mr. Olson for the reply. I asked because a 4 core adds more copper to the equation and was unsure if that changed the formula. I found using 3 primary windings is a must. I could not get a 2 primary to work.Even on a single core. I’m using a 2450 ohm 3 watt resistor as a dummy load for testing. I then test with resistor at the end of the feedline run. Then with antenna connected to transformer slightly above ground. No matter which configuration of the 49:1 I build it never works like the videos online !!!! I’ve used the mfj-259, a Rigexpert AA-35 and have a nano vna-4h arriving this week. Swr’s are way above what the online builders achieve. Would it be ok to attach photos here? I’m new here. Sincerely Craig KY4PI

Ps I’m using 130 feet of #14 stranded wire for radiator. 100 feet of Rg-8 flex coax and 100 PF 20 kv cap. Btw I have an old Ultramax 49:1 2 turn primary which with 100 PF that gives a great sweep of 3-30 mhz on the 2450 dummy load. Craig ky4pi

 

[AC9KH]

Thank you Mr. Olson for the reply. I asked because a 4 core adds more copper to the equation and was unsure if that changed the formula. I found using 3 primary windings is a must. I could not get a 2 primary to work.Even on a single core. I’m using a 2450 ohm 3 watt resistor as a dummy load for testing. I then test with resistor at the end of the feedline run. Then with antenna connected to transformer slightly above ground. No matter which configuration of the 49:1 I build it never works like the videos online !!!! I’ve used the mfj-259, a Rigexpert AA-35 and have a nano vna-4h arriving this week. Swr’s are way above what the online builders achieve. Would it be ok to attach photos here

Adding more cores will change the inductance of each coil a tiny amount, but it won't change the voltage step-up or impedance conversion ratio. What kind of ground are you using? An EFHW antenna does not need a counterpoise, but it does need a good earth ground or the feedline shield will become "hot" and radiate. You have two circuits to deal with; the primary is from the radio output thru the primary winding and the return path back to the radio is the shield of the coaxial feeder. The second circuit is a high-voltage on the secondary, one end of which feeds the radiator, the other going to earth ground.

There's several ways to wind a matching transformer. It can be wound as an autotransformer with a tapped winding. It can be wound as an isolation transformer where there is no electrical connection between the primary and secondary. It can be wound with a bifilar primary (the "classic book design") where the primary is twisted together with the start of the secondary winding, this introduces capacitive coupling of the primary and secondary. It can be wound as a reverse polarity transformer, which if you look at my qrz page is what I'm using because I'm switching it with a relay to convert the antenna from a 80m EFHW to a 1/4 wave Marconi on 160, which requires an extensive groundplane radial system. You can also eliminate the capacitor across the primary. The only reason to stick a capacitor in there is to lower the SWR on 10m - again, look at my qrz page and you'll see I don't use one.

Testing it with a 2450 resistor is not a good way to test it because that is a purely resistive load, while an EFHW is a reactive load with complex impedance. How you position the wire (height above earth ground, vertical or horizontal, inverted V configuration, etc) will change its feedpoint impedance and will typically require a custom-wound transformer.

So what I would suggest is install the antenna and transformer where you intend it to be. Unhook the feedline and put a short piece of ~2ft coax on it and see what you get, tune it up accordingly on the fundamental. Now put your feedline back on it and see what you get in the shack. On the fundamental the length of the feedline shouldn't make any difference. But a feedline is also a transformer with a reactive load at odd 1/4 wave multiples and you may need to tune the length of the feedline to get the desired result on the harmonics. This is because the operating frequencies are not exact harmonics. The middle of the 80m band is 3,750 kHz. The second order harmonic of that is 7,500 kHz, which is not in the 40m band, and so on. If the antenna system is low-Q it will be broad-banded. If it is a high-Q design it's going to tune really sharp and a high-Q antenna has better radiation efficiency. A high-Q antenna will put out greater field strength than a low-Q at the same input power.

So instead of striving to build a low-Q antenna, build one that's high-Q on the fundamental and do what most other people who have run EFHW's for years do - put the antenna tuner on it for the other bands. That's what they make antenna tuners for - besides the radio it is the single most useful piece of equipment in the shack. If you don't own a tuner either build or buy one. If you have 4:1 SWR on 40m with an 80m EFHW, don't worry about it - just tune it and run with it. Any standing wave losses on the feeder at HF are so slow that you won't be able to measure them.

 

[Crowling]

Thanks again for the informative reply! I read your qrz page. Some of the information is admittedly overwhelming. I have some photos I’d like you to see but the server says it’s to large? When you reference a sharp cutoff maybe that’s what I’m getting on my attempts. These are with a resistive load of 2450-2460 ohms on output. Craig ky4pi

 

[Crowling]

I don’t see any email on your qrz page otherwise I’d send you a couple photos. Craig

 

[AC9KH]

I have some photos I’d like you to see but the server says it’s to large?

Full sized 10 or 12 megapixel photos will normally be too large for email or web. Do you have a photo editor of some sort that will downsize the photos so they can be posted?

 

[AC9KH]

When you reference a sharp cutoff maybe that’s what I’m getting on my attempts.

If you are getting a sharp cutoff on SWR as you sweep the frequencies, that's actually good assuming the antenna can be tuned for resonance on the fundamental with a decent SWR and still be tuned on the harmonics with a tuner. That's the way my 80m EFHW works - I'm using a reverse polarity matching transformer so the primary can be switched out without adding inductance to the secondary, which acts as a choke to isolate the ground and "hot" sides of the antenna on 160. I designed the antenna specifically for 75/80 and 160 only. Which is not likely your goal but it does demonstrate how a matching transformer can be wound different ways to achieve a desired result with an antenna that has a feedpoint impedance around 2,500 ohm on one band and 50 ohm pure resistive on another by simply switching the transformer configuration with a relay.

 

[Crowling]

I’ll try again

 

[prcguy]

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance. Its better to use a pair of FT-240-61 mix toroid's and for three stacked use FT-240-52 mix. A single FT-240-61 mix toroid works great down to 40m and I think better for the upper bands but I have not tested it on 80m.

I had a conversation with Danny at MyAntennas a few years back trying to get his ferrite recipe and the only thing he would say is he doesn't use 43 mix in his EFHW transformers.

 

[Crowling]

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance. Its better to use a pair of FT-240-61 mix toroid's and for three stacked use FT-240-52 mix.

Ok. I modeled my own using guidance from videos and articles using stacked 240-43. They all claim and show low swr sweep of the hf bands. Mine…well not so much.

 

[AC9KH]

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance.

Which can be actually desirable for the lower HF frequencies down to MF. As the frequency goes up you get a phase delay in the alignment of the magnetic fields. This permeability phase shift becomes evident at around 7,000 kHz with stacked -43 cores. At the lower frequencies the high inductive reactance of stacked -43's requires no real amount of power to establish core magnetization and there are no eddy current losses, making them highly efficient for 3,500-4,000 kHz. Not so great for much above 7,000 kHz, however, and unusable above 20 MHz. When building a high-power transformer for 80m with stacked -43's I figure around 8uH per core and leakage inductance will be lower with two or three stacked vs a single core.

But.....this all depends on how you wind the secondary. If it overlaps the primary, or is bifilar with the primary, it's going to increase the frequency response of the core and make it low-Q, which is undesirable if you're building a dedicated 75/80m antenna.

This "stigma", or whatever you want to call it, for EFHW's being an all-band antenna is somewhat skewed IMO. Yeah, you can build a transformer to make it work but that doesn't mean the antenna is "good" on the harmonics. It simply means it works but the radiation pattern is not all that great (as with any so-called "all-band" antenna). So I personally gave up on the EFHW being an "all-band" antenna years ago and started concentrating on transformer designs that make them more efficient on the fundamental. Put up a shorter antenna for the higher frequencies and you'll get better results.

 

[prcguy]

Which can be actually desirable for the lower HF frequencies down to MF. As the frequency goes up you get a phase delay in the alignment of the magnetic fields. This permeability phase shift becomes evident at around 7,000 kHz with stacked -43 cores. At the lower frequencies the high inductive reactance of stacked -43's requires no real amount of power to establish core magnetization and there are no eddy current losses, making them highly efficient for 3,500-4,000 kHz. Not so great for much above 7,000 kHz, however, and unusable above 20 MHz. When building a high-power transformer for 80m with stacked -43's I figure around 8uH per core and leakage inductance will be lower with two or three stacked vs a single core.

But.....this all depends on how you wind the secondary. If it overlaps the primary, or is bifilar with the primary, it's going to increase the frequency response of the core and make it low-Q, which is undesirable if you're building a dedicated 75/80m antenna.

This "stigma", or whatever you want to call it, for EFHW's being an all-band antenna is somewhat skewed IMO. Yeah, you can build a transformer to make it work but that doesn't mean the antenna is "good" on the harmonics. It simply means it works but the radiation pattern is not all that great (as with any so-called "all-band" antenna). So I personally gave up on the EFHW being an "all-band" antenna years ago and started concentrating on transformer designs that make them more efficient on the fundamental. Put up a shorter antenna for the higher frequencies and you'll get better results.

Yes, if your EFHW is for 160 through maybe 40m then three stacked 43 mix cores are the way to go. For most of us who want 80-10m and high power the recipe changes. On properly designed and made cores like from MyAntennas you can connect a pair back to back and the loss over the entire 80-10m range is incredibly low, so the resulting antenna can be very efficient. You will have lots of gain lobes and nulls on the higher bands which can seem like the antenna is not working well but it really is, just not in the direction you installed it.

 

[AC9KH]

You will have lots of gain lobes and nulls on the higher bands which can seem like the antenna is not working well but it really is, just not in the direction you installed it.

Yep, I prefer verticals which are a little more sensitive to radiation pattern and takeoff angles than horizontals are. A vertical off its fundamental radiates equally poor in all directions.

 

[Crowling]

Yes, if your EFHW is for 160 through maybe 40m then three stacked 43 mix cores are the way to go. For most of us who want 80-10m and high power the recipe changes. On properly designed and made cores like from MyAntennas you can connect a pair back to back and the loss over the entire 80-10m range is incredibly low, so the resulting antenna can be very efficient. You will have lots of gain lobes and nulls on the higher bands which can seem like the antenna is not working well but it really is, just not in the direction you installed it.

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance. Its better to use a pair of FT-240-61 mix toroid's and for three stacked use FT-240-52 mix. A single FT-240-61 mix toroid works great down to 40m and I think better for the upper bands but I have not tested it on 80m.

I had a conversation with Danny at MyAntennas a few years back trying to get his ferrite recipe and the only thing he would say is he doesn't use 43 mix in his EFHW transformers.

And Danny has the equipment to make perfectly form

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance. Its better to use a pair of FT-240-61 mix toroid's and for three stacked use FT-240-52 mix. A single FT-240-61 mix toroid works great down to 40m and I think better for the upper bands but I have not tested it on 80m.

I had a conversation with Danny at MyAntennas a few years back trying to get his ferrite recipe and the only thing he would say is he doesn't use 43 mix in his EFHW transformers.

I don’t have the equipment Danny has to produce perfectly shaped wondings around the cores so they look pretty bad lol.

The typical home made 49:1 or 64:1 uses a single FT-240-43 mix toroid that will handle up to about 400w SSB. You don't want to stack that mix as it adds up to too much inductance. Its better to use a pair of FT-240-61 mix toroid's and for three stacked use FT-240-52 mix. A single FT-240-61 mix toroid works great down to 40m and I think better for the upper bands but I have not tested it on 80m.

I had a conversation with Danny at MyAntennas a few years back trying to get his ferrite recipe and the only thing he would say is he doesn't use 43 mix in his

Which can be actually desirable for the lower HF frequencies down to MF. As the frequency goes up you get a phase delay in the alignment of the magnetic fields. This permeability phase shift becomes evident at around 7,000 kHz with stacked -43 cores. At the lower frequencies the high inductive reactance of stacked -43's requires no real amount of power to establish core magnetization and there are no eddy current losses, making them highly efficient for 3,500-4,000 kHz. Not so great for much above 7,000 kHz, however, and unusable above 20 MHz. When building a high-power transformer for 80m with stacked -43's I figure around 8uH per core and leakage inductance will be lower with two or three stacked vs a single core.

But.....this all depends on how you wind the secondary. If it overlaps the primary, or is bifilar with the primary, it's going to increase the frequency response of the core and make it low-Q, which is undesirable if you're building a dedicated 75/80m antenna.

This "stigma", or whatever you want to call it, for EFHW's being an all-band antenna is somewhat skewed IMO. Yeah, you can build a transformer to make it work but that doesn't mean the antenna is "good" on the harmonics. It simply means it works but the radiation pattern is not all that great (as with any so-called "all-band" antenna). So I personally gave up on the EFHW being an "all-band" antenna years ago and started concentrating on transformer designs that make them more efficient on the fundamental. Put up a shorter antenna for the higher frequencies and you'll get better results.

I called Mouser and asked if it was possible that they shipped the wrong ferrites. Was told to measure ID and OD and thickness. All 4 I ordered matched the FAIRITE company specs on Mouser’s website. However….. I have an identical core I salvaged from somewhere with the same OD and ID and thickness. One would think it’s a 240-43. I was reading how to identify unknown ferrite values and the site suggests using an ohm meter. Using the probe’s scratch gently into the core and read ohm meter. If it measures extremely high resistance it’s usable for hf or vhf purposes and reads in the upper k ohms or lower it’s not. Well my stray core reads and the 240-43 does not. So the theory of measurement in mm or inches does not fly with me. What say you all?? Craig ky4pi

 

[AC9KH]

That might be a way to test it since a good core shouldn't conduct current. But otherwise the only way I know of to test a core for suitability for a step transformer vs just being a choke core is to wind it, put it on the scope and measure the phase shift vs frequency thru the winding. That will give you an idea of what the frequency response of the core is.

 

[AC9KH]

Another way I just thought of to test a core, since you have some of known value, is to wind say a 3 turn winding on it. Measure the inductance of the winding on the known cores vs the unknown. That should give you an idea of what the cold permeability of the core is.

 

[Crowling]

That might be a way to test it since a good core shouldn't conduct current. But otherwise the only way I know of to test a core for suitability for a step transformer vs just being a choke core is to wind it, put it on the scope and measure the phase shift vs frequency thru the winding. That will give you an idea of what the frequency response of the core is.

I have a scope and did wind a 2 primary. I couldn’t tell a phase shift as I’m not an engineer. I have a nanovna coming today. A 4H version. I’ll test it on a Rigexpert AA-35 zoom and get a sweep. Meanwhile I wish I had a way to send a picture of my sweeps. Craig

 

[Crowling]

I have a scope and did wind a 2 primary. I couldn’t tell a phase shift as I’m not an engineer. I have a nanovna coming today. A 4H version. I’ll test it on a Rigexpert AA-35 zoom and get a sweep. Meanwhile I wish I had a way to send a picture of my sweeps. Craig

To clarify I wound the “mystery” core and will report back lol.

 

[AC9KH]

I have a scope and did wind a 2 primary. I couldn’t tell a phase shift as I’m not an engineer. I have a nanovna coming today. A 4H version. I’ll test it on a Rigexpert AA-35 zoom and get a sweep. Meanwhile I wish I had a way to send a picture of my sweeps. Craig

You need to probe both ends of the winding and you'll see on the scope screen when the phase starts to shift from one end of the winding to the other as the frequency is increased. If it is a -43 core you should start to see a noticeable phase shift around 7,000 kHz.

Edit: to look at this on the scope you don't need a primary and secondary. I would wind like 14 or 15 turns on it with a single winding, probe both ends of the winding and look at the sine waves. If they are super-imposed on one another there is no phase shift. But as frequency goes up one sine wave will start to shift from the other one. You don't need to measure the phase angle, just a visual representation of where the input and output start to get out of phase. Different ferrite mix cores have different response to frequency and I know from experience that a quality -43 will start to exhibit phase shift around 7,000 kHz, which is a way to identify the mix that was used in the core. You can test both your "known" and "unknown" cores to see what the difference is and if the "unknown" core doesn't match the "known" ones, it's not suitable to be stacked with the "known" cores.

You can also put a winding on the core and measure the inductance of the winding with a LCR meter or analyzer and see how the "unknown" stacks up with the "known" ones. Measuring the inductance of a winding on it will give you an idea of the initial permeability of the core when it's cold. A -43 core has quite a bit higher inductance than a -52 or -61.