Construction of a 4:1 BALUN Why is it so ?

VK2RK

Member
I blame VK3TJS for this, I was quite happy with my inverted V fan dipole for 80 and 40, but as I have been looking into off center fed dipoles, trying to understand the common mode current issues some are having, I thought why not build one, it could be a better alternative to the fan dipole currently in use by me.

The OCF antenna is an evolution of a Windom antenna, it differs in that its coax fed via a BALUN, all other operation principles are the same, the feed point is moved from the center point to give an impedance of 200 Ohms, thus the harmonics of the higher frequencies will give the same impedance, making the antenna a multi-band antenna, However the down side is that at the higher frequencies the propagation pattern will no longer be that of a dipole, with many lobes.
There is modelling on the web that shows this.

The first thing that struck me is that the quality of the BALUN is very important in obtaining a constant match to both the fundamental and the harmonics, ideally the matching component (BALUN) must have a flat response, this to provide the 4:1 transformation to 50 Ohms. BUT a big but, this point will exhibit a very reactive value that many show as 200 Ohms, none of the research I have done shows the complex value eg( 200 J-something) so the conclusion is that I am not going to spend time calculating this as there exists many unknowns that vary considerably with external factors, thus the assumption is that of 200 Ohms reactive, ( Here is why we have the common mode current issue. )
A transmission line or system, (I include the BALUN) at the feed point we expect to see that the voltage and current to be in phase, but a big BUT the point chosen is not going to give us a pure resistivity load, by the nature that its not at the center, its phase difference is going to be very different to a center fed dipole, take this a bit further and the resistivity value of the transmission line will only be so at the exit of the cable not at the connection to the dipole.
With an off center fed dipole this situation is worst, so it can be expected to have larger common mode currents, purely due to the reactive (J) values of the complex impedance.
Concluding, a very flat responding BALUN is a must to reduce and keep common mode currents to a minimum, but this is not easy to achieve.

I decided to construct a 4:1 BALUN dual core, here I would like some comments from those that may have already been down this road I am travelling on.

BALUN Construction:

Core T200-2 (Brown Colour) Happen to have these in my junk box
Construction is that of Common Mode Choke
8 turns total 4 by 4 opposite phase
Output in series, input in parallel

IMAG3423-20200909-084056265.jpg

IMAG3427-20200909-084056806.jpg

The following figures have been measured with an VNA
200 OHMS resistor on the load
3 MHZ 39.5 0.8J VSWR 1.26
7 MHZ 51.5 0.15J VSWR 1.03
14 MHZ 55.1 0.11J VSWR 1.102

The above figures show some reactive values, albeit very small but their presence indicates an unrated variable, this was expected due to the large bandwidth involved, but the return loss at 3 MHZ is exessive in my opinion .
The part the I cant reconcile is the large impedance dip at 3 MHZ, not sure why, is it the lack of turns or the external complex factors in the test I am conducting.

I welcome any comments please.

Note the VNA was calibrated...

IMAG3429-20200909-093607213.jpg
 

VK3YNV

Moderator
Interesting result at the low end, as you said it might be not enough turns for that ferrite material at 3.5 MHz. I think j+ve means capacitative so it needs more inductance.

I'm waiting on some teflon wire to make up some baluns.
I got these toroids from farnell https://au.element14.com/fair-rite/5961002701/ferrite-core-toroid-61/dp/3282208

I note that you did it right and used separate cores for the two halves, the common mode currents are going to be different in each half so you can't cancel common mode currents with one core only.

I'll be interested to see how it goes on the Club net tonight.
 

VK2RK

Member
Interesting result at the low end, as you said it might be not enough turns for that ferrite material at 3.5 MHz. I think j+ve means capacitative so it needs more inductance.

I'm waiting on some teflon wire to make up some baluns.
I got these toroids from farnell https://au.element14.com/fair-rite/5961002701/ferrite-core-toroid-61/dp/3282208

I note that you did it right and used separate cores for the two halves, the common mode currents are going to be different in each half so you can't cancel common mode currents with one core only.

I'll be interested to see how it goes on the Club net tonight.
LOL, have not built the antenna yet ..... here you are looking for a report this evening net..

I did take a turn off then added a turn to measure the effect. The result is even more surprising, making no sense as to what is going on..

Reference at 3 MHz 31.6 Ohms 1.53uH 0.39J
One turn less 36.1 Ohms 1.42 uH 0.33J
One turn more 27.9 Ohms 1.59 uH 0.43J

What I think is going on is that the wire I have used been electrical 3 AMP wire, having thick insulation does not have enough capacitance between turns and to the core material.
Going to rewind using 1 mm enameled wire....

In the good old days without the fancy measuring gear, I would have put the thing up in the air and pondered at the strange VSWR at 3.5 MHz
cutting wire for best VSWR :)
 

VK2RK

Member
I moved away from the toroids I was using, and adopted the Duratech from Jaycar (LO1238)
Originally I used enamel copper wire, aside from been a bitch to wind as the wire is super stiff, the results obtained had been very variable, just placing the hand near the core stack changed the impedance, it was way too variable, the old toroids with the wire I had used was far more stable.
So decided to wind with electrical plastic covered wire, this reduces the capacitance between turns.

I have obtained a good reliable set of numbers, the toroid is exhibiting some capacitive reactive component at the higher frequencies, the antenna wire will be inductive (I Hope) this should cancel this out.

Ideally the match should have no reactance as reactance creates inefficient power transfer.

Next get a housing (PCV tube and end caps) some wire and get the thing up in the air and see what it gives.

IMAG3433-20200910-160524656.jpgIMAG3431-20200910-150141605.jpg
VNA Plot.jpg
 
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VK3YNV

Moderator
Hi Rob,

Interesting results. Ferrite properties are a bit of a mystery to me. I suspect the only way to find out for sure what works is to do what you've done and build a balun and test it to see how it performs over the frequency range you want.
 

VK2RK

Member
Hi Ray.

yes indeed are a mystery to me also, i looked at the specs for the new ferrites and are rated at a max of 2Mhz
the ones I had used went to 30 MHz
 

VK2RK

Member
Why does the VNA tell me lies.... ??
YES Lies, well in fact its my interpretation of what is going on at fault, in fact the VNA was telling me the truth.

The test environment is causing the reactive values to be distorted from the actual true values.
The image below shows that by suspending the unit the measured values differ.

The other thing I have tested using an R value closer to what a 1/3 2/3 feed point will give a t 15 meters above the ground (235 Ohms)
my test resistor is 239 Ohms (Close enough)

The reactive components are still due to the test environment, overall I am happy with the BALUN

Final Test BALUN.jpg
 
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VK3YNV

Moderator
Those balun scans look pretty good. The NanoVNASaver screenshot is a good way of presenting the data.
 

VK2RK

Member
With my head bowed in shame I am admitting defeat.

I could not get the rotten wire to tune as expected, the first BALUN failed due to heating of the ferrite, the effect was when hot the VSWR went above 3 : 1, if let it cool then system returned to normal values. This was with Jaycar obtained ferrites, clearly the loss was creating the heat in the core, the wire was fine and showed no heat stress. Note that another problem with this core was poor performance at 14 MHz

I wound a second BALUN, using mix 61 (T-200-61) this is the original core that gave a poor response at 3 MHz. When attempting to tune the length of the wire (Keeping the ratio constant) I got a strange result, no matter how long or how short I could not move the 80 Mtr resonant point it remained at 3.2MHz, then I realized that a hump in the curve was moving, this at first I thought it was common mode current effect, in fact it was the wire resonance, so what was causing the other dip in the curve, after giving this some thought plus the fact that the movement of the coax cable was showing changes in the curve amplitude (Not frequency) I concluded that the Steel tower plus the coax and the kind of transformer used had a natural resonance at 3.2 MHz with a very low impedance at about 40 Ohms, since the others would have presented higher values to the system showed little effect to changes.

Conclusion is that don't use a metal pole to support the apex (Balun) of the antenna, it becomes part of a resonant circuit.
in my opinion this makes the inverted V concept not a very good permanent solution, maybe as a field antenna to be used with a squid pole will be ok.

If you wish to use this type of antenna then use two support points not a center one, unless it's non metal.

SORRY I HAVE FAILED....
 
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VK2RK

Member
Found this Power Point presentation that explains at least to me some of the effects I had observed.
Since I cant publish the file on here I will provide a link.

Thanks Ray, file now local to forum.
 

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BillC

Member
Notice Rg load impedance is in series with the source when testing . I think that would define the input impedance.
 

VK2RK

Member
Notice Rg load impedance is in series with the source when testing . I think that would define the input impedance.
Rg is shown in the way it has (Convention) to show that the source has an output impedance of a value "Rg" a generator without Rg is assumed to be infinite by convention, when Rg=Rload maximum power transfer from the generator to the load takes place.
 

VK3YNV

Moderator
Found this Power Point presentation that explains at least to me some of the effects I had observed.
Since I cant publish the file on here I will provide a link.
Hi Rob, I've added PPT as an acceptable file type, so you can upload direct.
 
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VK2RK

Member
LO1238.jpganalisis.jpg

IMAG3455-20200924-100206620.jpg

Test of the Jaycar LO1238 Ferrite, Load 50 Ohms

There is little information available on this ferrite, the above test explains the issues I suffered with the heating and the high VSWR at 14 MHz
This material is really not that good for a broadband BALUN
 

VK2RK

Member
As I am waiting for my new VNA to arrive, I have been doing more research and reading.
There is lots of information to be found, but its very confusing and contradictory, my work so far as indicated that the ferrite material used is extremely important, raising the question "what is the best material for the required application"
Well finally I found a document that answers exactly this question, and the information with in aligns with what I had experienced.
The Jaycar ferrite (LO1238) that now we have identified as mix 41, thanks to Ray Gardner's work, was the one that gave the best result at 80 MHz, but fell off at the higher frequencies plus it overheated. Now I have a better understanding of what is going on.

What material.jpg
 

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