I just wound what I thought would be a 350? to 23k? AM BCB transformer. I put a 23k?n the secondary and the primary measures R=14? and X= -43? at 1MHz. I used a #73 Binocular core. Looking for about 8, 9, or 10 to 1 turns ratio. The inductances measure correct and the step up voltage measures correct. I'm guessing the problem is winding capacitance. But, I'm not sure. Any tricks to winding high ratio, high impedance RF transformers? I might try two lesser ratios in series.
Mikek
Maybe it didn't have enough turns for each winding?
Anyhow, I found that the only way to wind RF transformers is multifilar. Unfortunately that usually requires a whole lot of solder connections to get all the windings on the hi-Z side in series.
-- Regards, Joerg http://www.analogconsultants.com/
To reduce self-capacitance, one must segment the windings. There are many ways to do this. The traditional RF way was a series of adjacent Pi windings connected in series aiding.
On a pot core, use a two-section winding, connecting the sections in series. This reduces the self capaciotance to one quarter the value of the same total winding count all in one section.
Joe Gwinn
Measure the self-resonant frequency. It might be well below 1 MHz.
Why do this?
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Did you have enough inductance: what did you get for R and X with the secondary open?
Does your 23K dummy load measure 23K at 1MHz? It's only 1 MHz so it probably does but if in doubt measure.
If you don't have enough inductance you may need to use more turns or a larger core, or stack several small cores end to end.
-- Jasen.
Before I wound the tarnsformer I ran a test winding of 6 turns and it was 550uH. (1900 ? @ 550kHz) That's a factor of 5.4. I think that's enough, probably could reduce it a turn. An odd thing, I have a DE-5000 LCR meter that I'm using to test inductance. It will test at 100Hz, 120Hz 1000Hz, 10,000Hz and 100,000Hz. It measures 550uh on all frequencies except 100,000Hz where it measures 1522uH. I wound six turns a on another similar core, I get
575uH and at 100,000Hz it is 564uH. I have to assume it is the secondary that is causing this inductance increase at 100,000Hz.Ya, I thought about doing that, but then I realized how many connections that would be on this #36 wire. (9 connections) I would do it if I thought it would solve the problem, but with my limited understanding, I think, all that would do is help at high frequencies. Do you see it differently?
Mikek
Ya, can't I can't physically make a pi winding.
I have some pot cores, I don't have any sectioned bobbins, but I can fabricate something. Not sure the AsubL is large enough. More I think, I don't think they have the AsubL needed. Mikek
Am 31.05.20 um 20:14 schrieb amdx:
I think 23 KOhm wideband in the AM band is hopeless. I have tried Z=1:16 and Z=1:32 by rewinding Pulse CX2049 and similar (Macom) under the microscope with 50u wire. I was even flexible to accept a quite narrow response peak without caring too much where it was.
Wideband works only as a transmission line transformer and you cannot get that impedance ratio right. Soldering the ends of the quadrifilar wires together gives just so much stray inductance.
Even when I rewound them to 1:4 they were worse than the original. It is not as easy as it seems. I also tried cascading two transformers. Did not work at high impedance, as told by the VNA.
MCL has some 1:16 transformers ( from 50 Ohm system) that are somewhat acceptable.
Winding these things under the microscope is a highly contemplative / meditative job. But I did not achieve nirvana.
Cheers, Gerhard
Well below 1MHz, the primary is resonant at 123.5kHz (with 15pf probe capacitance) The secondary resonates at 90kHz. I don't get those numbers, It seems like they should have widely different frequencies, i.e. the primary should have a much higher resonant frequency. Does the secondary capacitance somehow get involved?
I don't really know, but it does tell me The secondary has about 50pf - 15pf = about 35pf.
I don't know what happens when a transformer is resonant below it's frequency of use. Mikek
If the coupling is good, the difference in srf is an artifact of the measurement.
If srf is around 100K, it's going to be useless for AM.
But why have a transformer? I assume this is your AM receiver front-end.
The AM band is so noisy, you don't get any advantage from a big antenna or a low-noise front-end amp.
A directional antenna might help, but a 1 MHz yagi would be a project. The rotator would be even more interesting.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Some of the MiniCircuits transformers (MCL?) could do a modest step-up, and diff-to-single-ended conversions, at 1 MHz.
Transformers don't actually have an impedance.
Most AM radios have a tuned network first, to match impedances but mostly to add some selectivity.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Am 01.06.20 um 01:55 schrieb snipped-for-privacy@highlandsniptechnology.com:
That's also the opinion that U. Rohde formulates in his communications receivers book. There is simply too much man made noise.
A friend of mine did this, but on 432 MHz:
But one could always play with the phasing of some antennas a few
100 m apart. Creating notches can really help. The antennas don't need to be big, but a rural environment is needed.Gerhard
I do understand that an amp in the BCB is of no use, unless you have an antenna that has a very low output signal, which many AM BCB antennas do have. Some are down 40 db from a dipole. It is true that the antenna I have is not a low output antenna. I'm just testing it to see how it acts. I did listen to a beacon last night and it did help seem to help on a 200kHz signal when I turned on the amp. But I'm just lousy at understanding Morse code, I can listen 10 times and still not get the call letters.
In most situations, turning on the amp moves the S meter but doesn't help the S/N at all. The only help you get in the AM band are directional antennas, often these are two antennas phased together. Mikek
John, I'm still trying to get something for nothing. The two methods, feeding a high input impedance amp (low output impedance) from a low impedance source, 'vs' using a matching transformer. The matching transformer does two negative things, it loads the antenna (you lose 1/2 of your signal) and it has a step down, on some antennas that could leave you with only
25% of your signal. (800? to 50?) Now, my thought is, what if I put a step up transformer between the antenna and the amp. I still have an unloaded antenna and I'm avoiding the step down of of the usual matching transformer, but the freebie as I see it, by adding the step up transformer, I can multiply my signal voltage by 4 to 10 times. I'm probably missing something, because you don't usually get something for nothing, but I don't know what it is I'm missing. This describes how the amp is used pretty well.All of this is subject to not over driving the amp, and actually being able to build a transformer that works. btw, the amp impedance is not as high as I thought. @ 550kHz R=122k? C= 37pf @1700kHz R= 52k? C= 37pf. My antenna impedance is about 350 ohms. Mikek
One could in theory synthesize a directional antenna from some number of small antennas some distant apart, namely some useful fraction of a wavelength.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
He is using such oddities such as Beverage on the ground (BoG), which has a very low output voltage but quite stable impedance (around 400 ohms) and in this case slightly capacitive in the MF BC band. Tuning out the capacitive reactance with some inductance would make the antenna nearly purely resistive.
A 9:1 step _down_ transmission line transformer transforming the 400 ohm down to 50 ohm and an ordinary 50 ohm coax could be used to feed a
50 ohm receiver input. Almost ideal power match.If the interest is medium wave DX, the vertical polarization is of interest. A few short vertical antennas a few wavelengths apart and some phasing should do the trick. A short vertical is highly capacitive, so one alternative is to use a FET source follower to match the antennas to 50 ohm coax for feeding the signal into the phasing unit.
In general, impedance has an upper limit that falls as frequency rises. You can make a 23kohm winding at 10kHz, you'll probably struggle at 100k, and you'll probably fail above 1MHz.
So you did...what... that binoc core is probably not much more than 1 uH/t^2, so 350 ohms at 1MHz is at least 50uH, or 7, call it 8 turns -- primary. And the secondary is then 8-10 times or at least 64 turns.
What do you suppose the Zo of those windings is? That is, considering the wire diameter and spacing, what's the transmission line impedance of a given turn with respect to its surroundings (assumed ground for an instantaneous wave at a point on the turn in question)?
Now, real multilayer windings aren't straight transmission lines, they're a jumble of lines stacked in series, both in the sense of the wire going around for multiple turns but also in that there are layers of transmission lines all the way down. (An equivalent circuit might use ideal single transmission lines, stacked so that all the port 1's are in series, and the port 2's are skew connected to the port 1's because that's how turns work.) Point is, Zo can be much higher for a multilayer winding, indeed for a wave of the hand it's around N_layers * Zo. But also dispersive, because such a structure doesn't have a constant velocity or impedance.
So, if the 64 turns are arranged in a square of 8x8 turns, they might be around 50 ohms for a given turn, or 8*50 = 400 ohms for the whole thing. That's a long ways away from 23k.
Which means equivalent capacitance will dominate, a proportional distance away from cutoff.
Cutoff being approximately the 1/4-wave electrical length of the winding (wire length).
If one turn is, oh I don't know, maybe 2cm, then 64 turns is 1.28m, or a 1/4 wave of 60MHz (at say 0.7c). 23kohm is 57 times above 400, so the cutoff at that impedance we should expect to be more like 1.04MHz.
Probably you're very near the peak, or just past it. Or maybe pretty far into it, I don't know. The -j impedance says it's capacitive, in any case.
Also, note that the above design exercise assumes nearly zero bandwidth -- the SRF Q would turn out pretty modest after all (probably < 1), but that wasn't an assumption I used! To cover the whole AM BCB, you need at least an octave more inductance (40% more turns), which will drop the HF cutoff by about 40% (40% more winding length!).
How to fix it?
Reduce winding length, apparently by at least 50%.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ "amdx" wrote in message news:rb0s6l$jek$1@dont-email.me... >I just wound what I thought would be a 350? to 23k? AM BCB transformer. > I put a 23k?n the secondary and the primary measures R=14? and X= -43? > at 1MHz. I used a #73 Binocular core. Looking for about 8, 9, or 10 to 1 > turns ratio. > The inductances measure correct and the step up voltage measures correct. > I'm guessing the problem is winding capacitance. But, I'm not sure. Any > tricks to winding high ratio, high impedance RF transformers? > I might try two lesser ratios in series. > > Mikek >
sformer.
and X= -43?
Transmission line transformers do work at RF frequencies.
shows 2:1, 3:1 and 4:1 ratios. The impedance of real transmission lines is never all that high, so I don't like your chances of finding something that could give you what you seem to want. Even 4:1 with 300R spaced-pair aeria l feed line line elements would only delivers 4.8k.
-- Bill Sloman, Sydney
You are seeing the effect of stray capacitances and leakage inductances. Due to stray capacitances, you are not going to achieve the impedance levels you are aiming to, unless you're using tuned circuits to compensate for the strays.
-- -TV
For wide-band transformers, the mechanical considerations limit the practical line impedance to around 120 ohms, like CAT6 twisted pair. The need for magnetic coupling to a core limits the mechanical size of the wire, and the impedance rises very slowly with the cable wire separation, relative to log of diameter ratio.
In MF, there is no sense to strive for minimal noise figure in the front end, as the band background will override it. What should be aimed to is maximal linearity to not get swamped by the large signals on band.
-- -TV
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