EMRFD Message Archive 7937
Message Date From Subject 7937 2012-11-22 23:56:49 Vojtech two questions on HF transformers Hello.
I have two questions on HF transformers.
I experimented with magnetic loops for reception, mainly for hotel indoor usage. I tried various circuits to match the loop to a 50 Ohm coax first, but the signal levels were relatively low with an ATS-3b transceiver (2xNE612 based superhet). The receiver has a relatively lossy matching of the antenna to the first mixer, as one more likely needs to attenuate the signal with a long wire antenna and the Gilbert cell mixer than to amplify it. Then I got an idea to match the input of the NE612 mixer to the indoor loop directly. The ATS-3b transceiver has one of the NE612 input legs connected to the removable band module, which is a perfect place to connect the loop antenna to. I just made a custom band module with a loop feeder connected to it.
Now the question is, how can I match a loop antenna with an impedance of around 7kOhm in resonance across the tuning capacitor to an asymmetric 1.5kOhm input of the NE612? I want to match the loop at the high impedance side to be able to have the heavy capacitor at the table and not having any additional wires hanging from the top of the loop.
Is a transmission line transformer a good idea? I was thinking of a classic Guanella 4:1 Balun, which would mandate impedance of each of the two bifilar windings 10x750 Ohm, that is 2x 20 turns bifilar on FT50-43 for 40m band. Is this a good idea?
The second question. For a transformer, when is a toroidal core better than a binocular core? For example, if a crystal filter in Elecraft K1 receiver is matched to an input of a NE612 detector with a 5:20 transformer on FT37-43, the same impedance is achieved with let's say 10:3 BN-43-2402, although the impedance transformation is slightly different.
Thanks, Vojtech OK1IAK, AB2ZA
7939 2012-11-23 08:24:21 jor Re: two questions on HF transformers Hello, Vojtech OK1IAK, AB2ZA
If you want to use a loop only for receiving and not TX look in my PE1KTH files. I use this design a long time with good results.
It is a wibeband active loop antenne aprox from 300KHz up to 160 MHz
and have a gain of +23dB. Made with a 20 mm wide aluminium strip 120 cm loop diameter. Receiving mostly the magnetic component.
Has a good strong signal behave.
One's a kit bud I have no longer PCB's or parts.
Fitted against the wooden roof in the loft.
May be you need a Pi anttenuator between the antenne for not to overload the 1e NE612 mixer. All stations in the 160 Mhz wide band are comming in.
73' Joris PE1KTH
7941 2012-11-23 11:46:22 kb1gmx Re: two questions on HF transformers 7942 2012-11-24 13:25:58 Vojtech Re: two questions on HF transformers > No, the transmission line would have to have a working impedance in the 2-5K range, not easily achieved. A conventional transformer may work but you need enough turns to get the impedance higher than 1.5K and 7K without unwanted resonance again difficult. the best bet isI would like to avoid the L network, which is a single band solution.
> an L network.
I wound a Guanella 1:4 balun on two FT50-43 cores 30 turns bifilar. I am aware of the losses when using a low impedance transmission line for a high impedance TLT balun. The transformer seems to be working to some extent, the loop is lot less loaded, tunes sharply and I get ample of signal on 40m to the NE612 mixer. I have no idea of the losses though, it would mean to build two sets of 50 Ohm : 1500 Ohm transformers, one more balun and sweep them back to back with my miniVNA.
I have a question on resonated transformers. For a resonated transformer, there is much lower impedance of the winding required for a high impedance ratio. What is the trick to resonated transformers and when to use them?
> The core does not set the transformation ratio to turns ratio does.The FT37-43 and BN-43-2402 are about the same price. For the high impedance matching, it requires considerable lower number of turns on the binocular core than on the FT37-43. The turns on the binocular core are much more bunched one over the other, not so nicely distributed as on the toroidal core. I suppose there is some difference, like lower self capacitance for the nicely distributed winding? There must be some difference.
> It's more a matter of the permeability of the core material that
> sets the minimum number of turns needed as well as the type of transformer. For the crystal filter to 612 case I might use either
> but turns ratio would be identical.
73, Vojtech OK1IAK
7943 2012-11-24 13:33:49 Vojtech Re: two questions on HF transformers Thanks Joris for the design.
I will try how far I could go with the passive circuit. There is a beauty in the simplicity of a passive circuit and it keeps the portable hotel setup simple.
In case I decide to go with your design, would it be possible for you to share the printed circuit board layout of your elegant circuit?
Thanks, Vojtech OK1IAK
7944 2012-11-24 14:18:49 Helge Kyndbo Re: two questions on HF transformers Please,where can I find the PE1KTH files, for the loop ?
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7945 2012-11-24 14:30:34 Alberto I2PHD Re: two questions on HF transformers 7946 2012-11-24 15:32:57 dx11 Re: two questions on HF transformers And there is more in:
However it is in dutch, so I suppose you won't be able to read it.
7947 2012-11-24 16:01:10 Alberto I2PHD Re: two questions on HF transformers 7948 2012-11-24 19:40:25 kb1gmx Re: two questions on HF transformers 7949 2012-11-25 01:00:47 dx11 Re: two questions on HF transformers A comparable loop, the WL1030, was designed in the Netherlands in the same
period as the PE1KTH loop.
It still has a website of its own, after registering, you can find all data
on it, including a PCB layout. A couple of years ago a kit was made for it,
I got still one laying near my desk waiting to be built.
> /And there is more in:Well, my wife is Dutch (born in Amsterdam), so, perhaps, with the promise of
> However it is in dutch, so I suppose you won't be able to read it.
> Cor Beijersbergen/
a dinner out, I could convince her to translate that article for me.... :-)
But the problem is that neither that article reports the PCB layout...
/*73 Alberto I2PHD*/
7950 2012-11-25 08:45:28 jor Re: two questions on HF transformers Hello Alberto I2PHD,
Pay the bill for the dinner Hi....
I put the PCB files in the Photo's map.
7951 2012-11-25 10:56:56 Alberto I2PHD Re: two questions on HF transformers 7952 2012-11-26 00:03:22 Vojtech Re: two questions on HF transformers Thanks a lot, Joris, for the PCB design.
Would you please give me the dimensions to scale the board to? We have a cheap garage PCB company here in Prague, which makes basic PCBs quite cheap, but I need to give them clear instructions.
Thanks and 73, Vojtech OK1IAK
7953 2012-11-26 00:14:41 Vojtech Re: two questions on HF transformers > The problem with answering questions is often the full criteriaSure thing. Often when one knows the proper question he himself is very near to the right answer.
> is not fully known or at best guessed at. So one makes suggestions that may work for the situation as understood. Doesnt' mean the suggestions meets the questioners needs. ;)
Actually the LC network may not be a bad idea after all. There is a classic way to match the magnetic loop with a "voltage divider" tuning capacitor connected to the loop at the "hot" side, to one leg of the tuning capacitor. There are symmetric designs with two tuning capacitors available indeed, but it is difficult to achieve synchronized action of the two capacitors. Actually the "voltage divider" is a misnomer, both capacitors are part of the same resonant circuit I believe and they both tune with the same sharpness.
I sketched a tuned loop matching circuit with 4 varactors yesterday, which requires two tuning voltages, one for tuning to resonance, the other for matching. The only problem is the relative low Q of the varactors. I have to prototype this circuit.
> > I have a question on resonated transformers. For a resonated transformer, there is much lower impedance of the winding required for a high impedance ratio. What is the trick to resonated transformers and when to use them?The right question is, what are the rules to design a tuned transformer. How to design a tuned transformer for a given transform ratio? There is a simple rule for an untuned transformer, but the tuned one remains a mystery to me.
> When is hard to know as its specific to the application.
> However a tuned transformer will require a different core than
> type 40 (maybe T50-2) for good Q.
> Two differences I can think of; the binocular core will need veryYou are right, I found myself yesterday trying to press 18 turns of AWG30 into a tiny binocular core :-)
> fine wire to do many turns through the small holes. the other is
> for cases needing the highest coupling with lowest leakage reactance
> the binocular core seems better. Most of this is second and third order stuff.
73, Vojtech OK1IAK
7954 2012-11-26 05:24:38 Vojtech Re: two questions on HF transformers 7955 2012-11-27 19:48:05 jor Re: two questions on HF transformers Vojtech,
The dimentions are under the PCB picture in the Photos map.
All the files are also in the "Files" map PE1KTH.
Be aware thad the single sited power PCB use the metal box bottom as the groundplane to keep the 50 ohm inpedance of the stripline.
A metal box for the power is inportand to prevent emision of output ennergy back to to loop at close distance preventing oscilati