I think the spec on 73 material permeability is also conservative. Few EMI
suppressing customers suffer if the impedance is higher than guaranteed. The
Fair-Rite 73 curves also show permeability peaking up even higher in the 500
kHz region.
W8JI has in the past made some remarks about stray winding capacitance which
would get worse with more turns. I believe his drawing shows some “dummy turns”
put in on the far side of ground, to cancel the stray capacitance?
An important advantage of fewer turns, is that you can use ordinary plastic
insulated hookup wire or kynar wire wrap wire to put the small number of turns
through the holes. If I use enamel magnet wire with these cores to get more
turns I end up nicking the enamel insulation on the core corners.
Tim N3QE
> On Jul 15, 2018, at 8:49 PM, Chuck Hutton <charlesh3@msn.com> wrote:
>
> Yes, I think we all agree on the meaning of the 4X rule and the other basics.
>
> The mystery to me remains that a 1 turn transformer was good to 270 kHz in
> the Clifton data. I calculate at 500 kHz:
>
> 1 turn on a BN73-202 with Al = 8500 gives 9 uH
>
> 9 uH is only 28 Ohms
>
> For reference, 2 turns = 34 uH and 107 Ohms. 3 turns is 77 uH and 242 Ohms.
>
>
> So theory seems to tell me I need 3 turns.
>
>
> Chuck
>
>
>
> ________________________________
> From: kd9sv <kd9sv@comcast.net>
> Sent: Sunday, July 15, 2018 4:43 PM
> To: 'Chuck Hutton'
> Subject: RE: Topband: Beverage transformers tht work down to 630 m
>
> Guys, the 4x rule is to my understanding that the measured impedance at the
> lowest frequency to be used at should be at least 4 times the operating
> impedance. In other words a 50 ohm system would require 200 ohms open
> circuit impedance measured at the primary winding of the xfmr. If 2 turns
> only measures about 100 ohms then 3 turns would likely be close enough and 4
> turns would also work and would measure 400 ohms which is 8 times the
> operating impedance of the antenna system. My test equipment can only
> measure down to about 450khz so below that I cannot give an opinion.
>
> 73, de gary...ps: the BN202-73 will likely work well with two/6 turns and
> 3/9 for a 9:1 system for 50 ohms
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Chuck
> Hutton
> Sent: Sunday, July 15, 2018 7:26 PM
> To: Tim Shoppa
> Cc: topband@contesting.com
> Subject: Re: Topband: Beverage transformers tht work down to 630 m
>
> Tim:
>
>
> Thanks for digging that out. It makes me worry much less about using
> BN73-303's with 2 or 3 turns in the primary at 630m.
>
>
> My only problem is that I don't understand why the low end is so good for
> the 1 turn primary. Using the "4x" rule for the transformer, 4 turns should
> be needed.
>
> Since I don't understand the response and I don't care about the high end
> response, I'm still tempted to use 4 turns and be sure.
>
>
> Perhaps part of the answer is that reality and theory do not coincide.
> According to the published Al, 2.7 turns is need at 500 kHz. to have 64 uH
> and satisfy the 4X rule. Yet my 3 turn windings measure 108 uH and 120 uH.
> That explains a good bit of the low end response.
>
>
> Chuck
>
>
>
> ________________________________
> From: Tim Shoppa <tshoppa@gmail.com>
> Sent: Sunday, July 15, 2018 3:50 PM
> To: Chuck Hutton
> Cc: topband@contesting.com
> Subject: Re: Topband: Beverage transformers tht work down to 630 m
>
> I agree the usual rule of thumb (Transformer winding Z should be several
> times larger than nominal line impedance) would cause you to think you
> should have more turns.
>
> The old Clifton Labs website is no more. But an archived page of
> measurements of transformers shows that the frequency response extends well
> below what you might think, from the rule of thumb. Archived page:
> https://groups.io/g/BITX20/attachment/27529/0/clifton%20Labs%20IMD%20in%20Br
> oadband%20Transformers.pdf
> Clifton Laboratories 7236 Clifton Road Clifton VA 20124
> ...<https://groups.io/g/BITX20/attachment/27529/0/clifton%20Labs%20IMD%20in%
> 20Broadband%20Transformers.pdf>
> groups.io
> Clifton Laboratories 7236 Clifton Road Clifton VA 20124 tel: (703) 830 0368
> fax: (703) 830 0711 E-mail: Jack.Smith@cliftonlaboratories.com
>
>
>
> He finds that BN73-202 transformers wound with a single turn winding, have a
> -3dB point at 270kHz.. A two turn winding would be good 4 times as low. So
> the rule of thumb seems very conservative.
>
> I have made step-up power converters using these cores and have been super
> impressed how well they work at frequencies well below the rule of thumb. I
> have run 30+ watts through these dinky cores with them just barely getting
> warm.
>
> Tim N3QE
>
> On Sun, Jul 15, 2018 at 6:29 PM, Chuck Hutton
> <charlesh3@msn.com<mailto:charlesh3@msn.com>> wrote:
> The commercially available Beverage transformers I have seen are like the
> W8JI model: 2 primary turns on a BN73-202 core.
>
> My calculations say 4 turns are needed at 630 m.
>
>
> Does anyone know of commercially available transformers with isolated
> windings that operates well down to 630m?
>
>
> Chuck
>
>
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