Topband: Beverage transformers tht work down to 630 m

[Chuck Hutton]

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 at gmail.com>
Sent: Sunday, July 15, 2018 3:50 PM
To: Chuck Hutton
Cc: topband at 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 But an archived page of measurements of transformers shows that the frequency response extends well below what you might think, from the rule of thumb.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%20Broadband%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 E­mail: Jack.Smith at 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 at msn.com<mailto:charlesh3 at 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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