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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