Topband: Beverage transformers that work down to 630 m

Sun Jul 15 22:03:44 EDT 2018

Greetings Top Banders,
   Well we cant all agree on a 4X transformer primary rule. It depends on what you want to do with the transformer. In this case a Beverage for 630 meters. Driving on with the 4X rule will make you think all is well until you decide one day to add a second Beverage and try to phase them. The phase relationship between the two different Beverages will be somewhat dependent on the AL of the transformer cores when using too few turns. AL is not guaranteed or for that matter very temperature or unit to unit stable. When using transformers for a single system a few turns will work well providing small signal losses. One must thoroughly check transformer phasing especially when phasing 8 antennas together like the Hi-Z 8A receiving antenna. Different cores have different phase and amplitude responses including toroid and binocular when used in different circuits such as a Magic Tee & impedance transformation.
So don't be tripped up by a hard and fast rule unless you are using a transformer in a single antenna use.
 I recently ran some VNA plots on the Hi-Z 50 to 75 ohm transformer. I tested 2 of them back to back using the 50 ohm ports of a VNA.
The results were, and remember a single transformer will have 1/2 of the values below that I measured for 2 back to back transformers.
These transformers are made as an autotransformer where the primary is 4 turns on a BN73-202 core with another turn added to the 4 for a 4 to 5 turns ratio. Yes, its 50 to 78.1 ohms as close as you can get with a small number of turns. Too many turns and you lose high frequency response.
100KHz    -1.12 dB  at +25.9 degrees phase shift
600KHz   -.15 dB     at +3.3 degrees phase shift  some 4 degrees off values at 160 meters.
1.6 MHz  -.22 dB    at -0.77 degrees phase shift
7.1 MHz  -.29 dB   at -9.0 degrees  phase shift
Remember single transformers are 1/2 these values.
Single antenna losses only are quite acceptable at 630 meters and 4 turns primary on a BN73-202 as loss wise one transformer is down less than 1 dB at 100 KHz. 

 Lee  K7TJR  OR

-----Original Message-----
From: Topband <topband-bounces at contesting.com> On Behalf Of Chuck Hutton
Sent: Sunday, July 15, 2018 5:49 PM
To: kd9sv <kd9sv at comcast.net>; topband at contesting.com
Subject: Re: Topband: Beverage transformers tht work down to 630 m

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

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

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%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 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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