Topband: (2wire) Beverage transformers

[Lee STRAHAN]

Hi Mike,
   Assuming that you did not miss that this was a tapped winding and not separate windings as Csaba mentioned. I see on reflection that he measured 1:1.16 on one of his tests. In reality it wont likely get much better than that. That test was likely the 3:12 he mentioned using. The high impedance side of these transformers are a little unpredictable using simple formulas with winding capacitance and magnetizing inductance added in the mix.
  Sometimes I use wire wrap wire if it is not going to be used outside otherwise I use #27 high temp motor winding class insulation wire which helps keep from shorting the wires to the core. I have the benefit of many part spools of motor winding wire scraps from a best friend and Ham in the Motor rewinding business. By the way, Norton amplifiers require 1:11:4 which is the same problem to solve as they are separate windings in the ones I use. I also fit shrink tubing in the Norton amp cores for insulation first. I don’t use Teflon because it has a dielectric constant around 5 which increases the capacitance from the wire to the core. Its tedious but can be done easily. And in the case of the Norton amp it leaves room for a larger wire on the 1 turn winding. Yes 4 AND16 for 20 total can be done but yes it takes time and lots of patience. For those turns counts I go to # 75 material toroid cores which have slightly more winding room but require more turns usually for 160 meter stuff.  All this probably more than you wanted to know. HNY
Lee  K7TJR  OR

From: Mike Waters <mikewate at gmail.com>
Sent: Saturday, January 2, 2021 11:44 AM
To: Lee K7TJR <k7tjr at msn.com>
Cc: HA3LN <list at ha3ln.hu>; topband <topband at contesting.com>
Subject: Re: Topband: (2wire) Beverage transformers

Lee,

What kind of wire do you use that allows that many turns (4t and 16t)?

73 Mike
W0BTU

On Sat, Jan 2, 2021, 1:37 PM Lee STRAHAN <k7tjr at msn.com<mailto:k7tjr at msn.com>> wrote:
Hello Csaba,
   I approach this problem this way your impedance ratio is 745/50 ohms or 14.9 . To get turns ratio use the square root of that which is 3.86 . So round that up to 4 as a good turns ratio.
  On a BN73-202 core I usually use a minimum of 4 turns on the 50 ohm side for 160 meters, so the secondary would need 4 turns ratio times that for 16 turns. Therefore 16 turns tapped at 4 turns should work for you. Some will say the 3 turns on the 50 ohm side should work and the secondary then would be turns ratio 4 times that or 12 turns. Therefore 12 turns tapped at 3 turns should work well also. Sorry, I do not follow your formula as shown but you can use the above and it will work fine as an 800 ohm load to the 745 ohm source. This will reflect 745/16 or 46.6 ohms to your cable. SWR for that at the 50 ohm cable  is 50/46.6 or 1.07 using resistance only for evaluation.
Lee   K7TJR  OR

-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com at contesting.com<mailto:msn.com at contesting.com>> On Behalf Of HA3LN
Sent: Saturday, January 2, 2021 4:59 AM
To: topband at contesting.com<mailto:topband at contesting.com>
Subject: Topband: (2wire) Beverage transformers

Hi All and HNY for 2021.

Preparing for the CQ160m with new (2 coax) 2-wire beverages to cover the missing azimuthal gaps based on LBDX. The first 2x Bevs worked great back in last Jan.

Now I have difficulties with reaching good imped match with the
T2 transformer (responsible to transform the 745 Ohms wire impedance to 50 Ohm coax). I use n1=3T/n2=12T tapped @6T transformer (2m high, 20cm wide with 0.8mm wire)

What can be the reason for the impedance transformation is rather off to the calculated value?

This is the T2 transformer from 2019:
http://ha3ln.hu/VNA_190116_230811.jpg
...and this from yesterday:
http://ha3ln.hu/VNA_210101_153241.jpg

I have
- same wire with the diam (even from the same roll)
- same BN73-202 cores (tried to use several cores from different
   sources to eliminate the possible mix inconsistencies)
- same winding method (including n2 tapping)
- created a low inductance test resistor network for 744 Ohms

...tried to wind
- lousy, and precise (crossing windings vs. side-by-side, bunched
   wires, etc.)
- n1 first and n2, after n2 first and n1, of course no difference.
- without the tapping, same as above.
- difference turning ratios (3/12, 2/12, 1/12, 3/11, etc.) to see
   the change


The best I could reach now on 160m is
- SWR: 1:1.29 (Rs=40.4 Ohms, Xs=-5.4 Ohms) vs. in 2019:
- SWR: 1:1.16 (Rs=43.2 Ohms, Xs=-1.6 Ohms)

I know, Beverages are really die hard antennas and this increased mismatch might have zero effect on performance but still, the engineer part of me...