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@contesting.com> On Behalf Of HA3LN
Sent: Saturday, January 2, 2021 4:59 AM
To: topband@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...
Thanks for the responses.
73!
Csaba HA3LN
http://ha3ln.hu/
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