You have the necessary instrumentation to check it out and it's easy to do.
First, 800 Ohms to 50 Ohms is a 16:1 ratio, so I don't get your 34:8 turns
ratio. From 50 Ohms that would match 900 Ohms. If you really need the
inductance of 8 turns, then use 32 turns on your secondary. That alone could
explain some of your results. Measure the match over 2 to 30 MHz with the
800 Ohm resistor.
Next take 5 lengths of your #24; long enough to get 3 turns on the core plus
a little for connections. Twist them all together either by hand or in an
electric drill. The twists-per-inch will affect the impedance, but for this
experiment, a few tenths of a dB will never be noticed. Wind 3 turns on the
core. Distribute the winding over the core; don't bunch them all together.
Use one 3 turn length for the 50 Ohm side and connect the others in series
for the 800 Ohm side. You will get a much better match over the 2-30 MHz
band. If you need to get lower than 2 MHz, use 4 turns or even 5 turns. The
tradeoff is less high-end with more turns, but lower low-end. Incidentally,
you will get better performance from the binocular core for both designs
because of the better coupling. You may need smaller wire to fit in the
core's holes.
The T16-6 is only rated for one Watt and could easily be popped near a
transmitter. A homemade TLT on a large binocular core will not have a power
problem.
-Doug, AE6SY
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Pete Smith N4ZR
Sent: Thursday, February 02, 2012 3:30 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Flag Antenna Transformer
Is this necessarily true? I have a couple of transformers built by ON4UN.
They use separate windings and a binocular core, and behave exactly as
expected.
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com, spots at telnet.reversebeacon.net, port 7000 and
arcluster.reversebeacon.net, port 7000
On 2/1/2012 4:58 PM, Doug Ronald wrote:
> You are getting these results because you wound a conventional
> transformer where a Transmission Line Transformer (TLT) is required.
> The core is fine, but twist the primary with the secondary using 12
> turns and 3 turns. Here is a reference to some theory on what you need:
>
> http://home.earthlink.net/~christrask/TraskTLTTutorial.pdf
>
> and the author (Chris Trask) has several other TLT articles on his web
> site including precise winding tips.
>
> Here is another helpful link to the way your finished transformer
> should
> look:
>
> http://www.qsl.net/wa1ion/bev/bb_antenna_matching.pdf
>
> Hope that helps,
>
> -Doug, AE6SY
>
>
>
>> I have a flag antenna which has worked well for several years. I used
>> a
> Minicircuits T16-6 transformer to couple the ~800 ohm termination
> resistance of the antenna to 50 ohm line. Recently I decided to do
> some upgrades, so I followed the recommendations of many (here and
> elsewhere) and made a matching transformer using an FT-140-43 core and
> 34:8 turns, secondary and primary on opposite sides of the core. I
> didn't have any #28 wire so I used #24. I tested this with a miniVNA,
> terminating the antenna side with an 800 ohm noninductive resistor.
> The impedance does not look right at all; there is a lot of inductive
> reactance and the resistive part starts at 125 ohms and climbs steeply
from 1.7 MHz. No on-the-air results yet.
>
>
>
> I'm wondering if this is right or not - measurements indicate the DIY
> transformer is much worse than the T16-6 in terms of presenting a good
> match.
>
>
>
> This would not be the first case of theory saying it won't work but
> practice says it will - just looking for a reality check here. Seems
> to me like the core material is all wrong...
>
>
>
> -Tony, K1KP
>
>
>
> _______________________________________________
>
>
>
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>
>
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