Re: BL2 -- Antenna Balun Usage

Posted by k6dgw on May 21, 2017; 8:15pm
URL: http://elecraft.85.s1.nabble.com/BL2-Antenna-Balun-Usage-tp7630889p7630894.html

Hi Phil,

(1) The MFJ-259x series of antenna/feedline analyzers havean N coax
connector. Using the BL2, you can connect it to the balanced ladder
line. How were you planning to connect it without the balun?  As you
state it, it does not sound like a reasonable experiment design.

(2) The impedance ratio of both the DXE and BL2 is the square of the
turns ratio.  Thus, the 4:1 DXE is a 16:1 ratio in complex ohms.

You can measure the complex impedance at the end of the balanced line
using a very short coax jumper from the BL2 to the analyzerand then
compute the impedance at the balanced terminal using the above. Be
careful of BL2 ratios that give extreme results, they're likely not
really valid.

(3) Transformers are rated for both power and breakdown voltage. My DXE
balun was rated at 11 KV and 10 KW ... although cramming 10 KW into an
SO-239 seems a bit unrealistic. [:-)  The transformer will have
lossesmanifested in heat. If it gets too hot, it will do what everything
else does whenit gets too hot.
Depending on the impedance ratio and the complex impedance you're
feeding, the voltagecan exceed the breakdown voltage, creating
well-known effects.

If the applied power saturates the transformer core, power peaks above
saturation will be clipped creating a huge RFI generator. Balun cores
are ferrite of various compositions. Each composition has a specific
Curie temperature, above which the core loses it's ferrimagnetic properties.

The MFJ-259x and VA1 are almost guaranteed to give non-identical
results. If they are large differences, youhave some other problem.  If
they are small, they are essentially meaningless for your application.
Hope this all helps.

73,

Fred ("Skip") K6DGW
Sparks NV USA
Washoe County DM09dn

On 5/21/2017 11:00 AM, Phil Hystad wrote:

> I am planning to use my BL2 to measure the impedance of my antenna+feedline at the point of entry into my balun (DX Engineering) which then feeds coax to my K-Line.
>
> The feedline is 450 ohm ladder (window style).  The antenna is an 80-meter, 136 feet, antenna up in the air about 50 to 60 feet.
>
> I have two antenna analyzers and I will likely use both of them to see the difference.  I will connect the 450-ohm feed line to the balanced side and a BNC to UHF adapter for attaching to my antenna analyzer [ (1) MFJ 259B, (2) Autek Research VA1].
>
> Questions:
>
> (1) How much difference would I expect in using the balun verses measuring the balanced line directly with my antenna analyzers (actually, I plan to do both but wondering if anyone else has done this with what results).
>
> (2) I bought the BL2 for this particular purpose as it is switchable between 1:1 and 4:1 transformations.  I wanted to figure out which would be best to use, a 1:1 or a 4:1 balun (DX Engineering and I have one of each).  Due to the location of the balun (DX Engineering that is) it is easier for me to do this measurement first before physically replacing the one balun with the other.  Right now, I am using a 4:1 balun.
>
> A third un-related (to above) question:  The BL2 is rated for maximum of 250 watts.  What are the bad effects of pushing more than 250 watts through the device, I mean a lot more like double or triple.  My guess is that the ferrite transformer will overheat causing possibly run-away thermal non-linear changes to the transforming.  What else will likely happen?
>
> 73, phil, K7PEH
>
>
>
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