Re: BL2 -- Antenna Balun Usage

Posted by Vic Rosenthal on May 22, 2017; 3:31am
URL: http://elecraft.85.s1.nabble.com/BL2-Antenna-Balun-Usage-tp7630889p7630908.html

Balun ratings are for a 1:1 SWR. My experience has been that when the
impedance gets very reactive, baluns heat up more. I had a 5 kW DXE
balun get too hot to touch with around 1 kW in such a situation.
Canceling the reactance on the open-wire line side of the balun made it
run cool.

Measuring the impedance of an open line with an antenna analyzer is
tricky. If you use a balun, it will act as an extra piece of
transmission line and transform the impedance. I tried to do it without
a balun with an Autek VA1 (battery operated) sitting on a plastic stand
-- it was difficult to adjust and I didn't feel the results were reliable.

What you can do if you know the length of the feedline and the details
of the antenna accurately is to use EZNEC to get the impedance at the
feedpoint and then TLW or a similar program to compute what it will look
like at the shack end of the feedline. This worked for me.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/


Vic

On 21 May 2017 21:00, Phil Hystad wrote:

> I am assuming that a question about the Elecraft BL2 is On-Topic (OT)
> as opposed to Off-Topic (OT)…
>
> 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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