[SADXA] Output Impedance of Baluns

[W7EXG (Bill)]

Hi Wes, 

I actually ordered 3 Baluns from Balun Design.

WHAT WAS TESTED

1) 1/1 (for whatever) model 1115
2) 1.5/1 (for 75/50) ohm model 15113
3) 1/2 (for 25/50) ohm model 1213

OUTPUT IMPEDANCE TEST SET UP

1/1- I use a Resistive Dummy load at the input (both for 75 ohm and 50 ohm
input tests). 

1.5/1 and 1/2 - I used a 50 ohm resistive dummy load at the input. 

I terminated the output to close external stud terminals with a BNC
connector coupled closely between the outputs. 

OUTPUT IMPEDANCE MEASUREMENT

I used a RIG EXPERT AA-54 to measure both the [R + JX] and [R // JX]
equivalent models.

Frequency sweep range was from 3 MHz to 54 MHz.

Basically I was treating the balun as a black box by studying the input
impedance with appropriate output resistive termination, and the output
impedance with the appropriate input resistive terminations. From Classic
two port network analysis. 

I independently verified the resistive loads over the anticipated frequency
test range. 

RESULTS

All 3 baluns had different output impedance. Although the real part was
anticipated somewhat within an expected range, the imaginary part was
substantially different, from capacitive (-JX) to inductive (+JX) reactance.


As mentioned in my original comments, the 1/1 balun was inductive and the
75/50 ohm balun was capacitive. So what is substantial? 

On a 10m fixed element rotatable dipole at 40 feet, the resonant frequency
of the 1/1 balun with 75 ohm coax was (JX=0)28400 kHz, while the same
antenna/height using the 75/50 ohm balun with 50 ohm input coax had a
resonant frequency (JX=0) of 29600 kHz, or 4.2% resonant frequency shift. 

Although the percent change was small, for me, the over 1 MHz shift in
resonant frequency made me a very unhappy camper. 

This is what led me to characterize the output impedance of all the baluns I
had ordered. I suspected at that time, the reactance of each balun was not
the same. 

IMPLICATION: 

When someone has a fixed element antenna and replaces the original balun
with another, it is possible the resonant frequency of the antenna will
shift, if the output impedances of each balun were not the same. 

Worst case -- is when one buys a balanced fixed element antenna without a
balun, and purchases a balun without the manufactures recommendation. The
resonant frequency of the fixed element antenna will be a function of the
balun purchased, not the element length (AS IT SAYS IN THE ARRL ANTENNA
BOOK!!) Hi Hi 
(Of course one could compensate by tweaking the element lengths, but on a 5
element Yagi, perhaps one should just employ the correct balun replacement).


SUGGESTED MANUFACTURE'S SPECIFICATION

This is the reason I suggest that ALL balun manufactures should specify the
output impedance for any design ratio, and specify the input conditions
under which this impedance was measured. This is the ONLY way one would know
what balun to choose for their application. This would insure no resonant
frequency change will occur with the NEW balun attached to their OLD fixed
element antenna. 

I hope this answers your question?

Best 73's
Bill W7EXG

-----Original Message-----
From: sadxa-bounces at mailman.qth.net [mailto:sadxa-bounces at mailman.qth.net]
On Behalf Of Wes Stewart
Sent: Saturday, December 22, 2018 8:11 AM
To: sadxa at mailman.qth.net
Subject: Re: [SADXA] Output Impedance of Baluns

Bill,

Where did you get these data?

Wes

On 12/21/2018 12:50 AM, W7EXG (Bill) wrote:
> These comments were made by Bill Davis W7EXG at the 12-20-18 SADXA
meeting.
>
>
> OUTPUT impedance of Baluns is not alike.
>
> For example, the output impedance of a 1:1 Balun [from Balun Design], is
> Inductive with input terminated on 50 or 75 ohms.
>
> The output impedance of a 1.5:1 balun (75/50 ohm Balun) (from Balun
Design)
> is capacitive with input terminated on 50 ohms.
>
> Thus, for a fixed rotatable dipole element length, the resonant frequency
of
> the dipole using a 1:1 Balun (75 ohm Coax) will be substantially different
> than for this same dipole element length with the 75/50 ohm balun (50 ohm
> Coax).
>
> BOTTOM LINE: Output impedance of Baluns is important, yet this important
> characteristic is typically not specified.
>
> 73's
> Bill W7EXG

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