[Antennas] Balun Measurements

2002年1月06日 19:40:09 -0500

Hi All,
I would like some input on some measurements I am making of a balun.
I have built several baluns and have used them on some balanced 
transmission lines with SWRs of up to 10:1 feeding a W3EDP and a G5RV 
antennas. The balun didn't get warm or blowup as some predicted. However, 
I want to measure the loss in a particular balun in a little more 
quantative manner other than to place one's finger on it while applying 
power. The balun is a small 1:4 current balun that I have run up to 100 
watts, wound on a FT114-43 toroidal core. I wanted to first determine the 
loss at a matched condition of 50 ohms to 200 ohms.
So I took a plastic thermos jug, added some additional insulation, and 
placed the balun inside with short lengths of small coax for the input and 
a twisted pair for the output. I also inserted a thermocouple and ran it 
to a temperature meter. Outside, the 200 ohm side of the balun connected 
to the 200 ohm side of a large commercial 1:4 balun whose 50 ohm side 
connected to a dummy load with a digital readout in watts. On the input 
side going from the transceiver is a Z11 Auto Tuner, a 150 watt, watt meter 
and an OHR milliwatt meter meter for reading reverse power in the milliwatt 
range and low power when needed. After first letting temperatures 
stabilize, I applied power of 40 watts from a Ten Tec Omni VI transceiver 
for 10 minutes, off for 10 minutes and on for 10 minutes then I let 
temperatures stabilize again. You can reduce the power of the Ten Tec down 
to zero and up to 100 watts, but the finals overheat if the power is run 
continuously for the entire 20 minute period. This power cycle produced a 
temperature rise of 9 degrees. I removed the balun and replaced it with a 
non-inductive 50 ohm 15 watt resistor, and ran low power to it on 
successive runs until I got the same temperature rise in the same time 
intervals as before. The power to the resistor was 1.5 watts. Thus the 
loss in the balun is 3.75% or .166 dB, and efficiency is 96.25%. The SWR 
into the combination of baluns was 1.15:1 at 21.1 MHz measured by the OHR 
milliwatt meter.
That was the easy part. Now to measure the loss at high impedances, I am 
planning to use a Johnson Matchbox with the 50 ohm side connected to the 
dummy load and the balanced transmission line side connected to the 200 ohm 
side of the balun under test. I should be able to change the impedance of 
the Johnson Matchbox while tuning for a non-inductive load on the balun. I 
also can use the large 1:4 balun to increase the Zo up or down. To add 
some reactance, I can detune the matchbox a little. I will measure the 
loss as before. To determine what the load is, I will have to read from 
the 50 ohm side of the balun under test using a Autec VA1 antenna 
analyzer. It will only read Z up to 500 ohms and a SWR of about 15:1. I 
can use a non-inductive resistor in place of the match box to verify the 
reading.
One problem is that the temperature meter reads in degrees with no 
fractions, so that limits the accuracy of measuring the temperature rise.
So comments, questions. Have at it.
TNX for your assistance.
73, Chas, W1CG