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write your own review of the AS-25-DIN 2:1 50 to 25 ohm Balun.
May 17, 2011 10:15
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Time owned: 0 to 3 months
As part of an overseas High Power HF project, a SteppIR DB-36 was modified for high power using an external balun made by Array Solutions. The Balun was ordered under part number AS-25-DIN Custom Balun 7/16 DIN. This balun was designed to handle 10 KW PEP and 5-8 KW CW. After a two month wait the balun arrived. Initial inspection reviewed some disappointments. The 7/16 DIN connector had been mounted to high on the plastic enclosure and the top lid had to be filed to provide clearance for the lid gaskets to seal properly and keep the moisture out. Also the free leads for connection to the dipole element of the SteppIR DB-36 beam did not have lock washers on their connection points inside the balun enclosure and with time would become lose. The proper lock washers were added for the installation. But before installation, we tested the balun.
The dipole leads were disconnected and a 25 ohm resistive load was placed across the balun output. I ran the VSWR across the band with an AIM-4170 (Array Solutions) analyzer and the VSWR curves are shown below.
Input Frequency MHz-VSWR-Resistance measured at input.
1.8 -1.2:1 -55 Ohms
3.8 -1.2:1-57 Ohms
7.2 -1.2:1 -55 Ohms
10 -1.3:1 -60 Ohms
18.1-1.3:1 -54 Ohms
24.8 1.4:1 45 Ohms
28.4 1.5:1 40 Ohms
The balun had a 2:1 dual core balun for the 50 ohm to 25 ohm transformation and the second dual cores are for a choke balun. When we received the balun two 33 pF 6KV caps were placed across the 25 ohm output to balance the VSWR input response. This is a problem for the capacitors must dissipate their reactive component as well as the antenna as the load for the power applied to the balun. I tested the series capacitance to ground at the output of the 2:1 portion of the balun. As you can see from the above chart the balun had a definite reactive component above 7 MHz. A variable trimmer was added to the 2:1 output before the choke balun input and the reactance was tuned out with the addition of 70 pF across the 33 pf. The output caps (66 pF) was removed from the output choke balun and added to the original 33 pF for a total capacitance of 99 pF at the output of the 2:1 balun. The new VSWR curves are shown below.
Input Frequency MHz- VSWR-Resistance measured at input.
1.8 -1.2:1 -49 Ohms
3.8 -1.2:1 -49 Ohms
7.2 -1.1:1 -50 Ohms
10 -1.1:1 -51 Ohms
14.2 -1.1:1 -52 Ohms
18.1 -1.1:1 -53 Ohms
21.3 -1.1:1 -52 Ohms
24.8 -1.1:1 -49 Ohms
28.4 -1.2:1 -43 Ohms
With the balun looking good on the network analyzer it was time to put some RF to the balun and look at the temperature coefficient of the design and application of the components.
This balun was to be operated well above 5 KW and I did not have the amplifier available for the test so we would have to do a best case test. A modified amplifier running a pair of 4CX1000Aís was used and 2500 watts CW was applied to the balun on 40, 20 and 10 meters. The load was a pair of 1000 watt bird 50 ohm loads in parallel for a terminated resistive impedance of 25 ohms.
The first test was run with the capacitors in their original location on the output of the choke balun. After 60 seconds the 66 pF capacitors measured 170 degrees F. The second test was run with the capacitors removed from the output of the choke balun and placed at the output of the 2:1 balun and after 60 seconds the temperature was just a bit over 102 degrees F. Toroid temperature was just a little warm about 110 degrees F after a 60 second test. The test was run several times over a ten minute period and the balun component temperature never got above 140 degrees F.
I realize this is not the same as a 5 KW test but it gives a window into the performance of the balun in general. Remember the balun will have the top on and heat dissipation will be much less and it will be located in a hot environment. As for VSWR, the SteppIR antenna will tune any capacitive or inductive reactance by adjusting the element length.
Over all the construction of the balun cores and wiring were top notch. My disappointment had to do with the in attention to the final product hardware mounting and to letting the product ship without adjusting the balun for absolute best performance knowing it would have very high power applied.
The balun was supposed to have an analyzer print out of the VSWR, reflection loss, displayed reactance etc. over the band of operation. About a week later one was sent to me. As a result of the above snafuís I felt I could only give it a 4 out of 5 on the review. I am sure these small teething issues will be taken care of in the production units.
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