Physically, the device is contained in a PVC tube approximately 34mm in diameter and 135mm long.
The information that came with it states the following:
- Matches 50-75 ohms coax to 200-300 ohms balanced.
- Broadband 1.8-40 MHz
- 2000 watts PEP max.
- withstands 650 pound pull
At the time I purchased this balun, the Unadilla website stated:
The W2AU is the preferred Balun of Amateurs worldwide. W2AU's operate at 1.1:1 or less VSWR from 1.8 to 40 mHz, and handle powers up to 2 KW PEP (at SWRs of less than 3:1). W2AU Baluns are transformer designs with ferrite cores to provide 95% minimum coupling efficiency. Each has a built-in lightning arrestor, hang up hook, and 600 lbs of pull-apart strength in a weatherized, rugged package using only stainless steel hardware and quality materials. Two models are available. The 1:1 matches 50(75) ohms unbalanced coax to 50(75) balanced dipoles. The 4:1 matches 50(75) ohm unbalanced coax to 200(300) ohm folded dipoles.When I made some initial measurements I found the SWR with a 200 ohm load to be in excess of the claimed 1.1:1 or less. When I complained to my supplier abount this discrepancy, he passed on my complaint to the manufacturer. Not long after this the website was changed to say "
W2AU's operate at a low VSWR from 1.8 to 40 mHz". Obviously, the interpretaion of low can mean just about anything! My personal opinion is that 2:1 or better is acceptable and low is anything that allows my tranceiver (ICOM IC-7400) to operate at full power without using an ATU (about 1.5:1 or better).
The table below shows the results of some measurements I made. The SWR was measured using an MFJ-259B antenna analyser. This was connected to the balun using a short length (40cm) of RG58. The balun output was connected to a 200 ohm non-inductive load. The insertion loss was measured with a HP8754A network analyser (no measurement on 1.8 MHz because it's outside the range of this device).
|Insertion Loss||Balun Loss (watts)|
|Frequency||SWR||db||%||@ 100w||@ 2Kw|
A minimum efficiency of 95% requires that the insertion loss be no more than 0.22db. In the case of my balun, this is only achieved on the 40m, 30m and 20m bands.
While I regard the insertion loss as acceptable on the HF bands, I would question the ability of this device to survive with 2,000 watts going into it, especially on 15m, 12m and 10m.
Of more concern is the mismatch introduced by the balun itself. Given the manufacturer's claim that this device functions up to 40 MHz, one can only assume that they considers 4.7:1 as a low SWR.