OWA Yagi Versus Low Impedance Yagi
I am very much a fan of the OWA (Optimised Wide-band Array) Yagi as the benefits far out weigh the negatives. For the the narrow minded (or serious DX'er with no other consideration other than Gain and Front to Back Ratio) the low impedance antenna is the way to go. Below I have listed the benefits and negatives of each.
What are the OWA benefits?
Whilst it is highlighted else where on this site, I have presented the main benefits of an OWA antenna below:
- Wide Bandwidth meaning one antenna and service large portions (or all) of the required band
- Very low SWR meaning minimum return losses
- Near 50 Ohm impedance and therefore limited radiation or losses in 50 Ohm feed coax
- No Matching arrangement at the feed point of the antenna meaning easy construction with direct coaxal feed
- Having no match, no power limits for the antenna. Power input is limited by the feed coax only
- Very easy construction and self build capabilities as a result of the above
- Last and maybe most important - Due to its design, small errors when building WILL NOT drastically affect final performance
- Minimal losses, maximum radiation efficiency
What are the OWA Negatives
Well there are only two but fundamental ones. Forward gain and front to back ratio. Front to back being a really big issue with antennas with less than 4 elements. But, is this really such a big dis-advantage? With the very low loses seen with an OWA and the matching and narrow (shifting) bandwidth of the low impedance antennas, whilst it may not make a huge difference to Front to back ratio, is there really a big difference in forward gain?
What are the benefits low impedance Yagi?
- Better on paper forward gain over an OWA per metre boom length
- Far superior front to back ratio especially on antennas with 4 elements or less
What are the low impedance Yagi negatives?
- Narrow bandwidth
- Very small acceptable VSWR curve
- Has a tendency for tuning to be affected by close by object such as houses, other antennas etc
- Affected quite often by changing weather conditions, rain playing a big prt in moving or altering the tuning of the antenna
- Small errors in construction will result in an in-effective antenna system
- Matching arrangement required
- If matching arrangement is installed without testing the antenna first, major errors could be hidden by the match device and performance lost
- losses in matching arrangements are hard to measure and calculate. These losses could result in forward gain being no better than an OWA
So is there an alternative to the OWA and Low Impedance Yagi?
YES, the new G0KSC LWA (Low Impedance Wide-band Array) Yagi. This range of antennas have been designed to carry all the benefits of the OWA plus include the forward gain and front to back properties of the low impedance Yagi. How can this be done? The antennas have been designed with a feed impedance of 28 Ohms. The feed impedance has become popular by the use of the K6STI program Yagi Optimizer which optimises Yagi designs for best performance. Within YO these antennas normally finish up around 28 Ohms. DK7ZB being one such user has made this design very popular and has enhance popularity with a simple coaxial matching unit bringing the feed point (at the match) to near 50 Ohms.
The G0KSC LWA designs use this simple DK7ZB match arrangement to match a 28 Ohm antenna that has been optimised for wide-band use. In so doing, the Yagi loses the 'twitchy' nature of the low impedance antenna and adopts the many benefits of a traditional OWA Yagi with just a very small shortfall in forward gain and front to back ratio.
Interested in taking a look? The first designs are coming online now. If you have a particular design you are looking for, mail me with your requirements and I will see what I can do.
73 Justin G0KSC