What makes a good Yagi great?
2 of 4 x 30 element Crossed LFA Yagis installed on the science block of the Princeton University NJ, USA (W2PU)
There is so much conflicting information on the Internet these days with regard to antennas, I thought I'd layout a few pointers here to what makes a good Yagi great and why the majority of modern antenna designers choose to model Yagis with a direct, natural 50 Ohm impedance that has good bandwidth. Often pattern shape and the impact on overall performance of narrow-band Yagis is over-looked during the antenna selection process. Hopefully, this passage will help you to understand the basics and help you select a more appropriate antenna for your needs.
What is 50Ohm impedance and why is it important?
Any antenna system has a feed impedance which will vary depending on the way it is made and its position in respect of ground. Most modern-day transceivers are made with a 50Ohm impedance in order that modern 50Ohm coax cable can be used to connect these transceivers to any antenna system. Naturally, the ideal feed impedance of any antenna connected to this feed line and transceiver is 50Ohm, this ensures the transmitting part of the transceiver remains safe and undamaged and at the same time, maximum transfer of power from the transceiver to antenna takes place.
Why do all antennas not have a natural 50Ohm feed impedance?
Many years ago, early designers of Yagis implemented a 'cut and try' method of optimisation. This is one where the elements are manually trimmed and then the antenna tested on an antenna testing range to see what changes have been made. While there was a certain amount of intelligence and prediction applied to this method of optimisation, it was painstakingly slow. Additionally, as more elements (and their respective positions are changed and re-sized) feed impedance will change. To achieve performance while at the same time achieving a direct, 50Ohm feed impedance was near impossible and therefore, a method of impedance matching needed to be implemented.
Is there any disadvantages to using matching devices?
Yes. A matching device is a transformer, it transforms the impedance of the antenna to the impedance the coax and transceiver needs in order to operate. If at this point we refer back to basic physics, we will know that there is no transformer known to science that is 100% efficient. In this case that inefficiency is where energy in the desired form changes to an unwanted form in in an antenna, these unwanted forms appear at noise (receive side of the antenna operation) and heat (transmit side of the antennas operation).
It is largely for this reason that any antenna you have with matching devices has a power limit to its function as so much of the input power (Watts) are lost as heat, that the matching device itself will break-down and could cause damage to the transmitter.
Can this issue of matching be overcome?
Yes. Design the antenna in order that it has a 50Ohm direct feed impedance without matching devices! In my opinion (and with modern antenna design and optimisation software being so sophisticated nowadays) a long time has passed since any Yagi antenna needs to be anything other than 50Ohm. Modern methods mean that the shape of the driven element can be altered to achieve a 50Ohm direct feed instead of adding a third-party matching device after the design phase is complete which would them provide all the disadvantages listed above.
What is the difference between using a matching device or driven element with bends or loop (LFA or folded dipole)?
This is an area were even respected designers fall short on understanding! Although very few amateur antenna designers who publish designs on the Internet use the latest available antenna modelling software, even the most modern of packages provide the ability to model accurately the performance and impact of matching devices when placed within an antenna system and therefore, these are added after model. A good example of such a failing is the associated performance of linear-loaded Yagis that are still insure today. Antenna designers over the last 20 year or so, designed and built antennas using linear-loaded methods that were so inaccurate, performance of the resulting antennas was very poor.
To this end, designers add matching devices in the real world (rather than in the software model design phase) and yet present performance characteristics of their antennas from the antenna software model and thus, the results are usually inaccurate and misleading to the potential user. These results will show show better results than they otherwise would if the matching device where place in software model.
When an LFA loop, folded dipole or bent element arrangement is used in order to achieve a Yagi with a direct 50Ohm feed, the whole antenna, as it will be used in the real world is modelled, predicted and confirmed in software and thus, the results presented will be far more accurate than when a matching transformer is added after model. This and the fact that usually, the only limiting factor to power handling will be restricted by the rating of the feed line only!
Do the negative results of matching devices remain constant at any frequency?
No. While the effects of the transformation of impedance are one thing we need to consider, the physical structure of the matching device itself is completely another and one which really comes into play as we more into upper VHF and into UHF, especially when crossed Yagi are used but why is this?
The matching device itself normally extends away from the driven element itself, normally outward in to the opposing plane (on a crossed Yagi example) or towards either the reflector or first director. This 'new' metallic object and it's associated proximately (which has not been modelled in software) no provides a de-tuning effect on the antenna which can lead to a performance degrade and additional loss.
So should I select a Yagi which uses a matching device or transformer?
If you are making a selection knowing the facts and that if the prediction plots you are presented with may not be accurate and the likely performance is less and the power restriction side of the arrangement is great than your need, then you have no issues. However, if your antenna selection is VHF or UHF, you should consider the noise generating in the receive path too, especially if your antenna is intended to be used for weak signal applications such as EME (Earth Moon Earth). G0KSC designs still rank at the top of the tree in terms of low noise on independent comparisons such as the VE7BQH list. View the link and compare sky temperature (lower is better) and G/T (the more positive the figure, the better) and compare for yourself.
Are you presented with all the facts?
In my opinion, there is no reason to use anything other than a directly fed Yagi with a natural 50Ohm feed point impedance, there are many benefits to this method, some of which have been discussed here. There are other considerations that should be made as a part of any selection such as pattern (to help reduce unwanted noise) and bandwidth (to provide a more constant, predictable performance) which we have not discussed here but these are important factors all the same.
For example, a badly designed Yagi will have large azimuth and elevation lobes which could result in receiving additional unwanted noise or causing interference to others. additionally (and perhaps more important) is the consistency of the performance the Yagi will provide. Often, Yagis are optimised for highest possible gain and element spacing is wider than perhaps it should be. This can result in 'ski-slop' type impedance curves meaning gain and F/B (front to back ratio) are not constant within their given passband. When I present gain and front to back figures, they are at a spot frequency (for example 14.10MHz or 50.10MHz). However, due to the above issue existing in many designs, others choose to present 'peak gain' and 'peak F/B' figures (which are normally at opposite ends of the band from one another) which again many would consider misleading.
Make an informed choice!
Hopefully the above information will enable you to be better informed when making a selection and deciding what antenna is best for you. Consider all parameters and compare all of the facts in any design or comparison to ensure you see the delivery of expected performance across a wide bandwidth enough bandwidth for your intended use and don't be mislead by outright performance figures at a spot frequency. If gain and F/B are not quoted at several points within your given band of choice, as the designer/provider for this information. Additionally, ask what would be the performance and functionality of the design during wet weather or icy conditions as many fall short in this area.
A good Yagi consists of a package of a number of parameters. For me, the most important of all is the optimisation of a Yagi for signal to noise ratio rather than gain, a factor that designers of more conventional style Yagis are fast to dismiss. I urge you to take a look around the Internet at the vast majority of users that see excellent results, results that provide constant, wideband performance with a lower level of unwanted noise that they saw previously. Sure, there will always be the exception to the rule, those with hidden agendas and otherwise. However, in the main, the results and benefits are clear to see.
One such example is provided below, VK2GGC installed a 6 element 28MHz LFA Yagi along side a TH6 multibander for comparison during a contest. While (as expected) the monoband LFA Yagi has noticeably more gain than the TH6, the difference in background noise between the two is also marked. View the video and make your own mind up!
If you need help selecting one of my designs be in self-build on this site or one of my latest designs through InnovAntennas or Force 12, Email me and I will be happy to discuss this with you.
All the best, Justin G0KSC - justin @ G0KSC .co. uk