Don't have the wool pulled over your eyes

Recently I found an article on the Internet claiming 'state-of-the-art' antenna designs for 2m. While reading through the document I winced and gasped at the claims and play on words. I do not normally react to things like this but due to the way this document was written, readers may very well be mislead into believing 'facts' that are either not true, unproven or are a 'bending of the truth'.

The primary focus of the above article seems to be to encourage the reader to believe that the antenna design discussed is 'as good as it gets' but does not highlight the issues of such a design and why most designers will simply not explore antennas designed in this way.

The antenna in question is quoted as 50Ohm and 'natural impedance'. This is despite the fact there is a huge variation of impedance throughout the bandwidth of the antenna with only a very small portion being anywhere near 50Ohm. This 'Ski Slope' impedance curve will lead to antenna instability and is one of the issues of this antenna style (boom too long for the number of elements in use) which I have covered several times within my published articles.
The author states the antenna was designed within the 25 year old Yagi Optimizer (YO) by K6STI which is quite obvious to see as the issues and pitfalls associated with antennas produced within YO are clear to see.

The antenna in question (and indeed a selection of other designs on the same site I viewed) have been designed with the 'gain, gain, gain' mind-set with YO being left to adjust the boom length to whatever it wishes . This has led to YO choosing the easy option for gain (lengthening the boom) to such a degree, the antenna has become extremely unstable due to its steep angle impedance plot.

A Ham lead to Believe Ohmic loss is more relevant than it is

Within the pages of DUBUS magazine earlier this year, I presented the OWL (Optimised Wideband Array) and proved a low impedance antenna does not need to be narrowband. Further, I proved that this misconception about low impedance Yagis came from the abuse of YO and the instabilities caused by spacing being far too wide between each element, especially the driver cell (first 3 elements).

My article went on to discuss the fact that with low impedance antennas, element density should be higher (per metre of boom) than with 50Ohm versions, not less dense which is a common misconception and the primary reason low impedance antennas have this narrow band tag.
It also discusses that if correct boom lengths are used, low impedance antennas can achieve much better wideband performance (in all aspects) than higher impedance antennas (per metre of boom) and discusses how important a constant impedance is to the production of a stable, reliable and quiet antenna.

The 'state-of-the-art' design in question has been extended in length (with a design bias on gain) by such a degree that only a very small portion of its bandwidth is indeed 50Ohm. In fact, the impedance varies almost 30Ohm within this very narrow bandwidth (from around 40Ohm to around 70Ohm). What impedance should the coax be feeding such an antenna? What would be the resulting losses (feeder and otherwise) if such an antenna were deployed as part of a 50Ohm transmitting station? Furthermore, without having a constant impedance across the antennas pass-band, huge instability within wet weather conditions (and were other object are nearby such as other antennas, houses, trees etc.) would exist.

It is safe to assume that when damp, an over-length Yagi of this nature would move well into 143MHz (from 144MHz) and prevent the ham from using his antenna until the antenna was dry once more. This very real fact is not discussed within the article at all. Like those who have contacted me about such designs, readers of the article who adopt these 'theories' could end up finding these flaws the hard way.

The article also claims 'World record gain for a 7 element'. Antenna gain comparisons are made against boom length, not the number of elements an antenna has. The reason no other 7 element Yagi produces this level of gain is the fact that not too many respected antenna designers would allow an antenna with that number of elements to be so grossly over-sized. With 8 elements within the design boom length, a far more stable antenna could be produced with excellent stability, much better F/B and better side lobe suppression. All for the sacrifice of perhaps .1 or maybe .2dB gain.

The author goes on to compare against a wideband low impedance antenna (of his own design so not optimal) and despite this antenna having a much better impedance curve, rates it second due to being .2dB gain less than the unstable version (6dB in a [True or not?] single S point on the signal metre so how much difference does this .2dB make?).

This said, his OWL version is still far too long for a low impedance Yagi and as discussed above, should be shorter than any 50Ohm equivalent. As a result, F/B and F/R (front to rear) for the low impedance antenna are less than the high, variable impedance antenna he promotes. If correct boom spacing was adopted on both antennas, better gain per metre of boom would be seen in addition to antenna stability. Sadly, It is this fixation upon 'gain, gain, gain' that blinkers would-be antenna designers into producing and publishing models such as these.

There are many more claims relating to Yagis and feed systems which play on the truth along with the use of 'the power of suggestion' meaning again hams could be mislead into believing what they are seeing is something other than just smoke and mirrors.

My advice when choosing a Yagi is this; Speak with independent and well-known hams and technical writers and ask their opinions on what is right, wrong, good or bad. Lionel, VE7BQH who compiles the 2m G/T tables recommended his friend VE7XDT to build 4 x 8el LFA yagis , Dino has seen excellent results and receives better than far bigger stations. GM3SEK who has been a technical writer for the RSGB's RadCom magazine for many years, asked me to design his 28MHz Yagi. Mitsubishi Development in Florida USA concluded after software simulation and practical testing at the Northwestern University antenna range that the LFA was a significant step forward in the development of the Yagi. Need I say more?

Conclusion

Within my many published articles I have always maintained that anyone with 5 minutes and YO can produce an antenna with gain. Skill, Dedication, Optimiser and Antenna Understanding are required in order to maintain gain while producing a wideband, stable, low noise antenna.

Putting gain into perspective once again, 6dB is required in order to see 1S unit increase in signal. My design criteria has always been to reduce gain and increase the user experience along with the ability for the antenna to receive weak signals.

My antennas are not just wideband from an impedance perspective, their performance is wide too. Although not an exact science, for each .1dB in gain reduction around 1dB less noise from every direction (other than the direction the antenna is facing) will be seen. Modelling in this way has seen some fantastic results.

Read the experiences of G3WOS and G4CCZ who migrated to the LFA Yagi from their former narrow band antennas. With the extremely fast take up of my designs around the world and the stunning EME successes users have seen, it is quite clear the direction I take is considered the right one by many.

There is nothing state-of-the-art about a high-gain, unstable, over-sized antenna with a 20KHz (or so) variable (dependant on weather conditions) bandwidth.
'Designers' have been making these very same mistakes for many years and I am sure will continue to do so until they allow themselves to be educated otherwise.

Recognition and Respect come from doing something different, something new and something not achieved before. Conducting experiments and practical tests with an open mind also help. Deciding there is only one way to design Yagis WILL limit self-learning and will not be conducive to producing premium antenna designs.

Perhaps if the author of this article finds my comments, he will use them constructively to better himself and ultimately, the antenna designs he produces.

Justin, G0KSC

Full Story: www.g0ksc.co.uk

Tweet