The importance of low noise antennas on 6M

Why DK7ZB is wrong low-noise antennas on 6m

It was recently brought to my attention the Martin, DK7ZB suggests on his website that 'you do not need low noise antennas on 6m'. He also quote Lionel VE7BQH as saying the following:

' Very low side lobes and F/R on this band produce no specific improvement in G/T based on pure sky noise and ground noise. Assuming reasonable side lobes, F/R and good VSWR bandwidth, Gain is the most critical parameter on 6M."

Martin then goes on to say you do not need low noise designs on 6m and strange feeding systems. I guess the following will be an education for both you the reader and DK7ZB.

First of all, let us analyze Lionels statement. 'Very low sidelobes and F/R (front to rear) on this band have no specific imporvement on G/T based on pure sky noise and ground noise...'

Lionel's reference towards low noise is not applicable to 6m for sure. Lionel refers to sky temperature and the fact this assumption is based on 'pure sky noise and ground noise' (NOT man-made noise form TV's ADSL Routers, Power supplys etc.). The G/T figure he refers to is a mean opinion score created by a calculation between the antennas sky temperature and it's gain. The resulting G/T (Gain/Temperature) figure is the antennas ability to recieve signals. This form of measurement is now widely used by 2m/70cms and up EME users in order to calculate an antennas receive effectiveness. However, this G/T and sky/ground noise has no effect on 6m. Infact it is only just starting to come into play on 2m and as a result it is purely the G/T figure which is important at 2M not gain or sky temperature individually. However, on 70cms an antennas sky temperature really is important in addition to the overal G/T figure. This was highlighted in a recent DUBUS article when one YU7EF low noise 70cms Yagi was able to hear signals than FOUR DK7ZB antennas (of a similar boom length) could not. Simply for the fact that design considerations had not been taking into account for the band the antennas were being modelled for.

 Refering once again to Lionel's comment '...based on pure sky noise and ground noise..' We know this measurement has no place on 6m and should not be a factor in an antennas design. However, we do know the considerations for noise reduction on 6m take on a whole new meaning and a different set of parameters should be met in order to achieve the best 'User Experience' for the 6m Ham. The most important of these parameters being the ability to reduce 'Real World' noise for which 6m is renowend. OK, you may be wondering exactly what this is if you live in a remote farm somewhere. The 'Urban' or city noise consists of almost everything electrical. In addition to the items mentioned at the begining of this paragraph are heating systems, computers, alarm clocks even. Almost everything around the house has the ability to produce noise and quite often, it ends up on the 6m band!

Next, let us look at the final section of Lionel's statement 'Assuming reasonable side lobes, F/R and good VSWR bandwidth, Gain is the most critical parameter on 6M.' So, While giving not set parameters, Lionel suggests that if an antenna has low side lobes, F/R (Front to Rear) and SWR bandwidth are good, then gain is what counts. Albeit this is very general and does not segregate between city and country locations for the 6m station let us work with this. Below I have produced a cut down version of the VE7BQH list including just these important parameters that in Lionel's opinion, determine whether or not you can except gain as the most important parameter on 6m. With one exception. I have increased the bandwidth on 6m to measure 500Khz rather than Lionel's 300KHz. I beleive it is pretty much accepted that the 50 to 50.500MHz is the section of 6m that an antenna should cover effectively. As an antenna with a 1:1.5 SWR banwidth over 300KHz could be very twitchy an unreliable, judging antennas over such a short bandwdith could present users with information that and antenna is good when perhaps it is not.

Antennna                                                  Worst SWR (500Khz)         F/R                   F/B                           Worst forward Side Lobe      Notes

DK7ZB 3el 12.5Ohm 1.9m Long               1.89                                13.81dB         19.63 dB                    None 

DK7ZB 3el 28Ohm 1.9m Long                  1.36                                10.51dB         24.37dB                     None

G0KSC 3el 50Ohm 1.94m Long               1.28                                14.58dB         26.64dB                     None

G0KSC 4el 50Ohm 2.08m Long               1.16                                12.93dB         21.25dB                     None

YU7EF 4el 50Ohm 2.35m Long                1.13                                16.05dB         17.85dB                     None

G0KSC 4el 50Ohm 2.46m Long               1.065                              13.51dB         16.27dB                     None

DK7ZB 3el 28Ohm 2.5m Long                  1.76                                17.09dB         15.27dB                     None                                            Error in published figures? SWR did nto match website

DK7ZB 4el 12.5Ohm 2.85m Long            3.85                                 13.12dB        23.47dB                      Elevation plane 105 deg           

With anything I do I set out an SoW (Statement of Work). Establish what the requirements are, what are the pit falls and disadvantages of any existing solutions, what (if any) parameters that do not exist in existing designs which would be nice to have and then look to establish a solution that will produce as many positivies and as few negatives as possible.

Below I have detailed the parameters that helped me the 6m antennas that I beleive will lead to reduced man-made noise and static that most hams are plagued with. I always beleive it is best practice to detail why rather than to say something is not needed without explaination.

3 Dimentional Pattern

Many antenna 'designers' model their antenna within YO (Yagi Optimiser) a very old program designed by K6STI which is no longer available. The package requires very little user input or experience to produce an antenna with Gain and as such many antennas have been designed, produced and published which fullfil a gain requirement. However, with a little more time, consideration, foresight and patients a much better alround antenna could be achieved with perhaps just .1 or .2dB gain the sacrifice. One of these such parameters is that of the elevation pattern.

Most hams simply look at the Azimuth pattern. This is the holistic view as if looking down upon the antenna. The gain, F/B (front to back ratio) and side lobes can be seen from this plot. It is also the chosen position for many to model their antennas in the Azimuth plane and this is clear to see if the respective elevation plots are seen whether or not this is the case.

As any 6m enthusiast will know, one of the biggest 'killer's of 6m is noise. Not noise from the sun or earth, but noise from house hold appliances. Plasma TV's, ADSL Routers, Switched mode power supplies and so on. Additional issues come from close by power lines and even rain static which can deem the band unusable. Now look again at the antenna you are conisdering to build. Look at the elevation plot and take note of the elevation lobes. If these are just 15dB down on your forward lobe congratulations, You Neighbours plasma TV will be just over 2S points down on the DX you want to receive! Furthermore, consider the potential interference you could give too with the huge levels of RF being presented to these nearby appliances.

Not all of my antennas have super-high suppressed elevation lobes and these antennas would not be recommended in 'Urban' or city locations. However, ALL my antennas have been modelled to include elevation lobe suppression and therefore most antennas you will find on this site will have the high levels of RF in the elevation other designs on the Internet will give you.

Side and rear lobe suppression

Another consideration for me is high side and rear sidelobe suppression. For many year 20dB F/B is consdered a target. For me this is not good enough having signals from behind the antenna just 3S points down on those in front. However, I will not produce a very high F/B in my antennas if it will result in unacceptable elevation lobes, so there has to be a balance.

The LFA (Loop Fed Array) is a low impedance antenna with a 50Ohm input. The loop itself is both the matching deivce and radiator in one. As well as limiting losses in matching devices, it provides a very flexible basis form the computer optimisation tools to work with. Rather than having a set impedance, the optimiser is able to adjust the loop in all directions in order to maintain a 50Ohm input impedance and therefore, the optimiser can be much more flexible and achieve better results. If an antenna is optimised with a loop installed, very good levels of side suppresion can be achieve and side lobes kepts to an absolute minimum. Afterall, if you have side lobes just 20dB down on your main lobe, you may still receive noise from a nearby appliance well after your beam has turned away from that direction. How good it that extra .2dB or gain now? If you can not hear them, you can not work them.

When Joe, CT1HZE first installed an LFA for 6m, his first comment on the ON4KST was this 'The front to back on this antenna is amazing !!' Later Joe said to me 'It is so nice not having to listen to the rest of Europe now when I am working North America. Joe now has installed a much larger 10 element LFA for 6m and a 9 element LFA for 4m.

High levels of side and rear suppression DO make a difference to what you can hear and ultimately work. If your antennas pattern is huge in every direction, every bit of noise or stations in other directions are much more likely to trouble you when you are listening to those weak DX stations

The Closed loop

Or should I say 'strange Yaig feed system' as Martin DK7ZB calls it! Has been aroudn for a very long time. It is the radiator of choice (in a folded dipole form) for many DX'er on VHF and UHF due to the low noise characteristics it possess as they are much less susceptable to man-made noise and static then a split dipole its. This is one of the reasons Quad lovers, love quads!

the only dissadvantage with a folded dipole is tend to slightly distort the elevation pattern due to the fact they are fed one side of the other and or are not inline with the parasitic elements. This is an issue overcome by the LFA loop being laid flat on the boom. In addition to the above mentioned, adding a grounded Pawsey stub (there is an earth connection to the boom from the shield of the coax 1/4W/L back) to the loop fed Yagi provides a DC ground to the whole radiator and as a result, further suppression of any noise picked up by the antenna (and potentially by the coax too) is seen. Mohamed A92GR said '..The powerline noise to the North East has completely dissapeared' while JO OE4VIE commented '..I no longer have rain static! I can use 6m no matter what the weather!'. These being just 2 of the many comments I receive on this subject so I am sure it is not just me that these antennas are low noise!

Ohmic Losses and return loss

There are certain areas which are diffuclt to measure and quite often Theory comes into play on what happens in certain circumstances. The introduction of YO lead to mass producing of bad antenna designs through the abuse of the tool, the user not really knowing how to use it or what parameters should be aimed at to produce a good antenna. Furthermore, these deisngers just looked to what had been done before rather than what could be better.One thing is for sure that there are a number of losses that can and do occur within an antenna and it's feed system which do not just lead to loss, they can result in noise too.

one such area is that of impedance. You maybe looking at a 28Ohm antenna or a 50Ohm antenna but for how much of it's bandwidth is it 50Ohm? If you are to look at the impedance 'curve' in many instances this resembles a 'Ski lope' the the actual antenna impedance being 40Ohm at 50.300Mhz and perhaps 18Ohm at 50Mhz for example. The actual point at which this antenna is 20Ohm is very small indeed and all manor of difficult-to-measure losses could occur inbetween. This slope in impedance is one of the main reasons for wet weather instability too.

Most LFAs have a constant impedance with a small variation through the antenna bandwith. Therefore, the 50Ohm feedline from your shack connects to a 50Ohm antenna, not something 10 or 20Ohms either side of this (after respective matching in some cases). Noise induced in this way maybe small but answer this question; Why have it when you don't need it?