4 x 17 LFA yagi assistance please

[VK7DX]

Hi Leigh,

Have a look here
www.grantronics.com.au/docs/stkyagis.pdf

cheers
Frank

[VK2OMD]

...
1 There has been some negative discussion regarding the "pseudo pawsey stub" feed system from the designer of the LFA antennas. I see all references to this have now been removed from his website - an interesting move? Would be interested to know if this feed system is the one you are using.
...

You mean like http://www.vk1od.net/balun/G0KSC/index.htm !

AFAIK, he still has his description of his bodged interpretation of the Pawsey balun in his article linked from my page, but he "no longer recommends" it.

I did write some notes on the flawed explanation of the loop and radiation cancellation, see http://www.vk1od.net/antenna/Innova/lfa.htm .

This doesn't mean they are bad antennas, it just questions their knowledge of fundamentals.

Owen

[VK2OMD]

Can we assume you are using a choke 1:1 balun as indicated as a requirement.....in addition to anything on the antenna..

The seller offers an Ugly Balun with no performance data, modelled or measured. G0KSC shows how to make one at http://www.g0ksc.co.uk/creatingabalun.html (right down the bottom), but no discussion of performance.

Symetric drive current is critical to achieving the improvments that highly optimised designs claim, it beggars belief that there is so little attention to ensuring that the balun is effective.

Owen

[ZL1RS]

Hi Leigh,

Have a look here
http://www.grantronics.com.au/docs/stkyagis.pdf

cheers
Frank

The section "Users of store bought Yagis" is misleading. The formula given is for individual 1/4 wl transformers for each Yagi in 2, 4, 6 and 8 Yagi arrays that are then all paralleled at a center point to give 50 ohms to match the main feedline. The diagram is correct for connecting 2 Yagis, but to use a 1/2 wave power divider as illustrated for 4, 6 and 8-way matching requires different impedances and D/d ratios in each of the 1/4 wl sections ...

A 4-way halfwave divider (2 Yagis connected at each end of the divider illustrated) requires two 50 ohm 1/4 wave sections and the correct D/d ratio to achieve this in air is 2.30.

A 6-way halfwave divider (3 Yagis connected at each end of the divider illustrated) requires two 40.82 ohm 1/4 wave sections and the correct D/d ratio to achieve this in air is 1.98

An 8-way halfwave divider (4 Yagis connected at each end of the divider illustrated) requires two 35.35 ohm 1/4 wave sections and the correct D/d ratio to achieve this in air is 1.80

73,

[ZL1RS]

The three horizontal members of the H frame are a worry. The middle one can be metal but if the top and bottom members are also metal then this will surely affect the pattern.. 37dB is a tremendous F-B and I'm sure it would not take much pattern distortion to collapse that value. ....

Modelling different stacking frames (on another Yagi) tends to support this worry:

no stacking frame

hmmm ...

ooops ...

And then there is the metalwork of that impressive tower stuffed into the bottom 1/3 of the array ...

However, given the other comments about the side lobes and main lobe, I can't help but feel there is something else awry in the array as well.

73,

[VK2OMD]

Hi Leigh,

Have a look here
http://www.grantronics.com.au/docs/stkyagis.pdf

cheers
Frank

The section "Users of store bought Yagis" is misleading.
...

For those searching, the URL is actually http://www.grantronics.com.au/docs/StkYagis.pdf .

Bob, that paper has several errors.

Another is in Fig 6 where "unequal lengths" is used in an overly restricted way.

The late Keith Malcolm cited Fig 6 to proved that my stack of two 50Ω Yagis with 1/4λ of 75Ω line to one Yagi, and 3/4λ of 75Ω line to the second inverted Yagi broke the "rules"... the rules according to Gordon. The only downside of my arrangment was that there was 0.068dB of additional loss in one leg which is inconsequential in the real world.

I guess that is what happens when you try to give people solutions rather than concepts.

I did write Gordon, but he was not interested in fixing the document at the time.

Owen

[VK2OMD]

...

Steve & Alan, tnx, no one has told me about any + or - and was nothing in instructions, which didn't give a great deal of detail. All i can say is that I have the coax connections set up the same on all 4 antennas.

These cost me quite a lot of money as I wanted something that was going to work extremely well, with no mucking around and reliably stay up there as I have to get a crane to lift all this up, looks like I might have to pull it all down again

Maybe it is just impossible to reverse the polarity because of the way the baluns are made (assuming you bought Justin's Ugly Baluns). If they were from Justin, perhaps you can ask him if there was any chance you could reverse the polarity.

I suspect that this is not an issue, but having one array out of phase will fairly mess things up... so it is worth covering off!

If you aren't 100% sure that the polarity of the connections is correct, you could try in-situ to verify that by feed pairs of Yagis using one as the common reference to see that they all exhibit the same pattern behaviour in pairs. This assumes you have safe access to the power divider with the system in-situ.

BTW, I didn't see a report of VSWR which might give confidence that ALL four antennas are connected.

Owen

[VK4BG]

Hi Guys,

I tried a 6 meter, 6 element LFA Array from this manufacturer. It was well made, and went together OK...after I chased up the assembly instructions as none were provided with the shipped antenna...same as Kevin , VK4UH's experience. Mine had the pawsey stub type matching thingo, and I made an ugly balun as per the instructions. It appeared to work OK, but I could not see any difference in performance compared to my previous 5 element homebrew yagi for 6.

Running a VNA over it, came up with nothing like the matching claimed on the website...granted, the website provides " models " of what it MAY achieve in real life performance. Removing the pawsey stub appeared to make absolutely no change to anything.

Rain static was not reduced one iota, but the antenna did appear to " work " reasonably well...but all the hype about the LFA...I'm not convinced there is any benefit. It has greater windage compared to a normal split dipole yagi, due to the large loop, has four weak points on the sliding junctions for the loop tuning given some time out in the fresh salt air and sunshine up here, and was mechanically not as strong as my previous yagi...movement in high winds was impressive.

Yep..it worked, but my rather unscientific tests did nothing to convince me it was worth all the cost and effort. I'm presently using an Ultrabeam on 6, and it is certainly not that crash hot on this band...but, bugger me, I'm still getting good reports from the JA's even on some very weak recent openings. If I had plenty of room like Lee, I'd love to fiddle a bit more with the LFA, but my small backyard makes antenna testing a bit of an educated guess most days, as comparisons are difficult if not impossible to do.

Good luck with it all Lee..I'll be interested to see what you eventually discover.

73
Glenn
VK4BG

[VK2KRR]

Thanks all for the input, im a bit pressed for time this morning so cant respond just yet, but I'll just add that I did get a reply from Justin in response to my initial email, he did ask about the positions of the 3 horizontal cross bars in the H frame, and he also mentioned -
"Second is having one or more antennas transposed wired. This is the most common issue. Just one antenna being reverse-wired will throw the system out"

I pressume this may relate to something like what others have said with the + & - side of the loop and the article VK7DX posted.

[VK4OX]

Hi Bob, ZL1RS,

Thank you for confirming me apprehension re- the horizontal, triple strut H frame.

Leigh,

When I first saw the pictures of you fantastic array, i thought, what a pity you don't have elevation control. Not only could you do EME but you could check out the performance of the array, (at least the frontal pattern) much more reliably with Sun noise measurements.

If the array has to come down, it might be worth considering temporarily erecting the array close to the ground with fixed az and el such that the trajectory of the sun will cross the boresight of the array during it's daily passage. Elevation should be at least 30 degrees to eliminate ground noise. Get a baseline noise measurement when the Sun is well away from the boresight. As the Sun approaches, plot the sun noise as a function of time then convert the time scale to angular degrees and you will get a plot of the main front lobe, first sidelobe and possibly the second sidelobe... The plot will not be of the E or the H plane but of some intermediate plane depending on the array's orientation to the Sun's path for that day. For accurate results, your rx should be linear, (AGC off). An RMS, audio voltmeter with dB scale connected across the rx audio output should be a good enough detector. You will have to take into account the usual (S+N)/N "problem". I can provide further details if you wish. Perhaps Owen, VK1OD, has an opinion/advice on this idea.

73, Adrian. VK4OX.

[VK2OMD]

The plot will not be of the E or the H plane but of some intermediate plane depending on the array's orientation to the Sun's path for that day. For accurate results, your rx should be linear, (AGC off). An RMS, audio voltmeter with dB scale connected across the rx audio output should be a good enough detector. You will have to take into account the usual (S+N)/N "problem". I can provide further details if you wish. Perhaps Owen, VK1OD, has an opinion/advice on this idea.

Hello Adrian,

In my experience receivers are likely to overload (ie become non-linear) not much above the onset of AGC. This is more marked on those with DSP detectors, overload of the ADC converter is sudden and severe. So, I know that it is popular to turn AGC off to improved linearity, but I would argue that it doesn't do that, and you have lost the warning that movement of the S meter provides of non-linearity.



The plot above shows that the error measuring RF power with a TS2000 with AGC off, is gross above -102dBm, AGC threshold is -104dBm.

So, the options are to use the receiver below the onset of AGC (by using an external attenuator as necessary), or to reduce RF gain so that the S meter NEVER moves during the measurements. The latter degrades rx NF, though only a little for S meter deflection below about S8 on many receivers.

At the end of the day, if you are going to make measurements with a receiver, prove any propositions by measuring the receiver behaviour rather than depending on folk lore.

Yes, measurements of the audio output should ideally be true power, and you do need to back the noise out of the total power to get a plot of S power over time whilst the Sun transits your bore sight.



Above is a plot of Rex's (7MO) antenna gain which I created from data he obtained using NFM (http://vk1od.net/software/nfm/). NFM allowed the quite high resolution measurements of true power (look at how the dots fall on the curve fit), and automated the data capture. You are unlikely to obtain this accuracy without some automation of measurement.

The curve provided confirmation of the main lobe beamwidth which is a good cross check on expected gain of the antenna. You cannot get gain without directivity, this proves the directivity so if the system is efficient (ie low loss), gain follows.

To save some responses, this level of measurement and quantitative analysis is not everyone's cup of tea... I know that, we all know that.

Owen

[ZL1RS]

... Bob, that paper has several errors.

Another is in Fig 6 where "unequal lengths" is used in an overly restricted way.

The late Keith Malcolm cited Fig 6 to proved that my stack of two 50Ω Yagis with 1/4λ of 75Ω line to one Yagi, and 3/4λ of 75Ω line to the second inverted Yagi broke the "rules"... the rules according to Gordon. The only downside of my arrangment was that there was 0.068dB of additional loss in one leg which is inconsequential in the real world. ...

Owen no doubt had the "polarity" of one feed point transposed to counter the 180 degree phase shift through the extra 1/2 WL of 75 ohm phasing line in the 3/4 WL leg. The same feeding/matching arrangement for short boom Yagis using 1/4 WL + 3/4 WL of 75 ohm coax and a 180 degree feed point phase change was carried in the VHF antennas section of the ARRL handbook (since the 1970's?) and used on the previous 6-over-6 6m LFA array here. It seemed to work OK (1.2:1 overall array SWR and audible EME echos at moonrise indicating there was a good amount of gain from the two stacked 6 element LFAs).

The diagram presented in a less restrictive way ...

73,

[VK2OMD]

Owen no doubt had the "polarity" of one feed point transposed to counter the 180 degree phase shift through the extra 1/2 WL of 75 ohm phasing line in the 3/4 WL leg. The same feeding/matching arrangement for short boom Yagis using 1/4 WL + 3/4 WL of 75 ohm coax and a 180 degree feed point phase change was carried in the VHF antennas section of the ARRL handbook (since the 1970's?) and used on the previous 6-over-6 6m LFA array here.

I didn't for a moment think that I had invented something new, just applied my mind to solving a problem. In this case, an extra half wave of coax that the birds can attack.



In the above, the two Yagis are identical, but the lower one is rolled over 180°, you will note the gamma arms are on opposite sides and so there is the 180° phase adjustment at the feed point. The coax here is neatly contained in PE tube... because the cockies just love vinyl jacket.

Owen

[ZL1RS]

FWIW, a simulation of the same small array I used for the H-frame comparison above with a single thick cross boom 300mm in diameter (perhaps similar to using a lattice tower section?) ...

with 300mm thick cross boom

(note: there was a "segment error" warning displayed with this simulation)

And with two 2" dia cross booms spaced 400mm ...

with 2 cross booms

73,

[VK2OMD]

FWIW, a simulation of ...

All plots interesting Bob.

They do show that some of the claimed benefits of a single array (especially the very tidy rear end) are not likely to be realised on a practical n-way support structure, let alone in the real world.

Owen

[ZL1RS]

... last one, and I will shut up

Left: 4 Yagi array with 3 cross booms, all Yagis in phase (front lobe 19.9dBi gain)
Right: 4 Yagi array with 3 cross booms, one Yagi out of phase (front lobe 13.7dBi gain, skewed back lobe only 20dB down)

Having just one Yagi's feed point "polarity" wrong results in 6.2 dB of forward gain lost (more than all of the expected 5.6dB or so 4 Yagi array gain), and the front/rear has degraded about 10dB down on the already degraded f/r of the "3 cross booms" model as compared to a "free space" model.

73,

[VK2FAK]

Hi All...

Before ya go..

I was looking at the patterns on the site given for the antenna......and I was wondering if it is normal that with a Free Space plot you get beamwidth of around 25deg for both elevation and azimuth but with the Plot that is suppose to be with the antenna up 15m the beamwidth is shown as 1.9 deg at the -3db points.. and why in the 15m high plot is there no F/B given, when its shown on all other plots.......normal ?

John

[ZL1RS]

Hi All...

Before ya go..

I was looking at the patterns on the site given for the antenna......and I was wondering if it is normal that with a Free Space plot you get beamwidth of around 25deg for both elevation and azimuth but with the Plot that is suppose to be with the antenna up 15m the beamwidth is shown as 1.9 deg at the -3db points.. and why in the 15m high plot is there no F/B given, when its shown on all other plots.......normal ?

John

http://www.innovantennas.com/antennas-a ... egory_id=1

The elevation plot with the antenna up at 15m is the resultant radiation pattern including the ground reflection effect at the particular height (according to what sort of theoretical "ground" characteristics the software takes into account). This pattern will change significantly according to antenna height, with fewer and broader lobes at lower heights and with the peak of the lowest lobe (Slice Max Gain ... = 2.0 deg) "pushed" higher in elevation at lower heights. And so the old adage: the higher the antenna, the lower the radiation angle. The "perfect" form in the simulation will never be seen in real life at a "normal" QTH with buildings and assorted vegetation in all directions extending out from the antenna causing all sorts of odd reflections.

No F/B as such is given because there is a null at 0 degrees resulting again from the ground reflection effect. Of some interest is the pattern of the rearward lobes, in this case all suppressed by some 35dB (meaning less noise pick up from the rear ... a promotional point for antenna manufacturers). As with the over ground elevation pattern this would not be realised in real life unless the antenna were suspended by balloons over dead flat ground ... ... but that doesn't matter, an antenna with 30 dB F/B quoted must be better than one with 28dB F/B, right?!

I'm sure all of us, myself included, are guilty of paying too close attention to these simulations ... but they are useful as broad comparisons e.g. one would notice some difference between an antenna with 20dB f/b and another with 35dB f/b, but in reality the transition from simulation (especially the free space simulations) to the real world makes a monkey out of these pretty pictures. (Not as severe as Lee's case because there is a suspicion of an error in construction somewhere).

Cynical hat off ...

73,

[VK2KRR]

Thanks everyone for your input on this topic. Always amazed at the technical knowledge of our fellow amateurs

Bob, tnx for the plots, very interesting.

Owen, I've not had the analyzer here to look at the total system SWR, but I have had my PA on a couple of times at 400W and it has not indicated any reflected power. Also the 910 tends to limit output if the SWR is bad, and I have not seen this either.
There are no baluns except the so called choke balun wound into the start of the coax, about 4 turns. The coaxes have also been electrically measured for equal length.

Glenn, has been no noticeable change in noise level here either.

Adrian, my horizon to the west is at 0 (zero degrees), so I can get a reasonable idea of the pattern using sun noise. I did do some measurements of the elevation pattern, just using the radio S meter. I still have them written here, I only checked from fairly low levels, but in summary in deg and signal -
3.80 - S5
3.40 - S2
3.00 - S1-0
2.60 - S1-2
2.50 - S2
2.40 - S3
2.30 - S4
2.20 - S4
2.10 - S5
2.00 - S5
Through to -
1.40 - S5
1.30 - S4
1.20 - S4
1.10 - S4
1.00 - S3
0.90 - S2
0.80 - S1
0.70 - S1-0
0.60 - S1
0.50 - S3
0.40 - S3-4
0.20 - S4
0.10 - S5
0.05 - S5
0.00 - S5
Sun starts dipping into horizon
-0.1 - S4
-0.2 - S4
-0.3 - S3
-0.4 - S3
-0.5 - S2
-0.6 - S1
-0.7 - S0
Sun gone

So noted there were lobe peaks at 3.80 deg, then from 2.10 to 1.40, then another started 0.10 to 0.00 sun starts setting. These were all the same signal strength, I would have thought there should have been a bigger peak somewhere ?
I can say that the array appears to be slightly front heavy and as such is leaning forward somewhat, so the front is pointing into the ground, I think could be as much as 5 deg ? its hard to tell. But I did mention this to Justin and he claimed that due to the effects of the ground the lobes should still be coming up over the horizon? But to me, looking at that last peak starting at 0.10 deg, im wondering if there is another lobe that would show better if I put some counter weight on the H frame to help pull it more level.

I should add that I have worked Franco I2FAK via EME with this array as I test, where he peaked -6dB. While this is good, Franco does have a big set up and I could have done something similar with the single 17b2. The -6dB occurred a few times in the moon decent, but I would have expected a big peak on a main lobe close to the horizon, but this did not happen, similar I guess to what was seen in the sun noise tests.

Over past few days Ive been running with single yagi put through to the main feeder, have not noticed any real loss in signal from the front of the yagi, not had other signals on to check f/b etc.

[ZL1RS]

... So noted there were lobe peaks at 3.80 deg, then from 2.10 to 1.40, then another started 0.10 to 0.00 sun starts setting. These were all the same signal strength, I would have thought there should have been a bigger peak somewhere ? ...

How high is you array on the tower Leigh? and how high is the tower on the hill from the flatter ground to your west that will be producing the ground gain lobes? Given you QTH, I suspect the effective height of the array is very high and the resultant lobes will be many and narrow. Note the difference in the strength of the ground gain lobes in the over ground elevation pattern on the Innov web site. The first 3 lobes are only a couple of dB different ... but as any antenna is raised higher the lobes become closer together and more even in signal strength. I've seen what you describe with a portable array about 200m above the ocean ... all EME signals coming and going every 5 minutes or so as the moon descended through the sharp lobes and finally set into the ocean.

.. I can say that the array appears to be slightly front heavy and as such is leaning forward somewhat, so the front is pointing into the ground, I think could be as much as 5 deg ? its hard to tell. But I did mention this to Justin and he claimed that due to the effects of the ground the lobes should still be coming up over the horizon? But to me, looking at that last peak starting at 0.10 deg, im wondering if there is another lobe that would show better if I put some counter weight on the H frame to help pull it more level.

No, the ground gain lobes will not change significantly due to down tilt on the array, they are still the vector sum of the radiation from the array and the ground reflections of that radiation. The lobes will still be above the horizon.

I should add that I have worked Franco I2FAK via EME with this array as I test, where he peaked -6dB. While this is good, Franco does have a big set up and I could have done something similar with the single 17b2. The -6dB occurred a few times in the moon decent, but I would have expected a big peak on a main lobe close to the horizon, but this did not happen, similar I guess to what was seen in the sun noise tests.

What you saw with the -6dB signal peaks a few times during the moon descent was to be expected (as noted above). There will be no single big peak (unless EME conditions like Faraday rotation peak or coincident ground gain peaks at both ends cause it), just a series of lobes getting progressively stronger as the moon elevation drops ... again, just like the diagram of the array at 15m on the Innov web site (+- differences in real life due to your array's actual height and the effect of any ground clutter).

As long as the 4 choke baluns formed on the ends of the phasing lines were connected with the same "polarity" to the four LFA loop feed points (this does need to be checked), one has to lean more towards the array pattern being screwed up by the particular arrangement of the H frame ... maybe a single cross boom of some 4-6" dia and some Parafil or Kevlar cattenery "wires" to "guy" it and the vertical risers?

73,