Can't find the low SWR point

[VK2FAK]

Hi all...

making a second beam for 70cm.....now the swr is showing around 1.4....but tuning up or down I can' t find the freq where it is at its lowest and does not rise more than about 0.2 swr .....ie it can't be that flat across the 70cm band....any ideas..

[VK3DXE]

Do you have access to an analyser? That'll tell you pretty quickly what, if any, the issue is.

[VK2FAK]

Hi all...

No...no fancy gear here..hihi......done the basic...checked cable...tested the setup with another little 5 ele beam...every thing tunes fine on that,,,swr goes up and down with different freq.....Just very little movement on the big beam..

[VK4TI]

Hi all...

making a second beam for 70cm.....now the swr is showing around 1.4....but tuning up or down I can' t find the freq where it is at its lowest and does not rise more than about 0.2 swr .....ie it can't be that flat across the 70cm band....any ideas..

how is the matching being done , folded driven and 41 coax or something else ?

[VK2FAK]

Hi all.....

A straight split dipole....28ohm with 2 parallel cables bring it back to 50ohm...

I can accept it may not be tuned right the first go....thats standard ....but would expect some more swr changing than is showing when tuning across the band...

[ZL1RS]

.....ie it can't be that flat across the 70cm band....

Well, actually, it could be. What is the design? ... "28 ohms" suggests DK7ZB?

Please tell us more John ... and a photo of the DE area and element mounting method. The last 70cm DK7ZB Yagi with SWR trouble had coax tails about 1" long ...

73,

[VK2FAK]

Hi all....

Well Bob.....before I go further I will take a look...at those tails as you call them......but even if they are to long....why wouldn't that show up in swr.....and not just show pretty much flat across the band...with swr of around 1.4......

[VK4TI]

Hi all.....

A straight split dipole....28ohm with 2 parallel cables bring it back to 50ohm...

I can accept it may not be tuned right the first go....thats standard ....but would expect some more swr changing than is showing when tuning across the band...

I had one like that on a 6m antenna , wouldn't tune , no idea to this day but changed to a coax 4:! and it worked , I would be checking the tails etc for a short or loop

[ZL1RS]

Hi all....

Well Bob.....before I go further I will take a look...at those tails as you call them......but even if they are to long....why wouldn't that show up in swr.....and not just show pretty much flat across the band...with swr of around 1.4......

Depending on the arrangement of the elements in the Yagi design, it COULD have a fairly flat SWR curve right across the band. This is why I asked which design you used. For example:

DK7ZB 28 ohm 70cm Yagi

7-swr.gif (3.37 KiB) Viewed 3359 times

You mention an SWR variation of 0.2, so I assume it does change from 1.4 to 1.6 across the band. The tails effectively add to the DE length which could change the SWR in the manner you describe.

Also, if the elements were mounted in a different manner than the original the frequency response could be pulled out of the band and you are looking at 'something else' than was originally intended.

The other possibility is the "75" ohm coax. I have found that some TV grade RG-6 coax is nothing like 75 ohms.

[VK2FAK]

Hi all.....

I shortened those tails down to bare min.....now the swr sits around 1.2 ....


yes it is a DK7ZB design....and so far , it is possible the extra may go away...with a 1:1 on the feed line...I used only the standard matching ......so will check when I put a 1:1 on....so something similar...

Ok now...if you have a piece of plastic touching the element...that is beyond the boom width (10mm or so)...does that have a shortening effect or other..

[ZL1RS]

Hi all.....

I shortened those tails down to bare min.....now the swr sits around 1.2 ....


yes it is a DK7ZB design....and so far , it is possible the extra may go away...with a 1:1 on the feed line...I used only the standard matching ......so will check when I put a 1:1 on....so something similar...

Ok now...if you have a piece of plastic touching the element...that is beyond the boom width (10mm or so)...does that have a shortening effect or other..

While you were doing that I simulated another DK7ZB 70cm Yagi that had a fairly low SWR curve across 430-440MHz (maximum SWR 1.2:1), but simulated with the DE 4mm too long as might be the case if the coax 'tails' were too long ...

DK7ZB 19 element 70cm Yagi

What was a very low SWR curve across the 70cm band is still fairly flat without a specific dip, but the SWR has increased to 1.4 to 1.6:1 over the band. As your SWR improved from 1.4:1 down to 1.2:1 by shortening the 'tails' it would be a fairly safe bet that shaving a mm or so off the DE might get you where you want to go. (Usual disclaimer )

Note that some Yagi designs may not show an SWR dip low in frequency just because the DE is a bit too long. If you play with enough Yagis in nec2 antenna simulator (and build them to verify), you will find some that actually do the opposite ... and in some the SWR just goes up and down with or without showing a specific dip or a change in dip frequency. It is all to do with the mutual coupling between the elements in the particular design (dependent on the particular element lengths and spacings). Note that in DK7ZB's SWR 'tuning' advice on some of his pages, he never says words to the effect "if the SWR dip is low, shorten the DE", but usually words to the effect "adjust the DE for minimum SWR" which are two different things. Poking a bolt into each end of the DE and leaving the head poking out a mm or two at each end will demonstrate what a DE too long will do, and can be a guide.

Yes, generally plastic element mounts will tend to shorten the element length. Independent practical experiments on 2m Yagis both here and in Germany using Nuxcom element mounts on a very narrow SWR bandwidth test Yagi showed a shortening effect which raised the SWR dip up the band by over 1MHz. On 6m band Yagis the effect is less, but still measurable. I have only built one 70cm Yagi where those element mounts were used, and it 'moved up the band' by about 3MHz.

73,

[VK2FAK]

Hi all....

While you were doing all that, I was doing a short 2km run.....

ok...well if the Freq is in the area I want, I would say the the plastic did not have to much effect....but I will try your ADVICE(no disclaimers here) and shave with a file little bits off the DE and see what happens...

Well see, I assume looking for a dip and a rise, as most of the time I have found them......but then I have not made many beams to see the other side effects...

[VK5APN]

John

I agree with Bob.

I recently built a 21 element DK7ZB 70cm yagi for portable work. With 50w have been having some fun off the moon. Including 1 CW QSO.

This yagi packs down to no longer 1500mm, ideal for aircraft travel etc.

Anyhow, when building it I cut the DE about 2.5mm long per side. It is always easier to take off material than put it back on.

I am using TV antenna style insulators. Will try and find a pic for you.

Built the matching section from the best 75ohm coax I could find (multistrand inner and braid outer - not the foil stuff). To the dimensions given on his website.

Checked SWR and found like you that the SWR dip was very small and hardish to find. But was not 1.1, more like 1.2

So I removed the DE and filed some off it, reassembled, checked, filed some more...........

Eventually could see the small dip approach the frequency I wanted it to be and left it like that.

Performs amazingly.

GL in your adventure

Regards

Wayne

[VK2FAK]

Hi all.....

Hi Wayne....good to hear your having fun with the longgg 70cm beam...

I am in the process of filing now...seemed to come down just a tab.so will keep going...

[ZL1RS]

Hi all....

While you were doing all that, I was doing a short 2km run.....

ok...well if the Freq is in the area I want, I would say the the plastic did not have to much effect....but I will try your ADVICE(no disclaimers here) and shave with a file little bits off the DE and see what happens...

Well see, I assume looking for a dip and a rise, as most of the time I have found them......but then I have not made many beams to see the other side effects...

The plastic probably does have an effect, it is just that you can't "see" it due to the particular SWR characteristics of that Yagi. I know one antenna designer who purposely designed a 2m Yagi with an incredibly flat SWR curves over several MHz for this exact reason (<1.05:1 across 2MHz is possible). Small changes introduced while the Yagi is built due to the element mounting method or element diameter variations or slight measurement/drilling errors are masked by his design and the builder ends up with a Yagi that looks good on an SWR meter. e.g. 3/8" and 10mm diameter elements look the same but are not the same ... 3/8" = 9.53mm, and this can move a 2m Yagi several hundred kHz along the band which would be a major problem in other very narrow band 2m Yagis designs.

I think that with this Yagi you don't look specifically for a SWR dip and rise. The point I am trying to make is that each Yagi design is different, and in this case if the SWR is low across the whole band in the particular design you built, then you may only see a change in SWR level ... e.g. 1:4 across the band, or 1.2:1 across the band, or 1.1:1 across the band as the DE length is changed. On the other hand, you might find a dip somewhere in the band at a certain DE length (with no guarantee that the dip will move in frequency as the DE is changed further), or indeed no 1:1 SWR dip anywhere at any DE length! Another Yagi design may have the 'traditional' SWR dip that moves in frequency with DE length change.

This still assumes the 75 ohm coax used to make the 1/4 WL matching section is actually 75 ohms, and it is really an electrical 1/4 WL. Including element mounting and 'coax tails', there are at least 4 'unknowns' in your Yagi, hence my disclaimer

Also keep in mind that achieving the 1.2:1 SWR at 70cm that you have already is already a pretty good result ...

73,

[VK5APN]

Found a pic

Yes your right Bob, 1.2 is very good, just thought I see if I could get it better. Did it affect the performance? Would be hard to pick it.

Wayne

[ZL1RS]

... one more thing. When a Yagi's feed point impedance is not 50 ohms and non-reactive the feedline length will change what the SWR meter is seeing. If you truly have a 50 ohm, non-reactive feedpoint at the Yagi then changing the feed line length will not change the SWR. Once you get a 1:1 SWR indicated, add a short length of feedline (or a few coax adapters) and measure again, if the SWR stays 1:1 then likely you have nailed it.

GL and 73,

[ZL1RS]

Wayne's comment ...

1.2 is very good, just thought I see if I could get it better. Did it affect the performance? Would be hard to pick it

... set me on a little reminiscing.

My first 2m EME array built in 1983/84 had an SWR of 1.7:1. Due to the nature of the DE (a curious tapered, unequal diameter folded dipole of sorts) I had no idea how to go about fixing it, and it didn't occur to me then to experiment with 'tuning' one Yagi first! Anyway, I decided to try and use it for EME as it was (the PA was a valve job, so just tune for maximum power with sensible grid and anode current, and "go"!) Despite the SWR, the expected level of audible CW echos were received for the size of antenna array (4 x 2.2WL Yagis), power level and preamp being used ... so the SWR in itself was not 'a game breaker'. It was used just like that for the next year or so with about 50 EME contacts made in CW mode before moving to the next job/country.

[VK2FAK]

Hi all....

Stopped for some dinner....watched some News....plenty of trees down in the area...some very strong winds here this afternoon..

Now after some filing have the swr down to 1:1.1 might try get it lower(as an exercise) ....but thats way better than when I first posted..

Oh yes, I understand the plastic has some effect, just not showing its ugly head...

Then to make sure the pattern is as it should be and not gone off the rails so to speak..

P.S down to about 1.05 swr now....so wont tempt my luck farther,,..

[VK3PY]

ZL1RS wrote:

When a Yagi's feed point impedance is not 50 ohms and non-reactive the feedline length will change what the SWR meter is seeing. If you truly have a 50 ohm, non-reactive feedpoint at the Yagi then changing the feed line length will not change the SWR. Once you get a 1:1 SWR indicated, add a short length of feedline (or a few coax adapters) and measure again, if the SWR stays 1:1 then likely you have nailed it.

Come again? Are you saying the feed line length affects the SWR? Please explain........

Chas
VK3PY