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On Wed, 4 Nov 2020 at 18:03, <aleatorii.yg@...> wrote:

Thanks for the reply, attached are some photos. Note this was sold in the
UK as a 13cm antenna (I think by Sandpiper, but I may be wrong), many years
ago.
I've never actually built a helix design myself, so am not that clued up
on any theory of them.

The impedance of a helical antenna is about 140 ohms. To match that to 50
ohms, you can use a quarter wave of transmission line of

sqrt(50 * 140) = 84 Ohms.

The bending of the conductor near the bottom is some attempt to achieve
that, but it is probably not working well. Soldering a bit of copper on
should help.

That groundplane looks very small, with the helix not in the centre. You
have a large support structure, which is sure to mess up the normal
equations, making it impossible to predict the performance without some
sort of EM simulation package.

In short, it looks quite a poor design to me.

Dave

On Thu, 5 Nov 2020 at 08:20, I0MOM via groups.io <morelli=
[email protected]> wrote:

Hi Gunter,

I made the measurements in the shack, the antenna was connected to the VNA
via 28 meters of RG-213 Coaxial cable.
As you suggested I’ll try to make direct measurement directly at the
antenna without the coax and see what does change.

Thanks for You interest,

73 de I0MOM Marco
Inviato da iPad

The usual technique would be to calibrate to have the nanoVNA in the shack,
but perform the calibration on the end of the coax. Then when you connect
the antenna to the coax, you will make an accurate measurement.

Of course, if the coax is faulty, all bets are off.

Dave

Dear Aleatrii,

Regarding the group, I was not making a whinge at you, just a general frustration that there are three different groups here covering nanovnas and I try to follow all three.

I certainly agree that most tasks the nano devices are put to could be undertaken on most of the various types and therefore it should not matter. The two groups of SAA2, which has a different architecture, were formed by a split because of disagreements between a few people regarding certain matters which you can find out about with a little research.

And I think you are right; the SAA2 groups are going on a lot about hardware, firmware etc, rather than practical use. I am guilty of this myself to a degree, but you can learn a lot about vnas etc from it because there are some very clued-up professionals and semi-professionals on those two groups.

And by the way, I am learning sonething about helical aerials.

Steve L. G7PSZ

WONRP, The current charts are on a new factory ended 100ft length of RG8X now.

I also think they must be something wrong with the balun as well since there is not much
that could be wrong with the wires on it, although i wonder if it could also be the fact that
there is extra wire on the spools which according to windcamp does not effect it.

I just received 2 - T130-2 cores today just need to find a schematic to wire up an new balun
with them at 1:1 to test it again before i see if i can open the factory balun on it. I think i need
150 watts for the kenwood.

Hi Gunter,

I made the measurements in the shack, the antenna was connected to the VNA via 28 meters of RG-213 Coaxial cable.
As you suggested I’ll try to make direct measurement directly at the antenna without the coax and see what does change.

Thanks for You interest,

73 de I0MOM Marco
Inviato da iPad


Inviato da Posta per Windows 10

On Thu, Nov 5, 2020 at 12:26 AM, Roy Appleton wrote:

Interesting that a book written in 1950 is the authority on the helical
antenna. From the stand point of theory or practical application?

Nearly everything we know and use is based on science from the past few centuries. It's easy to think that everything we create and use today has been discovered//designed in the past 20 or so years, but that's just not the case. All we've really done recently is made things smaller and quieter and sometimes easier to use (or harder), but todays tech hasn't really changed much over the past 50 years if you think about it, some say we're stagnating as far as tech goes (which is not good at this point considering how much waste and damage we create).

Your mobile phone uses tech from the 1950's and older (the transistor, plastic, glass etc), the internal combustion engine in just about every vehicle is still the same as it ever was in the 1860's (a little smaller and little more efficient but still the same dirty explosive tech). Your car runs on round things that were discovered who knows when! Battery tech stalled virtually a few decades after it was first discovered (the lithium battery is not as advanced from the original lead acid battery as you might think), it's hardly advanced at all as far as tech goes (much like the engine).

A younger (20 odd years old) member of my distant family laughed at me recently about using a walkie talkie and pressing a button to talk, he told me it's a very old fashioned box (a modern yaesu it was), I said to him his mobile phone is just a radio that presses the PTT for him a few 10's of times a second whilst he's talking, he doesn't believe me and doesn't believe it's a radio at all (but can't explain how peoples voices are transferred with it) .. this is unfortunately the level of knowledge the average human being has these days, easy to think otherwise but that's how things are.

I don't know who you are as you have not identified yourself.

But yes but the author did not explain himself.

1.??? Take ONE end of the _electrically half wave-length of 75 ohm coax_ (RG-79 type) and connect the center conductor to the center conductor on one side of the antenna.

2.??? Disconnect the soldered shield of the feeder coax from the other side of the antenna.

3.??? Solder the shield of the electrically half wave length of 75 ohm coax to the shield of the feeder coax.

4.??? Solder the shield from the OTHER side of the electrically half wave length of 75 ohm coax to the other two shields. Isolate the coax ends from the other connections.

5.??? Solder the enter conductor of the OTHER end of the electrically half wave length of 75 ohm coax to the other side of the antenna.

There you have a 4:1 coaxial balun matching the 75 ohm feeder coax to the 300 ohm impedance of the antenna (for that band only).

How it works...

If you look at the point where the two center conductors are connected together, that is where the feeder RF current is *split*.

If you look at the point where the OTHER side center conductor of the electrically half wave length of 75 ohm coax is connected to the other side of the antenna, it is where RF voltage is equal to the feeder voltage but OPPOSITE in phase. This contributes to TWICE voltage in respect to the other side of the antenna.

So, the NET feed point to the antenna is twice the voltage but half the current gives us a 4:1 (2 / 0.5) Impedance transfer.

Read below for more info...



Good luck, who ever you are...

Larry, K4MLA

Don't overlook the simple stuff. Disconnect the coax from the antenna and use an ohmmeter to check for shorted coax, especially if you are not really confident in your quality of connector assembly. Verify infinite resistance between the shell and center pin. Verify very low resistance (~1 ohm) between the center pins at both ends, and between the shells at both ends.

More than likely the antenna itself is OK. The info I saw shows it has a 1:1 balun, so if you can identify the ends of the windings, verify continuity of each winding and make sure there is isolation (infinite resistance) between them. Check the endpoints of the elements at the connector for shorts or opens.

The NanoVNA is a fantastic piece of gear, but it's easy to get caught up in trying to interpret the readings and overlooking fundamental tests.

On 11/4/20 4:26 PM, Roy Appleton wrote:

Interesting that a book written in 1950 is the authority on the helical
antenna. From the stand point of theory or practical application?

really both.

Sure, there's been innumerable variants of helical antennas developed over the years, and there's papers describing them, but the basics are pretty what Kraus figured out in the 40s. Kraus was very much a "try it and see, and then figure out the theory" kind of guy - when someone said that a helix wouldn't radiate axially (looking at a helix TWT for inspiration), he went down in the lab (at home), wound a helix, and tried it.
"That evening in the basement of my home I wound a 7-turn helical coil of 1 [lambda] in circumference and fed it via coaxial line and ground plane from my 12cm oscillator. I was thrilled to find it produced a sharp beam of circular polarized radiation off its open end."


He was someone who really had a deep understanding of electromagnetics, so I'm sure his first helical antenna wasn't any old diameter and pitch - he was looking at TWTs, so the whole idea of a wave propagating down a helix was something he was familiar with. And clearly, he understood how beams are formed (given he invented the W8JK pair of dipoles fed out of phase) and the Yagi Uda was well known at the time.

Chapter 7 is many, many pages of theory and practice on different conductor diameters, winding pitch, effects of circumference, length, etc, both as well developed theory and in nomograms and simple design formulas.

There are variants that are tapered, there are innumerable ground plane configurations flat, dished, with spikes, without them.


There's plenty of pictures of helix antennas in the textbook - The one for 10 MHz is quite impressive in scale.


The quad helix was developed by Kilgus in the late 60s, and is what is often seen for GPS (and earlier, for 137 MHz weather satellite receivers) (and, interestingly, on Mars Rovers for 400 MHz)

GPS folks use helibowls and helicones a lot to suppress sidelobes and multipath.

Yes, indeed, Mr. J. D. Krauss is the master of the helix!

Dave - W?LEV

On Thu, Nov 5, 2020 at 12:26 AM Roy Appleton <twelveoclockhigh@...>
wrote:

Interesting that a book written in 1950 is the authority on the helical
antenna. From the stand point of theory or practical application?

Roy
WA0YMH

On Wed, Nov 4, 2020, 4:14 PM Jim Lux <jimlux@...> wrote:

On 11/4/20 9:13 AM, aleatorii.yg@... wrote:

Have been testing various antennas on my NanoVNA V2 Plus4 and in all

but

one, the results appear roughly as expected.

The one that doesn't is a 2.4GHz helix, which shows a very poor match

there and actual resonance/low SWR is at about 3.4GHz.

I know the helix is an electronically shortened antenna, and may not

present an ideal 50ohm impedance, but that still doesn't make sense to me
when making these measurements.

Can anyone explain what's going on here?

The "natural" impedance of a multiturn helix is about 100 ohms. One
either does some sort of matching transformer, or, you adjust the
spacing of the turn closest to the ground plane to essentially make it a
broad band tapered transmission line transformer.

With a VNA, it's pretty easy to see the changes.

If it's a quad helix or something other than the classic axial
radiation, then I don't know what the normal Z is.

FOrtunately, you can find the book by Kraus, "Antennas" on line as a
pdf, and it is literally the book on helicals.


In neither axial nor other radiating modes is the helical a "shortened
antenna" - it's a totally different design.

--
*Dave - W?LEV*
*Just Let Darwin Work*

Interesting that a book written in 1950 is the authority on the helical
antenna. From the stand point of theory or practical application?

Roy
WA0YMH

True. It's a pretty bad antenna even at HF! When that much loss is shown
even in the first plot and progressively measures more and more as you step
upward in frequency, the loss is all there is. Especially the loss shown
at the lowest frequency with an 'average' return loss of only 5 dB (SWR of
3.6:1), most of the system is loss. The thing is about useless at 2-meters
(144 MHz)! I'd throw it out and start over, just my opinion.

Dave - W LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 11/4/20 11:08 AM, aleatorii.yg@... wrote:

Thanks for the reply, but what's confusing me is that, looking at the results of the VNA, this appears to be a 9cm antenna.

WiMax? there's a bunch of antennas made for 3.5 GHz

On 11/4/20 10:03 AM, aleatorii.yg@... wrote:

Thanks for the reply, attached are some photos. Note this was sold in the UK as a 13cm antenna (I think by Sandpiper, but I may be wrong), many years ago.
I've never actually built a helix design myself, so am not that clued up on any theory of them.

The general design concept is that the circumference of the helix is approximately a wavelength, and the "pitch angle" is around 12-14 degrees. and the spacing of the helix turns is around 1/4 the circumference. for 13 cm, that's 4 cm in diameter (13/pi) and 1 cm spacing

Yours looks closer to spacing = diameter.]
It does appear to be using the "adjust the last turn's spacing for match" technique.

On 11/4/20 9:13 AM, aleatorii.yg@... wrote:

Have been testing various antennas on my NanoVNA V2 Plus4 and in all but one, the results appear roughly as expected.
The one that doesn't is a 2.4GHz helix, which shows a very poor match there and actual resonance/low SWR is at about 3.4GHz.
I know the helix is an electronically shortened antenna, and may not present an ideal 50ohm impedance, but that still doesn't make sense to me when making these measurements.
Can anyone explain what's going on here?

The "natural" impedance of a multiturn helix is about 100 ohms. One either does some sort of matching transformer, or, you adjust the spacing of the turn closest to the ground plane to essentially make it a broad band tapered transmission line transformer.

With a VNA, it's pretty easy to see the changes.

If it's a quad helix or something other than the classic axial radiation, then I don't know what the normal Z is.

FOrtunately, you can find the book by Kraus, "Antennas" on line as a pdf, and it is literally the book on helicals.


In neither axial nor other radiating modes is the helical a "shortened antenna" - it's a totally different design.

Remember, resonance is not defined by lowest SWR, but at the frequency at
which the complex portion of the impedance goes to zero.

Helices are traveling wave antennas, not shortened 'rubber ducks'. \

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

Because of the nature of a helix, I found the precise measurement of the inner diameter open to error, so I'd ignore that.
The conductor gauge is 4.85mm.

Oh I see, I didn't realise this group was for SAA2 versions only. I chose here as it seemed the best place to ask as it's an actual practical application (the frequency range here is irrelevant here really, as the query is for a helix antenna in particular, rather than about the V2 Plus4 itself). Other NanoVNA groups here appear to largely be firmware related posts, from what I've seen.
As for 13cm, it's worth experimenting with, as is the 5.6GHz range (for radio amateur use), thanks to the very cheap drone FPV gear available from China. It's a shame that the latter range isn't presently catered for with any of the NanoVNA range, but hopefully it will be in time.

The diameter is about right, but 10 mm conductor is certainly out of the range forall of my charts.

Thanks again.

Details here: Loop spacing: 21.5mm
Loop diameter (inner): 36mm
Loop diameter (outer): 46mm
19 turns