Popular Antenna SWR measurements...

[Guest]

On 16/02/2020 at 12:06, The Admiral said:

I must admit this seemed a bit draconian to me, so I had to check it out. Hmm, seems that it is illegal unless you are licensed to do so. Wouldn't have believed it .

Ian

All it takes is one bampot determined to try to take a plane down, for terrorism reasons - whilst listening to the airband!  Then they'll not turn a blind eye anymore!

[The Admiral]

6 minutes ago, merlin100 said:

All it takes is one bampot determined to try to take a plane down, for terrorism reasons - whilst listening to the airband!  Then they'll not turn a blind eye anymore!

You are right of course, but like a lot of legislation there is no enforcement, but if you are caught for something else then they can charge you with that as well. Even using mobile phones whilst driving is barely enforced, unfortunately. There is also the practical element of how you'd prevent folk listening to airbands, unless all the messages were encrypted.

Ian

[Guest]

5 minutes ago, The Admiral said:

You are right of course, but like a lot of legislation there is no enforcement, but if you are caught for something else then they can charge you with that as well. Even using mobile phones whilst driving is barely enforced, unfortunately. There is also the practical element of how you'd prevent folk listening to airbands, unless all the messages were encrypted.

Ian

It would be expensive and difficult to encrypt.  They prefer the VHF Air Band to be 'in the clear' for safety reasons, as well as historical.  The argument was that it would be too costly for developing countries to afford to encrypt and that AM was still the best, simplest and clearest method of getting important information across.

[The Admiral]

26 minutes ago, Macavity said:

Reflecting on your SWR plot... I quickly learned (in practice frankly!) that
measuring SWRs, via a coax, superimposed a "sine wave" on everything!
It's "obvious", I guess! Maybe that's what you did in this instance tho'? 🤔

The TRUE reading of an antenna SWR is measured at the feed point of
the BARE antenna. Well, that's my story (now) and I'm sticking to it. lol 😛

It's "well known" that a dipole radiates at a third harmonic? 40m -> 15m.
Not sure about "quad" elements. Possibly at the second harmonic?!?! 🤔 

My "Aircraft Band" Antenna fed via 15m coax. Note the "sine waves":



I suspect "via coax" SWRs are still useful. At least you know your antenna
hasn't blown away in the WIND! lol 🤣

The thing is that in reality one needs a feeder to use an antenna, so any measurements should be on the system as a whole I feel. However, it would be interesting to compare the result with one directly on the antenna, if only to see if I can resolve the origin. It would need climbing into the loft, not the easiest of tasks, but easier than climbing a ladder outside in our present winds . I haven't a clue as to what harmonics a quad resonates at.

My current set-up has the antenna in the loft amongst the rafters, a short co-ax goes to the antenna with a couple of loops to form the balun (as per https://www.innovantennas.com/en/baluns-why-do-i-need-one), and terminated in a Type-N connector IIRC. This is connected to a length of co-ax that goes through the ceiling to the room below, where it terminates in another Type-N. The SDR dongle is connected to that via a Type-N to SMA converter cable. Unfortunate but necessary junctions. Do you know what impedance the nanoVNA input is? I assume that the outside of its SMA connector is common and presumably earthed via the USB connection on the computer.

Ian

Ian

[Macavity]

22 hours ago, The Admiral said:

The thing is that in reality one needs a feeder to use an antenna, so any measurements should be on the system as a whole I feel. However, it would be interesting to compare the result with one directly on the antenna, if only to see if I can resolve the origin.

For the sake of (minority? lol) interest here's the SWR measurement of the D777 Aircraft Antenna,
at feed point, with short SMA coax! I did this "for reference" before it was committed to the sky! 😛

Note the absence of the sine wave due to 15m coax feed. As to which is the more VALID measure
of antenna performance, I veer towards direct one! But the "whole system" idea is a valid point?



For fun, I show FAMED "Smith Cart" of the Coax Fed D777 Antenna:

This does show characteristic (pretty!) "resonace loop"(SWR close to 1.0)
amidst the general "coax-induced" circlings over a wider frequency range!

The "direct" Smith chart near resonance is a "boring" short ARC near 1.0!

I think I now have some sort of handle on all this! lol. It can be explained
with a little Antenna Theory. I am but a late-comer to all this good stuff...
But certainly the NanoVNA is a useful tool / expanded my knowledge! 😁

See e.g.  http://www.antenna-theory.com/tutorial/smith/chart.php

These days: That others understand the detail is good enough for me! 🤣

Edited February 23, 2020 by Macavity

[The Admiral]

Well I find it interesting! Can't explain it, but it does mess with my mind . That Smith Chart trace is bewildering. But why does the addition of the coax give rise to this?

I suppose that 'off-tune' the impedance of the antenna ceases to be purely resistive, and this must interact somehow with the reactive impedance of the co-ax. It might be interesting to see what the response of the co-ax would be on its own, say terminated with a 50Ω resistive load, or shorted, or whatever. I wonder what the natural resonances of the cable would be? I suppose that one should use a passband filter somewhere in the receiver, or ahead of it, to minimize breakthrough from the signals either side of the desired band.

Ian

[Macavity]

2 hours ago, The Admiral said:

Well I find it interesting!

So what does that say about the both of us? Don't answer that! lol 🤣

I had a broken connector on a budget coax, so I snipped that off, and
attached the remaining good connector to a T-piece between the two
inputs on the MicroVNA (As suggested). This is the LogMag S12 plot:



Or indeed, as you suggest, the basis of a potential "filter"etc. etc. 😺

The actual length of the coax (measured) was 2.60 Metres.... "Open"
at the far (cut) end... So an "electrical" quarter wave?!?! The first dip
is at 19.15MHz. The cable is (probably) RG58, so the Velocity Factor
is 66% etc. If you put the figures into the (lambda/4) Stub Formula:

75 / f(Mhz) ) * VF =  Length (Metres)

75 / 19.15 * 0.66 = 2.58 Metres !!!

i.e. pretty close to 2.60 allowing for unknown connector effects! 😎

So I can also measure Velocity Factors as well...

P.S. No detailed comments. lol. But we have the same ideas re.
Smith Charts! The coax is effectively a "(complex) impedence
transformer" yadayada... But at ONE frequency! So the variable
frequency results in the convoluted loop/circle seen in practice! 😏

Have Fun... 🙃

Edited February 23, 2020 by Macavity

[The Admiral]

15 hours ago, Macavity said:

Or indeed, as you suggest, the basis of a potential "filter"etc. etc.

I think you are giving me more credit than is due! I hadn't actually taken the step to think of the cable as the filter .

All (still) interesting! But I still don't have my head around Smith charts - that'll need some serious little grey-cell study.

Not sure that I follow the T-piece connections though, I thought when measuring cable properties it connected between the two terminals, but if you are checking out the resonance nature you'd connect just to S11. Need to look up the tutorials again. What I've done so far has all been a bit rushed as time hasn't been plentiful recently.

Keep up the good work .

Ian

[Macavity]

I sense there are a variety of ways to tune filter stubs using a VNA. 🤔

You can use just use the S11 input on the VNA and test the whole
system *inline* with the antenna. (It is the more obvious perhaps!)



http://www.hb9amo.net/hfcoaxialfilter.php

Or you can connect the ENDS between the S11 & S22 inputs. That
was perhaps the one I came across first!?! You are only looking for
the frequency of the resonance curve while you snip the stub coax? 😏

Edited February 24, 2020 by Macavity

[radiostationx]

Interesting discussions !

A coaxial bazooka dipole for Macavity...wxsatuser has an rsp1a by sdr play (nice radio !), and a diamond d-777 was mentioned (a 5/8 wave mil over a 5/8 wave civil co-linear with matching stub sleeve)...hmmm 

Airband fans..I like that stuff too !

You may find some more info on my website / blog 

http://www.merseyradar.co.uk/airband-radio/sdrplay-rsp1a-review/

http://www.merseyradar.co.uk/airband-radio/home-made-coaxial-dipole-antenna-for-civil-airband/

 

Regards all,

Mike

[radiostationx]

post error duplicated

Edited April 15, 2020 by radiostationx
duplicated post

[radiostationx]

post error duplicated

 

Edited April 15, 2020 by radiostationx
duplicated post

[wxsatuser]

4 hours ago, radiostationx said:

Interesting discussions !

A coaxial bazooka dipole for Macavity...wxsatuser has an rsp1a by sdr play (nice radio !), and a diamond d-777 was mentioned (a 5/8 wave mil over a 5/8 wave civil co-linear with matching stub sleeve)...hmmm 

Airband fans..I like that stuff too !

You may find some more info on my website / blog 

http://www.merseyradar.co.uk/airband-radio/sdrplay-rsp1a-review/

http://www.merseyradar.co.uk/airband-radio/home-made-coaxial-dipole-antenna-for-civil-airband/

 

Regards all,

Mike

I actually have two RSP1As, one on the LEO weather satellites and the other one I showed earlier on the Qatar Oscar 100 satellite downlink.

They are nice radios, mine seem to handle a 100watts HF close by with no problems.