help with phasing stubs

[Russell Baker]

Hi All, 

I'm trying to design a colinear antenna and need a halfwave phasing line. The idea is that I have a halfwave radiator, then a 'non radiating' halfwave phasing line connecting to another halfwave radiator and repeat to get 4 radiating halfwaves and 3 phasing sections. It looks something like the below.

The phasing sections are 450 ohm ladder line each one quarter wave long (halfwave to the end and back). So far so good and I have modelled this in eznec and mmana-gal. Both agree on the correct lengths for each component. I have built two of these antennas, one for 2m and one for 4m. They work but not that well so I decided to try and measure the phase delay of the non-radiating sections. I'm using a calibrated nanovna and have tried measuring the phase delay of a section with an open end and it looks strange. The 'resonance' point is way too low. 

When I measure the phase delay through the section with a shorted end the phase reversal point is not as pronounced and is much higher

I'm rather new to this so if anyone can spot my mistake I'm open to ideas.

Russell

G4ZRZ

 

[Mike G4CDF]

Hi Russell,

How long are the halfwave stubs and what is the specified velocity facor of the 450 ohm line?  Also how are you supporting the stubs?  

I can help if you wish to send me the EZNEC model (on discord preferably)

HNY

73 Mike G4CDF

[Steve Thompson]

I'm a bit confused, could you do a couple of sketches showing what you're measuring? The first picture appears to be a reflection measurement, the second transmission.

Thanks
Steve G8GSQ 

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[Mike G4CDF]

Russell,

Just a few more points:

I would suggest putting a balun on the VNA as the stub is balanced and may not read properly on the VNA.

I looked up 450 ohm ladder line and the velocity factor was stated as 0.91.  At 4 metres (70.450 MHz)  this means your stub should be about 969 mm long.  Is this the case?

The second picture looks roughly right to me, but the phase jump at resonance is smaller (90 degrees) than expected (360 degrees)possibly because the VNA is not good at high impedances and/or the balance issue noted above. 

The phase shift on the first picture looks to be nearer to 360 degrees as expected but at about 38MHz. Not sure why it is so low. 

Hope this helps,

73 Mike G4CDF

[Mike G4CDF]

Just a thought:  Is your VNA calibrated at the end of the leads that you are using to connect with?

[Steve Thompson]

The 'resonance' around 35MHz in the first plot isn't as dramatic as it might appear. The glitch is due to the trace switching from -180 degrees to +180 (which are one and the same in phase terms). If you take out the glitch and join the trace back together there's just a smooth continuous curve. If you switch the format to polar you will see the trace following a circular path.

Something seems odd with the blue trace on the second plot. The values look way out from what I'd expect regardless of whether it's showing reflection or transmission.

Steve G8GSQ 

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On 29/12/2025, 14:12 'Russell Baker' via Reading and District Amateur Radio Group <radarc@googlegroups.com> wrote:

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[gordonrussell.baker]

I am expecting there to be a noticeable phase shift. I'm measuring the stub in two different ways, one open ended with one port of the nanovna, and one with the far end shorted using both ports.

I'm not sure if the phase shifts from each of these should actually match in frequency. Nevertheless it's actually the shorted measurement I'm interested in as I need a half wave stub. 

Russell 

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[Steve Thompson]

I came across some 450ohm ladderline so cut a piece approx 1m long and tried your measurements. Don't take my results too literally, I didn't redo calibration to allow for a bit of extra cable and some adaptors, but I think they should be representative of what you might expect to see.

Pics 3 & 4 show the reflection measurement with the far end open cct. Pic1 is with it in more or less open space, the phase does vary as it wobbles around a few inches away from the desk. Pic 2 is with it wound into a coil about 120mm dia., as you might do in practice as part of the aerial. It changes things significantly.

Pics 1 & 2 are likewise but for a transmission measurement of the shorted line (quite by chance pic1 shows 180deg phase shift spot on 70MHz!). With the line in the open there's a bit more difference between reflection and transmission frequency of 180deg phase that I'd have expected but I haven't revisited to double check.

Personally, I'd work with the transmission measurement as that's what you want from the finished item. Specifying it as a stub length is an easier thing to do if you're trying to adjust for the right length without  test equipment that can directly measure phase. Whichever approach you take, it's clearly important to make the measurements with the line arranged in it's final shape.

73, Steve G8GSQ

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[gordonrussell.baker]

Thanks Steve,

Your results are in the ballpark of what I'd expect. What I'm measuring is not. So I must be doing something wrong. I wonder if you can see anything wrong with my approach?

Firstly, I've got some of the leads you can see in the picture with sma on one end and crocodile clips on the other. To calibrate the vna I left the leads open for the open calibration, connected red/black for the shorted point, connected a 50 ohm load for the load and connected the two leads for the through point.

To make the measurements I connected the red leads to the near ends of the shorted piece of ladder line and connected the two black clips together.

The measurements I get are way off. I get a resonance a lot higher than I'd expect. For example a 114cm piece of ladder line has a phase shift at 79.840 MHz! And a 251cm piece of insulated wire has a phase shift at 82.720MHz.

Clearly this is wrong but why?

Any ideas?

[Mike G4CDF]

I suspect that the leads feeding the croc clips are far too long as movement of the leads will affect the impedance and hence the accuracy of the calibration.  You need to keep them very short. When I helped an amateur friend test his 160m matching circuit (high power) we found that the connecting leads were critical.

73 Mike G4CDF

[Mike G4CDF]

From memory what we did was use SMA sockets soldered to the Unit under test (UUT), then calibrated the VNA (my N2PK VNA) at the end of the SMA leads then plugged these leads into the SMA sockets mounted on the UUT.  Using this method we were able to measure the capacitance (900 pF) and Q (1000 ) raesonably accurately.  Even at 1.8 MHz it was tricky. 

[Steve Thompson]

I'd echo Mike's comments. My measurements were done with SMA leads to
BNC skt adaptors and I just pushed the ladder line wires into the BNC
centre tulip. Aim for a reliable 50ohm impedance as far as possible up
to the point where you connect to the ladder line (or whatever you're
measuring - it's a general rule to follow).

Steve G8GSQ

On 01/01/2026 16:52, 'Mike G4CDF' via Reading and District Amateur Radio

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